青藏高原与极地气象

青藏高原与极地气象研究进展

Progress in the Tibetan Plateau and Polar Meteorology Research

1 青藏高原气象

1 Tibetan Plateau meteorology

1.1 Precursory signals (SST and soil moisture) of summer surface temperature anomalies over the Tibetan Plateau

Understanding the variability of surface air temperature (SAT) over the Tibetan Plateau (TP) and its precursory signals is of great benefit to climate change adaptation and socioeconomic development. This study explores the precursory signals of summer SATs over the TP in oceanic and land boundary conditions. The results show that the summer eastern TP SAT is significantly correlated with three precursors in April: the high-latitude North Atlantic sea surface temperature (SST), the northern Indian Ocean SST, and the Indian soil moisture (SM). The April SST anomalies (SSTAs) in the high-latitude North Atlantic can exert a cross-season impact on the summer SAT over the TP through two processes. The SSTAs in the high-latitude North Atlantic maintain from April to summer and modulate atmospheric circulation over the eastern TP through exciting a downstream wave train during summer, and finally modulate the summer SAT over the eastern TP. In addition to the above process, the April SSTAs in the high-latitude North Atlantic may remotely regulate simultaneous SM in the Indian subcontinent through stimulating a downstream wave train pattern. Through a persistent SM-precipitation interaction, the April Indian SM anomaly can affect the local precipitation and associated condensation heating anomalies during the ensuing summer, which forces an anomalous cyclone-anticyclone pattern around the TP and accordingly affects the summer SAT over the eastern TP. Additionally, the SSTAs in the northern Indian Ocean can persist from April to summer and adjust the intensity and location of the western North Pacific subtropical high through the Kelvin-wave-induced Ekman divergence during summer, eventually affecting the summer eastern TP SAT. The three precursory signals, which synergistically contribute to the variability of the summer eastern TP SAT, can be applied in predicting the summer SAT over the eastern TP.(Wang Hui, Liu Ge, Wang Sai, He Kejun)

1.2 Retrieval of O3, NO2, BrO and OClO columns from ground-based zenith scattered light DOAS measurements in summer and autumn over the northern Tibetan Plateau

Ground-based zenith scattered light differential optical absorption spectroscopy (DOAS) measurements were performed in summer and autumn (27 May to 30 November) 2020 at Golmud (94°54′ E, 36°25′ N;2807.6 m altitude) to investigate the abundances and temporal variations of ozone (O3) and its depleting substances over the northern Tibetan Plateau (TP). The differential slant column densities (dSCDs) of O3,nitrogen dioxide (NO2), bromine monoxide (BrO), and chlorine dioxide (OClO) were simultaneously retrieved from scattered solar spectra in the zenith direction during the twilight period. The O3vertical column densities(VCDs) were derived by applying the Langley plot method, for which we investigated the sensitivities to the chosen wavelength, the a-priori O3profile and the aerosol extinction profile used in O3air mass factor (AMF)simulation as well as the selected solar zenith angle (SZA) range. The mean O3VCDs from June to November 2020 are 7.21×1018molecules m−2and 7.18×1018molecules m−2at sunrise and sunset, respectively. The derived monthly variations of the O3VCDs, ranging from a minimum of 6.9×1018molecules m−2in October to 7.5×1018molecules m−2in November, well matched the OMI satellite product, with a correlation coefficient R = 0.98.The NO2VCDs at SZA = 90°, calculated by a modified Langley plot method, were systematically larger at sunset than at sunrise as expected with a pm/am ratio of about 1.56. The maximum of the monthly NO2VCDs,averaged between sunrise and sunset, was 3.40×1015molecules m−2in July. The overall trends of the NO2VCDs were gradually decreasing with the time and similarly observed by the ground-based zenith DOAS and OMI. The average level of the BrO dSCD 90°‒80° (i.e., dSCD between 90° and 80° SZA) was 2.06×1014molecules m−2during the period of June–November 2020. The monthly BrO dSCD 90°–80° presented peaks in August and July for sunrise and sunset, respectively, and slowly increased after October. During the whole campaign period, the OClO abundance was lower than the detection limit of the instrument. This was to be expected because during that season the stratospheric temperatures were above the formation temperature of polar stratospheric clouds. Nevertheless, this finding is still of importance, because it indicates that the OClO analysis works well and is ready to be used during periods when enhanced OClO abundances can be expected. As a whole, ground-based zenith DOAS observations can serve as an effective way to measure the columns of O3and its depleting substances over the TP. The aforementioned results are helpful in investigating stratospheric O3chemistry over the third pole of the world. (Cheng Siyang, Ma Jianzhong, Zheng Xiangdong)

