灾害天气

灾害天气研究进展

Advances in Research on Severe Weather

1 灾害天气监测

1 Severe weather monitoring technology

1.1 Establishment and preliminary application of the forward modeling method for Doppler spectral density of ice particles

Owing to the various shapes of ice particles,the relationships between fall velocity,backscattering crosssection,mass,and particle size are complicated.This affects the application of cloud radar Doppler spectral density data in the retrieval of the microphysical properties of ice crystals.In this study,under the assumption of six particle shape types,the relationships between particle mass,fall velocity,backscattering cross-section,and particle size were established based on existing research.Variations of Doppler spectral density with the same particle size distribution (PSD) of different ice particle types were discussed.The radar-retrieved liquid and ice PSDs,water content,and mean volume-weighted particle diameter were compared with airborne in situ observations in Xingtai,Hebei Province,China,in 2018.The results show the following:(1) For the particles with the same equivalent diameter (De),the fall velocity of the aggregates was the largest,followed by hexagonal columns,hexagonal plates,sector plates,and stellar crystals,with the ice spheres falling two to three times faster than ice crystals with the same De.Hexagonal columns had the largest backscattering crosssection,followed by stellar crystals and sector plates,and the backscattering cross-sections of hexagonal plates and the two types of aggregates were very close to those of ice spheres.(2) The width of the simulated radar Doppler spectral density generated by various ice crystal types with the same PSD was mainly affected by the particle’s falling velocity,which increased with the particle size.Turbulence had different degrees of influence on the Doppler spectrum of different ice crystals,and it also brought large errors to the PSD retrieval.(3)PSD comparisons show that each ice crystal type retrieved from the cloud radar corresponded well to aircraft observations within a certain scale range,when assuming that only a certain type of ice crystals existed in the cloud,which could fully prove the feasibility of retrieving ice PSDs from the reflectivity spectral density.(Ding Han,Liu Liping)

1.2 Improvement of X-band polarization radar melting layer recognition by the bayesian method and its impact on hydrometeor classification

Using the melting layer (ML) and non-melting layer (NML) data observed with the X-band dual linear polarization Doppler weather radar (X-POL) in Shunyi,Beijing,the reflectivity (ZH),differential reflectivity(ZDR),and correlation coefficient (CC) in the ML and NML are obtained in several stable precipitation processes.The prior probability density distributions (PDDs) of the ZH,ZDR and CC are calculated first,and then the probabilities of ZH,ZDR and CC at each radar gate are determined (PBB in the ML and PNB,in the NML) by the Bayesian method.When PBB > PNB,the gate belongs to the ML,and when PBB < PNB,the gate belongs to the NML.The ML identification results with the Bayesian method are contrasted under the conditions of the independent PDDs and joint PDDs of the ZH,ZDR and CC.The results suggest that MLs can be identified effectively,although there are slight differences between the two methods.Because the values of the polarization parameters are similar in light rain and dry snow,it is difficult for the polarization radar to distinguish them.After using the Bayesian method to identify the ML,light rain and dry snow can be effectively separated with the X-POL observed data.(Ma Jianli,Hu Zhiqun,Yang Meilin)

1.3 Integrated correction algorithm for X band dual-polarization radar reflectivity based on CINRAD/SA radar

The values of ratioaof the linear relationship between specific attenuation and specific differential phase vary significantly in convective storms as a result of resonance scattering.The best-linear-fit ratioaat X band is determined using the modified attenuation correction algorithm based on differential phase and attenuation,as well as the premise that reflectivity is unattenuated in S band radar detection.Meanwhile,the systemic reflectivity bias between the X band radar and S band radar and water layer attenuation (ZW) on the wet antenna cover of the X band radar are also considered.The good performance of the modified correction algorithm is demonstrated in a moderate rainfall event.The data were collected by four X band dual-polarization (X-POL)radar sites,namely,BJXCP,BJXFS,BJXSY,and BJXTZ,and a China’s New Generation Weather Radar(CINRAD/SA radar) site,BJSDX,in Beijing on 20 July 2016.Ratio a is calculated for each volume scan of the X band radar,with a mean value of 0.26 dB/(°) varying from 0.20 to 0.31 dB/(°).The average values of systemic reflectivity bias between the X band radar (at BJXCP,BJXFS,BJXSY,and BJXTZ) and S band radar(at BJSDX) are 0,−3,2,and 0 dB,respectively.The experimentally determinedZWis in substantial agreement with the theoretically calculated ones,and their values are an order of magnitude smaller than rain attenuation.The comparison of the modified attenuation correction algorithm and the empirical-fixed-ratio correction algorithm is further evaluated at the X-POL radar.It is shown that the modified attenuation correction algorithm in the present paper provides higher correction accuracy for rain attenuation than the empirical-fixedratio correction algorithm.(Wang Chao,Wu Chong,Liu Liping)

1.4 Method to retrieve aerosol extinction profiles and aerosol scattering phase functions with a modified CCD laser atmospheric detection system

Vertical distributions of ambient aerosols and their corresponding optical properties are crucial to the assessment of aerosol radiative effects.Traditionally,ambient aerosol phase function is assumed as a constant of input parameter in the retrieval of the vertical distribution of aerosol optical characteristics from remote sensing measurements (e.g.Lidar or camera-laser based instruments).In this work,sensitivity studies reveal that using constant aerosol phase function assumptions in the algorithm would cause large uncertainties.Therefore,an improved retrieval method was established to simultaneously measure ambient aerosol scattering phase functions and aerosol scattering function profiles with a modified charge-coupled device-laser aerosol detection system (CLADS),which are then combined to yield vertical profiles of aerosol extinction coefficients.This method was applied and evaluated in a comprehensive field campaign in the North China Plain during January 2016.The algorithm showed robust performance and was able to capture temporal variations in ambient aerosol scattering phase functions and aerosol scattering function profiles.Aerosol extinction coefficients derived with simultaneously measured aerosol phase functions agreed well with in-situ measurements,indicating that uncertainties in the retrieval of aerosol extinction vertical profiles have been significantly reduced by using the proposed method with the modified CLADS.The advantage of this modified CLADS is that it can accomplish these aerosol measurements independent of other supplementary instruments.Benefiting from its low cost and high spatial resolution (similar to 1 m on average) in the boundary layer,this measurement system can play an important role in the research of aerosol vertical distributions and its impacts on environmental and climatic studies.(Bian Yuxuan,Xu Wanyun,Hu Yaqiong)

1.5 Operational evaluation of the quantitative precipitation estimation by a CINRAD-SA dual polarization radar system

In this paper,a quantitative precipitation estimation based on the hydrometeor classification (HCA-QPE)algorithm was proposed for the first operational S band dual-polarization radar upgraded from the CINRAD/SA radar of China.The HCA-QPE algorithm,localized the Colorado State University-Hydrometeor Identification of Rainfall (CSU-HIDRO) algorithm,the Joint Polarization Experiment (JPOLE) algorithm,and the dynamic Z-R relationships based on variational correction QPE (DRVC-QPE) algorithm were evaluated with the rainfall events from March 1 to October 30,2017 in Guangdong Province.The results indicat that even though the HCA-QPE algorithm did not use the observed rainfall data for correction,its estimation accuracy was better than that of the DRVC-QPE algorithm when the rainfall rate was greater than 5 mm h−1; and the stronger the rainfall intensity,the greater the QPE improvement.Besides,the HCA-QPE algorithm worked better than the localized CSU-HIDRO and JPOLE algorithms.This study preliminarily evaluated the improved accuracy of QPE by a dual-polarization radar system modified from CINRAD-SA radar.(Chen Chao,Liu Liping,Hu Sheng)

1.6 Performance of a radar mosaic quantitative precipitation estimation algorithm based on a new data quality index for the Chinese polarimetric radars

The quality of radar data is crucial for its application.In particular,before radar mosaic and quantitative precipitation estimation (QPE) can be conducted,it is necessary to know the quality of polarimetric parameters.The parameters include the horizontal reflectivity factor,ZH; the differential reflectivity factor,ZDR; the specific differential phase,K-DP; and the correlation coefficient,rho (HV).A novel radar data quality index (RQI) is specifically developed for the Chinese polarimetric radars.Not only the influences of partial beam blockages and bright band upon radar data quality,but also those of bright band correction performance,signal-to-noise ratio,and non-precipitation echoes are considered in the index.RQI can quantitatively describe the quality of various polarimetric parameters.A new radar mosaic QPE algorithm based on RQI is presented in this study,which can be used in different regions with the default values adjusted according to the characteristics of local radar.RQI in this algorithm is widely used for high-quality polarimetric radar data screening and mosaic data merging.Bright band correction is also performed to errors of polarimetric parameters caused by melting ice particles for warm seasons in this algorithm.This algorithm is validated by using nine rainfall events in Guangdong Province,China.Major conclusions are as follows:ZHZDR,and K-DP in bright band become closer to those under bright band after correction than before.However,the influence of K-DP correction upon QPE is not as good as that ofZHandZDRcorrection in bright band.OnlyZHandZDRare used to estimate precipitation in the bright band affected area.The new mosaic QPE algorithm can improve QPE performances not only in the beam blocked areas and the bright band affected area,which are far from radars,but also in areas close to the two radars.The sensitivity tests show the new algorithm can perform well and stably for any type of precipitation occurred in warm seasons.This algorithm lays a foundation for regional polarimetric radar mosaic precipitation estimation in China.(Zhang Yang,Liu Liping,Wen Hao)

1.7 Statistical analysis of microphysical and dynamical parameters for clouds and precipitation over Naqu Tibetan Plateau in summertime using Ka-band cloud radar

A Chinese Ka-band solid-state transmitter cloud radar (CR) was employed to investigate clouds and precipitation over the Tibetan Plateau (TP) in summer 2014.After performing quality control and merging the output from the three work modes of CR,the Doppler spectral density data were analyzed to estimate the vertical air motion (Vair) and particle fall speed (Vfall) in clouds.The reproduced radar reflectivity was employed to retrieve the ice water content (IWC) and ice effective radius (Re).The cloud type classification algorithm was developed to classify cloud conditions into eight different categories.The vertical and daily variations of reflectivity,Vair,Vfall,IWC,andRewere then analyzed,and as a result,four conclusions were formulated.First,the clouds with reflectivity less than −10 dBz were mainly located above 7 km,and the precipitable particles were formed below this layer (rain echo top) due to the abundant supercooled water therein.Second,the weak updraft in the range of 2−4 km caused the high occurrence of weak clouds during 04:00−12:00 Beijing local time (BT),and the rain echo top did not exceed 7 km due to the existing downdraft in 4−6 km.After 14:00 BT,convective clouds quickly developed,and the averaged updraft,reflectivity,and the echo top notably increased.Third,particular cloud types commonly exhibited weak reflectivity,low IWC,and less vertical variations,while the others more obvious vertical variations with larger IWC andRe.Last,compared with the radar sensitivity and range from radar,Vairbiases that were introduced by the Doppler spectra broadening due to turbulence,wind shear,and radar beam width,could be neglected.(Liu Liping,Gao Wenhua)

1.8 Uncertainties in retrieving microphysical properties of rain profiles using ground-based dualfrequency radar

Dual-frequency radar (DFR) could improve the accuracy of estimating the microphysical properties of rain profiles.Unfortunately,factors that cause inaccuracies in retrieved rain profiles include uncertainties in raindrop size distribution (DSD) parameterizations,DFR retrieval methods and radar measurement errors.The primary objective of this study was to assess the uncertainties in retrieving rain profiles to offer insight into application considerations for ground-based DFRs in China.The uncertainties caused by DSD models are assessed by comparing attenuation coefficients and rain rates from DFR algorithms with those directly derived from DSD spectra from disdrometers.Then,based on a DSD model,the impacts of retrieval methods,radar range resolution,measurement error and errors in temperature on DFR retrievals are explored by comparing the rain profiles obtained using DFR algorithms with those directly output from the Weather Research and Forecasting model.Overall,the impact of measurement error on the DFR retrievals is relatively significant for both the forward retrieval method and the iterative backward retrieval method and should be eliminated in practical applications.At an individual uncertainty level,the impact of DSD parameterizations on the retrievals from the dual-frequency technique is less than ± 10% bias when the shape factorμof the gamma distribution ranges from 2 to 4.The retrieved rain profiles from the forward retrieval method are relatively sensitive to radar range resolution and the retrieval performance can be improved with increasing the range resolution,while the iterative backward retrieval method exhibits stable retrieval performance.The impact of temperature errors on the retrieved rain profiles is negligible.(Wang Gaili,Liu Shengnan,Liu Liping)

1.9 Vertical structure and dynamical properties during snow events in middle latitudes of China from observations by the C-band vertically pointing radar

Data from the continuous observations of four shallow snow events (echo top 8 km) and two deep events( 10 km) were obtained using the C-band vertically pointing radar with frequency-modulation continuouswave technology with extremely high resolution during the winter of 2015−2016 in middle latitudes of China.Snow-generating cells (GCs) were found near the cloud top in each event.Reflectivity (Z),radial velocity,and the vertical gradients ofZ(dZ/dh,wherehis the vertical distance) andVr(dVr/dh) showed different vertical distribution characteristics between the upper GC and lower stratiform regions (St regions).Fall streaks (FSs)associated with GCs were embedded in the St regions.In the deep events,the proportions of GC regions were slightly larger,but the average contributions to the growth ofZ(33%) were lower than those in the shallow events (42%).The average dZ/dhvalues were usually two to three times larger inside GCs and FSs compared to outside.Bimodal Doppler spectra were used to establish the relationships betweenZand the reflectivityweighted particle fall speed (Vz) for the two regions.The vertical air velocity (Wa) andVzwere then retrieved,and the results show that both the updraft and the downdraft were alternately observed in GC regions.GC locations were usually accompanied by strong upward air motion,with average speeds mostly distributed around 1.2 m s−1,whereas downward air motion often appeared between GCs.In the St regions,the speeds ofWawere mainly within 0.5 m s−1.The upper areas of the St regions consisted primarily of weak upward motion,whereas weak downward motion dominated the lower areas.There was no apparent difference inWainside and outside the FSs.The averageVzwas slightly larger inside GCs and FSs compared to outside,with a difference of 0.1−0.3 m and 0.2−0.4 m s−1,respectively.(Cui Ye,Ruan Zheng,Wei Ming)

1.10 DSD参数对双频雷达估测降水的影响研究

双频雷达通过估计降水的雨滴谱分布（Rain Drop Size Distribution，DSD）参数来估测降水，相比多普勒天气雷达利用Z-I关系估测降水的方法，估测结果更加准确。真实的雨滴谱分布随时空不断变化，难以准确描述。本研究的目的是评估将其描述成Gamma分布模型对双频雷达估测降水造成的误差。研究中利用架设在广东龙门和西藏那曲的雨滴谱仪观测资料，计算Ku/Ka双频雷达的反射率因子，利用双频雷达算法反演降水率R和衰减系数k，并将其与雨滴谱仪直接计算的结果进行比较，探讨双频雷达反演算法中DSD模型对反演结果造成的影响，并对比反演误差的地区差异性。研究结果表明：DSD参数对双频雷达反演降水的影响在那曲和龙门地区有一定的差异。从双频技术反演的质量加权的粒子直径(Dm)、Ka波长衰减系数（k_Ka）、波长衰减系数（k_Ku）和降水率（R）来看，当Gamma分布的形状因子μ分别取2、3和4时，龙门地区对四个参数的反演都能得到较为稳定准确的结果，相对误差基本 10%。而就Ka和5～40 mm/h的降水率而言，那曲和龙门有很大的不同，在龙门当μ取3时相对误差最小，都在0值附近波动，而那曲则要需要μ值在4和6附近才能使得相对误差达到最小。(刘胜男，王改利)

1.11 Ka/Ku双波段云雷达探测云降水滴谱和空气垂直运动速度的能力模拟分析

由于湍流、雷达探测灵敏度等对单波长云雷达探测回波强度谱密度的影响，造成了云雷达探测空气垂直运动速度和雨滴谱的误差，而双波长云雷达利用Mie散射造成的不同粒子后向散射大小差异来提高空气上升速度探测精度，从而提高反演雨滴谱的能力，并且可提高订正雨区衰减的能力。为此中国气象科学研究院研发了Ka/Ku双波段云雷达，并于2019年4月开始在广东龙门进行了云降水观测。本文针对该双波段云雷达观测模式和灵敏度等参数，在Gamma滴谱假设条件下，模拟分析了Ka、Ku波段功率谱及其比值与云降水参数、温度和湍流的关系，研究了雷达灵敏度、湍流对空气垂直速度、雨滴谱反演和衰减订正的影响，并利用个例数据进行了风场反演试验，讨论了双波段探测微降水动力和微物理参数的优势。结果表明：（1）温度只能影响两个波段功率谱比值(Ratio)的大小，对其峰值位置基本没有影响，而湍流对其峰值位置的影响不超过0.5 m/s；（2）湍流、雷达灵敏度对单波段云雷达探测空气垂直速度的影响比较明显，湍流使空气上升速度被高估，雷达最小可测回波强度随高度的增加而增加使该参数被低估，其影响远远大于温度和湍流对双波段云雷达反演空气垂直速度的影响；（3）对于单波段雷达来说，雷达灵敏度和湍流明显影响雨滴谱、含水量和衰减系数的探测，湍流使得雨滴谱拓宽，低估含水量和衰减系数；而雷达灵敏度却使反演的雨滴谱变窄，增加小粒子数浓度，并高估了含水量和衰减系数；（4）选取2019年4月15—16日的个例进行空气上升速度的反演，并与模拟分析的结果进行对比。结果显示实际观测数据反演的空气上升速度与模拟分析结果中的趋势较为一致。这项工作为单波段和双波段云雷达的多普勒功率谱数据分析和云降水微物理和动力参数的反演可提供参考。(郑晨雨，刘黎平)

1.12 双偏振雷达水凝物相态识别算法的参数改进

双偏振雷达的水凝物相态识别算法基于模糊逻辑方法建立，针对方法的可靠性和稳定性问题，利用2016—2017年暖季广州S波段双偏振雷达数据，从3个方面找出影响识别效果的关键因素并改进。使用模糊逻辑的累加值为检验依据，找出不合理的模糊规则，通过相态特征统计和权重矩阵修改加以改进。使用误差敏感性检验法系统，评估误差对识别效果的影响，发现反射率因子的误差在−0.5～+0.5 dBz、差分反射率因子的误差在−0.1～+0.1 dB、雷达相关系数的误差在0～0.02、差分相移率的误差在−0.3～+0.9 dB的范围内，识别结果稳定性较好。此外，相态时空分布统计中发现底层冰雹面积异常增加，通过空间一致性检验可订正异常结果。(徐舒扬，吴翀，刘黎平)

2 青藏高原天气研究

2 Research on weather over the Tibetan Plateau

2.1 第二次青藏高原综合科学考察研究

根据第二次青藏高原综合科学考察研究任务一“西风—季风协同作用及其影响”年度计划，中国气象科学研究院灾害天气国家重点实验室为牵头单位，2020年任务一组织6个专题的科学家、专家组成的科研团队，开展了数百人次科学考察与观测试验，其中包括：（1）冰川、黄土、湖泊历史演变环境指标多类型综合科学考察：考察队进入巴基斯坦喀喇昆仑、三江源冰川、西昆仑区结则茶卡冰前湖、亚洲水塔区冰川地貌与湖泊、天山—帕米尔黄土区域，高质量地完成历史演变环境指标多类型（石笋、树轮、冰芯钻取等）科学考察任务；（2）藏东南区域飞机云降水过程探测试验：在三江源、祁连山与藏东南等地组织云降水过程飞机综合观测，获取复杂地形下气溶胶、云、降水宏微物理特征飞行探测及无人机组网观测；实施了地面雷达、探空火箭、自动站、降水现象仪、微波辐射计等多种观测设备的综合观测；（3）构建多圈层地气相互作用立体综合观测试验网络：在六个关键区（河湖源区、喜马拉雅山区中段、江湖源区、中巴经济走廊、藏东南区、三江流域及横断山区）实施西风—季风区多圈层地气相互作用立体综合观测组网建设级观测（包括大气边界层塔、双波段闪烁仪、风温廓线仪组网、微波辐射计、探空组网、风吹雪、土壤温湿度组网观测等）；（4）西风—季风影响下多尺度地形特征——对流结构观测：在墨脱、珠峰、雅鲁藏布江河谷区等水汽输送通道关键区布设了相控阵雷达超级站、云雷达等设备，结合卫星遥感与探空、微波辐射计、边界层通量站、GPS水汽观测仪和自动气象站等，初步构成了雅鲁藏布江河谷区天—地—空—体化水汽输送与云降水及水汽输送通道的综合观测系统；（5）青藏高原气候变化影响关键区科学考察：在天山—帕米尔高原等地区共开展十余次野外科学考察；科考船完成1次印度洋科考计划；（6）青藏高原环境生态综合科考：2020年共派出17支科考分队约160人次，开展西风—季风协同作用下青藏高原亚洲水塔区和祁连山—阿尔金区水环境、植被环境、碳氮循环和大气环境的野外考察。

基于上述综合与立体观测考察研究，评估了青藏高原及周边地区过去和未来的气候变化，发现西风与季风协同作用下青藏高原冰川历史演化规律，揭示了东亚夏季风在中世纪—小冰期过渡期快速衰退现象，并探讨了过去2万年青藏高原及其毗邻地区冰川演化规律及可能机制，其中“喀喇昆仑冰川科考与冰芯钻取”成功入选中国科学院2020年第2季度科技创新亮点成果；研究了近代冰川变化与降水变率、水储量变化格局的关联性；获得了藏东南飞机与地面综合探测三维立体云物理观测数据集；提出极端天气气候变异与生态环境影响效应，高原西风减弱或北移对青藏高原极端天气气候灾害具有决定性作用；首次定量给出青藏高原湖泊年蒸发总量，揭示出高原湖泊群在大气水分循环过程中蒸发的巨大贡献，揭示了亚洲水塔热力驱动下青藏高原水汽输送“窗口”全球效应；首次评估了青藏高原对流尺度模拟的降水及其对积雪的影响；研究高原科考关键区对流云高分辨率结构动态演变特征及其特殊规律，揭示出“世界屋脊”空气低密度条件对高原对流云的触发效应，加深了对冰川与边界层大气间湍流的动量及热量输送的认知；揭示出西风—季风协同作用变化对植被环境趋好的影响效应。上述研究成果为川藏铁路工程提供数值预报新技术与决策服务。(徐祥德，徐柏青，马耀明，陆日宇，郭学良，周广胜，马伟强，张胜军)

2.2 Climate modulation of summer rainstorm activity in eastern China based on the Tibetan Plateau spring heating

Rainstorm intensity and frequency in the eastern China are closely associated with the Asian summer monsoon,one of the largest and most pronounced circulation systems on Earth.Changes in monsoon strength are linked to mechanical and thermal forcing by the Tibetan Plateau (TP),which,as an extensive elevated landmass,forms a vast heat source that protrudes into the free atmosphere.However,the impact of TP heating variability on rainstorm behavior in the eastern China remains unclear.By analyzing 50 years of climate data,significant interannual variability was identified in the occurrence of rainstorms (precipitation amounts of 50 mm),with decreased trend in the northern China and increased variability in the southern China.Results show that these changes are closely related with the springtime TP heating,indicating that recent changes in rainstorm distributions in the eastern China are consistent with interdecadal variability in TP thermal forcing.(Huang Wubin,Zhao Yang,Sun Chan)

2.3 Enhancement of the summer extreme precipitation over North China by interactions between moisture convergence and topographic settings

This study investigates the roles of atmospheric moisture transport under the influence of topography for summer extreme precipitation over North China (NC) during 1979−2016.Based on the rain gauge precipitation reanalysis and data,38 extreme precipitation days in NC during the 38 years were selected and associated moisture fluxes were estimated.The results show that there is a dominant moisture influx of 311.8 kg m−1s−1into NC along its southern boundary from tropical oceans,and a secondary influx of 107.9 kg m−1s−1across its western boundary carried by mid-latitude westerlies.The outflux across the eastern boundary is 206.9 kg m−1s−1and across the northern boundary is 76.0 kg m−1s−1,giving a net moisture gain over NC of 136.8 kg m−1s−1.During extreme precipitation days,the moisture flux convergence (MFC) was much larger,exceeding 4×10−5kg m−1s−1.The MFC maximum core,the pronounced moisture transport,and the striking extreme precipitation zone over NC are all anchored to the east of the steep slopes of the surrounding topography.Moreover,a remarkably high humidity and strong upward motion also occur near steep slopes,indicating the critical role of the adjacent topography on the extreme precipitations.Simulations with and without the topography in NC using the Weather and Research Forecasting model for the six selected out of the 38 extreme precipitation days demonstrate that the surrounding topography reinforces the MFC over NC by 16% relative to the case without terrain,primarily through enhanced wind convergence and higher moisture content,as well as stronger vertical motion induced by diabatic heating.The interactions between moisture convergence and topographic settings strengthen the extreme precipitation over NC.(Zhao Yang,Chen Deliang,Li Jiao)

2.4 Impact of “blocking” structure in the troposphere on the wintertime persistent heavy air pollution in northern China

In the winters of 2012−2018,a total of 25 “ultra-long” (≥6 days),wide-ranging pollution events occurred in the northern China.The results show that the atmospheric circulation pattern corresponding to 62.5% of the persistent “most serious” and “more serious” air pollution events in the northern China were the blocking structures,and that 43.75% of the 500-hPa atmospheric circulation anomalies in the middle and high latitudes of Eurasia were “dual-blocking”,18.75% of them were “single-blocking”.The abnormally stable blocking situation provided a special circulation background for the occurrence and maintenance of persistent heavy air pollution in the northern China.The Okhotsk blocking is significantly positively correlated with the persistent “most serious” air pollution events.“Stagnation” of the blocking system and its dynamic effect play an important role in regulating atmospheric environmental capacity and accelerating the accumulation of aerosols during the persistent heavy pollution episodes.Due to the synergy between the weak wind effect of the leeward slope on the eastern side of the Loess Plateau in this region and the downward airflow of the largescale blocking system,the effect of sustained suppression of atmospheric pollutant diffusion in the northern China is more significant.The downward air flow along the eastern leeward slope of the Loess Plateau is very important for accumulation of air pollutants,which is controlled by the tropospheric blocking high.In addition,the “subsidence (temperature) inversion” effect produced by the synergy between the downward airflow of the eastern leeward slope of the Loess Plateau and the large-scale blocking system creates a continuous and stable“warm-cover” structure in the middle of the troposphere on the eastern Loess Plateau; this effect strengthens the radiation effect of aerosols in the atmospheric pollutants,as well as the “two-way feedback” mechanism between adverse meteorological conditions in the boundary layer and atmospheric pollutants.(Cai Wenyue,Xu Xiangde,Cheng Xinghong)

2.5 Influence of satellite observation angle to tropical cyclone intensity estimation using the deviation angle variance technique