1.3 Monsoon clouds control the summer surface energy balance on east Rongbuk glacier (6,523 m above sea level), the northern of Mt. Qomolangma (Everest)

To identify the atmospheric controls of the summertime glacier surface energy balance in the Himalayas,in situ meteorological data collected at 6,523 m above sea level during May–July 2005 were obtained and analyzed. Our results showed that net shortwave radiation (103 W m−2) and turbulent sensible heat flux(12 W m−2) acted as energy sources, and net longwave radiation (−62 W m−2) and turbulent latent heat flux(−20 W m−2) represented heat sinks. Cloud cover controlled the summer surface energy balance. During the active period of the South Asian summer monsoon, the frequent cloud coverage increased the incoming longwave radiation more than it decreased the incident solar radiation. Intensification (weakening) of the South Asian summer monsoon strengthened (suppressed) surface melting. The melt energy measured during the nonmonsoon period was small due to the energy consumption associated with glacier volume warming,energy loss from sublimation, and large heat loss through net longwave radiation due to the low amount of incoming longwave radiation caused by the low cloudiness. The comparison of glacier surface energy balances on the Tibetan Plateau shows that on continental glaciers, net radiation is lower and accounts for a smaller contribution to energy sources, and the dominant energy sinks are sublimation and evaporation, rather than melting, which is the primary energy sink for maritime/subcontinental glaciers. This implies an important spatial variability in glacial sensitivity to different climatic conditions on the Tibetan Plateau. (Liu Weigang,Zhang Dongqi, Ding Minghu)

1.4 Links between the thermal condition of the Tibetan Plateau in summer and atmospheric circulation and climate anomalies over the Eurasian continent

We examine the links between the thermal condition of the troposphere over the Tibetan Plateau with the atmospheric circulation and climate over the Eurasian continent. The temperature of the troposphere over the Tibetan Plateau is higher than the temperature in other regions at the same latitude and is consistent with the temperature of the Eurasian troposphere on an interannual timescale. The higher temperature of the troposphere over the Tibetan Plateau leads to anomalous south–north temperature gradients from mid-latitudes over the Eurasian continent to its two flanks, accompanied by anomalous easterly and westerly winds in the upper troposphere in the subtropics and at higher latitudes. Anomalous anticyclonic circulations and subsidence motions appear between the anomalous easterly and westerly winds and contribute to the high surface air temperature over West Asia, Central Asia and East Asia via anomalous vertical temperature advection in the troposphere and change in the amount of solar radiation incident on the surface. The enhanced East Asian summer monsoon associated with the high temperature of the troposphere over the Tibetan Plateau also partly contributes to the high surface air temperature over East Asia via horizontal temperature advection. The westerly wind anomalies in the north of the mid-latitudes over the Eurasian continent indicate the enhancement and northward shift of the mid-latitude westerly jet. This is related to anomalous upward motion and higher precipitation in Northeast China and North China. Sensitivity experiments based on an atmospheric model verify the impact of anomalous tropospheric heating over the Tibetan Plateau in summer on the atmospheric circulation over the Eurasian continent. (Nan Sulan, Zhao Ping, Chen Junming, Liu Ge)

1.5 台站建设

2021年4月，于西藏自治区墨脱县新建方舱式风廓线雷达（型号CFL-03，北京无线电测量研究所）观测站，位于墨脱县气象局观测场旁，海拔1305 m，地理环境为郊外山腰，可实现0～6 km风廓线的探测。2021年7月，于云南贡山新建微波辐射计（型号MP-3000A，Radiometer）观测站，位于贡山县气象局楼顶，海拔1590 m，地理环境为郊外山腰，测量从地面至天空10 km高度垂直剖面上的温度、湿度廓线，液态水含量，云参数及有无降水等大气信息。2021年8月，于西藏自治区嘉黎县新建边界层梯度通量观测系统，位于嘉黎县气象局观测场旁，海拔4489 m，地理环境为河谷，观测要素包含5层空气温湿、5层土壤温湿、5层风速、风向、光合有效辐射、净辐射、土壤热通量、地表温度、雨量、雪深、CO2/H2O汽通量。

1.6 青藏高原大气环境立体综合观测试验

根据中国气象科学研究和西藏自治区双边协议，联合推进青藏高原野外科学试验。双方在拉萨站开展了多轴差分吸收光谱（MAX-DOAS）和大气水汽—气溶胶激光雷达地基主被动遥感观测，提升了提升青藏高原地区大气环境多组分垂直廓线观测能力，为开展青藏高原气候变化研究、青藏高原立体生态环境研究、青藏高原臭氧低谷研究等重大前沿科学问题奠定了数据基础。首次在青藏高原北部（西宁—达日—玉树线）和南部（拉萨—林芝—波密线）同时协作开展基于车载平台的青藏高原大气环境多组分三维结构动态遥感探测试验，有望对三江源地区和藏东南水汽通道上的大气环境有新认识。

1.7 第二次青藏高原水环境科学考察

2021年进行了一次青藏高原水环境科学考察，在青海曲麻莱、西藏错那的冻土安装了冻土温度、水热通量和气象观测仪器；在西藏浪卡子枪勇冰川和申扎甲岗山冰川安装了水位计和自动气象站；在在青海玉树隆宝湿地安装了水位计并进行观测。根据西风季风作用范围，选取了3个位于不同气候区的典型站点（错那、若尔盖、普兰）进行降水采样，将通过分析降水样品的同位素来确定西风—季风作用时间，并在甘肃省玛曲县进行水汽稳定氢氧同位素在线观测。参加了“藏东南大峡谷河谷关键区野外观测现场考察”和“高原气候资料稀缺地区气候变化及其影响应对”的野外考察，对沿途冰冻圈及其变化情况进行了文献查阅和现场考察。

1.8 推进青藏高原区域气候监测、诊断与预测业务系统建设

中国气象科学研究院与国家气候中心基于青藏高原相关科研成果，筛选、构建出了在我国气候诊断、预测中具有实际业务应用意义的系列指标和预测模型，共同建立了“青藏高原区域气候监测诊断预测系统（TPMAPS）” ，为国家气候中心汛期气候趋势预测业务提供了参考，发挥了积极作用。2021年11月9日，经专家论证，TPMAPS进入准业务运行。

2 极地气象

2 Polar meteorology

2.1 Two atmospheric responses to winter sea ice decline over the Barents-Kara Seas

The intraseasonal atmospheric responses to winter sea ice decline over the Barents-Kara Seas are examined by dividing rapid sea ice decline events into two categories, based on the direction (upward vs.downward) of the anomalous surface turbulent heat flux (ASTHF) after the sea ice loss. The upward ASTHF events, which could potentially have a large impact on the overlying atmosphere, are characterized by anomalously negative total column water and surface air temperature minus skin temperature, and anomalously positive surface wind speed following the sea ice loss. The downward ASTHF events show opposite features.Both types of events are linked to the Madden-Julian oscillation and subsequent circulation anomalies. This result indicates that on the intraseasonal time scale, not all sea ice decline events influence the atmosphere. (Jiang Zhina, Steven B. Feldstein, Sukyoung Lee)

2.2 Potential mechanisms governing the variation in rain/snow frequency over the northern Antarctic Peninsula during austral summer