The deviation angle variance (DAV) method was developed to objectively estimate tropical cyclone(TC) intensity from geostationary infrared (IR) brightness temperature data.Here,we demonstrate that improvements of 25% root mean square error (RMSE) in major hurricane intensity estimation (relative to best track) can be obtained by considering the pixel-by-pixel satellite view angle in the estimation.Using data from the Chinese Fengyun 2E and 2F satellites for Super Typhoon Soudelor (2015),we demonstrate how the satellite observation angle can reduce the accuracy of intensity estimation,especially for the strongest TCs.Based on these results,an improved DAV estimator is developed using 12-year (2004−2015) Geostationary Operational Environmental Satellite (GOES)-East satellite IR images over the North Atlantic basin.(Hu Liang,Ritchie Elizabeth A,Tyo J Scott)

2.6 Influences of atmospheric ventilation on the composition of the upper troposphere and lower stratosphere during the two primary modes of the South Asia high

There are two key modes of the South Asia high (SAH) during the boreal summer:the Iranian Plateau (IP)and Tibetan Plateau (TP) modes.The anomalies of chemical constituents in the upper troposphere and lower stratosphere (UTLS) region within the SAH area largely depend on this bimodality.To better understand the underlying mechanisms of this dependence,the ensemble of 30-day backward trajectories,initialized in the UTLS region within the SAH,was simulated by a Lagrangian model FLEXPART.The comparative diagnostic was performed from the perspective of atmospheric ventilation.The results show that vertical transport from the lower troposphere to the UTLS during the TP mode was very efficient,resulting in tropospheric air mass transported into the UTLS within a shorter timescale than during the IP mode.Furthermore,the effect of SAH isolation during the TP mode was stronger than during the IP mode.This stronger trapping is likely to force the tropospheric air mass to reside in the SAH area for a longer period of time.In addition,compared to the IP mode,near-surface air mass sources during the TP mode overlapped more with areas of severe air pollution (CO emissions).The above three factors associated with the processes of atmospheric ventilation,i.e.,the efficiency of vertical transport,the strength of the SAH isolation,and the boundary layer sources,provide potential explanations as to why the anomalies of atmospheric constituents in the UTLS are different between the TP and the IP mode.(Yang Shuai,Wei Zhang,Chen Bin)

2.7 Remote moisture sources for 6-hour summer precipitation over the southeastern Tibetan Plateau and its effects on precipitation intensity

Using the Lagrangian moisture tracking methodology for 37-year reanalysis data,this study presents a quantitative climatology of remote moisture sources for 6-hour summer precipitation over the southeastern Tibetan Plateau (SETP) during the boreal summer for the 1980−2016 period.The connections between moisture sources and regional-scale precipitation intensity are also explored.Different from previous viewpoints,this study finds that the terrestrial and oceanic sources contribute approximately 70% and 30% to the moisture supply for the summer precipitation over the SETP,with the Indian continent and Arabian Sea acting as the most important providers,respectively.Enhanced regional-scale precipitation,particularly for extreme events,can largely be further attributed to the increased moisture uptake originating from these two predominant sources with transport time scales of 2−5 days prior to the onset of precipitation.The intensified precipitation events are more likely to be associated with the enhanced water vapor transport conveyed by the large-scale atmospheric circulations,together with the land evaporation over the remote source regions.(Yang Shuai,Zhang Wei,Chen Bin)

2.8 Roles of oceanic moisture exports in modulating summer rainfall over the middle-lower Yangtze River Basin:Inter-annual variability and decadal transition

Precipitation over the mid-lower reaches of the Yangtze River Basin (YRB),which is a typical East Asian monsoon region in China,is strongly impacted by oceanic moisture exports (OMEs) and feature variability at multiple spatial-temporal scales,resulting in frequent floods and droughts.We focus on two key issues of OME and its relevance to the summer rainfall variability over the YRB:determination of the individual contributions of OME from three specific oceanic sub-regions:the Arabian Sea (ARS),Bay of Bengal (BOB),and South China Sea (SCS),and their roles in the inter-annual variability and decadal changes in YRB summer rainfall.Using a Lagrangian forward trajectories tracing approach,we create a catalogue of OME-based diagnosed precipitation fields for three sub-regions spanning the summer seasons (May to August) of 1980−2013 with 6-hour resolution using ERA-Interim data.The results show that the pattern of the diagnosed OME-based precipitation resembles the observed pattern over the YRB in terms of climatology and temporal variation.Climatologically,the SCS region accounts for the largest relative contribution (60.7%),followed by the ARS(30.8%),whereas the contributions of the BOB (8.5%) are almost negligible.On the inter-annual scale,the variation in OME-based rainfall originated from the ARS,and the BOB is out of phase with that of the SCS.In contrast to climatology,the inter-annual changes in rainfall over the YRB are primarily modulated by the variation in the OME from the ARS.However,for the decadal changes,particularly in 1992/1993 over the YRB,the SCS and ARS are equivalent contributors to the summer precipitation over the YRB.These findings thus provide new insights into our understanding of the multi-scale variability of summer precipitation over the YRB region.(Chen Bin,Zhang Wei,Yang Shuai)

2.9 Short-term tropical cyclone intensity forecasting from satellite imagery based on the deviation angle variance technique

The deviation angle variance (DAV) is a parameter that characterizes the level of organization of a cloud cluster compared with a perfectly axisymmetric tropical cyclone (TC) using satellite infrared (IR) imagery,and can be used to estimate the intensity of the TC.In this study,the DAV technique is further used to analyze the relationship between satellite imagery and TC future intensity over the North Atlantic basin.The results show that the DAV of the TC changes ahead of the TC intensity change,and this can be used to predict short-term TC intensity.The DAV-IR 24-h forecast is close to the National Hurricane Center (NHC) 24-h forecast,and the bias is lower than those of NHC and other methods during weakening periods.Furthermore,an improved TC intensity forecast is obtained by incorporating all four satellite bands.Using SST and TC latitude as the other two predictors in a linear regression model,the RMSE and MAE of the DAV 24-h forecast are 13.7 and 10.9 kt (1 kt approximate to 0.51 m s−1),respectively,and the skill space of the DAV is about 5.5% relative to the Statistical Hurricane Intensity Forecast model with inland decay (Decay-SHIFOR) during TC weakening periods.Considering the DAV is an independent intensity technique,it could potentially add value as a member of the suite of operational intensity forecast techniques,especially during TC weakening periods.(Hu Liang,Ritchie Elizabeth A,Tyo J Scott)

2.10 Why are there more summer afternoon low clouds over the Tibetan Plateau compared to eastern China?

In this study,we analyze the relationships between summer afternoon low cloud cover and environmental conditions over the Tibetan Plateau (TP).Using in situ measurements,satellite data,and reanalysis,and based on theoretical analysis,we find that there is stronger thermal turbulence,lower temperature,and higher frequency of low cloud formation for the same surface relative humidity over the eastern and central TP compared with the eastern China.With the same sensible heat flux,the decreased air density enhances buoyancy flux,which increases the planetary boundary layer height and moisture vertical transport.At the same time,with the same near-surface relative humidity,the lower temperature over the TP decreases the lifting condensation level,which increases the probability of the air parcel reaching this level.Compared to the low-elevation region in the eastern China,these two mechanisms enhance low cloud occurrence in the afternoon over the TP.(Wang Yinjun,Zeng Xubin,Xu Xiangde)

2.11 1961—2016年秦岭山区冷季积雪日数变化特征及其影响因子

根据1961—2016年秦岭地区32个气象站点的气温、降水及积雪等相关数据，运用REOF、M-K检验和小波分析等方法，对秦岭地区冷季积雪日数的时空变化和影响因子进行分析。结果表明:秦岭地区冷季多年平均积雪日数表现为北坡比南坡积雪日数多。在全球气候变暖的背景下，海拔越高积雪日数减少得越多。秦岭冷季积雪日数呈现显著减少的趋势，5个区的积雪日数年代际变化特征显著，在20世纪末到21世纪初发生了由积雪日数偏多到偏少的突变。各区冷季积雪日数的周期变化主要集中在10～20 a，秦岭南坡同时也显示了较为明显的4 a左右的周期变化。西北太平洋海温阶段性增暖是导致秦岭冷季积雪日数减少的外强迫因素。秦岭地区冷季平均气温的显著增暖和冷季降水量的显著减少直接造成积雪日数的减少。秦岭冷季积雪日数减少的突变要比气温增暖的突变滞后4～7 a。(李茜，魏凤英，雷向杰)

2.12 2013—2017年气象条件变化对中国重点地区PM2.5质量浓度下降的影响

2013ü 2017年中国出台《大气污染防治行动计划》（简称“大气十条”），实施了系列污染减排措施，重点地区PM2.5质量浓度下降明显，这其中气象条件变化起到了多大作用，是政府和公众特别关心的问题。本研究基于各类气象要素观测、诊断、结合污染—气象条件指数等对PM2.5污染影响深入分析，发现“大气十条”实施后的2014ü 2015年中国重点地区气象条件相较2013年变差，2016和2017年气象条件相较转好。但在京津冀地区2017年相较2013年PM2.5质量浓度下降的39.6%中，仅有约5%（约占总PM2.5降幅的13%）是来自气象条件转好的贡献；在长江三角洲地区下降的34.3%中，有约7%（约占总PM2.5降幅的20%）是来自气象条件转好的贡献，由于气象条件改善程度明显低于此区域观测到的PM2.5降幅，显示出“大气十条”实施五年减排仍然发挥了PM2.5污染改善的主导作用，天气和气候变化因素虽有影响但没有起到控制性作用（本研究用PLAM指数来量化气象条件变好或变差）。在珠江三角洲地区，气象条件对2017年相较2013年的年均PM2.5浓度下降影响较弱，下降成效也主要来自减排的贡献。2017年冬季气象条件在京津冀和长江三角洲区域相较2013年分别转好约20%和30%，在两区域冬季PM2.5分别约40.2%和38.2%的降幅中起到了明显的“助推”作用。京津冀区域2016年冬季气象条件好于2017年冬季约14%，但2017年冬季PM2.5降幅仍大于2016年，显示出2017年更大力度的减排措施发挥了重要作用；在北京冬季持续性重污染期间选择气象条件相同的过程对比，也发现因减排导致的PM2.5下降幅度逐年增加，特别是2016和2017年下降的PM2.5浓度幅度更为明显，表明“大气十条”实施5年后空气质量改善的根本原因是各项控制措施取得了实质性进展，特别是2017年冬季污染物排放量得到了有效削减。中国大气PM2.5持续性重污染主要发生在冬季，冬季京津冀地区仅因气象条件不利就会导致PM2.5浓度较其他季节上升约40%～100%，这与冬季到达地面的太阳辐射下降有关，与中国华北冬季受青藏高原大地形“背风坡”效应所导致的下沉气流和“弱风效应”有关，与气候变暖导致的区域边界层结构日趋稳定有关。重污染形成是因为区域出现停滞—静稳的形势，高空环流型主要可分为平直西风和高压脊型，污染形成后不断累积的PM2.5污染还会进一步导致边界层气象条件转差、转差气象条件的反馈作用控制了PM2.5的“爆发性增长”现象，形成显著的不利气象条件与PM2.5累积之间的双向反馈。表明在中国现今大气气溶胶污染程度仍然居高的情况下，不利气象条件是持续性重污染形成、累积的必要外部条件。在重污染形成初期大幅降低区域污染排放，是消除和减少持续性重污染事件的关键手段。即使在有利气象条件下，也不宜无限制地允许排放，因为当污染累积到一定程度后会显著改变边界层气象条件、会“关闭”污染扩散的“气象通道”。（张小曳，徐祥德，丁一汇）

2.13 基于加密探空观测的成都市一次重霾污染过程中大气边界层气溶胶垂直结构分析

基于2017年1月4—7日成都市一次重霾污染过程的系留汽艇探测的低层大气气象要素和大气颗粒物垂直探空加密观测资料，分析了大气边界层结构及气溶胶垂直分布。结果表明，此次重霾污染期间，大气边界层昼夜变化特征基本消失。稳定边界层结构出现25次，对流边界层结构仅出现3次，大气边界层结构趋于稳定，边界层高度普遍在700 m以下。霾污染发生、维持及消散阶段大气边界层气溶胶垂直结构具有明显差异。霾污染发生阶段，大气边界层气溶胶粗、细粒子主要集中在300 m高度以下，近地面层大气气溶胶粒子累积触发霾污染事件；霾维持阶段，大气颗粒物粒子浓度数垂直方向趋于一致，大气边界层稳定结构中存在强的大气垂直混合作用；在霾消散阶段，较高处的气溶胶粒子浓度最先下降，且下降幅度最大，表明对流层自由大气作用对霾污染消散具有影响。大气边界层风速的增大加剧了大气传输扩散。温度与大气颗粒物浓度在近地层呈负相关关系，在100 m高度以上呈正相关关系。大气边界层低层偏冷、高层偏暖的稳定大气热力层结减弱了大气污染物的垂直扩散。相对湿度的增加有利于气溶胶粒子的吸湿增长和液相化学反应，加剧了霾污染。(曹蔚，赵天良，徐祥德)

2.14 台风“温比亚”(1818)影响辽东半岛的预报分析

2018年第18号台风“温比亚”北上引发辽东半岛普降大暴雨，局部特大暴雨，但业务数值预报模式在其路径、强度和降水预报方面均有一定偏差。利用中国气象局热带气旋年鉴、常规和非常规气象观测资料、FY-2G卫星云顶亮温（TBB）和欧洲中期数值预报中心ERA-Interim全球再分析资料(0.125°×0.125°)，对台风“温比亚”影响辽东半岛的预报进行分析。结果表明：（1）“温比亚”预报的难点是登陆后转向点及转向后路径的预报，西风槽和大陆高压东移阻挡了“温比亚”的西行;台风“苏力”的西北移，导致副高位置偏北，其与东北地区高压脊形成的高压带则有利于“温比亚”的北抬。（2）“温比亚”和“苏力”2个台风与副高之间所形成的东南风低空急流，提供了持续的水汽和能量，既有利于“温比亚”强度的维持，又诱发辽东半岛强降水的持续发生。（3）“温比亚”在变性过程中与西风槽以及低空急流相互作用有利于其北侧螺旋云系的发展。强降水落区与台风低层环流北侧辐合带内冷暖平流活动密切相关，冷暖平流交汇处的能量锋带对强降水有较好的指示作用。（4）数值预报模式对转向点和转向后的路径预报存在较大分歧，除了参考集合预报产品外，还可采用相似预报手段，对比分析相似个例和误差小的数值预报模式的大尺度环境场，借助于数值预报产品和相似个例进行订正。(梁军，冯呈呈，张胜军)

2.15 影响我国霾天气的多尺度过程

频发的霾天气是我国现阶段面临的最主要大气环境问题之一。霾期间高浓度大气细颗粒物（PM2.5）是多种物理化学过程综合影响的结果，包括排放、气—粒转化、大气边界层、局地环流、天气与气候等过程。上述过程的时空尺度跨越了几个数量级，在空间尺度上涵盖了纳米尺度至上千千米尺度。多尺度过程本身的复杂性以及不同过程之间的相互影响是目前大气环境领域面临的最严峻挑战，直接影响到对于霾天气形成机制的科学认识、预报技术与数值模式研发，以及相应的大气污染治理。文章综述了在影响我国霾天气的多尺度过程及其与气溶胶的相互作用领域取得的研究进展。研究表明：二次气溶胶已经成为我国大气气溶胶的主要部分，在霾过程后期，液相非均相过程对气—粒转化有重要贡献；PM2.5呈现多时间尺度周期性振荡，包括1，4～7 d以及40～60 d等，边界层、天气和气候等多尺度过程是造成上述周期性变化的主因；已有证据表明，我国高气溶胶已经影响到该区域大气光化学、大气边界层，甚至天气和气候过程。气溶胶与上述过程的相互作用进一步影响了气溶胶浓度及其空间分布，但是此问题极为复杂，尚存在很大不确定性。为此，今后需重点加强以下研究：加强包含气溶胶理化性质、大气光化学、气象要素在内的多要素协同观测，重点开展对流层内多要素协同垂直探测；增强跨学科领域研究，尤其是大气物理—大气化学—天气/气候等多学科间的交叉性研究；加强气溶胶与大气化学、边界层、天气气候等过程相互作用的数值模拟研究。(权建农，徐祥德，贾星灿)

3 暴雨和强对流研究

3 Research on heavy rainfall and strong convection

3.1 A dynamical-statistical-analog ensemble forecast model:Theory and an application to heavy rainfall forecasts of landfalling tropical cyclones

Combining dynamical models with statistical algorithms is an important way to improve weather and climate prediction.In this study,a concept of a perfect model,whose solutions are from observations,is introduced,and a dynamical-statistical-analog ensemble forecast (DSAEF) model is developed as an initialvalue problem of the perfect model.This new analog-based forecast model consists of the following three steps:(i) construct generalized initial value (GIV),(ii) identify analogs from historical observations,and (iii)produce an ensemble of predictands.The first step includes all appropriate variables,not only at an instant state but also during their temporal evolution,which plays an important role in determining the accuracy of each predictand.An application of the DSAEF model is illustrated through the prediction of accumulated rainfall associated with 21 landfalling typhoons occurring over South China during the years of 2012−2016.Assuming a reliable forecast of landfalling typhoon track,two different experiments are conducted,in which the GIV is constructed by including:(i) typhoon track only; (ii) both typhoon track and landfall season.Results show overall better performance in the second experiment than in the first one in predicting heavy accumulated rainfall in the training sample tests.In addition,the forecast performance of both experiments is comparable to the operational numerical weather prediction models currently used in China,the United States,and Europe.Some limitations and future improvements as well as comparisons with some existing analog ensemble models are also discussed.(Ren Fumin,Ding Chenchen,Zhang Da-lin)

3.2 An application of the LTP_DSEF model to heavy precipitation forecasts of landfalling tropical cyclones over China in 2018

Recently,a track-similarity-based Dynamical-Statistical Ensemble Forecast (LTP_DSEF) model has been developed in an attempt to predict heavy rainfall from Landfalling Tropical cyclones (LTCs).In this study,the LTP_DSEF model is applied to predicting heavy precipitation associated with 10 LTCs occurring over China in 2018.The best forecast scheme of the model with optimized parameters is obtained after testing 3452 different schemes for the 10 LTCs.Then,its performance is compared to that of three operational dynamical models.Results show that the LTP_DSEF model has advantages over the three dynamical models in predicting heavy precipitation accumulated after landfall,especially for rainfall amounts greater than 250 mm.The model also provides superior or slightly inferior heavy rainfall forecast performance for individual LTCs compared to the three dynamical models.In particular,the LTP_DSEF model can predict heavy rainfall with valuable threat scores associated with certain LTCs,which is not possible with the three dynamical models.Moreover,the model can reasonably capture the distribution of heavier accumulated rainfall,albeit with widespread coverage compared to observations.The preliminary results suggest that the LTP_DSEF model can provide useful forecast guidance for heavy accumulated rainfall of LTCs despite its limited variables included in the model.(Jia Zuo,Ren Fumin,Zhang Da-lin)

3.3 Boundary layer height as estimated from radar wind profilers in four cities in China:Relative contributions from aerosols and surface features

The turbulent mixing and dispersion of air pollutants is strongly dependent on the vertical structure of the wind,which constitutes one of the major challenges affecting the determination of boundary layer height(BLH).Here,an adaptive method is proposed to estimate BLH from measurements of radar wind profilers(RWPs) in Beijing (BJ),Nanjing (NJ),Chongqing (CQ),and Wulumuqi (WQ) of China,during the summer of 2019.Validation against simultaneous BLH estimates from radiosondes (RSs) yielded a correlation coefficient of 0.66,indicating that the method can be used to derive BLH from RWPs.Diurnal variations of BLH and the ventilation coefficients (VC) at four sites were then examined.A distinct diurnal cycle of BLH was observed over all four cities; BLH gradually increased from sunset,reached a maximum in the afternoon,and then dropped sharply after sunset.The maximum hourly average BLH (1.426 ± 0.46 km) occurred in WQ,consistent with the maximum hourly mean VC larger than 5000 m2s−1observed there.By comparison,the diurnal variation of VC was not strong,with values ranging between 2000 and 3000 m2s−1,likely owing to the highhumidity environment.Furthermore,surface sensible heat flux,latent heat flux,and dry mass of particulate matter with aerodynamic diameter≤2.5 μm (PM2.5) concentrations were found to somehow affect the vertical structure of wind and thermodynamic features,leading to a difference between RS and RWP BLH estimates.This indicates that the atmospheric environment can affect BLH estimates using RWP data.The BLH results from RWPs were better in some specific cases.These findings show great potential of RWP measurements in air quality research,and will provide key data references for policy-making toward emission reductions.(Liu Boming,Guo Jianping,Gong Wei)

3.4 Characteristics and causes of extreme rainfall induced by binary tropical cyclones over China

Binary tropical cyclones (BTC) often bring disastrous rainfall to China.From the viewpoint of the extreme of the BTC maximum daily rainfall,the characteristics of BTC extreme rainfall (BTCER) during 1960−2018 are analyzed using daily rainfall data,and some representative large-scale mean flows,in which the associated BTCs are embedded,are analyzed.Results show that the frequency of BTCER shows a decreasing trend (−0.49 per 10 years) and is mainly distributed within the BTC heavy rainstorm interval (100 mm≤BTCER 250 mm).BTCER occurs mostly from July to September with a peak in August.Three BTCER typical regions(Minbei,the Pearl River Delta (PRD),and Taiwan) are identified according to the clustering of stations with high BTCER frequency and large BTCER.A further analysis of the 850-hPa BTC composite horizontal wind and water vapor flux over the PRD region shows the existence of two water vapor transport channels,which transport water vapor to the western tropical cyclones.In the first of these channels,the transport takes place via the southwest monsoon,which accounts for 58% of the total moisture,and an easterly flow associated with the eastern tropical cyclones accounts for the remaining 42%.(Wang Mingyang,Ren Fumin,Xie Yanjun)

3.5 Characteristics and performance of wind profiles as observed by the radar wind profiler network of China

Wind profiles are fundamental to the research and applications in boundary layer meteorology,air quality and numerical weather prediction.Large-scale wind profile data have been previously documented from network observations in several countries,such as Japan,the USA,various European countries and Australia,but nationwide wind profiles observations are poorly understood in China.In this study,the salient characteristics and performance of wind profiles as observed by the radar wind profiler network of China are investigated.This network consists of more than 100 stations instrumented with 1290 MHz Doppler radar designed primarily for measuring vertically resolved winds at various altitudes but mainly in the boundary layer.It has good spatial coverage,with much denser sites in the eastern China.The wind profiles observed by this network can provide the horizontal wind direction,horizontal wind speed and vertical wind speed for every 120 m interval within the height of 0 to 3 km.The availability of the radar wind profiler network has been investigated in terms of effective detection height,data acquisition rate,data confidence and data accuracy.Further comparison analyses with reanalysis data indicate that the observation data at 89 stations are recommended and 17 stations are not recommended.The boundary layer wind profiles from China can provide useful input to numerical weather prediction systems at regional scales.(Liu Boming,Guo Jianping,Gong Wei)

3.6 Characteristics,physical mechanisms,and prediction of pre-summer rainfall over South China:Research progress during 2008-2019

The pre-summer rainy season (April to mid-June) over South China (SC) is characterized by a high intensity and frequent occurrence of heavy rainfall in the East Asian monsoon region.This review describes recent progress in the research related to this phenomenon.The mechanisms responsible for pre-summer rainfall consist of multiscale processes.Sea surface temperatures over the tropical Pacific and Indian Oceans are shown to have a great influence on the interannual variations of pre-summer rainfall over SC.Synoptic disturbances associated with regional extreme rainfall over SC are mainly related to cyclone- and troughtype anomalies.Surface sensible heating and mechanical forcing from the Tibetan Plateau can contribute to the formation and intensification of such anomalies.On a sub-daily scale,double rain belts often co-exist over SC.The northern rain belt is closely linked to dynamic lifting by a subtropical low pressure and its associated front/shear line,whereas westward extension of the western North Pacific high and intensification of the southwesterly monsoonal flows play important roles in providing high-equivalent potential temperature air to the west- and east-inland regions,respectively.The southern rain belt,with a smaller horizontal span,exists in the warm sector over either inland or coastal SC.The warm-sector rainfall over inland SC results from surface heating,local topographic lifting,and urban heat island effects interacting with the sea breeze.The warm-sector rainfall over coastal SC is closely associated with double low-level jets,land-sea-breeze fronts,and coastal mountains.A close relationship is found between convectively-generated quasi-stationary mesoscale outflow boundaries and continuous convective initiation in extreme rainfall events.Active warmrain microphysical processes can play an important role in some extreme rainfall events,although the relative contributions of warm-rain,riming,and ice-phase microphysical processes remain unclear.Moreover,to improve rainfall predictions,efforts have been made in convection-permitting modeling studies.(Luo Yali,Xia Rudi,Chan Johnny C L)

3.7 Convection-permitting regional climate simulation of warm-season precipitation over eastern China

Convection-permitting regional climate models have been shown to improve precipitation simulation in many aspects,such as the diurnal cycle,precipitation frequency,intensity and extremes in many studies over several geographical regions of the world,but their skill in reproducing the warm-season precipitation characteristics over the East Asia has not been robustly tested yet.Motivated by recent advances in computing power,model physics and high-resolution reanalysis,we use the convection-permitting weather research and forecasting (WRF) model configured with 3 km grid spacing to simulate the warm-season precipitation in the eastern China for 10 seasons (2008−2017).The hourly 31-km-resolution ERA5 reanalysis data are used to provide initial and boundary conditions for the simulations.The objectives are:(1) to evaluate the model skill in simulating warm-season precipitation climatology in the East Asian monsoon region,(2) to identify the promises and problems of the convection-permitting simulation,and (3) to investigate solutions for the model deficiencies.Results demonstrate that the 3-km-resolution WRF model reasonably reproduces the spatial characteristics of seasonal and sub-seasonal precipitation,the seasonal meridional migration associated with the summer monsoon activity,the diurnal variation phase and amplitude,and the propagating convection east of the Tibetan Plateau.The major deficiency is that the model overestimates precipitation amount,especially in the afternoon.Analysis and sensitivity experiments suggest that improved treatment of sub-grid cloud fraction and the aerosol effects may help to suppress the often-reported high precipitation bias.These results provide useful guidance for improving the model skill in simulating warm-season precipitation in East Asia.(Yun Yuxing,Liu Changhai,Luo Yali)

3.8 Elucidating the life cycle of warm-season mesoscale convective systems in eastern China from the Himawari-8 geostationary satellite

The life cycle of mesoscale convective systems (MCSs) in the eastern China is yet to be fully understood,mainly due to the lack of observations of high spatio-temporal resolution and objective methods.Here,we quantitatively analyze the properties of warm-season (from April to September of 2016) MCSs during their lifetimes using the Himawari-8 geostationary satellite,combined with ground-based radar and gauge measurements.Generally,the occurrence of satellite-derived MCSs has a noon peak over the land and an early morning peak over the ocean,which is several hours earlier than the precipitation peak.The developing and dissipative stages are significantly longer as total durations of MCSs increase.Aided by three-dimensional radar mosaics,we find the fraction of convective cores over the northern China is much lower when compared with those in the central United States,indicating that the precipitation produced by broad stratiform clouds may be more important for the northern China.When there exists a large amount of stratiform precipitation,it releases a large amount of latent heat and promotes the large-scale circulations,which favors the maintenance of MCSs.These findings provide quantitative results about the life cycle of warm-season MCSs in the eastern China based on multiple data sources and large numbers of samples.(Chen Dandan,Guo Jianping,Yao Dan)