Precipitation with different phases can exert different influences on the Antarctic mass balance. Using the observational rain and snow days from the Great Wall Station, ERA-interim reanalysis, and other data,this study investigates the mechanisms governing the year-to-year variability of precipitation phase (i.e.,rainfall and snowfall) over the northern Antarctic Peninsula (AP) during austral summer (December, January,and February; abbreviated as DJF) for the period 1985–2016. The results reveal that the rainfall and snowfall anomalies are controlled mainly by the change in the proportion of precipitation occurring as rain and snow,and the latter is strongly influenced by the change in air temperature. Through regulating the air temperature,different atmospheric circulation anomalies affect the variability of the rainfall and snowfall over the northern AP during summer. Specifically, a circulation pattern with an anomalous anticyclone over the Malvinas Islands and an anomalous cyclone over the Amundsen-Bellingshausen Seas (ABS) can increase summer rainfall,whereas an anomalous cyclone over the Weddell Sea facilitates more snowfall. The summertime atmospheric circulation anomalies, which modulate the variability of rainfall over the northern AP, are primarily caused by an atmospheric teleconnection pattern persisting from austral spring (September–November, SON) to summer. Such a persistent teleconnection pattern can be attributed to the long-time maintenance of sea surface temperature anomalies due to air-sea interaction processes. (Wang Sai, Liu Ge, Ding Minghu, Chen Wen,Zhang Wenqian, Lyu Junmei)

2.3 Impact of internal variability on recent opposite trends in wintertime temperature over the Barents-Kara Seas and central Eurasia

The large ensembles of the IPSL-CM6A-LR model output for the historical forcing experiment were employed to investigate the role of internal variability in the formation of the recent “warm Arctic-cold Eurasia” trend pattern in winter surface air temperature (SAT). The ensemble-mean SAT shows a positive trend over Arctic during 1990–2014, indicating a positive contribution of anthropogenic forcing to the warming Arctic. Over the region of central Eurasia, the ensemble-mean SAT trend is opposite to the observed trend.The winter SAT trends display remarkable inter-member diversity over the Barents-Kara Seas (BKS) region and central Eurasia, suggesting an important role played by internal variability. In addition to anthropogenic forcing, the results suggest that the barotropic anticyclone over northern Eurasia arising from internal variability can also contribute positively to the warming anomalies over the BKS region. On the other hand,through a fingerprint pattern matching method, it is found that the observed cooling trend over central Eurasia tends to be predominantly due to the internal variability. Finally, the results estimate that the internal variability can contribute to about 50%‒60% of the observed warming trend over the BKS region. (Wang Sai, Chen Wen)

2.4 Evaluation of multiple density-dependent empirical snow conductivity relationships in East Antarctica

Nine density-dependent empirical thermal conductivity relationships for firn were compared against data from three automatic weather stations at climatically different sites in East Antarctica (Dome A, Eagle, and LGB69). The empirical relationships were validated using a vertical, 1D thermal diffusion model and a phasechange-based firn diffusivity estimation method. The best relationships for the abovementioned sites were identified by comparing the modeled and observed firn temperature at a depth of 1 and 3 m, and from the mean heat conductivities over two depth intervals (1–3 and 3–10 m). Among the nine relationships, that proposed by Calonne et al. (2011) appeared to show the best performance. The density- and temperature-dependent relationship given in Calonne et al. (2019) does not show clear superiority over other density-dependent relationships. This study provides a useful reference for firn thermal conductivity parameterizations in land modeling or snow–air interaction studies on the Antarctica ice sheet. (Ding Minghu)

2.5 On the differences in precipitation type between the Arctic, Antarctica and Tibetan Plateau

Under the effect of global warming, more precipitation will shift to rainfall in cryospheric regions.Considering the influence of the precipitation type on surface energy and mass cycles, it is important to determine the specific precipitation features and to classify the precipitation type in key areas correctly. We analyzed the monthly distribution, variations in annual days of each precipitation type, and trends based on daily precipitation and air temperature observations from six tripolar stations. The results indicated that snow dominated the precipitation type at Zhongshan Station (69.4°S, 76.4°E) throughout the year, while the Greatwall Station (62.2°S, 59.0°W) exhibited a relatively diverse precipitation type distribution and significant seasonal cycles. Compared to the Greatwall Station, every precipitation type was less frequently encountered at the Barrow (71.3°N, 156.8°W), Coral Harbour (64.2°N, 83.4°W), Linzhi (29.6°N, 94.5°E), and Maqu stations(34°N, 102.1°E), in which all the sites demonstrated classical reverse seasonal variation. A consistent trend across the years was found regarding the trends of the different precipitation types, except at the Greatwall and Coral Harbour stations. Due to snow/rain conditions partly converting into sleet conditions, which may be related to air temperature changes and synoptic atmospheric activities, inconsistent increasing trends of the sleet days were observed compared to the snow/rain days. Furthermore, a hyperbolic parameterized model was also fitted to determine the air temperature threshold of precipitation type transitions in this paper. According to the threshold comparison results, a warm bias in the temperature threshold was found at the warm stations.We also proposed that high relative humidity and low freezing levels were the likely reasons for the ERA5 reanalysis datasets. Finally, this paper’s fitted parameterized model was proven to perform better than the ERA5 reanalysis datasets through validation. This preliminary research provides observational evidence and possible interpretation of the mechanism of precipitation type changes in tripolar areas. (Yang Diyi, Ding Minghu)

2.6 The shallowing surface temperature inversions in the Arctic

Temperature inversion plays an important role in various physical processes by affecting the atmospheric stability, regulating the development of clouds and fog, and controlling the transport of heat and moisture fluxes. In the past few decades, previous studies have analyzed the spatiotemporal variability of Arctic inversions, but few studies have investigated changes in temperature inversions. In this study, the changes in the depth of Arctic inversions in the mid-21st century are projected based on a 30-member ensemble from the Community Earth System Model Large Ensemble (CESM-LE) project. The ERA-Interim, JRA-55, and NCEP-NCAR reanalyses were employed to verify the model results. The CESM-LE can adequately reproduce the spatial distribution and trends of present-day inversion depth in the Arctic, and the simulation is better in winter. The mean inversion depth in the CESM-LE is slightly underestimated, and the discrepancy is less than 11 hPa within a reasonable range. The model results show that during the mid-21st century, the inversion depth will strongly decrease in autumn and slightly decrease in winter. The shallowing of inversion is most obvious over the Arctic Ocean, and the maximum decrease is over 65 hPa in the Pacific sector in autumn. In contrast,the largest decrease in the inversion depth, which is more than 45 hPa, occurs over the Barents Sea in winter.Moreover, the area where the inversion shallows is consistent with the area where the sea ice is retreating,indicating that the inversion depth over the Arctic Ocean in autumn and winter is likely regulated by the sea ice extent through modulating surface heat fluxes. (Zhang Lin, Ding Minghu, Lyu Junmei)

2.7 Characteristics of low-level temperature inversions over the Arctic Ocean during the CHINARE 2018 campaign in summer

Across the Arctic Ocean, the semi-permanent sea-ice layer insulates the atmosphere from the Arctic Ocean, thereby influencing the lower boundary conditions of the atmosphere in this region. The presence of a temperature inversion predominantly characterizes the Arctic lower troposphere throughout the year. The 9th Chinese National Arctic Research Expedition took place in the Pacific Sectors of the Arctic Ocean from July to September 2018, gathering high-resolution radiosonde data to provide a detailed structure of the lowlevel temperature inversion (LLI) over the summer months. Statistical analyses showed that LLIs occurred most frequently below 500 m and were generally weaker and shallower than those occurring during the winter months. Low-level inversions are influenced by both local meteorology and large scale synoptic conditions.The continual presence of low-level clouds or fog tends to lift the inversion layer at the local scale, transferring it from a surface-based inversion (SBI) to an elevated inversion (EI). By contrast, SBIs were found to dominate over regions of melting sea-ice. Regimes, where the cloud top penetrated the inversion base were more frequently observed than where regimes cloud capped by inversion. Inversions tended to be deeper, stronger,and with a lower base during cloud top penetrating inversion base regimes. At the synoptical scale, the intense poleward intrusions of warm air brought moist air to the top of low-level inversions, creating a robust thermal stratification between layers and promoting extensive and frequency occurrences of low cloud/fog.Moreover, the polar high contributed to establishing a multilayer inversion structure at relatively high altitudes by subsidence. These findings contribute to an improving understanding of low-level vertical temperature structures and their influence on a rapidly warming Arctic. (Zhang Lei, Li Jian, Ding Minghu )