3.9 Establishment of an objective standard for the definition of binary tropical cyclones in the western North Pacific

To develop an objective standard for defining binary tropical cyclones (BTCs) in the western North Pacific(WNP),two best-track datasets,from the China Meteorological Administration and the Joint Typhoon Warning Center,were adopted for statistical analyses on two important characteristics of BTCs-two TCs approaching each other,and counterclockwise spinning.Based on the high consistency between the two datasets,we established an objective standard,which includes a main standard for defining BTCs and a secondary standard for identifying typical/atypical BTCs.The main standard includes two requirements:two coexisting TCs are a pair of BTCs if (i) the separation distance is≤1800 km,and (ii) this separation maintains for at least 12 h.Meanwhile,the secondary standard defines a typical BTC as one,for which there is at least one observation when the two TCs approach each other and spin counterclockwise simultaneously.Under the standard,the ratio of typical BTCs increases as the BTC duration increases or the minimum distance between the two TCs decreases.Then,using the JTWC dataset,it was found that there are 505 pairs of BTCs during the period 1951−2014,including 328 typical BTCs and 177 atypical BTCs,accounting for 65.0% and 35.0% of the total,respectively.In addition,a study of two extreme phenomena—the maximum approaching speed and the maximum counterclockwise angular velocity in typical BTCs—shows that the configuration of the circulation conditions and the distribution of the BTCs favor the formation of these extreme phenomena.(Ren Fumin,Xie Yanjun,Yin Biwen)

3.10 Improvement in the forecasting of heavy rainfall over South China in the DSAEF_LTP model by Introducing the intensity of the tropical cyclone

The intensity of the tropical cyclone has been introduced into the Dynamical-Statistical-Analog Ensemble Forecast (DSAEF) for Landfalling Typhoon (or tropical cyclone) Precipitation (DSAEF_LTP)model.Moreover,the accumulated precipitation prediction experiments have been conducted on 21 target tropical cyclones with daily precipitation≥100 mm in South China from 2012 to 2016.The best forecasting scheme for the DSAEF_LTP model is identified,and the performance of the prediction is compared with three numerical weather prediction models (the European Centre for Medium-Range Weather Forecasts,the Global Forecast System,and T639).The forecasting ability of the DSAEF_LTP model for heavy rainfall (accumulated precipitation≥250 and≥100 mm) improves when the intensity of the tropical cyclone is introduced,giving some advantages over the three numerical weather prediction models.The selection of analog tropical cyclones with a maximum intensity (during precipitation over land) equaling to or higher than the initial intensity of the target tropical cyclone gives better forecasts.The prediction accuracy for accumulated precipitation is higher for tropical cyclones with higher intensity and higher observed precipitation,with in both cases positive linear correlations with the threat score.(Ding Chenchen,Ren Fumin,Liu Yanan)

3.11 Introducing TC intensity into the DSAEF_LTP model and simulating precipitation of supertyphoon Lekima (2019)

In this study,the Dynamical-Statistical-Analog Ensemble Forecast model for landfall typhoon precipitation (the DSAEF_LTP model),which has been developed by Ren et al.,in 2020,is applied to precipitation simulations for super-typhoon Lekima (2019),a tropical cyclone (TC) that produced heavy rainfall over the eastern China.A new variable,TC intensity,is introduced into the generalized initial value(GIV) of the DSAEF_LTP model.Two different groups of simulation experiments,one group including the new TC intensity variable in the GIV and the other group excluding it,have been conducted.Results show that,with TC intensity,the ability of the DSAEF_LTP model to forecast heavy rainfall (i.e.accumulated precipitation exceeding thresholds of 250 and 100 mm) for Lekima is improved.Its threat score (TS) ranks the second compared with those of three numerical weather prediction models (i.e.ECMWF,GRAPES and GFS).Further study shows that three factors prevent the DSAEF_LTP model from achieving more satisfactory results for Lekima:the historically rare occurrence of extreme precipitation in the northern China during Lekima,the increase of forecast track error when Lekima moved northwards,and the calculation of similarity between tracks of Lekima and historical TCs within an improper region (i.e.similarity region).To solve the third problem,new similarity region schemes are adopted and produce higher TS,in which the TS of precipitation greater than 100 mm (TS100) ranks first and the TS of precipitation greater than 250 mm (TS250) gets closer to the first place.(Jia Li,Jia Zuo,Ren Fumin)

3.12 Monsoonal influences on offshore rapid intensification of landfalling typhoons in a sheared environment over the South China Sea

Tropical cyclones (TCs) formed in the western North Pacific and South China Sea can undergo rapid intensification (RI) shortly before making landfall in China.Forecasting such offshore RI is a great challenge in operations.In this study,the offshore RI events in a sheared environment are examined for TCs that made landfall in China during 1979−2017.It is found that there were only three offshore RI events in a sheared environment,all of which occurred to the south of Hainan Island within the monsoon trough in early- to mid-July,coinciding with the termination of the Meiyu season.The specific geographic location and timing of the occurrence of the offshore RI in the sheared environment are associated with the adjustment of the East Asia summer monsoon system when the mei-yu season terminates in the Yangtze River valley.In addition to the adjustment favorable for TC intensification by enhancing the TC-trough interaction in the upper troposphere,this study suggests that two environmental factors also contribute to the offshore RI over the South China Sea in a sheared environment.One is the intrusion of dry air associated with the western North Pacific subtropical high (WNPSH) and the other is the penetration of the water vapor flux associated with the monsoon surge.The adjustment of the East Asia summer monsoon system allows the water vapor flux of the monsoon surge to penetrate the TC circulation and prevents the dry air of the WNPSH from intruding into the TC circulation.(Qiu Wenyu,Wu Liguang,Ren Fumin)

3.13 Novel models to estimate hourly diffuse radiation fraction for global radiation based on weather type classification

The diffuse radiation is well recognized as a key variable in solar energy assessment,albeit with sorely lacking ground-based measurements.Here,we proposed two novel models to estimate hourly diffuse radiation using the typical meteorological annual radiation data in Beijing as training samples.Model 1 was a combination of four classical models,including Liu & Jordan,Orgill & Hollands,Erbs and Reindl,in which the weight or coefficient was determined by weather types derived from clearness index.In Model 2,the weather type classification was refined by total cloud cover,and the principal component analysis (PCA) was further applied to determine the major meteorological variables for each weather type as model’s input,along with linear fitting.Using sub-typical annual radiation data as testing samples,the proposed models showed strong extrapolation ability with three statistical metrics:lower mean absolute percentage error and normalized root mean square error but relatively higher correlation coefficient,compared with other models.Finally,these models were verified by the observations in Wuhan.The results indicat that weather type classification and PCA effectively improved model’s performance by eliminating the collinearity between meteorological and environmental variables.Furthermore,both models performed better than any single classical model,irrespective of large-scale weather patterns.(Li Fen,Lin Yilun,Guo Jianping)

3.14 Roles of double low-level fets in the generation of coexistinginland and coastal heavy rainfall over South China during the presummer rainy season

In this study,the key dynamic factors influencing the formation of six heavy-rainfall events with coexisting inland and coastal rainfall over South China during May−June 2011−2017 are investigated using the global ensemble forecasts from the European Centre for Medium-Range Weather Forecasts.An ensemble sensitivity analysis and comparisons between the identified good and bad members are conducted.Results confirm the importance of the synoptic low-level jet (SLLJ) and boundary layer jet (BLJ),but with different contributions,in determining the generation of the inland and coastal rainbelts,respectively.The inland heavy rainfall is closely related to both SLLJ and BLJ,especially their meridional wind components.The more intense cold or quasi-stationary fronts,low-level vortexes,or shear lines accompanied by the stronger geostrophic southerly winds within both jets over inland and coastal South China are,the more rainfall over the inland region is favored.The ageostrophic southerly winds of the BLJ make limited contributions to the inland rainfall production,but exert greater impacts on the generation of extreme (yet localized) coastal rainfall.The geostrophic meridional winds and associated low-level fronts/vortexes also play a nonnegligible but indispensable role in producing the coastal rainfall.These findings add to our knowledge of heavy rainfall in the East Asian monsoon region.(Liu Xi,Luo Yali,Huang Ling)

3.15 Roles of synoptic to quasi-monthly disturbances in generating two pre-summer heavy rainfall episodes over South China

In this study,the power spectral analysis and bandpass filtering of daily meteorological fields are performed to explore the roles of synoptic to quasi-monthly disturbances in influencing the generation of presummer heavy rainfall over South China.Two heavy rainfall episodes are selected during the months of April−June 2008−2015,which represent the collaboration between the synoptic and quasi-biweekly disturbances and the synoptic and quasi-monthly disturbances,respectively.Results show that the first heavy rainfall episode takes place in a southwesterly anomalous flow associated with an anticyclonic anomaly over the South China Sea (SCS) at the quasi-biweekly scale with 15.1% variance contributions,and at the synoptic scale in a convergence zone between southwesterly and northeasterly anomalous flows associated with a southeastwardmoving anticyclonic anomaly on the leeside of the Yungui Plateau and an eastward-propagating anticyclonic anomaly from higher latitudes with 35.2% variance contribution.In contrast,the second heavy rainfall episode takes place in southwest-to-westerly anomalies converging with northwest-to-westerly anomalies at the quasimonthly scale with 23.2% variance contributions to the total rainfall variance,which are associated with an anticyclonic anomaly over the SCS and an eastward-propagating cyclonic anomaly over North China,respectively.At the synoptic scale,it occurs in south-to-southwesterly anomalies converging with a cyclonic anomaly on the downstream of the Yungui Plateau with 49.3% variance contributions.In both cases,the lowertropospheric mean south-to-southwesterly flows provide ample moisture supply and potentially unstable conditions; it is the above synoptic,quasi-biweekly or quasi-monthly disturbances that determine the general period and distribution of persistent heavy rainfall over South China.(Jiang Zhina,Zhang Da-Lin,Liu Hongbo)

3.16 Science and prediction of heavy rainfall over China:Research progress since the reform and opening-up of new China

This paper reviews the major progress on development of the science and prediction of heavy rainfall over China since the beginning of the reform and opening-up of new China (roughly between 1980 and 2019).The progress of research on the physical mechanisms of heavy rainfall over China is summarized from three perspectives:(1) the relevant synoptic weather systems,(2) heavy rainfall in major sub-regions of China,and (3) heavy rainfall induced by typhoons.The development and application of forecasting techniques for heavy rainfall are summarized in terms of numerical weather prediction techniques and objective forecasting methods.Greatly aided by the rapid progress in meteorological observing technology and substantial improvement in electronic computing,studies of heavy rainfall in China have advanced to investigating the evolution of heavy-rain-producing storms and observational analysis of the cloud microphysical features.A deeper and more systematic understanding of the synoptic systems of importance to the production of heavy rainfall has also been developed.Operational forecast of heavy rainfall in China has changed from subjective weather event forecasts to a combination of both subjective and objective quantitative precipitation forecasts,and is now advancing toward probabilistic quantitative precipitation forecasts with the provision of forecast uncertainty information.(Luo Yali,Sun Jisong,Li Ying)

3.17 Sideswiping tropical cyclones and their associated precipitation over China

Tropical cyclone (TC) precipitation (TCP) has attracted considerable attention in recent decades because of its adverse socioeconomic impacts.In particular,considerable effort has been devoted to quantifying TCP and investigating the precipitation of TCs that make landfall.However,precipitation over land induced by TCs that do not make landfall (i.e.,offshore),the so-called “sideswiping” TCs (STCs),is an important component of TCP but has attracted little attention from the research community to date.Here,best-track and daily precipitation data from the China Meteorological Administration during the 59 years of 1960−2018 are used to study STC precipitation (STP).Results show that:(i) the annual number of STCs fluctuates significantly from 3 to 17,with a mean frequency of 8.8 STCs per year; (ii) there are decreasing trends in STC frequency and STP amount over the past 59 years; (iii) both STC frequency and STP are high from July to October,with maxima in August; (iv) the distribution of STP,covering most of China’s coastal regions,is dominated by intense STCs,and the annual STP decreases from southeast coastal regions to northwest inland areas,with a maximum value over the islands of Taiwan and Hainan; and (v) extreme STP events could appear not only over the island and coastal areas,but also over inland areas such as Zhumadian of Henan Province due to the influences of local orography and favorable large-scale forcing.(Feng Tian,Ren Fumin,Zhang Lin)

3.18 Statistical characteristics of pre-summer rainfall over South China and associated synoptic conditions

In this study,the climatological characteristics of pre-summer (April to June) rainfall over South China(SC) and the associated synoptic conditions are examined using 1980−2017 hourly rainfall observations and reanalysis data.The amount,frequency,and intensity of rainfall show pronounced regional variations and substantial changes between pre- and post-monsoon-onset periods.Owing to the more favorable thermodynamic conditions after monsoon onset over the South China Sea (SCS),rainfall intensifies generally over SC irrespective of the rainfall-event durations.Increased rainfall amounts in longer-duration ( 6 h)events were found over a designated west-inland region (west of 111°E),which are partially attributed to enhanced dynamic instability.In addition,rainfall events occur more frequently over the west-inland region,as well as coastal regions to the west of 118°E,but less over a designated east-inland region.Inland-region rainfall is closely linked to dynamic lifting driven by subtropical synoptic systems (low pressure and an associated front or shearline).The westward extension of the western North Pacific high and the eastward extension/movement of the front or shearline,interacting with the intra-period intensification of the southwesterly monsoonal flows,play important roles in providing high-θe,(equivalent potential temperature) air to the westand east-inland regions,respectively.Warm-sector coastal rain fall is closely related to the deceleration of the southerly boundary layer (BL) air flow over the northern SCS and associated convergence of BL high-θeair near the coast.Meanwhile,the southwesterly synoptic-system-related low-level jet in the lower-to-middle troposphere to the south of the inland cold front can contribute to the coastal rainfall occurrence by providing divergence above the BL convergence near the coast.The BL flow often simultaneously strengthens with the lower-troposphere horizontal winds,suggesting a close association between the BL flow and the synoptic systems.The quantitative statistics provided in this study complement previous case studies or qualitative results and,thus,advance our understanding about pre-summer rainfall over SC.(Li Zhenghui,Luo Yali,Du Yu)

3.19 Temperature inversion and clouds over the Arctic Ocean observed by the 5th Chinese National Arctic Research Expedition

The Arctic is of great significance to global weather and climate systems.However,its atmospheric conditions have not yet to be fully understood,partly because of the lack of intensive observations.Here we reported on the characteristics of lower troposphere over the Arctic Ocean using high-resolution radiosonde data collected during the 5th Chinese National Arctic Research Expedition from July to September 2014.Statistical analyses showed that temperature inversion (TI) occurred most frequently below 700 m,without apparent temporal variability between morning and noon.In contrast,the occurrence frequency of morning surface-based inversion (SBI) was always higher than that at noon in the whole boundary layer,whereas the elevated inversion (EI) frequency seemed much lower in the morning compared with that at noon.More frequent deeper clouds tended to occur in the morning than at noon,which led to more frequent EI than SBI.SBI was found to dominate the clear-sky condition,whereas EI was generally observed under cloudy conditions.Regarding the seasonal variation,deep TI and clouds dominated during the first intensive observational period (IOP-1; July 21−31) because of warm advection in combination with heating effects induced by long-range transported black carbon (BC).During IOP-2 (August 1−18),synoptic scale subsidence prevailed,with frequent EI and SBI partly caused by BC accumulation over the Arctic Ocean.In comparison,more clouds and less TI occurred during IOP-3 (from August 27 to September 11) largely due to the outbreaking polar vortex.These observations contribute to a better understanding of vertical temperature and cloud structures under global warming.(Wang Ding,Guo Jianping,Chen Aijun)

3.20 The effects of cloud-aerosol interaction complexity on simulations of presummer rainfall over the southern China

Convection-permitting simulations are used to understand the effects of cloud–aerosol interactions in a case of heavy rainfall over the southern China.The simulations are evaluated using radar observations from the Southern China Monsoon Rainfall Experiment (SCMREX) and remotely sensed estimates of precipitation,clouds and radiation.We focus on the effects of complexity in cloud–aerosol interactions,especially the depletion and transport of aerosol material by clouds.In particular,simulations with aerosol concentrations held constant are compared with a fully cloud–aerosol-interacting system to investigate the effects of twoway coupling between aerosols and clouds on a line of organised deep convection.It is shown that the cloud processing of aerosols can change the vertical structure of the storm by using up aerosols within the core of line,thereby maintaining a relatively clean environment which propagates with the heaviest rainfall.This induces changes in the statistics of surface rainfall,with a cleaner environment being associated with lessintense but more-frequent rainfall.These effects are shown to be related to a shortening of the timescale for converting cloud droplets to rain as the aerosol number concentration is decreased.The simulations are compared to satellite-derived estimates of surface rainfall,a condensedwater path and the outgoing flux of short-wave radiation.Simulations for fewer aerosol particles outperform the more polluted simulations for surface rainfall but give poorer representations of top-of-atmosphere (TOA) radiation.(Kalli Furtado,Paul Field,Luo Yali)

3.21 The impact of reforestation induced land cover change (1990-2017) on flood peak discharge using HEC-HMS hydrological model and satellite observations:A study in two mountain basins,China

Understanding the effect of land use and land cover (LULC) type change on watershed hydrological response is essential for adopting applicable measures to control floods.In China,the Grain to Green Program(GTGP) and the Natural Forest Conservation Program (NFCP) have had a substantial impact on LULC.We investigated the effect of these conservation efforts on flood peak discharge in two mountainous catchments.We used a series of Landsat images ranging from 1990 to 2016/2017 to evaluate the LULC changes.Further to this,the hydrological responses at the basin and sub-basin scale were generated by the Hydrologic Modeling System (HEC-HMS) under four LULC scenarios.Between 1990 and 2016/2017,both catchments experienced an increase in forest and urban land by 18% and 2% in Yanhe and by 16% and 8% in Guangyuan,respectively.In contrast,the agricultural land decreased by approximately 30% in Yanhe and 24% in Guangyuan,respectively.The changes in land cover resulted in the decrease in flood peak discharge ranging from 14%in Yanhe to 6% in Guangyuan.These findings provide a better understanding on the impact of reforestation induced LULC change on spatial patterns of typical hydrological responses of mountainous catchment and could help to mitigate flash flood hazards in other mountainous regions.(Kabeja Crispin,Li Rui,Guo Jianping)

3.22 The response of warm-season precipitation extremes in China to global warming:an observational perspective from radiosonde measurements

Consensus has been reached that precipitation extremes vary proportionally with global warming.Nevertheless,the underlying cause and magnitude of these factors affecting their relationships remain highly debated.To elucidate the complex relationship between precipitation extremes and temperature in China during the warm seasons (May through September),a 60-year (1958−2017) record of hourly rain gauge measurements,in combination with surface air temperature,RH,precipitable water (PW),and convective available potential energy (CAPE) collected from 120 radiosonde stations were examined.Spatially,the scaling relationship between precipitation extremes and temperature exhibits a large geographic difference across China.In particular,the Clausius-Clapeyron (CC) and sub-CC relationships tend to occur in Northwest (ROI-N)and Southeast China (ROI-S),whereas the super-CC relationship is found to mainly concentrate in the central China (ROI-C).Additionally,the response of precipitation extremes to temperature becomes more sensitive as precipitation intensity increases,shifting from CC to super-CC at a certain point of inflection that varies by geographic regions.This shift occurs at approximately 15℃ in ROI-C and ROI-N,but at around 20℃ in ROI-S.Within the temperature range of the super-CC slope,the PW rises with the increases in temperature,whereas the CAPE decreases with rising temperature,which is contrary to the monotonic scaling of precipitation with temperature.From the perspective of interannual variation,the precipitation extremes correlate positively with temperature.This further confirms the notion that global warming,through jointly affecting PW and CAPE,is able to considerably regulate precipitation extremes.(Guo Jianping,Yan Yan,Chen Dandan)

3.23 Urbanization enhanced summertime extreme hourly precipitation over the Yangtze River delta

An extensive urban agglomeration has occurred over the Yangtze River delta (YRD) region of East China as a result of rapid urbanization since the middle 1990s.In this study,a 44-year (i.e.,1975−2018) climatology of the summertime extreme hourly precipitation (EXHP; greater than the 90th percentile) over the YRD is analyzed,using historical land-use data,surface temperature,and hourly rain gauge observations,and then the relationship between rapid urbanization and EXHP changes is examined.Results show significant EXHP contrasts in diurnal variation and storm type roughly before and after middle July.That is,tropical cyclones(TCs) account for 16.4% of the total EXHP hours,80.5% of which occur during the late summer,whereas non-TC EXHP accounts for 94.7% and 66.2% during the early and late summer,respectively.Increasing trends in occurrence frequency and amount of the non-TC and TC-induced EXHP are detected over the urban agglomeration.Statistically significant larger increasing trends in both the EXHP and surface temperature are observed at urban stations than those at the nearby rural stations.An analysis of 113 locally developed non-TC extreme rainfall events during 2011−2018 summers also suggests the contribution of the urban heat island effects to the more occurrences of EXHP,especially over a band-shaped urban region where several major cities are distributed.This study reveals a significant correlation between rapid urbanization and increased EXHP during the past two decades over the YRD region.The results have important implications for understanding the impact of urbanization on EXHP changes in a warming climate.(Jiang Xiaoling,Luo Yali,Zhang Da-Lin)

3.24 High-resolution simulations of heavy rainfalls in association with monsoon systems and typhoons using cloud-resolving models

Monsoon systems and typhoons are major causes of heavy rainfall events in East Asia and occasionally give rise to serious disasters.Most heavy rainfall systems are composed of intense convective clouds.Cloudresolving models are,therefore,indispensable for studies of their mechanisms and processes,and also for accurate and quantitative predictions of heavy rainfall.Advances in computing now make it feasible to use cloud-resolving models for studies and predictions of heavy rainfalls associated with monsoon systems and typhoons.This chapter reviews recent studies of heavy rainfall events using regional cloud-resolving models such as MM5,WRF,JMA-NHM,and CReSS.As an example,the details of the CReSS model are summarized and two simulation experiments of heavy rainfall events are introduced:one caused by the Baiu front in Japan in 2017 and the other accompanying a typhoon in 2004.These high-resolution experiments,which successfully simulated the structure of heavy rainfall systems and provided quantitative predictions of rainfall,were performed with a horizontal resolution of 1 km.These results indicate that cloud-resolving models are very useful and indispensable for mechanism studies and quantitatively accurate predictions of heavy rainfall associated with monsoon systems and typhoons.(Kazuhisa Tsuboki,Luo Yali)

3.25 The Southern China Monsoon Rainfall Experiment (SCMREX)

During the pre-summer rainy season (April–June),South China often experiences frequent occurrences of extreme rainfall,leading to severe flooding and inundations.Although scientific understanding of the processes responsible for heavy rainfall production continues to advance (Schumacher 2017),quantitative precipitation forecasting is still a great challenge over the world including South China.To improve the quantitative precipitation forecasting (QPF) skill of the pre-summer rainy season rainfall,the China Meteorological Administration (CMA) initiated a nationally coordinated research project,the Southern China Monsoon Rainfall Experiment (SCMREX).With the participation and strong support of international experts,the SCMREX was approved by the World Meteorological Organization (WMO) as a World Weather Research Programme (WWRP) Research and Development Project (RDP).The scientific objectives of the SCMREX are:(1) to better understand the physical mechanisms governing the initiation,evolution,morphology,organization,and duration of the heavy-rain-producing MCSs that determine the precise timing and location and accumulative amount of rainfall; (2) to better understand the microphysical and kinematic structures of the heavy-rain-producing MCSs that determine the instantaneous rainfall amount and are of relevance when validating parameterization schemes representing cloud-precipitation microphysical processes in numerical weather prediction (NWP) models; and (3) to improve fine-scale quantitative precipitation forecasting (QPF)skills by better understanding the multi-scale precipitation processes,assessing the impact of assimilating highresolution observations into convection-permitting (horizontal grid spacing of 1–4 km) numerical models,and evaluating and improving cloud microphysical parameterization schemes in convection-permitting models.This article reviews the progress of SCMREX in 2016-2017 and briefly discusses future research opportunities.(Luo Yali,Bao Xinghua,Wang Hui)

3.26 LTP_DSEF模型对2018年登陆中国热带气旋强降水预报的应用

最近，一种基于路径相似的登陆热带气旋降水动力统计集合预报（LTP_DSEF）模型被发展用来预报登陆热带气旋（LTC）带来的强降水。文章把LTP_DSEF模型应用于2018年登陆中国的10个热带气旋（TC）的强过程降水预报，通过测试模型的3452套预报方案确定了对这10个LTC的最佳方案，然后将其性能与3家动力模式（ECMWF、GFS和GRAPES）进行对比。结果表明：LTP_DSEF模型在预报LTC的较强过程降水方面与3家动力模式相比很有优势，特别是预报250 mm以上量级的过程降水；对单TC，LTP_DSEF模型预报LTC过程降水的能力优于或者略逊于3家动力模式，特别在3家动力模式对某些TC的强降水均无预报能力时，模型仍能提供宝贵的大于零的TS值；此外，虽然与实况相比该模型预测的强降水范围倾向偏大，但它在多数情况下能合理地捕捉到强降水的落区。初步研究表明，尽管LTP_DSEF模型只引入了TC路径和登陆时间两个相似性变量，但它已能为LTC的强过程降水预报提供非常有用的指导。（贾作，任福民，张大林）

3.27 1980—2017年南海季风爆发前后华南前汛期降水统计特征对比分析

利用1980—2017年华南地区303 个国家级地面气象站逐小时降水数据、ERA-Interim 再分析资料，分析华南前汛期（4—6月） 降水统计特征，定义站点上短时（1～6 h）、中等时长（7～12 h）和长时（ 12 h）降水事件，对比降水量、频次和强度在南海季风爆发前后的变化，以及所定义的西部内陆、东部内陆、沿海地区的异同。结果表明：（1） 南海季风爆发后，研究区域平均而言，3类降水事件的降水量增多、小时降水强度增强，短时、长时降水事件发生频次增多，而中等时长降水事件发生频次有所减少。（2） 从空间分布来看，南海季风爆发后，小时降水强度在整个华南地区均增强，西部内陆时长大于6 h 的降水事件尤为明显；降水事件的发生频次在西部内陆和沿海地区升高，而东部内陆时长大于6 h 的降水事件发生频次降低；因此，季风爆发后西部内陆和沿海地区的总降水量均显著增大，而东部内陆的总降水量变化不大。（3） 西部内陆降水事件主要在夜间开始发生，持续时间越长的事件越早开始，且由西向东逐渐推迟；东部内陆短时降水事件主要在14:00 （北京时，下同）左右开始，季风爆发后更为明显，而时长大于6 h的降水事件的开始时间和峰值时间无明显的分布规律；沿海地区短时降水事件在季风爆发前主要于05:00—08:00时开始，季风爆发后，在海岸线约50 km 以内仍然如此，而较远离海岸线的短时降水事件主要于14:00开始，沿海地区长时降水事件在季风爆发前、后都倾向于在夜间开始，并在日间出现峰值。（李争辉，罗亚丽）

3.28 1982—2016年云南省不同强度降水气候态及其变化

基于1982—2016年云南省67个气象站逐小时降水资料，利用百分位阈值法定义极端小时降水（大于第95百分位）、强小时降水（第80至95百分位之间）、中等—弱小时降水（小于第80百分位），并分析其35 a平均气候态和变化趋势。结果表明：（1）云南省年降水强度大值中心位于南部，年降水频次大值中心位于西北部，分别对应年降水量大值和次大值中心。云南省降水主要集中在夏秋季，同时云南省西北部存在春汛期，该地区2—4月降水频次高、强度大、降水量多。（2）全省极端小时降水、强小时降水的阈值表现为从南向北、从西向东减小。（3）过去35 a全省大部分地区年降水量和小时降水频次均表现为下降趋势，且随着降水强度等级降低而增强，秋季最为明显，而小时降水强度表现为增强趋势，夏秋两季极端小时降水的增强趋势通过显著性检验。（于淑婷，罗亚丽，李建）

3.29 地形作用下低空急流的演变与强降水对流风暴系统的相互作用

利用雷达、卫星、风廓线雷达和地面加密区域自动气象站等观测资料，分析了2016年入梅后发生在鄂东地区一次极端强降水事件的中尺度对流系统发生发展过程、结构演变及其传播特征，旨在揭示造成强降水过程中的3个中尺度对流系统（MCS）的触发、发展、维持机理以及它们之间内在的中尺度动力学关系，尤其是地形作用下的低空急流的演变与强降水对流风暴系统相互作用过程。研究表明：（1）与大多数梅雨锋上的强降水带与低空切变线平行分布不同，此次极端强降水雨带呈倾斜的“n”字形，其中两条主雨带近乎与低空切变线垂直；此次极端强降水分别由大别山迎风坡上西北—东南向MCS、湖北中东部平原地区西北—东南向MCS和桐柏—大洪山东侧东北—西南向MCS造成。3个MCS移动缓慢，都具有后向传播的特征。（2）大别山迎风坡上MCS初始雷暴是低空急流下边界不断向下扩展过程中在地形抬升作用下触发的，而湖北中东部平原地区的MCS和桐柏—大洪山东侧MCS的触发、发展、加强都与大别山迎风坡上MCS形成的冷池加速推进形成的出流边界与环境气流形成的强烈辐合抬升作用有关。（3）垂直于大别山的边界层西南急流对山坡上的对流冷池产生的顶托作用不仅平衡了冷池密度流产生的向下作用力，而且进一步强化了山区的辐合抬升强度，使得大别山迎风坡上强降水风暴系统得以长时间维持和发展；当山坡上的对流冷池堆积到足够厚度，或者由于低空急流的下边界迅速抬升时，这种平衡被打破，大范围的冷池俯冲下山并在平原地区快速推进，造成了湖北中东部平原地区大范围的雷暴大风和MCS发展加强，并沿冷池前沿逐步组织化，形成平原地区东南—西北向的强降水带。（黄小彦，孙继松，刘文婷）

3.30 海南岛热带气旋极端降水的特征及其成因

围绕海南岛热带气旋（TC）极端降水的特征及其成因，利用国家气象信息中心提供的海南岛台站日降水数据、上海台风所的TC最佳路径数据集和NCEP/NCAR再分析数据，通过气候统计、天气诊断相结合的方法，探讨了1958—2013年海南岛TC极端降水的空间分布和时间变化特征，并进一步诊断分析了极端降水产生的可能成因。结果表明，海南岛TC极端降水在西北部出现最多，东南部出现较少。从长期趋势看，无论从TC极端降水量还是从TC极端降水频数，TC极端降水的极端趋势均在增加。充足的西南水汽通量输送是产生TC极端降水的重要条件；当南亚高压和副热带高压相距较近，即南亚高压偏东偏强，副热带高压偏西偏强时，海南岛容易出现TC极端降水事件；在TC路径和低层风场结构的配合下，海南岛地形对极端降水的落区有关键影响。（蒋贤玲，任福民，蔡亲波）

3.31 华北地区持续性极端暴雨过程的分类特征

利用1960—2015年日降水资料，筛选出华北地区56次持续性极端暴雨过程。基于距平相关系数的客观聚类分析方法和天气学检验，将它们进行分类，并使用NCEP（2.5°×2.5°）再分析资料进行分类合成，对比分析不同环流背景下华北地区持续性极端暴雨过程的基本特征。结果表明，这些持续性极端暴雨事件按照环流背景可分为经向型、纬向型、减弱的登陆热带气旋型和初夏型4类。它们一般都与不同天气系统配置结构下的锋面动力学过程有关，由于锋面结构特征、环境大气层结状态以及与低空急流有关的暖湿气流输送通道和强度不同，造成不同环流形势背景下，暴雨日的高频站点与过程平均累计降水量在空间分布上存在差异。（1）纬向型对应的锋区强度明显强于经向型，但是其对应的层结稳定度与整个夏季状态相当，而经向型存在弱的层结不稳定异常，这表明，纬向型的对流活动一般不如经向型强，持续性锋面降水特征更清晰，造成站点上日降水量超过50 mm的最大频率明显低于经向型，但是过程累计平均最大降雨量却比经向型大。（2）从水汽输送通道来看，源于西太平洋副热带高压南侧的水汽通道只在纬向型环流主导下的华北区域持续性极端暴雨过程中起主导作用。初夏型以及减弱的登陆热带气旋与西风带系统相互作用造成的极端暴雨过程中，活跃的印度季风造成25°N以南异常强盛的纬向低空西南气流携带充沛的水汽，穿过中南半岛后以西南低空急流或者通过减弱的登陆热带气旋“中转”，是这两类暴雨区的主要水汽供应方式；经向型环流背景下的水汽输送也与这支源于青藏高原南侧的西风气流异常有关。这可能是华北地区夏季降水与印度季风降水的相关显著强于中国东部其他地区的主要原因。（3）减弱的登陆热带气旋与西风带系统相互作用造成的极端暴雨事件同样由经向型环流主导，但是，更充沛的水汽输送、更强的上升运动和更深厚的大气不稳定层结状态是它比一般的经向型强度更大的直接原因；此外，中高纬度弱冷空气侵入对减弱的登陆热带气旋顶部形成持续性极端暴雨过程非常重要。（周璇，孙继松，张琳娜）

3.32 强热带风暴“碧利斯”（0604）极端降水研究回顾

强热带风暴“碧利斯”于2006年7月14日在我国福建省登陆，登陆后一路西行，在福建、浙江、湖南、江西、广东、广西多个省份产生强降水，其陆上维持时间之长、影响范围之广、降水极端性之强，在历史上极为罕见。“碧利斯”引发的降水主要有3个极值中心，分别对应着降水的3个阶段，其中第2阶段降水极端性最为突出，在湖南、江西和广东交界处发生了暴雨增幅，造成严重的洪涝灾害。为了更加全面地认识此次过程，本文对“碧利斯”产生极端降水的机制做了回顾总结，分别从有利的大尺度形势、中尺度系统的影响、地形作用、云微物理过程以及动力因子的诊断分析5个角度进行，最后对“碧利斯”极端降水区别于其他极端降水个例的机理以及进一步可能的研究方向进行了讨论。（马蕴琦，任福民，冯恬）

3.33 影响中国双台风活动气候特征研究

在双台风判定客观标准的基础上提出影响中国双台风的定义：在双台风活动期间，双台风中至少有一个台风对中国大陆或2个大岛——海南岛和台湾岛之一造成降水的双台风，称为影响中国双台风。然后利用台站逐日降水资料和台风最佳路径资料，采用热带气旋（TC）降水天气图客观识别法（OSAT）和上述定义，对1960—2017年的影响中国双台风进行研究。结果表明：1960—2017年影响中国双台风共有255对，年均4.4对，占西北太平洋双台风总数的60.6%。影响中国双台风年频数表现为显著下降趋势。影响中国双台风的影响期最长可达10天，主要集中在1天、2天和3天，分别占18.8%，29.4%和24.3%。在地理分布上，影响中国双台风主要分布在112°～138°E、12°～30°N范围，频发区在菲律宾北部附近洋面；受双台风影响的年均频次和年均降水均表现为由东南沿海向西北内陆阶梯递减，主要影响区为中国的台湾岛、东南沿海和华南沿海，其中台湾岛受影响最大。进一步分析发现，影响中国双台风影响期内出现单站最大日降水当日两TC平均位置主要表现为东—西向分布，西台风和东台风正好分别位于东亚夏季风的西南风水汽通道和副热带高压西南侧东南风暖湿气流中，不仅有利于西台风从西南风水汽通道中获得水汽，而且有利于东台风向西台风的水汽输送，从而有利于西台风对中国台湾岛、东南沿海和华南沿海造成的强降水。（谢彦君，任福民，李国平）

3.34 中国暴雨的科学与预报：改革开放40年研究成果

总结了改革开放以来中国学者在暴雨科学与预报领域取得的重要研究进展和主要成果。其中，暴雨机理研究成果从重要天气系统、中国主要区域的暴雨、台风暴雨等3个方面分别进行综述，而暴雨预报技术研发与应用则从中国数值天气预报发展和暴雨预报客观方法两方面进行归纳。（罗亚丽，孙继松，李英）

4 台风研究

4 Typhoon research

4.1 A study of the effects of anthropogenic gaseous emissions on the microphysical properties of landfalling typhoon Nida (2016) over China

Using the Weather Research and Forecasting model with chemistry module (WRF-Chem),Typhoon Nida(2016) was simulated to investigate the effects of anthropogenic gaseous emissions on the vortex system.Based on the Multi-resolution Emission Inventory for China (MEIC),three certain experiments were conducted:one with base-level emission intensity (CTRL),one with one-tenth the emission of SO2(SO2_C),and one with one-tenth the emission of NH3(NH3_C).Results show that the simulations reasonably reproduced the typhoon’s track and intensity,which were slightly sensitive to the anthropogenic gaseous emissions.When the typhoon was located over the ocean,a prolonged duration of raindrop growth and more precipitation occurred in CTRL run.The strongest updraft in CTRL is attributed to the maximum latent heating through water vapor condensation.During the landfalling period,larger (smaller) differential reflectivities in the main-core of the vortex were produced in NH3_C (SO2_C) run.Such opposite changes of raindrop size distributions may lead to stronger (weaker) rainfall intensity,and the ice-related microphysical processes and the relative humidity in the lower troposphere were two possible influential factors.Moreover,additional ten-member ensemble results in which white noise perturbations were added to the potential temperature field,indicated that the uncertainty of the thermodynamic field in the current numerical model should not be ignored when exploring the impacts of aerosol on the microphysics and TC precipitation.(Deng Lin,Gao Wenhua,Duan Yihong)

4.2 Analysis of an ensemble of high-resolution WRF simulations for the rapid intensification of super typhoon Rammasun (2014)

Diagnostics are presented from an ensemble of high-resolution forecasts that differed markedly in their predictions of the rapid intensification (RI) of Typhoon Rammasun.We show that the basic difference stems from subtle differences in initializations of (a) 500−850-hPa environmental winds,and (b) midlevel moisture and ventilation.We then describe how these differences impact the evolving convective organization,storm structure,and the timing of RI.As expected,ascent,diabatic heating and the secondary circulation near the inner-core are much stronger in the member that best forecasts the RI.The evolution of vortex cloudiness from this member is similar to the actual imagery,with the development of an inner cloud band wrapping inwards to form the eyewall.We present evidence that this structure,and hence the enhanced diabatic heating,is related to the tilt and associated dynamics of the developing inner-core in shear.For the most accurate ensemble member:(a) inhibition of ascent and a reduction in convection over the up-shear sector allow moistening of the boundary-layer air,which is transported to the down-shear sector to feed a developing convective asymmetry;(b) with minimal ventilation,undiluted clouds and moisture from the down-shear left quadrant are then wrapped inwards to the up-shear left quadrant to form the eyewall cloud; and (c) this process seems related to a critical down-shear tilt of the vortex from midlevels,and the vertical phase-locking of the circulation over upshear quadrants.For the member that forecasts a much-delayed RI,these processes are inhibited by stronger vertical wind shear,initially resulting in poor vertical coherence of the circulation,lesser moisture and larger ventilation.Our analysis suggests that ensemble prediction is needed to account for the sensitivity of forecasts to a relatively narrow range of environmental wind shear,moisture and vortex inner-structure.(Li Xun,Davidson Noel E,Duan Yihong)

4.3 Autumn tropical cyclones over the western North Pacific during 1949-2016:A statistical study

We used the tropical cyclone (TC) best track data for 1949−2016,provided by the Shanghai Typhoon Institute,China Meteorological Administration (CMA-STI),and a TC size dataset (1980−2016) derived from geostationary satellite infrared images to analyze the statistical characteristics of autumn TCs over the western North Pacific (WNP).We investigated TC genesis frequency,location,track density,intensity,outer size,and landfalling features,as well as their temporal and spatial evolution characteristics.On average,the number of autumn TCs accounted for 42.1% of the annual total,slightly less than that of summer TCs (42.7%).However,TCs classified as strong typhoons or super typhoons were more frequent in autumn than in summer.In most years of the 68-year study period,there was an inverse relationship between the number of autumn TCs and that of summer TCs.The genesis of autumn TCs was concentrated at three centers over the WNP:the first is located near (14°N,115°E) over the northeastern South China Sea and the other two are located in the vast oceanic area east of the Philippines around (14°N,135°E) and (14°N,145°E),respectively.In terms of intensity,the eight strongest TCs during the study period all occurred in autumn.It is revealed that autumn TCs were featured with strong typhoons and super typhoons,with the latter accounting for 28.1% of the total number of autumn TCs.Statistically,the average 34-knot radius (R34) of autumn TCs increased with TC intensity.From 1949 to 2016,164 autumn TCs made landfall in China,with an average annual number of 2.4.Autumn TCs were most likely to make landfall in Guangdong Province,followed by Hainan Province and Taiwan Island.(Yao Xiuping,Zhao Dajun,Li Ying)

4.4 Dependence of superintensity of tropical cyclones on SST in axisymmetric numerical simulations

This study revisits the superintensity of tropical cyclones (TCs),which is defined as the excess maximum surface wind speed normalized by the corresponding theoretical maximum potential intensity (MPI),based on ensemble axisymmetric numerical simulations,with the focus on the dependence of superintensity on the prescribed sea surface temperature (SST) and the initial environmental atmospheric sounding.Results show a robust decrease of superintensity with increasing SST regardless of being in experiments with an SSTindependent initial atmospheric sounding or in those with the SST-dependent initial atmospheric soundings as in nature sorted for the western North Pacific and the North Atlantic.It is found that the increase in either convective activity (and thus diabatic heating) in the TC outer region or theoretical MPI or both with increasing SST could reduce the superintensity.For a given SST-independent initial atmospheric sounding,the strength of convective activity in the TC outer region increases rapidly with increasing SST due to the rapidly increasing air–sea thermodynamic disequilibrium (and thus potential convective instability) with increasing SST.As a result,the decrease of superintensity with increasing SST in the SST-independent sounding experiments is dominated by the increasing convective activity in the TC outer region and is much larger than that in the SST-dependent sounding experiments,and the TC intensity becomes sub-MPI at relatively high SSTs in the former.Due to the marginal increasing tendency of convective activity in the TC outer region,the decrease of superintensity in the latter is dominated by the increase in theoretical MPI with increasing SST.(Li Yuanlong,Wang Yuqing,Lin Yanluan)

4.5 Differences in western North Pacific tropical cyclone activity among three El Niño phases

The impacts of El Niño on tropical cyclone (TC) activity over the western North Pacific (WNP) are examined through investigation of three types of tropical Pacific warming episodes according to where the maximum sea surface temperature (SST) anomalies occur in the equatorial Pacific:the eastern Pacific El Niño(EPE),the central Pacific El Niño (CPE),and the mixed El Niño (ME).More TCs form over the eastern part of the WNP in all the three El Niño types,whereas the frequency of TCs over the western part of the WNP increases as the peak SST anomalies migrate from east to west.Although TCs more frequently recurve at higher latitudes during EPE and CPE,the most frequent region for recurving is much closer to the East Asian continent in CPE years than in EPE years.In contrast,more TCs track westward and threaten the Philippines in ME years.The increased TC genesis over the western part of the WNP can be explained by enhanced lowlevel relative vorticity,reduced vertical wind shear,and increased maximum potential intensity during CPE and increased mid-level moisture during EPE and ME.This increase is further related to updraft anomalies near the date line driven by an anomalous Walker circulation and an anomalous low-level cyclonic circulation over the WNP.The TC track differences among the different El Niño types are linked to the east-west shift of the western Pacific subtropical high,possibly caused by an anomalous Hadley circulation from 120° to 130°E that is strongly coupled with the anomalous Walker circulation.(Song Jinjie,Klotzbach Philip J,Duan Yihong)

4.6 Different responses of tropical cyclone tracks over the western North Pacific and North Atlantic to two distinct sea surface temperature warming patterns

How future tropical cyclone (TC) activity could change under global warming is enormously important to society,which has been widely assessed using state-of-the-art climate models.However,these models were predominantly based on the projection of an El Niño-like warming pattern.Recent studies suggest that a La Niña-like warming pattern is also possible.Here we compare the responses of TC track density (TCTD) over the western North Pacific and North Atlantic to the two distinct global warming patterns.We find that the La Niña-like warming pattern reduces western North Pacific TCTD except in the South China Sea and along China coast and increases NA TCTD,while the El Niño-like warming pattern generally reduces TCTD in both basins.This is due to different responses of large-scale dynamic/thermodynamic conditions to the distinct zonal sea surface temperature gradients associated with the two warming patterns.These results help better understand potential future change in TC tracks.(Zhao Jiuwei,Zhan Ruifen,Wang Yuqing)

4.7 Effects of terrain and landmass near Fujian Province of China on the structure and propagation of a long-lived rainband in typhoon Longwang (2005):A numerical study

A similar to 14-hour long-lived spiral rainband in Typhoon Longwang (2005) produced catastrophic rainfall in Fujian Province of China on 2 October 2005.In this study,the effects of terrain and landmass near Fujian on the structure and propagation of this rainband are investigated through high-resolution numerical simulations.Results show that although the terrain and landmass near Fujian played a marginal role in the formation of the rainband,both greatly affected the structure and propagation of the rainband.Namely,convection in the upwind sector of the rainband tended to be maintained and locked up near the coastline in the control experiment with both the terrain and landmass near Fujian retained,but shrank more inland with the terrain near Fujian flattened,and further inland with the landmass near Fujian replaced by the virtual ocean.It is found that due to the land-sea surface roughness contrast,the upstream tangential winds from ocean would be substantially decelerated over land and thus induced a local subgradient force onshore near the coastline.The radially inward agradient force and the subsequent surface friction helped maintain the moisture convergence,and thus convection and the cold pool in the upwind sector of the rainband near the coastline.Although the orographic lifting and blocking effects were found to be marginal to the moisture convergence in the rainband,the terrains near Fujian enhanced the deceleration of surface winds,enhancing the effect of landsea surface roughness contrast on low-level moisture convergence and thus the lockup of the upwind sector of the rainband.(Li Yuanlong,Wang Yuqing,Lin Yanluan)

4.8 Factors affecting the weakening rate of tropical cyclones over the western North Pacific

In this study,based on the 6-hour tropical cyclone (TC) best track data and the ERA-Interim reanalysis data,statistical analyses as well as a machine learning approach,XGBoost,are used to identify and quantify factors that affect the overwater weakening rate (WR) of TCs over the western North Pacific (WNP) during 1980−2017.Statistical analyses show that the TC rapid weakening events usually occur when intense TCs cross regions with a sharp decrease in sea surface temperature (DSST) with relatively faster eastward or northward translational speeds,and move into regions with large environmental vertical wind shear (VWS) and dry conditions in the upshear-left quadrant.Results from XGBoost indicate that the relative intensity of TC (TC intensity normalized by its maximum potential intensity),DSST,and VWS are dominant factors determining TC WR,contributing 26.0%,18.3%,and 14.9% to TC WR,and 9,5,and 5 ms−1day−1to the variability of TC WR,respectively.Relative humidity in the upshear-left quadrant of VWS,zonal translational speed,divergence at 200 hPa,and meridional translational speed contribute 12.1%,11.8%,8.8%,and 8.1% to TC WR,respectively,but only contribute 2−3 m s−1day−1to the variability of TC WR individually.These findings suggest that the improved accurate analysis and prediction of the dominant factors may lead to substantial improvements in the prediction of TC WR.(Fei Rong,Xu Jing,Wang Yuqing)

4.9 How much does the upward advection of the supergradient component of boundary layer wind contribute to tropical cyclone intensification and maximum intensity?

Although the development of supergradient winds is well understood,the importance of supergradient winds in tropical cyclone (TC) intensification is still under debate.One view is that the spinup of the eyewall occurs by the upward advection of high tangential momentum associated with supergradient winds from the boundary layer.The other view argues that the upward advection of supergradient winds by eyewall updrafts results in an outward agradient force,leading to the formation of a shallow outflow layer immediately above the inflow boundary layer.As a result,the spinup of tangential wind in the eyewall by the upward advection of supergradient wind from the boundary layer is largely offset by the spindown of tangential wind due to the outflow resulting from the agradient force.In this study,the net contribution by the upward advection of the supergradient wind component from the boundary layer to the intensification rate and final intensity of a TC are quantified through ensemble sensitivity numerical experiments using an axisymmetric TC model.Results show that consistent with the second view above,the positive upward advection of the supergradient wind component from the boundary layer by eyewall updrafts is largely offset by the negative radial advection due to the outflow resulting from the outward agradient force.As a result,the upward advection of the supergradient wind component contributes little (often less than 4%) to the intensification rate and but it contributes 10%−15% to the final intensity of the simulated TC due to the enhanced inner-core air-sea thermodynamic disequilibrium.(Li Yuanlong,Wang Yuqing,Lin Yanluan)

4.10 Improved prediction of landfalling tropical cyclone in China based on assimilation of radar radial winds with new super-observation processing

This work explores the impact of assimilating radial winds from the Chinese coastal Doppler radar on track,intensity,and quantitative precipitation forecasts (QPF) of landfalling tropical cyclones (TCs) in a numerical weather prediction model,focusing mainly on two aspects:(1) developing a new coastal radar super-observation (SO) processing method,namely,an evenly spaced thinning method (ESTM) that is fit for landfalling TCs,and (2) evaluating the performance of the radar radial wind data assimilation in QPFs of landfalling TCs with multiple TC cases.Compared to a previous method of generating SOs (i.e.,the radially spaced thinning method),in which the density of SOs is equal within the radial space of a radar scanning volume,the SOs created by ESTM are almost evenly distributed in the horizontal grids of the model background,resulting in more observations located in the TC inner-core region being involved in SOs.The use of SOs from ESTM leads to more cyclonic wind innovation,and larger analysis increments of height and horizontal wind in the lower level in an ensemble Kalman filter data assimilation experiment with TC Mujigae (2015).Overall,forecasts of a TC’s landfalling position,intensity,and QPF are improved by radar data assimilation for all cases,including Mujigae and the other eight TCs that made landfall on the Chinese mainland in 2017.Specifically,through assimilation,TC landing position error and intensity error are reduced by 33% and 25%,respectively.The mean equitable threat score of extreme rainfall ( 80 mm per 3 hours)forecasts is doubled on average over all cases.(Feng Jianing,Duan Yihong,Wan Qilin)

4.11 Increasing destructive potential of landfalling tropical cyclones over China

This study investigates the trend in destructive potential of landfalling tropical cyclones (TCs) in terms of power dissipation index (PDI) over mainland China in the period of 1980−2018.Results show that both the accumulated PDI and averaged PDI after landfall show significant increasing trends.The increasing trends are found to be contributed primarily by the increasing mean duration of TCs over land and the increasing TC intensity at landfall.Further analyses indicate that the increase in landfalling TC intensity prior to and at landfall,the decrease in intensity weakening rate after landfall,and the northward shift of landfalling TC track density all contribute to the longer duration of TCs after landfall.Moreover,the conducive largescale conditions,such as the increases in coastal sea surface temperature and land surface temperature and soil moisture,the decrease in low-level vertical wind shear,and the increase in upper-level divergence,are all favorable for intense landfalling TCs and their survival after landfall,thus contributing to the increasing destructive potential of landfalling TCs over China.(Liu Lu,Wang Yuqing,Zhan Ruifen)

4.12 Increasing trend in rapid intensification magnitude of tropical cyclones over the western North Pacific

Rapid intensification (RI) refers to a significant increase in tropical cyclone (TC) intensity over a short period of time.A TC can also undergo multiple RI events during its lifetime,and these RI events pose a significant challenge for operational forecasting.The long-term tendency in RI magnitude of TCs over the western North Pacific is investigated in this study.During 1979−2018,a significant increasing trend is found in RI magnitude,which primarily results from the significant increasing number of strong RI events,defined as 24-hour intensity increases of at least 50 kt.Furthermore,there are significantly more (slightly fewer) strong RI occurrences west (east) of 155°E in 1999−2018 than in 1979−1998.Significant increases in strong RI occurrences are located over the region bounded by 10° similar to 20°N,120° similar to 150°E.These changes are likely induced by the warming ocean but appear uncorrelated with changes in the atmospheric environment.By contrast,there are slight decreases in strong RI occurrences over the region bounded by 12.5° similar to 22.5°N,155° similar to 170°E,likely due to the offset between RI-favorable influences of the warming ocean and RI-unfavorable influences of increasing vertical wind shear (VWS).(Song Jinjie,Duan Yihong,Klotzbach Philip J)

4.13 Infrared precipitation estimation using convolutional neural network

Infrared (IR) information is fundamental to global precipitation estimation.Although researchers have developed numerous IR-based retrieval algorithms,there is still plenty of scope for promoting their accuracy.This article develops a novel deep learning-based algorithm entitled infrared precipitation estimation using a convolutional neural network (IPEC).Based on the five-channel IR data,the IPEC first identifies the precipitation occurrence and then estimates the precipitation rates at hourly and 0.04°×0.04° resolutions.The performance of the IPEC is validated using the Stage-IV radar-gauge-combined data and compared to the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks-Cloud Classification System (PERSIANN-CCS) in three subregions over the continental United States (CONUS).The results show that the five-channel input is more efficient in precipitation estimation than the commonly used one-channel input.The IPEC estimates based on the five-channel input show better statistical performance than the PERSIANN-CCS with 34.9% gain in Pearson’s correlation coefficient (CC),38.0% gain in relative bias (BIAS),and 45.2% gain in mean squared error (MSE) during the testing period from June to August 2014 over the central CONUS.Furthermore,the optimized IPEC model is applied in totally independent periods and regions,and still achieves significantly better performance than the PERSIANN-CCS,indicating that the IPEC has a stronger generalization capability.On the whole,this article proves the effectiveness of the convolutional neural network (CNN) combined with the physical multichannel inputs in IR precipitation retrieval.This endto-end deep learning algorithm shows the potential for serving as an operational technique that can be applied globally and provides a new perspective for the future development of satellite precipitation retrievals.(Wang Cunguang,Xu Jing,Tang Guoqiang)

4.14 Location of the preferred region for tropical cyclogenesis in strong monsoon trough pattern over the western North Pacific

We examined the processes of tropical cyclogenesis in strong monsoon trough pattern over the western North Pacific (WNP) using reanalysis data and numerical experiments.Composite analysis showed that more tropical cyclones are likely to form in the central WNP (130°−165°E) and fewer tropical cyclones appear in the western (120°−130°E) and eastern (165°−180°E) WNP when monsoon trough extends southeastward.Numerical experiments with the same weak artificial vortices inserted into eight different regions of the monsoon trough showed that weak tropical disturbances tend to develop more rapidly in the central WNP near 140°−160°E,particularly near 150°−155°E when the monsoon trough extends eastward,whereas weak tropical disturbances tend to develop more slowly in the eastern WNP near 165°−170°E and do not form in the western WNP near 120°−137.5°E.Our modeling results are consistent with the observational analyses.The failure of tropical cyclogenesis in the western WNP is due to the decrease of the moisture and heat (including the sensible and latent heat) from the underlying ocean,whereas large vertical wind shear and dry conditions in the upper-level of the vortex reduce the gradient of intensification of tropical disturbances in the eastern WNP when the vortices have a similar initial intensity.(Cao Xi,Wu Renguang,Wei Na)

4.15 Occurrence and development of an extreme precipitation event in the Ili Valley,Xinjiang,China and analysis of gravity waves

We used observational data and the results from a high-resolution numerical simulation model to analyze the occurrence and development of an extreme precipitation event in the Ili Valley,Xinjiang,China on 26 June 2015.We analyzed the horizontal wavelength,period,speed,ducting,energy propagation and feedback mechanism of inertial gravity waves.A low-level convergence line was formed in the valley by the northerly and westerly winds as a result of Central Asian vortices and the trumpet-shaped topography of the Ili Valley.There was sufficient water vapor in the valley for the precipitation event to develop.A mesoscale vortex formed and developed on the low-level convergence line and the rainfall was distributed either near the convergence line or the mesoscale vortex.The low-level convergence line and the uplift caused by the terrain triggered convection,and then the convection triggered waves at lower levels.The combination of ascending motion induced by the lower level waves and the mesoscale vortex led to the development of convection,causing the precipitation to intensify.When the convection moved eastward to Gongliu County,it was coupled with the ascending phase of upper level waves,causing both the convection and precipitation to intensify again.We applied spectral analysis methods to verify that the waves were inertial gravity waves.The upper level inertial gravity waves propagated westward at a mean speed of -12 m s−1with periods of 73−179 min and horizontal wavelengths of 50−55 km.The lower level inertial gravity waves propagated eastward at a mean speed of 8 m s−1with periods of 73−200 min and a horizontal wavelength of 85 km.The more (less) favorable waveguide conditions determined whether the gravity waves persisted for a long (short) time and propagated for a longer(shorter) distance.Based on the mesoscale Eliassen-Palm flux theory,the wave energy of inertial gravity waves had an important effect on the maintenance and development of convection and precipitation by affecting wind strength and wind divergence.Feedback was mainly through the meridional and vertical transport of zonal momentum and the meridional transport of heat.(Huang Xin,Zhou Yushu,Liu Lu)

4.16 Recent increase in tropical cyclone weakening rates over the western North Pacific

This study investigates annual mean changes in 24-hour weakening rates of western North Pacific tropical cyclones (TCs) and annual number of rapid weakening (RW) events from 1982 to 2019.There is a significant increasing trend in the mean weakening rate,which correlates with a significant increase in RW number.Because stronger TCs are more likely to experience greater weakening rates and thus undergo RW,the increase in the ratio of intense TCs to all TCs can cause an increase in the weakening rate and RW frequency.Furthermore,the weakening rate and the RW number significantly increase over the region of 25°−35°N,120°−150°E from 1982−2000 to 2001−2019.Over this region,there are stronger sea surface temperature gradients in more recent years,likely contributing to a higher probability of TCs undergoing RW.Other environmental variables do not exhibit significant changes.Our study highlights inhomogeneous Pacific warming that may favor RW of TCs.(Song Jinjie,Klotzbach Philip J,Duan Yihong)

4.17 Relationship between spring tropical cyclone frequency over the western North Pacific and El Niño-Southern Oscillation

The number of tropical cyclones (TCs) over the western North Pacific (WNP) during spring (March-May)has a significant inverse correlation with concurrent El Niño-Southern Oscillation (ENSO) conditions during the period from 1979 to 2018.This relationship is different from the previously-documented weak relationship between TC frequency and ENSO during the climatologically most active portion of the TC season.In general,TCs seldom occur in El Niño years during March-May,whereas they frequently form over the western part of the WNP,particularly to the southeast of the Philippines,in La Niña years.This difference can be largely explained by ENSO-driven differences in the genesis potential index as derived from environmental variables.In La Niña years,the abnormally moist mid-troposphere,which relates to the strengthened vertical transport of water vapour induced by the enhanced Walker circulation,primarily favours TC development,while increased sea surface temperatures and positive low-level relative vorticity anomalies appear to play a lesser role in impacting TC formation.(Song Jinjie,Klotzbach Philip J,Duan Yihong)

4.18 Reply to “Comments on ‘How much does the upward advection of the supergradient component of boundary layer wind contribute to tropical cyclone intensification and maximum intensity?’”

This is a reply to the comments by Smith et al.(2020,hereafter SGM20) on the work of Li et al.(2020,hereafter LWL20) recently published in the Journal of the Atmospheric Sciences.All the comments and concerns by SGM20 have been well addressed or clarified.We think that most of the comments by SGM20 are not in line with the intention of LWL20 and provide one-sided and thus little scientifically meaningful arguments.Regarding the comment on the adequacy of the methodology adopted in LWL20,we believe that the design of the thought (sensitivity) experiment is adequate to address the scientific issue under debate and helps quantify the contribution by the upward advection of the supergradient component of boundary layer wind to tropical cyclone intensification,which is shown to be very marginal.Note that we are open to accept any alternative,better methods to be used to further address this scientific issue.(Li Yuanlong,Wang Yuqing,Lin Yanluan )

4.19 Revisiting the relationship between tropical cyclone size and intensity over the western North Pacific

This study revisits the nonlinear relationship between tropical cyclone (TC) size and intensity over the western North Pacific (WNP).Using best track data from the Japan Meteorological Agency for 2007−2019,we find that the statistical turning point on a fitted size-intensity curve only exists for TCs with greater lifetime peak intensities (e.g.,typhoons).The track and evolution type are two factors determining the statistical size-intensity relationship.The nonlinear size-intensity relationship is only observed for recurving typhoons reaching their peak intensities and sizes at different times,due to a nonmonotonic size change during the weakening stage.The slight size decrease in the initial weakening stage can be linked to a TC weakeninginduced shrinkage balanced by a baroclinic expansion when recurving typhoons move into higher latitudes.Finally,different evolution types for WNP typhoons can be determined by the ratio of intensity/size to its theoretical maximum.Tropical cyclone (TC) size and intensity are two primary metrics determining the destructive potential of a TC.Previous studies have reported a nonlinear size-intensity relationship,in which TC size substantially increases with intensity to an upper bound and then remains constant or even decreases with increasing intensity.Using best tracks from the Japan Meteorological Agency (JMA) for 2007−2019,we find that this statistical nonlinear size-intensity relationship only occurs for TCs with stronger lifetime maximum intensities (e.g.,typhoons).Whether the size-intensity relationship is linear or nonlinear is jointly determined by the track that the TC takes and its evolution type on a size-intensity diagram.There is a linear size-intensity relationship for recurving typhoons reaching their peak intensities and sizes simultaneously and for typhoons that are relatively straight moving.For other recurving typhoons,the nonlinear size-intensity relationship is primarily caused by a nonlinear size decrease during the weakening stage.We speculate that,as recurving typhoons move poleward,weakening TCs often begin to shrink,while the interaction between the TC and the midlatitude circulation leads to a baroclinic expansion of TC size.Finally,different evolution types for WNP typhoons can be determined by the ratio of intensity/size to its theoretical maximum.(Song Jinjie,Duan Yihong,Klotzbach Philip J)

4.20 Statistical and comparative analysis of tropical cyclone activity over the Arabian Sea and Bay of Bengal (1977-2018)

A statistical comparative analysis of tropical cyclone activity over the Arabian Sea (AS) and Bay of Bengal (BoB) has been conducted using best-track data and wind radii information from 1977 to 2018 issued by the Joint Typhoon Warning Center.Results show that the annual variation in the frequency and duration of tropical cyclones has significantly increased over time over the AS and insignificantly decreased over the BoB.The monthly frequency of tropical cyclones over the AS and the BoB shows a notable bimodal character,with peaks occurring in May,October and November,respectively.The maximum frequency of tropical cyclones occurs in the second peak as a result of the higher moisture content at mid-levels in the autumn.However,the largest proportion of strong cyclones (H1−H5 grades) occurs in the first peak as a result of the higher sea surface temperatures in early summer.Tropical cyclones over the AS break out later during the first peak and activity ends earlier during the second peak,in contrast with those over the BoB.This is related to the onset and drawback times of the southwest monsoon in the two basins.Tropical cyclones over the AS are mainly generated in the eastern basin,whereas in the BoB the genesis locations are meridionally (zonally) distributed in May−June (October−November) as a result of the seasonal movement of the low-level positive vorticity belt.The Arabian Sea is dominated by tropical cyclones that track west and northwest,accounting for about 74.6% of all the tropical cyclones there,whereas the tropical cyclones with a NE track account for only 25.4%.The proportions of the three types of tracks are similar in the BoB,with each accounting for about 33% of the tropical cyclones.The mean intensity and size of tropical cyclones over the AS are stronger and larger,respectively,than those over the BoB and the size of tropical cyclones over the North Indian Ocean in early summer is larger than that in the autumn.The asymmetrical structure of tropical cyclones over the North Indian Ocean is affected by topography and the longest radius of the 34 kt surface wind often lies in the eastern quadrant of the tropical cyclone circulation in both sea areas.(Fan Xiaoting,Li Ying,Lyu Aimin)

4.21 The effect of warm water and its weak negative feedback on the rapid intensification of typhoon Hato (2017)

Typhoon Hato (2017) went through a rapid intensification (RI) process before making landfall in Zhuhai,Guangdong Province,as the observational data shows.Within 24 hours,its minimum sea level pressure deepened by 35 hPa and its maximum sustained wind speed increased by 20m s−1.According to satellite observations,Hato encountered a large area of warm water and two warm core rings before the RI process,and the average sea surface temperature cooling (SSTC) induced by Hato was only around 0.73℃.Air-sea coupled simulations were implemented to investigate the specific impact of the warm water on its RI process.The results showed that the warm water played an important role by facilitating the RI process by around 20%.Sea surface temperature budget analysis showed that the SSTC induced by mixing mechanism was not obvious due to the warm water.Besides,the cold advection hardly caused any SSTC,either.Therefore,the SSTC induced by Hato was much weaker compared with that in general cases.The negative feedback between ocean and Hato was restrained and abundant heat and moisture were sufficiently supplied to Hato.The warm water helped heat flux increase by around 20%,too.Therefore,the warm water influenced the structure and the intensity of Hato.Although there might be other factors that also participated in the RI process,this study focused on air-sea interaction in tropical cyclone forecast and discussed the impact of warm water on the intensity and structure of a tropical cyclone.(Huo Zimo,Duan Yihong,Liu Xin)

4.22 Trends in landfalling tropical cyclone-induced precipitation over China

In this study,trends in landfalling TC-induced precipitation over China during 1980−2017 and the involved possible mechanisms are analyzed.Consistent with previous studies,it is found that the total annual TC precipitation shows a distinct spatial distribution with a significant increasing trend in the southeastern China but a decreasing trend in the southern China.This characteristic is found to be related to the increase in both the annual TC precipitation frequency and the precipitation intensity per TC over the southeastern China but to the decrease in the annual TC precipitation frequency over the southern China.A noticeable northward shift of total landfalling TC-induced annual precipitation has been identified.It is shown that the precipitation induced by strong TCs (STCs) significantly increased in the southern China,whereas that induced by weak TCs (WTCs) increased in the southeastern China,with the latter dominating the northward shift of total landfalling TC-induced precipitation over mainland China.The increasing trend of STC-induced precipitation in the southern China is found to be closely related to sufficient water vapor supply and the increase in average duration and intensity of STCs after landfall.The increasing trend of WTC-induced precipitation in the southeastern China is related to the northward shift of the average landfalling position of WTCs and changes in the environmental conditions that are more favorable for TC maintenance and precipitation.(Liu Lu,Wang Yuqing)

4.23 川藏地区雷暴大风活动特征和环境因子对比

利用2010—2017年中国气象局重要天气报、地面观测和探空资料以及欧洲中期天气预报中心ERA-Interim再分析资料，对川藏地区雷暴大风的活动特征、环境因子和环流形势进行统计分析，并对其中高原（海拔高度不低于1 km）和盆地（海拔高度低于1 km）区域雷暴大风活动进行对比。结果表明：川藏高原区域雷暴大风频次呈5—6月和9月双峰型分布，主要发生在午后；盆地区域主要发生在夏季，午后和夜间均较活跃。高原站雷暴大风年平均频次约为2次/站，在雷暴和大风中分别约占4.5%和8%。盆地站年平均频次仅为0.4次/站，雷暴中仅占1.5%，但在大风中约占60%。高原站雷暴大风的中低层环境温度递减率较大，一般呈上湿下干的逆湿垂直结构；而盆地站雷暴大风通常具有上干下湿的垂直结构。分别对5—6月和9月高原站雷暴大风两个峰值时段的环流形势进行合成分析，发现5—6月受高空西风槽影响，中层有弱冷平流侵入，高层位于高空急流入口区右侧，环境垂直风切变较大；而9月受副热带高压边缘影响，中高层较干，低层暖湿气流明显。这些均有利于雷暴大风发生。（王黉，李英，宋丽莉）

4.24 华东登陆热带气旋降水不同分布的对比分析

利用CMORPH降水资料，将热带气旋（TC）登陆后的降水分为路径左侧降水（L型）和右侧降水（R型）两类，并针对登陆华东地区TC的L型和R型降水的大气环流场、环境水平风垂直切变以及台风环流内的动热力条件进行对比分析，结果表明：2005ü 2014年间登陆华东地区的20例TC中包括12例L型和8例R型。总体来看，大气环流因子对于登陆华东TC降水分布起主要作用。L型降水TC高层南亚高压主要呈纬向带状分布，在登陆过程中路径左侧维持偏东风高空辐散气流，中层西风槽偏东，西太平洋副热带高压（简称副高）偏南，环境水平风垂直切变指向西南。R型降水TC高层南亚高压断裂，呈经向分布。TC路径左侧风场较均匀，右侧东南风高空辐散气流明显。副高的位置偏北呈块状，同时环境水平风垂直切变指向东北，有利于路径右侧降水。台风环流内，低层冷暖平流输送以及水汽辐合与降水落区也有较好对应关系。L型TC低层暖平流的输送使TC西南象限低层增暖，大气稳定度降低。同时水汽辐合区也主要位于西南象限，有利于TC路径左侧降水。而R型TC副高位置偏北可将南侧的东南暖湿气流向台风环流更西部输送，东北象限维持暖平流，有利于路径右侧降水发生。（卜松，李英）

4.25 热带气旋过台湾后再次登陆的路径强度变化统计分析

登陆台湾后再次登陆大陆的热带气旋（TC）由于受复杂下垫面及中低纬天气系统的共同影响，过岛后在海峡内的路径、强度及结构变化复杂，导致登陆大陆的精确化定位、定强及预报难度大。分析了1949—2017年二次登陆的81个热带气旋路径及强度变化特征，并对上海台风所（CMA/STI）、美国联合台风警报中心（JTWC）及东京区域台风中心（RSMC-Tokyo）的热带气旋最佳路径数据中过岛热带气旋的定位定强进行对比分析。结果表明：二次登陆大陆热带气旋强度以减弱为主，少数热带气旋在海峡内增强；过岛后热带气旋路径多数会发生明显偏折，但3家最佳路径资料判断的偏折趋势不一致；由于热带气旋过岛时结构遭到破坏，定位定强难度增大，导致3个业务中心对其定位定强的差异较大，这种不确定性增大了其路径和强度监测预报的难度。（邢蕊，徐晶，林瀚）

4.26 热带气旋研究和业务预报进展——第九届世界气象组织热带气旋国际研讨会（IWTC-9）综述

2018年12月3—7日在美国夏威夷召开的第九届世界气象组织热带气旋国际研讨会对国际热带气旋研究和预报预警业务的近期进展进行了充分回顾，并对未来的发展方向进行了系统研讨。过去4年（2015—2018年），在热带波动和对流对热带气旋生成的影响、复杂环境影响热带气旋路径转向的机理、对流和风垂直切变对热带气旋强度变化的作用机制、双眼墙形成和替换机理、热带气旋变性过程对环境条件的响应、热带气旋气候变化特征、热带气旋多平台观测技术等诸多方面的研究取得了显著进展。同时，在热带气旋生成概率预报、全球模式中热带气旋强度预报改进、热带气旋活动次季节业务预测技术发展以及热带气旋预报不确定性理论等方面也取得积极进展。在综述此次会议的相关议题后，简要讨论中国热带气旋研究和业务发展方向。（端义宏，方娟，程正泉）

5 雷电研究

5 Lightning research

5.1 雷电野外科学试验

雷电团队克服新冠疫情的不利影响，于2020年6月中旬开始组织人工触发闪电试验。虽然受试验期雷暴系统明显偏少的影响，未能成功触发闪电，但团队进一步强化了野外试验能力，包括：发展实现了低频电场探测阵列（LFEDA）的实时定位技术，扩充了广东地区闪电成像阵列（LMA）站网，设计了未来5年人工引雷试验场的建设规划和目标。另一方面，团队在3—10月组织广州高建筑物雷电观测试验，在广州粤大站补充了同步电磁场探测设备，在广东省气象局主站增设了雷电光谱观测项目，获得超过150例高建筑物观测个例，并通过对历史资料的整理，建立了包含有340余例高建筑物雷电的光学和电磁场同步综合观测数据集。（吕伟涛，张阳，郑栋，马颖，樊艳峰）

5.2 雷电探测技术研发

研制了新一代精细化低频全闪定位系统，形成了低功耗、一体化的闪电低频信号探测子站，通过云平台实现了全闪实时定位，定位数据丰富度相比业务系统有明显提升。提出了基于双向双面镜像延拓的集合经验模态分解算法，实现了优异的降噪性能，对弱信号的提取准确度大幅提升，进一步提升闪电定位系统性能。通过与TRMM/LIS闪电数据和WWLLN闪电数据的对比，揭示了FY-4A闪电成像仪（LMI）的闪电时空分布特征和光学辐射特性，评估了LMI的探测性能。客观评估了美国国家闪电探测网（NLDN）对高塔闪电的定位误差，发现沿海地区系统偏差较内陆大，定位结果整体偏向海洋一侧。（张阳，范祥鹏，刘恒毅）

5.3 雷电物理研究

发现当建筑物高度确定，闪电回击峰值电流越强，对应的闪击距离越大，并探讨了闪击距离与上行连接先导起始时间的相关性。改进闪电先导随机模式，模拟研究指出600 m高的建筑物被雷击的概率为100 m高的建筑物的3.6倍，高建筑物会使附近击中地面的闪电的接地点向建筑物方向偏移。发现触发闪电起始过程存在单次和多次两种类型，揭示了始发自持发展开始的两个关键物理过程——先驱放电簇和始发先驱放电簇的异同。提出长时序电流由击穿电流、电晕电流和准稳恒电流构成的3组分模型，建立了基于PSO算法的长时序触发闪电回击电流的分步式参数化重构方法，实现了在400 μs的长时间尺度上重构回击电流，并给出各个分量的参数特征。首次解析出触发闪电信号平静期内的磁场脉冲，明确了“梯级”传播形式是上行正先导的一般发展特征；发现了甚高频辐射脉冲簇出现在单个磁场脉冲产生的初始阶段，揭示出不同频段的辐射信号对应了不同的放电过程。研究了上行负先导发展过程与母体雷暴电荷结构的关系，估算了上行负先导的放电参数，明确了电荷结构以及初始云内过程对上行负先导始发的重要作用，提出了一种上行负先导的始发机理模型。（吕伟涛，樊艳峰，武斌）

5.4 雷暴电学研究

建立了云分档电模型并改进了融化起电物理过程的参数化方法，发现层云区融化层经常观测到的正电荷区是冰相粒子的非感应融化起电产生的，同时层云区电荷主要源于对流核区的输送。明确了中尺度对流系统层云区内闪电的主要激发和传播区域特征及其对应的垂直反射率结构，揭示了层云区闪电发生区域与回波亮带区域之间特殊且密切的空间关系。利用WRFDA-3DVAR同化系统，通过调整模式相对湿度的方法，同化闪电资料，并利用台风进行模拟检验，发现在台风快速增强阶段前期，同化眼壁区域闪电具有最好的预报效果。在暖云降水主导的暴雨过程中，发现主要闪电活动偏离对流和强降水核心区以及闪电位置主要对应干雪粒子的现象，建立了概念模型，从弱对流系统中电荷产生速率与电荷向外输送速率的对比关系上对发现进行了解释。（郑栋，张文娟，王飞）

5.5 雷电预警预报技术研发与应用

构建了融合WRF模式产品和闪电观测数据的ADSNet和LightNet两种雷电短时预报AI模型，设计并初步开发完成了雷电短时预报系统，通过对AI模型的调用，给出未来0～12 h内逐小时闪电活动落区预报，并实现短时预报产品的自动分析、生成、存储、传输及显示。经过对比评估，该系统相比与单独使用WRF_Elec模式，在4 km×4 km网格分辨率下，6 h闪电累积预报TS评分提高约3倍。（姚雯，孟青，徐良韬）

5.6 An improved hydrometeor identification method for X-band dual-polarization radar and its application for one summer hailstorm over northern China

Currently,the high-resolution severe storm observations obtained by vehicle-mounted mobile X-band dual-polarization radar (X-pol) are widely used in severe storm structure and dynamics studies; however,the identification of hailstones based on X-pol is still rare,leading to the limited application of X-pol in identifying hydrometeors in the microphysical processes of severe storms.Based on previous works,this study constructs an improved X-pol hydrometeor identification (HID) method for hailstorms; the hydrometeor categories include a rain-hail mixtures (RH) category indicating hailstones are melting or experiencing wet growth,and the correlation coefficient between horizontally and vertically polarized echoes is used to locate the local mixed phase region in storms to avoid misclassifications of hydrometeors resulted by environmental temperature in this local mixed phase region.The improved HID method can see liquid water information above the melting layer; it provides a possibility to observe the microphysical process of hailstones wet growth above the melting layer.Furthermore,this improved HID method was utilized to study the high spatiotemporal resolution data collected by the Institute of Atmospheric Physics of Chinese Academy of Sciences (IAP) X-pol for one summer hailstorm over the northern China.The results indicate that the identified RH is consistent with the surface hail record of National Weather Observatory of the China Meteorological Administration (CMA),the specific differential phase (K-DP) column is more sensitive than the differential reflectivity (ZDR) column for indicating the updraft zone movement,and we considered that the relative position between the key area of hail (KAH,where above theZDRcolumn,ZDRsimilar to -2 to 0 dBz,ZHsimilar to≥50 dBz) and the updraft zone (indicated by theZDR,K-DP column) will determine the evolution of hail.Furthermore,high-density graupels (HDGs) in the upper layer of the KAH may be the source of RH embryos,and these HDGs collect liquid droplets from the liquid water-rich (LWR) area located behind the KAH to form a large number of RH in the lower layer of the KAH.These characteristics are important for hailstone forecasting and warnings.(Zhao Chuanhong,Zhang Yijun,Zheng Dong)

5.7 Analysis of location errors of the US National Lightning Detection Network using lightning strikes to towers

The location accuracy of the US National Lightning Detection Network (NLDN) has been evaluated using as ground-truth rocket-triggered lightning data or video records but only at a few specific locations.In this study,by using the NLDN data for the events attributable to lightning strikes to towers,the location error of the NLDN across the entire contiguous United States was evaluated for the first time.We found that,on average,the NLDN median location error reduced from 198 to 84 m after the 2013 NLDN upgrade.The location error at the periphery of the network was significantly larger than that in its interior.In the coastal regions,there was directional location bias toward the water.Simulation results suggest that the bias is related to the lengthening of field waveform front due to electromagnetic wave propagation over lossy ground coupled with the asymmetrical sensor configuration relative to the strike point (lack of offshore sensors).(Zhu Yanan,Lyu Weitao,Cramer John)

5.8 Application of ensemble empirical mode decomposition in low-frequency lightning electric field signal analysis and lightning location

The application of empirical mode decomposition (EMD) in the analysis and processing of lightning electric field waveforms acquired by the low-frequency e-field detection array (LFEDA) in China has significantly improved the capabilities of the low-frequency/very-low-frequency (LF/VLF) time-of-arrival technique for studying the lightning discharge processes.However,the inherent mode mixing and the endpoint effect of EMD lead to certain problems,such as an inadequate noise reduction capability,the incorrect matching of multistation waveforms,and the inaccurate extraction of pulse information,which limit the further development of the LFEDA’s positioning ability.To solve these problems,the advanced ensemble EMD(EEMD) technique is introduced into the analysis of LF/VLF lightning measurements,and a double-sided bidirectional mirror (DBM) extension method is proposed to overcome the endpoint effect of EMD.EEMD can effectively suppress mode mixing,and the DBM extension method proposed in this article can effectively suppress the endpoint effect,thus greatly improving the accuracy of a simulated signal after a 25–500-kHz bandpass filter.The resulting DBM_EEMD algorithm can be used in the LFEDA system to process and analyze the detected electric field signals to improve the system’s lightning location capabilities,especially in terms of accurate extraction and location of weak signals from lightning discharges.In this article,a 3-D image of artificially triggered lightning obtained from an LF/VLF location system is reported for the first time,and methods for further improving the location capabilities of the LF/VLF lightning detection systems are discussed.(Fan Xiangpeng,Zhang Yijun,Paul R.Krehbiel)

5.9 Application of lightning data assimilation to numerical forecast of super typhoon Haiyan (2013)

Previous observations from World Wide Lightning Location Network (WWLLN) and satellites have shown that typhoon-related lightning data have a potential to improve the forecast of typhoon intensity.The current study was aimed at investigating whether assimilating TC lightning data in numerical models can play such a role.For the case of Super Typhoon Haiyan in 2013,the lightning data assimilation (LDA) was realized in the Weather Research and Forecasting (WRF) model,and the impact of LDA on numerical prediction of Haiyan’s intensity was evaluated.Lightning data from WWLLN were used to adjust the model’s relative humidity (RH) based on the method developed by Dixon et al.(2016).The adjusted RH was output as a pseudo sounding observation,which was then assimilated into the WRF system by using the three-dimensional variational (3DVAR) method in the cycling mode at 1-hour intervals.Sensitivity experiments showed that,for Super Typhoon Haiyan (2013),which was characterized by a high proportion of the inner-core (within 100 km from the typhoon center) lightning,assimilation of the inner-core lightning data significantly improved its intensity forecast,while assimilation of the lightning data in the rainbands (100−500 km from the typhoon center) led to no obvious improvement.The improvement became more evident with the increase in LDA cycles,and at least three or four LDA cycles were needed to achieve obvious intensity forecast improvement.Overall,the improvement in the intensity forecast by assimilation of the inner-core lightning data could be maintained for about 48 hours.However,it should be noted that the LDA method in this study may have a negative effect when the simulated typhoon is stronger than the observed,since the LDA method cannot suppress the spurious convection.(Zhang Rong,Zhang Wenjuan,Zhang Yijun)

5.10 Characterizing radio frequency magnetic radiation of initial upward leader stepping in triggered lightning with interferometric lightning mapping

In summer of 2019,the bandwidth of magnetic field sensor with relatively high sensitivity was extended to 1.2 MHz during the triggered lightning experiment of the Field Experiment Base on Lightning Sciences,the China Meteorological Administration (CMA-FEBLS) in Conghua,Guangdong Province.The measurements with the new magnetic fields reveal the presence of microsecond-scale magnetic pulses during the entire duration of upward positive leader (UPL),including the quiet stage when only few signals can be discerned in previous observations,which indicates that the UPL generally propagates in a stepwise manner during the initial stage of triggered lightning.Synchronous mapping observations from the broadband VHF interferometer shows that the VHF radiation corresponds to the onset of individual magnetic pulses,indicating that the VHF signals are radiated by the breakdown processes of individual stepping,and these breakdown events launch the meter-scale current pulses as the radiation source of individual magnetic pulses.(Fan Yanfeng,Lu Gaopeng,Zhang Yang)

5.11 Evolution characteristics during initial stage of triggered lightning based on directly measured current

The initiation of a leader is an important lightning discharge process,but how an upward positive leader(UPL) initiates is still not fully understood.The evolution characteristics of a UPL during its initial stage were systematically studied based on directly measured current data of 14 triggered lightning events in 2019.It was found that the initial stage of triggered lightning can be divided into two types:a single initial process form and a multiple initial process form,with percentages of 64.29% and 35.71%,respectively.Compared with the former,the latter usually lasts longer,and the corresponding lightning is often triggered under a lower groundlevel quasi-static electric field.In each initial process,precursor current pulses (PCPs),PCP clusters and initial precursor current pulses (IPCPs) are typical current waveforms,and the pulse durations and transferred charges of PCPs increase linearly with time.However,in the multiple initial process form,the pulse durations and transferred charges of PCPs will reduce significantly after each previous initial process and then continue to increase in the following initial process.In each initial process,when the pulse duration and transferred charge of a PCP increase to a certain extent,PCP clusters and IPCPs begin to appear.For the emergence of PCP clusters,the average values of the threshold are 3.48 µs and 19.53 µC,respectively.For the occurrence of IPCPs,the corresponding values are 4.69 µs and 27.23 µC,respectively.The average values of pulse durations and transferred charges of IPCPs are larger than those of PCP clusters.Compared with adjacent PCP clusters,IPCPs contain more pulses,with a critical range of 6−7.IPCPs also last longer,and have a critical range of 138−198 µs.(Chen Zefang,Zhang Yang,Fan Yanfeng)

5.12 FY-4A LMI observed lightning activity in super typhoon Mangkhut (2018) in comparison with WWLLN data

Using lightning observations from the Fengyun-4A Lightning Mapping Imager (FY-4A LMI),besttrack data from the Shanghai Typhoon Institute,bright temperature (TBB) data from Himawari-8 satellite,and composite reflectivity (CR) data from the South China radar network,we investigate the spatiotemporal distribution of lightning activity and convective evolution during the landfall of Super Typhoon Mangkhut,the strongest landing typhoon in China in 2018.Three stages of active total lightning are observed,and differences of lightning characteristics between the inner core and the outer rainbands are present.The onset of innercore lightning outbreak is about 4 hours ahead of the maximum intensity of the storm,providing indicative information on the change of typhoon intensity.Lightning rates in the outer rainbands increase rapidly 12 hours before the landfall,and lightning activity is mainly confined in the outer rainbands after the landfall.A good correlation in hourly variation is shown between lightning rates from the LMI and TBBs from the satellite.The averaged TBB within the inner core reaches its minimum (−80 ℃) when the inner-core lightning outbreak occurs,indicating the occurrence and enhancement of deep convection there.Lightning locations observed by the LMI has a good spatial correspondence with regions of low TBBs and high CRs,revealing the monitoring capability of the LMI to lightning activity and deep convection in landing typhoons.Comparisons between the World Wide Lightning Location Network (WWLLN) and the LMI reveal that the spatial distribution,temporal evolution,and radial pattern of lightning activity in Mangkhut observed by the two systems are consistent.Furthermore,due to the detection capability of total lightning,the LMI has advantages in revealing the higher ratio of intra-cloud lightning within the inner core in typhoon.The continuous and real-time observation of FY-4A LMI provides an unprecedented platform for monitoring total lightning and deep convection in landing typhoons in China,which will promote the generation of new research and applications in the future.(Zhang Wenjuan,Hui Wen,Lyu Weitao)

5.13 How does the melting impact charge separation in squall line? A bin microphysics simulation study

A new electrification and discharge model was developed based on a two-moment bin microphysical scheme coupled with the Weather Research and Forecasting (WRF) model.Based on the electrical model,the role of the noninductive charging mechanism associated with the melting processes of both snow and graupel(rimed particles) in the charge structure formation in the stratiform region of an organized convective system was examined.Our results show that the snow melting charging mechanism forms a substantial positive charge layer near and below 0 ℃ isotherm in the stratiform region of a squall line.It was also found that the graupel melting charging process mostly enhanced the positive charge layer in the convective region with little impact in the stratiform region.The in situ charging of noninductive collisional and melting processes and the charge transportation from the convective core all contribute to the charge structure formation in the stratiform region of a squall line.(Xu Liangtao,Xue Lulin,Geresdi Istvan)

5.14 Leader charges,currents,ambient electric fields,and space charges along downward positive leader paths retrieved from ground measurements in metropolis

An approach for retrieving the leader channel charge density,leader current,ambient electric field,and space charge along a leader channel path from ground observations with a complicated ground condition was presented.With this approach,properties of two downward positive leaders (DPL1 and DPL2) struck at highrise buildings were studied based on electric and optical measurements made on the roof of a 100 m high building in Guangzhou,China.It shows that the leader-produced electric field on the roof of the 100 m high building was about 4 times of that on flat ground.The 2-D speed for both leaders showed a general increasing trend as the leader going down,in the range of 1.8×105to 32.3×105and 1.7×105to 46.9×105m s−1,respectively.The channel line charge density for both leaders showed firstly a sharp increasing trend and then a decreasing trend as the leader going down,in the range of 0.4 to 8.6 and 0.4 to 15.2 mC m−1,respectively.The current of the two leaders varied in the range of 0.7 to 5.4 and 0.7 to 4.6 kA,respectively.The ambient electric field(downward is positive) for both leaders showed an alternating polarity along the leader path,in the range of −120 to +176 and −250 to +180 kV m−1,respectively.The space charge for both leaders showed also an alternating polarity along the leader path,in the range of −12.3 to +4.2 and −13.9 to +7.8 nC m−3,respectively,which may reflect the in-cloud electric structure and the corona charge distribution between the cloud and ground.(Gao Yan,Chen Mingli,Lyu Weitao)

5.15 Lightning activity and its associations with cloud structures in a rainstorm dominated by warm precipitation

Lightning activity and its associations with cloud structures during a rainstorm dominated by warm cloud precipitation were studied in Guangdong,China on May 7,2017,using three-dimensional lightning location and polarimetric radar data.The overall convection and lightning activities of the rainstorm were weak.The rainstorm generally showed a typical tripolar charge structure with the main negative charge core located between the −15 ℃ and −8 ℃ environmental isotherms in the first 4 hours.The height of the charge regions clearly decreased after this period,with the main negative charge core being below the −8 ℃ isotherm.Lightning discharges were more concentrated in areas featuring relatively weak convection and relatively low precipitation intensity.Most of the locations with lightning discharges were dominated by dry aggregated snow and weak updrafts and downdrafts.This investigation demonstrated that the lightning discharges were spatially separated from the area of origin of charging in this rainstorm.It is proposed that,with weak convection in the rainstorm,the charging rate was lower than the speed of charge transfer from the area of origin,causing a relatively low charge density and a low frequency of lightning in the area of origin of charging.Meanwhile,the aggregation of small charged particles in regions away from the area of origin of charging might be conducive to the formation of a relatively high charge density and therefore relatively frequent lightning flashes.This situation is different from a typical thunderstorm with strong convection.(Liu Ze,Zheng Dong,Guo Fengxia)

5.16 Lightning and deep convective activities over the Tibetan Plateau

Lightning characteristics and its relationship with convection over the plateau are of great interest to researchers.High-density lightning is located near Naqu and the northeastern Qinghai.About 95% of the flashes occur during May to September.The ratio of intra-cloud lightning to cloud-to-ground (CG) lightning in the midsouthern plateau is approximately four.The lightning over the plateau is typically of shorter duration,smaller horizontal extent,and weaker optical radiance than that over the inland eastern China.The deep convective systems (DCSs,with 20-dBz echo tops exceeding 14 km) over the Tibet Plateau are of smaller horizontal scale and weaker convective intensity.The DCSs around the Naqu area have a mean area of about 120 km2,with mean 20- and 40-dBz echo tops of approximately 15.8 and 9.9 km,respectively.The corresponding values for intense DCSs (IDCSs,with 40-dBz echo tops exceeding 10 km) are approximately 196 km2,16.5 km,and 11.3 km,respectively.The average area and height of precipitation clouds with lightning are an order of magnitude larger and 2–4 km higher than those without lightning,respectively.On average,the thunderstorms over the eastern plateau tend to have larger horizontal extent and those over the south-bywest and southeastern parts of the plateau tend to a have higher top.The plateau thunderstorm tends to have a larger-than-usual lower positive charge center (LLPCC),which is suggested to be associated with the weak convection and low freezing level of the plateau.(Zheng Dong,Zhang Yijun,Zhang Yang)

5.17 Parametric reconstruction method for the long time-series return-stroke current of triggered lightning based on the particle swarm optimization algorithm

The lightning research group of the Chinese Academy of Meteorological Sciences has carried out observations and experiments with artificially triggered lightning for more than ten years,and it accumulated thousands of examples of channel base current data on the return stroke of artificially triggered lightning prior to 2020.Based on the current data,this paper explores ways of improving the constructed function of a long time-series (400 µs) return stroke and the parametric reconstruction of the current waveform.The long timeseries current can be divided into three components:the breakdown pulse current,corona current and quasiuniform current.The corona current and quasi-uniform current are constructed by a Heidler function,while the breakdown pulse current is determined by the waveform characteristics of the current peak,which can be divided into the Heidler type and high-order exponential type.According to the three-component model of the return-stroke current,a two-step parametric reconstruction method for the long time-series lightning returnstroke current based on the particle swarm optimization (PSO) algorithm is proposed.In this paper,the results of the parameterized reconstruction of 14 return strokes are given,and the results of the multidimensional error analysis of 13 long time-series return strokes are given to illustrate the accuracy of the improved parameterized model of the return-stroke current and the effectiveness of the two-step reconstruction method based on the PSO algorithm.(Fan Xiangpeng,Yao Wen,Zhang Yang)

5.18 Preliminary observations from the China Fengyun-4A lightning mapping imager and its optical radiation characteristics

The Fengyun-4A (FY-4A) Lightning Mapping Imager (LMI) is the first satellite-borne lightning imager developed in China,which can detect lightning over China and its neighboring regions based on a geostationary satellite platform.In this study,the spatial distribution and temporal variation characteristics of lightning activity over China and its neighboring regions were analyzed in detail based on 2018 LMI observations.The observation characteristics of the LMI were revealed through a comparison with the Tropical Rainfall Measuring Mission (TRMM)−Lightning Imaging Sensor (LIS) and World Wide Lightning Location Network (WWLLN) observations.Moreover,the optical radiation characteristics of lightning signals detected by the LMI were examined.Factors that may affect LMI detection were discussed by analyzing the differences in optical radiation characteristics between LMI and LIS flashes.The results are as follows.Spatially,the flash density distribution pattern detected by the LMI was similar to those detected by the LIS and WWLLN.Highflash density regions were mainly concentrated over the southeastern China and northeastern India.Temporally,LMI flashes exhibited notable seasonal and diurnal variation characteristics.The LMI detected a concentrated lightning outbreak over the northeastern India in the premonsoon season and over the southeastern China in monsoon season,which was consistent with LIS and WWLLN observations.LMI-observed diurnal peak flash rates occurred in the afternoon over most of the regions.There was a “stepwise” decrease in the LMI-observed optical radiance,footprint size,duration,and number of groups per flash,from the ocean to the coastal regions to the inland regions.LMI flashes exhibited higher optical radiance but lasted for shorter durations than LIS flashes.LMI observations are not only related to instrument performance but are also closely linked to onboard and ground data processing.In future,targeted improvements can be made to the data processing algorithm for the LMI to further enhance its detection capability.(Hui Wen,Zhang Wenjuan,Lyu Weitao)

5.19 Quantification of precipitation using polarimetric radar measurements during several typhoon events in the southern China

Accurate quantitative precipitation estimation (QPE) during typhoon events is critical for flood warning and emergency management.Dual-polarization radar has proven to have better performance for QPE,compared to traditional single-polarization radar.However,polarimetric radar applications have not been extensively investigated in China,especially during extreme events such as typhoons,since the operational dual-polarization system upgrade only happened recently.This paper extends a polarimetric radar rainfall system for local applications during typhoons in the southern China and conducts comprehensive studies about QPE and precipitation microphysics.Observations from S-band dual-polarization radar in Guangdong Province during three typhoon events in 2017 are examined to demonstrate the enhanced radar rainfall performance.The microphysical properties of hydrometeors during typhoon events are analyzed through raindrop size distribution (DSD) data and polarimetric radar measurements.The stratiform precipitation in typhoons presents a lower mean raindrop diameter and lower raindrop concentration than those of the convection precipitation.The rainfall estimates from the adapted radar rainfall algorithm agree well with rainfall measurements from rain gauges.Using the rain gauge data as references,the maximum normalized mean bias (NMB) of the adapted radar rainfall algorithm is 20.27%; the normalized standard error (NSE) is less than 40%; and the Pearson’s correlation coefficient (CC) is higher than 0.92.For the three typhoon events combined,the NSE and NMB are 36.66% and −15.78%,respectively.Compared with several conventional radar rainfall algorithms,the adapted algorithm based on local rainfall microphysics has the best performance in the southern China.(Xia Qiulei,Zhang Wenjuan,Chen Haonan)

5.20 Quantifying the contribution of tropical cyclones to lightning activity over the Northwest Pacific

This study quantifies the impacts of tropical cyclones (TCs) to the geographical and seasonal lightning activity over the Northwest Pacific (5°−35°N,100°−160°E),and examines the connection between the occurrence of TC lightning and El Niño-Southern Oscillation (ENSO).Lightning data from the World Wide Lightning Location Network (WWLLN) and best-track data from the China Meteorological Administration are used to investigate 263 TCs for TC seasons (June−November) in the years 2005−2017.Results show that,on average,TCs account for about 4.9% of all lightning in the domain.The highest contributions occur in the northern South China Sea and the ocean northeast of the Philippines.The monthly TC contribution varies from 2.6% to 6.1% with the greatest in July and the lowest in June.The peak location of the lightning center contributed by TCs are observed a poleward shift from early summer (June−July) to late summer (August−September),and an equatorial shift from late summer to autumn (October−November).In terms of TC intensity,tropical storm strength TCs are the dominant lightning contributor (2.0%) over the Northwest Pacific.A greater likelihood of lightning activity in tropical storms than in typhoons is observed during the landfall,especially within the inner core region.Super typhoons produce lightning in more eastward longitude and more equatorward latitude than weaker strength TCs.The relationship between TC lightning and ENSO reveals that TCs contribute greater lightning during La Niña periods (5.0%) than El Niño periods (3.2%) over the Northwest Pacific.In El Niño years,super typhoons contribute the greatest lightning amounts (1.8%),while in La Niña years tropical storms contribute the greatest (2.2%).Infrared satellite observations indicate that the high contribution of lightning in tropical storms is due to the large occurrence of very deep convection in these storms during their landfall.(Zhang Wenjuan,Zhang Yijun,Zheng Dong)

5.21 Simulation of cloud-to-ground lightning strikes to structures based on an improved stochastic lightning model

An improved stochastic method for computer simulation of lightning leaders is developed based on the results from optical observation data.The development and attachment process of downward negative cloudto-ground lightning in the near-ground area is simulated.The distribution of lightning strike points influenced by tall structures is statistically analyzed.The results show that when downward negative leaders initiate at 1500 m height over a structure,the relative strike frequency for the structure increases at a decreasing rate as the structure height increases.The strike frequency for a 600-m tall structure is approximately 3.6 times that for a 100-m tall structure.Additionally,the structure may attract some lightning to hit itself and shift nearby ground strike points toward the structure.For taller structures,the deviation effect is more apparent.It is stipulated in this study that if the ground strike density in the vicinity of the structure is no more than 5% of the average density,then the structure has a sufficient protective effect on this area.The data indicate that there is a positive correlation between the protection distance and the height of the structure.The protection distances of structures of 100−600 m in height are 200 m,280 m,350 m,400 m,450 m,and 480 m approximatively,which show a declining rate of increase.(Jiang Ruijiao,Lyu Weitao,Wu Bin)

5.22 Vertical reflectivity structures near lightning flashes in the stratiform regions of mesoscale convective systems

Using an S-band Doppler radar and a very high frequency (VHF) radiation source location system,254 lightning flashes occurring in stratiform regions (stratiform lightning flashes) of 14 mesoscale convective systems (MCSs) in Chongqing,China,were observed and used to analyze the structure of vertical reflectivity in the area around the lightning channel.In most stratiform lightning flashes,the vertical reflectivity structures can be grouped into three general categories:single-layer,double-layer,and triple-layer.The single-layer structure flashes are the most numerous (65.0%),followed by the double-layer (26.8%) and triple-layer(1.2%) flashes.The regions with the single-layer structure and the double-layer structure are inferred to have mesoscale updrafts of different intensity in the middle of stratiform region.Around 62.3% of the singlelayer structure flashes initiate within 0−4 km above the vertical reflectivity core,and the main initiation range of these flashes widens with increasing value of the vertical reflectivity core.Around 49.2% and 12.7% of the double-layer structure flashes initiate within 3−6 km and 1−2 km above the vertical reflectivity core,respectively.The probability of lightning initiation at a greater distance to the vertical reflectivity core is higher than that at a shorter distance.This may imply that some electrifications occurring during the melting process are not as strong as expected and a non-riming non-inductive charging process occurring in a weak but broad mesoscale updraft in the middle of stratifom region may be more important for initiation of stratiform lightning flashes.(Wang Fei,Zhang Yijun,Liu Hengyi)

5.23 广州高建筑物雷电观测与研究10年进展

作为中国气象局雷电野外科学试验基地（CMA_FEBLS）的重要组成部分，广州高建筑物雷电观测站（TOLOG）始建于2009年，迄今已积累数百次高建筑物雷电资料。对于雷电连接过程，高建筑物会起到“放大镜”的作用：TOLOG的观测在国际上首次发现了连接过程中负—正先导之间“头部—侧面”连接的现象，给出了先导连接行为的两种基本形态；揭示了负先导梯级发展过程的精细化结构，给出了下行先导和上行先导的二维/三维发展特征；估算了不同高度建筑物上雷电的闪击距离。高建筑物对雷电电磁场具有“放大器”的作用，且建筑物越高增强效应越显著。高建筑物是下行和上行闪电的“汇集点”：对下行闪电的吸引作用可保护高建筑物附近的其他物体免遭雷击；正地闪的回击、延续电流和云内放电过程均可在高建筑物上触发负极性上行闪电。另外，高建筑物区域可作为闪电监测系统的“标校场”，TOLOG的观测资料在地闪定位系统探测效率和定位精度的评估方面也得到了应用。（吕伟涛，陈绿文，马颖）

5.24 广州两座高建筑物上闪击距离的二维光学观测

利用广州高建筑物雷电观测站的高速摄像机在2012—2018年拍摄到的发生在两座尖顶建筑物广州塔（600 m高，12次）和广晟国际大厦（360 m高，9次）上的21次下行地闪光学数据，结合广东电网闪电定位系统提供的回击峰值电流数据，统计建筑物高度和回击峰值电流强度对闪击距离的影响，并探讨闪击距离与上行连接先导起始时间的相关性。结果表明：更高的建筑物上雷电的闪击距离更长，广州塔闪击距离的中位数约是广晟国际大厦闪击距离中位数的2倍；对于确定高度的建筑物，闪击距离有随着回击峰值电流增强而变长的趋势，且建筑物越高，对应的回击峰值电流也越强；在下行与上行先导连接前0.1 ms内，二者的平均速率之比小于4，且速率比值在0～1这一区间的样本最多，占比约65%。（齐奇，吕伟涛，武斌）

5.25 人工触发闪电不同放电阶段电流特征关系

分析了2019年夏季在广州从化人工引雷试验场获取的14次人工触发闪电通道底部电流数据，以有无回击（RS）和初始连续电流（ICC）持续时间长短2个标准对数据进行分类，研究不同触发闪电和不同放电阶段的差异和规律。研究表明：相比无回击的触发闪电，产生回击的触发闪电具有更大的先驱放电脉冲（PCP）及初始先驱放电脉冲（IPCP）的平均峰值电流、更多的IPCP总体转移电荷量、更大的ICC平均电流和总体转移电荷量以及更长的ICC持续时间；初始连续电流持续时间是回击平均峰值电流大小、首次继后回击转移电荷量大小和首次继后回击峰值电流大小的重要影响因素，且长初始连续电流的触发闪电对应的PCP及IPCP平均峰值电流也更大、平均转移电荷量也更多；PCP和IPCP平均峰值电流与ICC持续时间相关性最强，是决定ICC放电持续时间的重要因素，未能产生初始连续电流的PCP脉冲簇其平均转移电荷量小于初始先驱放电脉冲簇，其转化的关键阈值之一是平均转移电荷量大于25.91 μC。（王敬轩，张阳，陈泽方）

5.26 人工触发闪电初始连续电流的中低频磁场特征

中国气象局雷电野外科学试验基地开展的人工触发闪电试验是研究闪电电磁辐射效应的有效手段，利用架设在试验场地周边的多套磁场天线所获取的高灵敏度磁场数据，针对初始连续电流阶段的中低频磁场特征开展研究。得益于磁场天线带宽的拓展，首次解析出了相对平静期内的磁场脉冲，单个脉冲的平均宽度约为1 μs，平均脉冲间隔约为14 μs，对应了该阶段中上行先导的小尺度击穿发展形式；在近、远距离磁场测量中均观测到了与先导通道头部击穿放电相关的爆发式磁场脉冲，其平均脉冲间隔（约为24.5 μs）明显大于平静期脉冲的统计值，而且在爆发式脉冲期间通道底部电流逐步增大到几十至上百安培，上述现象表明此时电场条件更加有利于上行先导的发展。此外，高灵敏磁场天线能够直观地呈现出初始连续电流脉冲（ICCP）的电荷传输过程，且ICCP期间观测到的规则磁场脉冲的脉冲间隔比其他类型的磁场脉冲小一个量级，可能体现了正极性击穿和负极性击穿的特征差异。（樊艳峰，陆高鹏，张阳）

5.27 双向先导正端突然延展现象的高速摄像观测

基于广州高建筑物雷电观测站的观测设备，于2016年6月4日在广州塔上发生的一次上行闪电过程中观测到双向发展的直窜先导正端在回击前、后突然延展的现象。利用高时间分辨率的光学和电场变化同步数据，分析双向先导正端突然延展现象的细节特征。结果表明：回击前直窜先导双向发展时正端可能会出现多次突然延展的现象；突然延展现象有时由双向先导的正端与已有悬空先导序列相连而引发，并促使双向先导正端传输至未击穿空气中；在一次继后回击后，通道正端头部也观测到两次突然延展现象，但未沿回击前正端伸展通道传输，而是通过开辟新通道进入未击穿空气；回击前直窜先导正端3次突然伸展的二维平均速率约为2.3×106m/s，伸展长度平均值约为115 m；回击后通道头部两次突然延展的二维平均速率约为4.3×106m/s，伸展长度平均值约为212 m。（武斌，吕伟涛，齐奇）

5.28 一次多回击触发闪电全过程的连续干涉仪观测

基于中国气象科学研究院自主研发的闪电连续干涉仪，对2019年6月11日在中国气象局雷电野外科学试验基地广州从化人工引雷试验场成功触发的一次多回击闪电放电全过程进行观测，结合通道底部电流数据和电场变化数据，共同揭示触发闪电全放电过程：连续干涉仪能够定位到最小为8 A的不连续的先驱电流脉冲辐射信号，初始先驱电流脉冲（IPCP）的平均转移电荷量约为先驱电流脉冲（PCP）的2倍；上行正先导连续发展后为初始连续电流（ICC）过程，最初正流光通道以105m/s量级的速度继续发展延伸，之后出现反冲先导放电；在ICC阶段出现的经典M分量，可由向前的106m/s量级速度的正流光（先导）产生，也可由已有通道头部产生的反冲先导产生，且整个M分量过程中，多个反冲先导维持了放电过程的持续；之后的回击间过程以反冲先导为主要放电形式，回击电流脉冲之前存在多次反冲先导过程，但多数未发展到接地通道，只处于企图先导阶段，直至成功的先导回击产生；而前两次回击具有超短的时间间隔，约为4.5 ms，这是由于两次回击前的先导来源于云内不同分支的反冲先导过程。（张阳，陈泽方，王敬轩）

5.29 一次暖云强降水主导的对流单体闪电活动特征

利用中国气象局雷电野外科学试验基地（CMA_FEBLS）三维闪电观测数据，结合广州双偏振雷达观测数据，分析了2017年5月7日广东一次暖云强降水对流单体的闪电活动及其与云降水结构的关系。该单体在4 h内产生1250个闪电，地闪比例约24%。绝大多数闪电出现在4～12 km高度，对应温度层为0 ℃～-40 ℃；闪电放电活动的峰值高度出现在8.5 km，对应环境温度约-19 ℃。分析的强降水单体宏观上呈现上正、中负、下正的三极性电荷结构，中部负电荷核心区约为-8 ℃～-15 ℃。在闪电活动区域中，由干雪粒子主导区域占比约82%，霰粒子主导区域占比约11%，且大部分与闪电活动关联的霰粒子主要位于4～8 km高度。总闪频数与30 dBz雷达回波顶高、-20 ℃温度层上大于20 dBz的回波体积具有较好的相关性。闪电活动的平均位置高度与20 dBz雷达回波顶高和-20 ℃温度层上大于30 dBz的回波体积具有较好的相关关系。闪电活动与最大降水强度之间具有较好的时序对应关系，单个闪电表征降水量的值为10～7 kg/fl量级。（刘泽，郭凤霞，郑栋）

5.30 粤港澳闪电定位系统对高建筑物雷电的探测

基于2016—2017年广州高建筑物雷电观测站获取的资料对粤港澳闪电定位系统（简称定位系统）的性能进行评估，并根据2014—2018年定位系统历史资料对广州高建筑物区域的雷电活动特征进行初步分析，结果表明：定位系统对闪电的探测效率为93%（214/229），对回击的探测效率为93%（449/481），对下行闪电首次回击、继后回击及上行闪电回击的定位误差的平均值（中值）分别为361 m（188 m）、252 m（167 m）和294 m（173 m）；当接地点高度低于200 m（不低于200 m）时，定位系统对下行负极性闪电首次和继后回击的云闪/地闪识别正确率分别为99%（80%）和93%（35%），有83%的上行负地闪回击被定位系统误判为云闪，广州高建筑物区域内绝大部分负云闪定位记录实际是高建筑物地闪；对定位系统得到的孤立高建筑物闪电密度中心进行分析后发现，广州塔（600 m）闪电密度中心200 m半径范围内年均回击次数约为中信广场（390 m）和广发证券大厦（308 m）的5倍，推测广州塔闪电的主要类型为上行闪电，而中信广场和广发证券大厦则为下行闪电。（陈绿文，吕伟涛，张义军）

6 模式和再分析资料

6 Model and reanalysis data

6.1 A 100-m-scale modeling study of a gale event on the lee side of a long narrow mountain

In this study,a gale event that occurred on the lee side of a long narrow mountain was investigated,together with the associated mountain flows,using a realistic-case large-eddy simulation (LES) that is based on the Weather Research and Forecasting Model.The mountain is located on the southeastern Tibetan Plateau,where approximately 58 gales occur annually,mostly in the afternoons during the winter season.Benefitting from realistic topography and high horizontal resolution as fine as 111 m,the LES can replicate features similar to the wind fields observed during the gale period.Investigation of the early morning wind structure over the mountain revealed that weak inflows were blocked,reversed,and divided in the upstream area and that some weak lee waves,rotors,and two clear lee vortices were evident downstream.As the upstream wind accelerated and the boundary layer developed during the daytime,the lee waves became amplified with severe downslope wind and rotors.The interaction and coherent structure of the downslope wind,rotor,and vortices were investigated to show the severe wind distribution.The mountain drags associated with blocking and amplified lee waves were displayed to show the potential impact on the large-scale model.The linear lee-wave theory was adopted to explain the wave evolution during this event together with a discussion of the uncertainty around low-level nonlinear processes.(Xue Haile,Li Jian,Qian Tingting)

6.2 An improvement of convective precipitation nowcasting through lightning data dynamic nudging in a cloud-resolving scale forecasting system

A lightning data dynamic nudging (LDN) method was designed to adjust the dynamic field in convective clouds based on the physical relationship between lightning and vertical velocity.Under this framework,vertical velocity data retrieved from the Guangdong-Hongkong-Macau lightning location system were assimilated by a nudging approach using the Global/Regional Assimilation and Prediction System in mesoscale model (GRAPES-Meso).The effect of the assimilation on short-term ( 6 h) precipitation forecast was evaluated by simulating the squall line event on 7 May and a continuous experiment from May to August 2018.Firstly,a series of sensitivity experiments was carried out to adjust the configuration of the assimilation system based on the squall line event.The results show that LDN expanded the spatial distribution of the positive vertical velocity ( 1 m s−1) at 700 hPa by about 2%,and thus enlarged the spatial distribution of severe rainfall ( 40 mm h−1).It should be noted that LDN did not significantly change the intensity of the updraft (i.e.,the maximum vertical velocity) of the squall line relative to the simulation without assimilation.Overall,the equitable threat score (ETS) and fractions skill score (FSS) were increased by 0.04 and 0.13,respectively,and the positive effect lasted for 2−3 h after the data were assimilated.In view of the data assimilation frequency,using two successive nudging procedures with an interval of 12 min had the best effect on the forecast of severe precipitation.For the assimilation of both lightning and radar observations,the asynchronous assimilation of lightning and radar measurements performed slightly better than the synchronous assimilation.LDN improved the accuracy of severe precipitation forecast ( 40 mm h−1and 20 mm h−1for the squall line case and the continuous run,respectively),whereas assimilating radar data improved weak precipitation forecast (1−20 mm h−1in the squall line case and 1−10 mm h−1in the continuous run).(Wang Hong,Chen Dehui,Yin Jinfang)

6.3 Analysis of short-term cloud feedback in East Asia using cloud radiative kernels

Cloud radiative kernels were built by BCC_RAD (Beijing Climate Center radiative transfer model)radiative transfer code.Then,short-term cloud feedback and its mechanisms in East Asia (0.5°−60.5°N,69.5°−150.5°E) were analyzed quantitatively using the kernels combined with MODIS satellite data from July 2002 to June 2018.According to the surface and monsoon types,four subregions in East Asia—the Tibetan Plateau,Northwest,temperate monsoon (TM),and subtropical monsoon (SM)—were selected.The average longwave,shortwave,and net cloud feedbacks in East Asia are −0.68 ± 1.20,1.34 ± 1.08,and 0.66 ± 0.40 W m−2K−1± 2 sigma),respectively,among which the net feedback is dominated by the positive shortwave feedback.Positive feedback in SM is the strongest of all subregions,mainly due to the contributions of nimbostratus and stratus.In East Asia,short-term feedback in spring is primarily caused by marine stratus in SM,in summer is primarily driven by deep convective cloud in TM,in autumn is mainly caused by land nimbostratus in SM,and in winter is mainly driven by land stratus in SM.Cloud feedback in East Asia is chiefly driven by decreases in mid-level and low cloud fraction owing to the changes in relative humidity,and a decrease in low cloud optical thickness due to the changes in cloud water content.(Wang Fei,Zhang Hua,Chen Qi)

6.4 Changes in anthropogenic PM2.5 and the resulting global climate effects under the RCP4.5 and RCP8.5 scenarios by 2050

Using an aerosolclimate model,we studied the temporal and spatial variations of anthropogenic PM2.5(aerodynamic diameter≤2.5 µm) and coarse particulate matter (CPM; aerodynamic diameter 2.5 µm) under Representative Concentration Pathway (RCP) 4.5 and RCP8.5 scenarios from 2014 to 2050.The corresponding radiative forcing and climate responses were also explored.The PM2.5burden decreases over most continents,especially East Asia.The CPM particles increase over the northern Asia,North America,and central Africa,in contrast to decrease over most regions of East Asia and North Africa.Relative to 2014,the global annual mean effective radiative forcing due to changes in PM2.5and CPM burden are 1.17 (1.10) and −0.06 (−0.07) W m−2under RCP4.5 (RCP8.5),respectively.The reduction in PM2.5burden leads to apparent warming,especially over high latitudes of the Northern Hemisphere,with global annual mean surface air temperature increasing by 1.25 K under RCP4.5,and 1.22 K under RCP8.5.In contrast,changes in CPM result in apparent cooling over North America and the northern Asia,with global annual mean changes in surface air temperature of 0.10 K for both scenarios.The Northern Hemisphere Hadley cell weakens and moves northward due to changes in PM2.5after 2014,whereas the corresponding circulation in the Southern Hemisphere is strengthened,with the Intertropical Convergence Zone shifting to 10°N.Global annual mean precipitation increases by 0.10 mm per day under both scenarios.Generally,anthropogenic PM2.5contributes significantly to future changes in radiative forcing and climate.(Yang Dongdong,Zhang Hua,Li Jiangnan)

6.5 Development of a coupled atmosphere-ocean typhoon regional assimilation and prediction system for operational typhoon forecasting by the Chinese Academy of Meteorological Sciences-Part I:Experiments of western North Pacific typhoons in 2016

Typhoons can interact intensely with oceans,and the atmosphere-ocean interaction has an enormous impact on the intensity and structure of typhoons.To consider the impact of typhoon-ocean interactions,a regional coupled atmosphere-ocean system for operational forecasting of the western North Pacific typhoons is established in this paper based on a public coupled atmosphere-ocean framework and in accordance with the operational typhoon forecasting conditions and system settings of the Chinese Academy of Meteorological Sciences.The new coupled system is fully validated and used for hindcasting experiments on all typhoons in 2016.The results show that the computational cost associated with the proposed coupled system is approximately 5.33 as much as times that of the atmosphere-only system.A comparison of the hindcasts of the 2016 typhoons using the atmosphere-only system and the proposed coupled system reveals that the use of the coupled system can obviously reduce the forecasted intensity error by 10%−20% but not the typhoon track error.In addition,some of the intrinsic structural characteristics of typhoons influence the effectivity of the coupled system.Specifically,when typhoons have a high intensity and a low translational speed,move in an East−West or turning direction,and are at 20°−25°N latitude,the forecasts can be relatively significantly improved by the coupled system; otherwise,the improvement is not obvious.(Liu Xin,Duan Yihong,Huo Zimo)

6.6 Effects of microphysical latent heating on the rapid intensification of typhoon Hato (2017)

A 72-hour cloud-resolving numerical simulation of Typhoon Hato (2017) is performed by using the Weather Research and Forecasting (WRF) model with the Advanced Research WRF (ARW) core (V3.8.1)on a horizontal resolution of 2 km.To enhance the background tropical cyclone structure and intensity,a vortex dynamic initialization scheme with a terrain-filtering algorithm is utilized.The model reproduces reasonably well the track,structure,and intensity change of Typhoon Hato.More specifically,the change trend of simulated maximum wind speed is consistent with that of best-track analysis,and the simulated maximum wind of 49 m s−1is close to that (52 m s−1) of the best-track analysis,indicating that the model has successfully captured the rapid intensification (RI) of Typhoon Hato (2017).Analyses of the model outputs reveal that the total microphysical latent heating of the inner-core region associated with enhanced vertical upward motion reaches its maximum at 9-km height in the upper troposphere during the RI stage.The dominant microphysical processes with positive latent heat contributions (i.e.,heating effect) are water vapor condensation into cloud water (67.6%),depositional growth of ice (12.9%),and generation (nucleation) of ice from vapor (7.9%).Those with negative latent heat contributions (cooling effect) are evaporation of rain (47.6%),melting of snow(27.7%),and melting of graupel (9.8%).Sensitivity experiments further show that the intensification speed and peak intensity of this typhoon are highly correlated to the dominant heating effect.A significant increase in graupel over 5−10-km height and snow at 10−14-km height in the inner-core region of Typhoon Hato corresponds well with its RI stage,and the latent heating from nucleation and depositional growth is crucial to the RI of simulated Hato.(Zhao Dajun,Yu Yubin,Yin Jinfang)

6.7 Establishment of high-resolution aerosol parameterization and its influence on radiation calculations

To accurately simulate the effects of aerosols on the atmospheric radiation budget,we here establish an aerosol optical parameterization scheme with high spectral resolution (974-band) for radiative transfer model(BCC_RAD); this scheme includes sulfate (SF),black carbon (BC),organic carbon (OC),dust (SD),and sea salt (SS) aerosols.The results of the 974-band scheme are compared with those obtained using a lowresolution 17-band scheme.Our results show the 974-band scheme more accurate in its representation of the aerosols optical properties.The largest relative differences in the volume extinction coefficient (K-ex) between the 974-band and 17-band schemes range from 5.87% to 54.64%,with BC (41.13%) and SF (54.64%) showing the largest values at 1.91 and 2.7 µm,respectively.The single scattering albedo of hygroscopic aerosols (SF,SS,and OC) show minimal values at 2.84 and 6.01 µm,respectively,in the 974-band scheme due to light absorption; however,the low spectral resolution of the 17-band scheme prevents these features from being resolved.Specifically,the relative difference between the two schemes approaches 72.72% at 2.84 µm for SF.Thus,improved spectral resolution may be required for some aerosols.We also compare the effects of aerosols on the radiation budget in East Asia simulated by 974-band and 17-band BCC_RAD models.Under clear-sky conditions,the simulation results indicate a relative difference in the radiation flux of 0.28%−5.93% for the two schemes.The 974-band and 17-band schemes show daily mean direct radiative forcings (DRFs) due to all aerosols of −16.97 and −17.13 W m−2at the surface and −7.12 and −6.93 W m−2at the top of the atmosphere,respectively.Additionally,the 974-band model generally simulates a larger DRF for BC and smaller DRFs for scattering aerosols such as SF; these differences become greater for smaller zenith angles.(Zhang Hua,Zhu Sihong,Zhao Shuyun)

6.8 Evaluation of Noah-MP land-model uncertainties over parsely vegetated sites on the Tibetan Plateau

The water budget and energy exchange over the Tibetan Plateau (TP) region play an important role on the Asian monsoon.However,it is not well presented in the current land surface models (LSMs).In this study,uncertainties in the Noah with multiparameterization (Noah-MP) LSM are assessed through physics ensemble simulations in three sparsely vegetated sites located in the central TP.The impact of soil organic matter on energy flux and water cycles,along with the influence of uncertainties in precipitation are explored using observations at those sites during the third Tibetan Plateau Experiment from 1 August 2014 to 31 July 2015.The greatest uncertainties are in the subprocesses of the canopy resistance,soil moisture limiting factors for evaporation,runoff (RNF) and ground water,and surface-layer parameterization.These uncertain subprocesses do not change across the different precipitation datasets.More precipitation can increase the annual total net radiation (Rn),latent heat flux (LH) and RNF,but decrease sensible heat flux (SH).Soil organic matter enlarges the annual total LH by similar to 26% but lessens the annual total Rn,SH,and RNF by similar to 7%,7%,and 39%,respectively.Its effect on the LH and RNF at the Naqu site,which has a sand soil texture type,is greater than those at the other two sites with sandy loam.This study highlights the importance of precipitation uncertainties and the effect of soil organic matter on the Noah-MP land-model simulations.It provides a guidance to improve the Noah-MP LSM further and hence the land-atmosphere interactions simulated by weather and climate models over the TP region.(Zhang Guo,Chen Fei,Chen Yueli)

6.9 Fourth-order conservative transport on overset grids using multi-moment constrained finite volume scheme for oceanic modeling

With an increase in model resolution,the compact high-order numerical advection scheme can improve its effectiveness and competitiveness in oceanic modeling due to its high accuracy and scalability on massiveprocessor computers.To provide high-quality numerical ocean simulation on overset grids,we tried a novel formulation of the fourth-order multi-moment constrained finite volume scheme to simulate continuous and discontinuous problems in the Cartesian coordinate.Utilizing some degrees of freedom over each cell and derivatives at the cell center,we obtained a two-dimensional (2D) cubic polynomial from which point values on the extended overlap can achieve fourth-order accuracy.However,this interpolation causes a lack of conservation because the flux between the regions are no longer equal; thus,a flux correction is implemented to ensure conservation.A couple of numerical experiments are presented to evaluate the numerical scheme,which confirms its approximately fourth-order accuracy in conservative transportation on overset grid.The test cases reveal that the scheme is effective to suppress numerical oscillation in discontinuous problems,which may be powerful for salinity advection computing with a sharp gradient.(Gu Juan,Peng Xindong,Dai Yongjiu)

6.10 On the extreme rainfall event of 7 May 2017 over the coastal city of Guangzhou.Part I:Impacts of urbanization and orography

In this study,a nocturnal extreme rainfall event induced by the urban heat island (UHI) effects of the coastal city of Guangzhou in South China on 7 May 2017 is examined using observational analyses and 18-hour cloud-permitting simulations with the finest grid size of 1.33 km and the bottom boundary conditions nudged.Results show that the model reproduces convective initiation on Guangzhou’s downstream side (i.e.,Huashan),where a shallow thermal mesolow is located; the subsequent back-building of convective cells as a larger-scale warm-moist southerly flow interacts with convectively generated cold outflows,and their eastward drifting and reorganization into a localized extreme-rain-producing storm near Jiulong under the influences of local orography.In particular,the model produces the maximum hourly,3- and 12-hour rainfall amounts of 146,315,and 551 mm,respectively,at nearly the right location compared to their corresponding observed extreme amounts of 184,382,and 542 mm.In addition,the model reproduces an intense meso−gamma-scale vortex associated with the extreme-rain-producing Jiulong storm,as also captured by Doppler radar,with organized updrafts along cold outflow boundaries over a semicircle.A comparison of sensitivity and control simulations indicates that despite the occurrence of heavier rainfall amounts without the UHI effects than those without orography,the UHI effects appear to account directly for the convective initiation and heavy rainfall near Huashan,and indirectly for the subsequent formation of the Jiulong storm,while orography plays an important role in blocking cold outflows and enhancing cool pool strength for the sustained back-building of convective cells over the semicircle,thereby magnifying rainfall production near Jiulong.(Yin Jinfang,Zhang Da-Lin,Luo Yali)

6.11 Positive definite advection transport algorithm for conservation law equations on nonuniform irregular grids

The multidimensional positive definite advection transport algorithm (MPDATA) is an important numerical method for the computation of atmospheric dynamics.MPDATA is second-order accurate,positive definite,conservative,and computationally efficient.However,the method is problematic in which it results in a loss of precision when computing a nonuniform irregular grid.Furthermore,research revealed two reasons for this problem.On the one hand,numerical discretization of boundary derivatives of the finitevolume method is incompatible with nonuniform meshes (or grids); on the other hand,the up-wind scheme of staggered grids is not applicable to the calculation of irregular grids.We overcome these two problems by using the multipoint Taylor expansion method to obtain a boundary derivative numerical approximation scheme that does not depend on the grid structure.Furthermore,combined with the well-balance central-upwind scheme,a positive definite advection scheme for irregular meshes is proposed.Then,the positivity of the new numerical scheme is analyzed.Finally,the result of this study is verified by numerical simulation.(Yuan Xinpeng,Xiong Chunguang)

6.12 Quantifying contributions of uncertainties in physical parameterization schemes and model parameters to overall errors in Noah-MP dynamic vegetation modeling

Quantifying contributions of errors in model structure and parameters to biases in a land surface model(LSM) is critical for model improvement.This paper investigated the uncertainties in parameterizations and parameters in the Noah with multiparameterization (Noah-MP) LSM with dynamic vegetation using eddy flux data.First,we conducted full factorial experiments of eight land subprocesses,followed by sensitivity analysis(SA) to identify the subprocesses for which possible parameterizations made significant difference.Then,based on the full factorial experiments and SA results,we selected the statistically optimal parameterizations combination and the most biased parameterizations combination.Lastly,we calibrated the parameters in two selected parameterizations combinations.The results showed that five subprocesses—surface exchange coefficient,soil moisture beta threshold,radiation transfer,runoff and groundwater,and surface resistance to evaporation—had significant influence on the model performances,and the interactions were generally low but contributed up to 80% of the variation in the performance at some sites.In the optimization period,following the criterion by Moriasi et al.(2007,https://doi.org/10.13031/2013.23153),the parameter optimization improved the performance of both parameterizations combinations at most sites,which was satisfactory,and the superiority between two parameterizations combinations was preserved; in the validation period,adjusting the parameterizations was more robust than parameter optimization in improving LSMs.Finally,we found that uncertainty in soil parameters was much higher than that in vegetation parameters because the optimal soil parameters were significantly different among sites with the same soil types and recommended that spatially calibrating the soil parameters was a major issue for Noah-MP dynamic vegetation modeling.(Li Jianduo,Chen Fei,Lu Xingjie)

6.13 Statistical characteristics and synoptic situations of long-duration heavy rainfall events over North China

The spatiotemporal characteristics of long-duration heavy rainfall (LDHR) events are statistically analyzed using hourly surface observations over the central-southern North China during the warm seasons of 2011−2018,revealing pronounced variabilities in the frequency and amount of LDHR.Two accumulated rainfall peaks are found in the western (WHRR) and eastern (EHRR) regions of the central-southern North China.The LDHR occurrence frequency decreases westward,and the peaks of the LDHR amount,frequency and intensity in the WHRR and EHRR are observed at nighttime (21:00−02:00 Beijing Standard Time) or in the early morning (03:00−07:00 Beijing Standard Time).The rainfall amount exhibits a bimodal diurnal variation in the WHRR (determined mainly by the rainfall intensity),whereas a single rainfall frequencyrelated peak is found in the EHRR.Four types of LDHR events corresponding to different flow patterns,synoptic systems,and moisture transport mechanisms are classified according to their locations.The first is heavy rainfall in the WHRR with an upper-level jet favorable for an ascending motion near Taihang Mountain;topographic blocking of southerly flow is crucial for heavy rainfall formation.The second describes heavy rainfall in the EHRR attributable to the favorable configurations of upper- and lower-level systems.Heavy rainfall occurs over both the WHRR and the EHRR in the third type,including topographic blocking and convergence associated with low-pressure systems or shear lines and a mesoscale low vortex or shear line related to topographic effects and positive vorticity advection in front of a westerly trough.The fourth shows a scattered distribution of LDHR stations and is generally not comparable to the first three types.(Kang Yanzhen,Peng Xindong,Wang Shigong)

6.14 The change of cloud top height over East Asia during 2000-2018

In order to know how regional averaged cloud top height (CTH) responded to the past climatic change,the spatial and temporal variations of CTH based on MODIS data (MOD03_08_v6.0) from March 2000 to February 2018 are analyzed.The results show that the CTH in East Asia was higher in the southwest and lower in the northeast.The annual mean CTH was found to increase in East Asia at the rate of 0.020 km per year,of which,0.035 km per year over the eastern land and 0.034 km per year over the eastern sea.The CTH changes were statistically correlated with the sea surface temperature changes over the eastern sea (r=0.68),which indicates that the CTH changes may be affected by the underlying surface.Statistically significant increasing trend of annual mean CTH was found in the latitude zone of 30°−40°N.Furthermore,CTH showed decreasing trends of about −0.030 km per year in the Middle-Lower Yangtze River Basin,Tarim Basin,Turpan Basin and northeastern Sichuan Basin in summer since more low cloud formations benefited to the decrease of CTH.In winter,the decreasing trend of CTH was found over the north of 40°N,whereas the increasing trend was found over the south of 40°N in East Asia.(Zhao Min,Zhang Hua,Wang Haibo)

6.15 Towards understanding multi-model precipitation predictions from CMIP5 based on China hourly merged precipitation analysis data

Large uncertainties still exist in the simulation and projection of precipitation from current climate models.Here,the newly released state-of-the-art China Hourly Merged Precipitation Analysis (CHMPA) data has been used to evaluate the ten models from the fifth phase of the Coupled Models Intercomparison Project(CMIP5).Particularly,the precipitation predictions under the Representative Concentration Pathways (RCP)4.5 and RCP8.5 scenarios in China are assessed for the period from 2008 to 2017.Interestingly,the ensemble mean precipitation under the two emission scenarios does not show systematic differences.Intercomparison analysis of precipitation between multi-model prediction and CHMPA yields a high correlation coefficient (0.85−0.95)on the annual timescale.However,most models tend to overestimate the precipitation in the northern China but to underestimate that in the southern China,due to the model-simulated monsoon precipitation extending to the north earlier.Relative to UKMO-HadGEM2AO model,other models overestimate precipitation at the southeastern edge of the Tibetan Plateau where the overestimation reaches up to 150%.In terms of the temporal evolution of predicted precipitation,the multi-model ensemble produces relatively small interannual variability except for more summer monsoon precipitation with biases over 0.3 mm/day,which indicates that models are not capable of reproducing the seasonal and meridional propagation of precipitation.Compared with the original model output,the precipitation corrected by quantile mapping algorithm better agrees with the observations for spatial and temporal distributions.The findings have great implications for better utilizing model-predicted precipitation in climate change studies.(Lyu Yanmin,Guo Jianping,Yim Steve Hung-Lam)

6.16 GRAPES_YY模式发展及其对梅雨锋降水模拟的性能检验

GRAPES_YY是在球面准均匀阴阳网格动力框架基础上发展起来的全球非静力模式，目前已耦合全套物理过程参数化方案，具备了中期天气预报能力。为了解模式对环流和降水的实际预报效果，检验模式对多尺度天气系统的预报能力和误差分布特征，以ERA-Interim资料和逐时地面融合降水产品为参考，对1个月（2018年6月中旬至7月中旬）批量试验结果和一次长江流域梅雨个例（7月4—7日）模拟结果进行分析。结果表明：在改进上边界条件基础上，模式有较好的稳定性，短期预报时效内对全球环流和降水刻画准确，模式24 h累计降水纬向平均成功再现了低纬度和中纬度地区的两个降水峰值，但对低纬度对流性降水和中纬度格点尺度降水预报偏强。其次，模式成功模拟出7月4—6日中国东部梅雨雨带位置、走向及南北摆动，整体上能正确反映主要天气系统的移动演变，但受分辨率限制以及湿物理过程影响，暴雨以上量级降水还存在强度预报偏弱、位置偏北的问题。GRAPES_YY模式模拟结果基本合理可信，对中低纬度梅雨锋降水的预报能力较GRAPES_GFS略有改善。（赵益帆，李晓涵，彭新东）

6.17 物理滤波初始化四维变分在临近预报中的应用

运用WRF（Weather Research and Forecasting Model）天气研究和预报模式以及WRFDA（WRF Data Assimilation）同化系统，探究采用物理滤波初始化四维变分同化方法提高数值预报在临近预报时效的预报能力的可能性。通过采用12 min同化窗，在不显著增加计算量的情况下，得到更协调的模式初始场，从而提高模式预报能力。选取2018年8月华北地区17个降水个例进行研究。结果表明，采用物理滤波初始化四维变分同化技术能够明显改进模式短时临近降水预报能力，明显提高对大量级降水预报的ETS评分，6 h累积降水大于25.0 mm量级的ETS评分由0.125提高到0.190，且6 h累积降水大于60.0 mm量级的ETS评分由0.016提高到0.081。研究还表明，同化雷达风场通过改进初始动力场使次网格尺度降水过程（积云参数化）快速响应，可提高短时临近时段的降水预报能力。（姜文静，梁旭东）

6.18 夏季白天中国中东部不同类型云分布特征及其对近地表气温的影响

本文利用2001—2017年ERA5再分析资料以及CERES卫星资料，探究夏季白天中国中东部不同类型云的云量及其光学厚度的时空变化特征，并利用一维辐射对流模式定量分析不同类型云对近地表气温的影响。观测结果表明：夏季白天中国中东部总云量及其光学厚度整体呈由南向北逐渐减小的分布特征，且中高云量占主导地位。总云量整体呈−0.3%/a显著减少趋势，其中低云的贡献（−0.27%/a）最大；总云光学厚度为0～0.1/a增加趋势，其中低云光学厚度（0.06/a）和中低云光学厚度（0.03/a）呈增加趋势，而中高云光学厚度（−0.08/a）和高云光学厚度（−0.03/a）呈减少趋势。模式结果表明：四种不同类型云的温度效应（Cloud Effect Temperature，CET）均为负值，表现为降温效应。低云、中低云、中高云和高云的年均CET值分别为−2.9 ℃、−2.7 ℃、−2.2 ℃和−1.7 ℃。其中，低云在华北平原降温可达−5 ℃；中低云和中高云在四川盆地和云贵高原降温可达−7.8 ℃。不同类型云温度效应与近地表气温的年际变化具有较好的一致性，具体表现为：2004年前（后）近地表气温呈现下降（上升）趋势，不同类型云的CET在此期间呈下降（上升）趋势，表现为云的降温效应增强（减弱）与近地表气温下降（上升）相对应，体现了夏季白天中国中东部4种不同类型云温度效应与近地表气温都呈正相关关系。特别地，夏季白天中国中东部中高云量占主导地位，其CET与近地表气温的相关系数高达0.63。综上，夏季白天中国中东部不同类型云温度效应对近地表气温的影响不同，但均呈正相关关系。定量分析不同类型云对近地表气温的影响可以为定量研究云反馈对区域增暖的作用以及合理预估未来区域增暖情景提供必要的科学参考。（游婷，张华，王海波）

7 卫星研究与应用

7 Satellite research and application

7.1 A multivariable approach for estimating soil moisture from microwave radiation imager (MWRI)

Accurate measurements of soil moisture are beneficial to our understanding of hydrological processes in the earth system.A multivariable approach using the random forest (RF) machine learning technique is proposed to estimate the soil moisture from microwave radiation imager (MWRI) onboard Fengyun-3C satellite.In this study,soil moisture operational products system (SMOPS) products disseminated from NOAA are used as a truth to train the algorithm with the input of MWRI brightness temperatures (TBs) at 10.65,18.7,23.8,36.5,and 89.0 GHz,TB polarization ratios (PRs) at 10.65,18.7,and 23.8 GHz,height in digital elevation model (DEM),and soil porosity.The retrieved soil moisture is also validated against the independent SMOPS data,and the correlation coefficient is about 0.8 and mean bias is 0.002 m3m−3over the period from 1 August 2017 to 31 May 2019.Our retrieval of soil moisture also has a higher correlation with ECMWF ERA5 soil moisture data than the MWRI operational products.In the western part of China,the spatial distribution of MWRI soil moisture is much improved,compared to the MWRI operational products.(Zhang Sibo,Weng Fuzhong,Yao Wei)

7.2 Advanced radiative transfer modeling system (ARMS):A new-generation satellite observation operator developed for numerical weather prediction and remote sensing applications

(1) Urgent requirements for developing Fengyun satellite observation operators; (2) Capabilities of ARMS; (3) Preliminary results from ARMS simulations on Fengyun satellite observations; (4) The pathway for transitioning ARMS into operation and the plan for future development.(Weng Fuzhong,Yu Xinwen,Duan Yihong)

7.3 Advanced radiative transfer modeling system developed for satellite data assimilation and remote sensing applications

An advanced radiative transfer modeling system (ARMS) has been developed for satellite data assimilation and remote sensing applications.It inherits the capability of other fast radiative transfer models developed mainly for US and European satellite programs but focuses on the radiative transfer components that are specific for the assimilation of the Fengyun satellites and those sensors not included in existing models.The ARMS forward operator includes a fast transmittance module,a new particle absorption and scattering look-up table,surface emissivity and polarimetric radiative transfer solver.The particle optical properties of aerosols and ice clouds are calculated based on a super-spheroidal T-matrix model.The polarimetric twostream approximation is adopted as the preferred solver to simulate the Stokes vector and their Jacobians.The simulations of ARMS are compared with those of other models and observations of Fengyun satellites,respectively.The comparison shows close agreement between ARMS and other existing fast radiative transfer models.Therefore,ARMS has robust simulation capabilities for existing satellite sensors.(Yang Jun,Ding Shouguo,Dong Peiming)

7.4 Application and evaluation of the small-angle approximation on forward radiative transfer model

The small-angle approximation is used in solving the radiative transfer equation when the singlescattering phase function has a strong forward peak.The original vector radiative transfer equation is decomposed into three equations:the forward,the regular,and the correction ones.The forward equation can be efficiently solved using the small-angle approximation.The solution of the regular equation is given by the adding-doubling method in this study.The correction equation including cross terms between the forward and the regular quantities is given and analyzed.The combined model associated with the forward and regular equations is further verified in the forward radiative transfer model,where the molecular absorptions with respect to the standard atmospheric profiles are taken into consideration using the line-by-line method.Great agreements are given between the combined model and the model using the straight adding-doubling method.(Sun Bingqiang,Ding Shouguo)

7.5 Climatology of passive microwave brightness temperatures in tropical cyclones and their relations to storm intensities as seen by FY-3B/MWRI

A new database,the tropical cyclones passive microwave brightness temperature (TCsBT) database including 6273 overpasses of 503 tropical cyclones (TC) was established from 6-year (2011–2016) Fengyun-3B (FY-3B) microwave radiation imager (MWRI) Level-1 brightness temperature (TB) data and TC besttrack data.An algorithm to estimate the TC intensity is developed using MWRI TB’s from the database.The relationship between microwave TB and the maximum sustained surface wind (Vmax) of TCs is derived from the TCsBT database.A high correlation coefficient between MWRI channel TB and Vmax is found at the radial distance 50–100 km near the TC inner core.Brightness temperatures at 10.65,18.70,23.8,and 36.5 GHz increase but 89 GHz TB’s and polarization corrected TB at 36.5 GHz (PCT36.50) and PCT89 decrease with increasing TC intensity.The TCsBT database is further separated into the 5063 dependent samples (2010–2015) for the development of the TC intensity estimation algorithm and 1210 independent samples (2016)for algorithm verification.The stepwise regression method is used to select the optimal combination of storm intensity estimation variables from 12 candidate variables and four parameters (10.65h,23.80v,89.00v and PCT36.50) were selected for multiple regression models development.Among the four predictors,PCT36.50 contributes the most in estimating TC intensity.In addition,the errors are lower for estimating 6-hour and 12-hour future Vmax than estimating the current Vmax.(Qian Bo,Jiang Haiyan,Weng Fuzhong)

7.6 Detection of wind corridors based on “Climatopes”:A study in central Ji’nan

Wind corridor blueprints are an essential reference for climate planning that takes into account zones of fresh cold air,air channels as well as heat island alleviation zones.This article addresses the matter of how to plan urban wind corridors more efficiently by proposing a “ventilation potential coefficient” (VPC) as well as providing an index for natural cold sources.The study additionally proposes a surface urban heat island (SUHI)index as a means of evaluating the severity of the heat island effect.Based on these three indexes,a new method of categorizing urban climatic zones,called “Climatopes”,is developed.Using the above methods,data from 2016 meteorological observation stations and Landsat8 satellites,as well as urban planning materials and basic geographical information on a scale of 1:2000 are used to create an Urban Climate Analysis Map(UCAnMap) and formulate a plan for the construction of wind corridors in the urban centre of Ji’nan.The results reveal that the intensity of urban heat islands in Ji’nan is closely related to a lack of natural cold sources,the specific layout of different types of land use as well as poor planning of urban “function zones”.Moreover,there is a significant correlation between the VPC and other factors such as surface roughness length,building height,the density of building frontal area and floor area ratios.The potential wind corridors could also be determined based on the VPC.Based on an UCAnMap composed of 11 Climatopes and background wind environments,this article ascertains that it would be possible to construct a network of 3-level one corridor(each over 500 m wide) and 11-level two corridors (each over 80-m wide).Additionally,regulatory measures and protective legislation are proposed for the corridors,as well as for the city’s overall development.(Liu Yonghong,Cheng Pengfei,Chen Peng)

7.7 Development and application of ARMS fast transmittance model for GIIRS data

The Fengyun (FY)-4A geosynchronous interferometric infrared sounder (GIIRS) fast transmittance model has been developed as part of the advanced radiative transfer modeling system (ARMS) and applied to quantitative uses of the GIIRS data.To determine the accuracy of the fast transmittance model,one dataset that includes 83 atmospheric profiles generated by the European Center for Medium-Range Weather Forecasts(ECMWF) and another dataset that includes 48 atmospheric profiles developed at the University of Maryland at Baltimore County (UMBC) are utilized for training and testing,respectively.The radiative transfer simulations at six viewing angles are compared with the line-by-line results for dependent and independent evaluations of the fast transmittance model.It is shown that the profile dataset chosen for coefficient training has a substantial effect on the accuracy of the fast transmittance model.ARMS simulations using the fast transmittance model are compared with RTTOV simulations using the fast transmittance model (DiDi RTTOV) as well as GIIRS observations.The biases between the ARMS simulations and GIIRS measurements are less than 1.0 K and 1.5 K(in absolute value) for most channels of GIIRS band 1 and 2,respectively,and the difference between the two models is less than 0.7 K,except those near the strong absorption bands.There are some GIIRS channels with large biases,which are unusable.Several issues are also found during the comparison.The Jacobians with respect to different physical parameters are also calculated using both ARMS and DiDi RTTOV and compared for consistency.It is found that the temperature Jacobians of both models are similar; however,the water vapor and ozone Jacobians differ substantially between ARMS and DiDi RTTOV.(Kan Wanlin,Dong Peiming,Zhang Zhiqing)

7.8 Evaluation of the in-orbit performance of the microwave temperature sounder onboard the FY-3D satellite using different radiative transfer models

The in-orbit performance of the microwave temperature sounder (MWTS) onboard the Fengyun-3D (FY-3D) satellite is evaluated using the community radiative transfer model (CRTM) and the radiative transfer for the television infrared observation satellite operational vertical sounder (RTTOV) model.Both radiooccultation data from the Constellation Observing System for Meteorology,Ionosphere and Climate (COSMIC)Data Analysis and Archive Center and ERA-Interim analysis data from the European Centre for Medium-Range Weather Forecasts are taken as inputs to the two fast radiative transfer models.The radio-occultation data are quality controlled and the collocation criterion between the radio-occultation data and the MWTS measurements is defined such that the spatial and temporal difference is 50 km and 3 hours,respectively.The biases of MWTS channels 5−10 produced by both CRTM and RTTOV agree well over the oceans under clear sky conditions between 60°S and 60°N from July to December 2018.The mean biases simulated by the radio-occultation and ERA-Interim data are negative and the absolute values of the biases are 0.6 and 1.5 K for channels 5−10 of the MWTS,respectively.The biases at channels 4 and 11 between the CRTM and RTTOV simulations are inconsistent and require further investigation of the transmittance difference between the fast models.The standard deviation of the biases from the radio-occultation and ERA-Interim data are 0.5 and 1.8 K for the limited amount of collocated radio-occultation data.Asymmetrical patterns are detected for the MWTS through the scan angle-dependent bias.The long-term mean MWTS bias shows only a weak dependence on latitude,which suggests that biases do not vary systematically with brightness temperature.(Han Yang,Hou Xueyan)

7.9 Examining Asian dust refractive indices for brightness temperature simulations in the 650−1135 cm−1 spectral range

In this paper,we modeled dust impacts on atmospheric radiative transfer in the thermal infrared region of 650−1135 cm−1by using the radiative transfer for TOVs model (RTTOV).We assessed three sets of Asian dust refractive indices (the Volz refractive index implemented in the RTTOV as default,and the Gobi and Taklamakan refractive indices) for brightness temperature simulations.The US 76 standard profile was employed to represent the atmospheric status.Three scenarios of high (532.6 similar to 575.2 hPa),middle(661 similar to 703.6 hPa) and low (786.3 similar to 825.8 hPa) dust layers and four cases of aerosol optical thickness (AOT) (0.1,0.5,1.0,and 1.5 at 1000 cm−1) were considered in the simulations.It is found that the refractive indices have a clear impact on simulating the values and the spectral slope of brightness temperature in the mid-thermal infrared band.Brightness temperature difference (BTD) for different refractive indices(Gobi/Taklamakan-Volz) increases with the height of dust layer and the AOT.The BTD reaches about 4 K for the high dust layer (532.6 similar to 575.2 hPa) and the AOT of 1.5.Most BTDs (0.5−4 K) are much larger than the noise-equivalent temperature difference of high-spectral resolution infrared sounders.Even for the AOT of 0.1,BTDs for different refractive indices could be close to or larger than 0.2 K.This study emphasizes the requirement of accurate refractive indices according to dust origins for improving brightness temperature simulations and provides valuable references for potential improvement of dust retrievals.(Bi Lei,Ding Shouguo,Zong Ruirui)

7.10 Impacts of cloud scattering properties on FY-3D HIRAS simulations

Cloudy sky spectral radiance at the top of the atmosphere has always been an important while difficult variable to simulate for fast radiative transfer models.In this paper,we focus on examining the impacts of cloud scattering properties on the spectral radiance signature of the high-spectral resolution infrared atmospheric sounder (HIRAS) onboard the Fengyun-3D (FY-3D) satellite by using the advanced radiative transfer modeling system (ARMS) and the community radiative transfer model (CRTM).Cloud scattering properties used in the radiative transfer models are critical for modeling the spectral radiance under cloudy sky,which involves choices of appropriate cloud particle models and particle size distributions,etc.Multiple FY-3D HIRAS observations over the southern China and Southeast Asia with ice or liquid water cloud cover on 6 May 2018 are examined,respectively.Vertical atmospheric profiles are derived from the modern-Era retrospective analysis for research and applications,version 2 reanalysis product.Cloud property retrievals from the moderate resolution imaging spectroradiometer are used.Cloud scattering property parameterization schemes based on spherical and nonspherical cloud particle shapes are implemented for liquid water and ice clouds in ARMS and CRTM,respectively.Results show that both ARMS and CRTM can well simulate the radiance at the HIRAS spectral ranges under liquid water cloud condition as compared with the HIRAS observation with mean absolute error (MAE) of brightness temperature of less than 1 K.However,for ice cloud conditions,ARMS model using assumed spherical ice properties exhibits large biases between simulation and observation.CRTM with nonspherical ice properties using 16-stream approximation shows MAE less than 1 K and MAE of about 1 K using 2-stream approximation.(Yi Bingqi,Ding Shouguo,Bi Lei)

7.11 Multisource assessments of the Fengyun-3D microwave humidity sounder (MWHS) on-orbit performance

The microwave humidity sounder (MWHS) onboard the Fengyun-3D satellite is providing the data for profiling atmospheric temperature and moisture and has become an important data source for improving the weather forecasts.In this article,three data sources are utilized for assessing the MWHS on-orbit performance,including global navigation satellite system occultation sounder (GNOS),ECMWF (European Centre for Medium-Range Weather Forecasts) re-analysis (ERA)-interim reanalysis,and advanced technology microwave sounder (ATMS) data.GNOS-retrieved atmospheric profiles and the reanalysis data are used as inputs to the community radiative transfer model (CRTM) for simulating the MWHS brightness temperatures at the top of the atmosphere in July 2018 for characterizing the instrument performance.Since ATMS is a well-calibrated microwave sounding instrument onboard both Suomi NPP and NOAA-20 satellites,its measurements are also collocated with MWHS data for a consensus analysis using the simultaneous nadir overpasses (SNOs) method.In comparing GNOS simulations,MWHS upper air temperature sounding channels (36) have relatively larger biases (less than 2.5 K) than the water vapor sounding channels.However,the standard deviation of the difference between observations and simulations (O-B) is larger for water vapor sounding channels.For ERA simulations,MWHS sounding channels exhibit negative biases similar to GNOS results but the standard deviation of O-B at the water vapor channels is much smaller.When compared with ATMS water vapor channels,MWHS biases are mostly negative and agree with those from ERA simulation.Thus,the large uncertainty in simulating MWHS water vapor sounding channels from GNOS could result from the poor input water vapor profiles and high water vapor variability in the lower troposphere.(Kan Wanlin,Han Yang,Weng Fuzhong)

7.12 Preface for the special issue of radiative transfer models for satellite data assimilation

Satellite observations have a major positive impact on the accuracy of numerical weather prediction(NWP).They are used in NWP data assimilation systems through variational,ensemble and hybrid methods.For direct assimilation of satellite observations from passive infrared and microwave instruments,a radiative transfer model is typically required to compute the radiance at the top of the atmosphere from atmospheric state variables.In the past two decades,many countries have invested in developing the fast radiative transfer models through their space and NWP programs.The development of RTTOV (radiative transfer for TOVS) fast model in European community started in the 1990s and released in 1999.In the United States,the community radiative transfer model (CRTM) project was initiated at the Joint Center for Satellite Data Assimilation in earlier 2000s and CRTM was released to the United States NWP community in earlier 2010 for satellite radiance assimilation and remote sensing applications.An advanced radiative transfer modeling system (ARMS) is now being developed in China and it is now ready for uses in more broader applications including data assimilations.On April 2 to May 3,2019,the Chinese Academy of Meteorological Sciences State Key Laboratory of Severe Weather,the European Center for Medium-Range Weather Forecast (ECMWF)and US Joint Center for Satellite Data Assimilation (JCSDA) jointly hosted an international workshop in Tianjin,China on radiative transfer models in support of satellite data assimilation.More than 100 scientists from China,US,UK,Germany and Japan attended the workshop.A science steering committee for radiative transfer (SSC4RT) was also formed and 10 distinguished scientists were nominated as the SSC4RT members.The participants reported the major progresses in developing the fast radiative transfer models for satellite data assimilations.In the past,the NWP community primarily had used RTTOV (Europe) and CRTM (US).Now,China is developing the advanced radiative transfer modeling system (ARMS) for more satellite data applications.The SSC recognized the significance of ARMS development and ARMS will become a third pillar supporting NWP satellite data assimilation,after RTTOV and CRTM.This special issue solicits papers documenting research either reported at the workshop or pertaining to the main topics at the Tianjin workshop in solutions of radiative transfer equations,state-of-the art methods for computing the optical properties of aerosols,clouds,and precipitation,and surface reflectivity and emissivity modeling.It also addresses,where appropriate,shortcomings in the models that fast models are trained on,such as line-by-line transmittance models and surface dielectric property models.(Weng Fuzhong,Benjamin T.Johnson,Zhang Peng)

7.13 The long-term trend of upper-air temperature in China derived from microwave sounding data and its comparison with radiosonde observations

Currently,the satellite Microwave Sounding Unit (MSU/AMSU) datasets developed from three organizations—remote sensing systems (RSS),the University of Alabama at Huntsville (UAH),and the NOAA Center for Satellite Applications and Research (STAR)—are often used to monitor the global long-term trends of temperatures in the lower troposphere (TLT),midtroposphere (TMT),total troposphere (TTT),troposphere and stratosphere (TTS),and lower stratosphere (TLS).However,the trend in these temperatures over China has not been quantitatively assessed.In this study,the decadal variability and longterm trend of upper-air temperature during 1979−2018 from three MSU datasets are first evaluated over China and compared with the proxy MSU dataset simulated from homogenized surface and radiosonde profiles (EQU) at 113 stations in China.The regional mean MSU trends over China during 1979−2018 are 0.22−0.27 (TLT),0.15−0.22 (TMT),0.20−0.27 (TTT),0.02−0.14 (TTS),and 20.33−20.36 (TLS) K decade−1,whereas the EQU trends are 0.31(TLT),0.19 (TMT),0.24 (TTT),0.07 (TTS),and 20.26 (TLS) K decade−1.The trends from RSS generally show a better agreement with those from EQU.The trends from both MSU and EQU exhibit seasonal and regional difference with a larger warming in TLT in February and March,and stronger cooling in TLS from late winter to spring.The TLT and TMT over the Tibetan Plateau and northwestern China show larger warming trends.The variabilities from MSU and EQU agree well except TLT in Tibet and the southern China.The major difference in regional mean temperatures over China between MSU and EQU is related primarily to the satellite instrument changes during 1979−1998 and the radiosonde system changes in China in the 2000s.(Guo Yanjun,Weng Fuzhong,Wang Guofu)

7.14 Comparing the thermal structures of tropical cyclones derived from Suomi NPP ATMS and FY-3D microwave sounders

Accurate information on the thermal structures of tropical cyclones (TCs) is essential for monitoring and forecasting their intensity and location.In this study,a scene-dependent one-dimensional variation (SD1DVAR)algorithm is developed to retrieve atmospheric temperature and moisture profiles under all-weather conditions.In SD1DVAR,the background and observation error matrix varies according to the scattering intensity.Specifically,the observation error matrix increases in precipitating atmospheres due to a larger uncertainty in the forward operator.With the data from the advanced technology microwave sounder (ATMS) onboard the Suomi National Polar-orbiting Partnership (NPP) satellite,SD1DVAR can retrieve better thermal structures in the storm life cycle than NOAA microwave integrated retrieval system (MIRS).Comparing with the aircraft dropsonde observations,the temperature and humidity errors from SD1DVAR are about 3 K and 20%,respectively whereas those from MIRS are around 4−5 K and 30%,respectively.SD1DVAR is also applied for the microwave temperature sounder (MWTS) and microwave humidity sounder (MWHS) onboard FY-3D satellite.The MWTS and MWHS datasets are first combined into a single comprehensive microwave suite (CMWS) data stream and then used to retrieve the hurricane thermal structures.It is shown that the hurricane structure from CMWS is very similar to that from ATMS.However,due to the availability of 118 GHz measurements from the CMWS,the hurricane temperature vertical structure is better resolved,and the humidity error is also reduced by about 5%.(Hu Hao,Han Yang)

7.15 Retrieval of oceanic total precipitable water vapor and cloud liquid water from Fengyun-3D microwave sounding instruments

Fengyun-3D (FY-3D) satellite is the latest polar-orbiting meteorological satellite launched by China and carries 10 instruments onboard.Its microwave temperature sounder (MWTS) and microwave humidity sounder (MWHS) can acquire a total of 28 channels of brightness temperatures,providing rich information for profiling atmospheric temperature and moisture.However,due to a lack of two important frequencies at 23.8 and 31.4 GHz,it is difficult to retrieve the total precipitable water vapor (TPW) and cloud liquid water path(CLW) from FY-3D microwave sounder data as commonly done for other microwave sounding instruments.Using the channel similarity between Suomi National Polar-orbiting Partnership (NPP) advanced technology microwave sounder (ATMS) and FY-3D microwave sounding instruments,a machine learning (ML) technique is used to generate the two missing low frequency channels of MWTS and MWHS.Then,a new data set named as combined microwave sounder (CMWS) is obtained,which has the same channel setting as ATMS but the spatial resolution is consistent with MWTS.A statistical inversion method is adopted to retrieve TPW and CLW over oceans from the FY-3D CMWS.The inter-comparison between different satellites shows that the inversion products of FY-3D CMWS and Suomi NPP ATMS have good consistency in magnitude and distribution.The correlation coefficients of retrieved TPW and CLW between CMWS and ATMS can reach 0.95 and 0.85,respectively.(Han Yang,Yang Jun,Hu Hao)

7.16 A preliminary study on the influence of Beijing urban spatial morphology on near-surface wind speed

In order to explore the actual impact of urban spatial morphology on real near-surface wind speed on the urban scale,taking Beijing,China as an example，this study adopted an urban-rural ratio method to construct a WsR (wind speed ratio) to quantify the impact of urbanization on the near-surface wind speed.The spatial morphological parameters such as building height,building density (BD),building standard deviation,floor area ratio (FAR),frontal area index,roughness length,sky view factor (SVF) and fractal dimension at the 500 m spatial resolution were calculated,and further the relationship of these morphological parameters and WsR was analyzed.Results show that due to urbanization,the minimum and maximum average annual wind speed of urban areas are 56% and 160% of the suburban wind speed,and urbanization can reduce the wind speed in urban areas at different times by 3% to 27%.On the urban scale,there are significant spatial correlations between the eight morphological parameters and WsR.Except for the quadratic curve of BD and WsR,the other parameters are significantly linearly related to WsR.FAR,FAI and SVF,the three most significant impact factors,can individually explain 51%,48% and 48% of the change of WsR.(Liu Yonghong,Xu Yongming,Zhang Fangmin)

7.17 城市生态气象监测评估初步研究与实践——以北京为例

气象条件作为影响生态系统最活跃、最直接的驱动因子，影响着生态系统的质量和人类生存的环境，关系着生态保护和建设的成果，而城市生态系统具有与其他系统不一样的气候特征，目前还未形成一套有关城市的生态气象监测评估方法。基于生态气象学理论，分别从城市气候环境、与气候相关的陆表环境、大气环境、人居环境以及城市高影响天气气候事件等5个方面选择不同的要素和指标开展了城市生态气象监测评估初步研究，并以北京为例，利用2018年国家和区域自动气象站资料、大气成分观测资料、2002—2018年MODIS卫星资料、Landsat及环境一号卫星资料，开展了2018年北京城市生态气象监测评估。监测评估显示：（1）2018年北京城市“热岛”和“干岛”气候特征明显，并在北京二环与五环之间存在一个“冂”形风速低值区。（2）2018年北京陆表生态环境、大气环境、人居环境进一步好转：其中植被覆盖度达61.6%，创2002年以来新高，气象条件贡献率达50%，生态涵养区植被生态质量处于正常偏好的面积比例达93.2%；中心城区陆表温度为2011年以来最低值；重要水源地密云水库、官厅水库水体面积均为2000年以来最大值；气溶胶光学厚度、霾日数、大气静稳指数分别较过去4年平均值下降14%、31%和8%，大气扩散条件偏好，对霾日减少贡献率达21%，外地污染传输对PM2.5贡献达到53%；城市生态冷源较2013年明显增加，城市“热岛”得到缓解。（3）历史罕见的夏季高温闷热、冬季阶段低温、极端强降水以及持续无降水等高影响天气气候事件给城市安全运行和生态环境带来不利影响。综合评估表明，2018年北京气象条件总体利于陆表生态环境改善，有利的气候条件提高了生态环境的质量，但城市生态质量仍面临着极端天气气候事件、城市热岛、低风速以及外来大气污染输送等风险。（刘勇洪，轩春怡，李梓铭）

7.18 基于1 km网格的北京暴雨洪涝灾害风险区划

目前许多城市暴雨洪涝灾害综合风险区划对暴雨在复杂地形下可能引发的山洪与地质灾害造成的高风险以及对城市交通安全风险估计不足，同时常规的气象观测资料已难以描述暴雨致灾危险性精细化分布。本文基于自然灾害风险评估理论，利用遴选的293个北京气象自动站2006—2017年逐时降水观测资料、北京2015年1:25万基础地理信息、2016年Landsat8晴空遥感影像、灾情资料以及网格化的社会经济资料，在承灾体暴露度基础上充分考虑了承灾体对暴雨引发的城市积涝、山洪与地质灾害灾损敏感性差异，从暴雨致灾危险性、孕灾环境敏感性、承灾体易损性3个方面开展了北京地区暴雨灾害1 km分辨率的精细网格化风险评估与区划，并结合实际案例进行了分析。结果显示：（1）基于高密度降水观测资料提取的网格化短历时暴雨频次和暴雨量能较为精细地评估致灾危险性；基于遥感与GIS提取的不透水盖度、地形起伏度与河网密度可有效评估暴雨洪涝孕灾环境敏感性；基于1 km网格化的GDP、人口密度和路网密度以及灾损敏感系数可有效评估暴雨引发的积涝、山洪与地质灾害对人员、财产和公路交通的易损性；（2）与已有成果比较，本次北京暴雨洪涝风险区划不但凸显了暴雨对城市的积涝风险，也凸显了暴雨引发的山洪与地质灾害风险，同时突出了暴雨对城市交通设施安全的影响；（3）风险区划结果基本反映了北京市暴雨灾害的潜在风险，北京暴雨洪涝灾害防御的重点区域应放在风险较高的3个区域。（轩春怡，刘勇洪，杨晓燕）