2.8 Estimation and long-term trend analysis of surface solar radiation in Antarctica: A case study of Zhongshan Station

Long-term, ground-based daily global solar radiation (DGSR) at Zhongshan Station in Antarctica can quantitatively reveal the basic characteristics of Earth’s surface radiation balance and validate satellite data for the Antarctic region. The fixed station was established in 1989, and conventional radiation observations started much later in 2008. In this study, a random forest (RF) model for estimating DGSR is developed using ground meteorological observation data, and a high-precision, long-term DGSR dataset is constructed. Then, the trend of DGSR from 1990 to 2019 at Zhongshan Station, Antarctica is analyzed. The RF model, which performs better than other models, shows a desirable performance of DGSR hindcast estimation with an R2of 0.984,root-mean-square error of 1.377 MJ m−2, and mean absolute error of 0.828 MJ m−2. The trend of DGSR annual anomalies increases during 1990–2004 and then begins to decrease after 2004. Note that the maximum value of annual anomalies occurs during approximately 2004/2005 and is mainly related to the days with precipitation(especially those related to good weather during the polar day period) at this station. In addition to clouds and water vapor, bad weather conditions (such as snowfall, which can result in low visibility and then decreased sunshine duration and solar radiation) are the other major factors affecting solar radiation at this station. The high-precision, long-term estimated DGSR dataset enables further study and understanding of the role of Antarctica in global climate change and the interactions between snow, ice, and atmosphere. (Zeng Zhaoliang,Ding Minghu)

2.9 The surface energy balance of Austre Lovénbreen, Svalbard, during the ablation period in 2014

The ability to simulate the surface energy balance is key to studying land-atmosphere interactions;however, it remains a weakness in Arctic polar sciences. Based on the analysis of meteorological data from 1 June to 30 September 2014 from an automatic weather station on the glacier Austre Lovénbreen, near Ny-Ålesund, Svalbard, we established a surface energy balance model to simulate surface melt. The results reveal that the net shortwave radiation accounts for 87% (39 W m–2) of the energy sources, and is controlled by cloud cover and surface albedo. The sensible heat equals 6 W m–2and is a continuous energy source at the glacier surface. Net longwave radiation and latent heat account for 31% and 5% of heat sinks, respectively. The simulated summer mass balance equals ‒793 mm w.e., agreeing well with the observation by an ultrasonic ranger. (Zou Xiaowei, Ding Minghu)

2.10 Skillful prediction of winter Arctic oscillation from previous summer in a linear empirical mode

The winter Arctic oscillation (WAO), as a primary atmospheric variability mode in the Northern Hemisphere, plays a key role in influencing mid-high-latitude climate variations. However, current dynamical seasonal forecasting systems have limited skills in predicting WAO with lead time longer than two months.In this study, we design a linear empirical model using two effective precursors from anomalies of the Arctic sea ice concentration (SIC) and the tropical sea surface temperature (SST) initiated in preceding late summer(August) which are both significantly correlated with WAO in recent four decades. This model can provide a skillful prediction of WAO at about half-year lead started from previous summer and perform much better than the dynamical models. Such a significantly prolonged lead time is owed to the stable precursor signals extracted from the SIC and SST anomalies over specific areas, which can persist from previous August and be further enhanced through autumn months. Validation results show that this model can produce a 20-year independent-validated prediction skill of 0.45 for 1999–2018 and a 39-year cross-validated skill of 0.67 for 1980–2018, providing a potentially effective tool for earlier predictions of winter climate variations at midhigh latitudes. (Ren Hongli, Yu Nie)

2.11 南极冰盖地表能量平衡的研究进展

南极冰盖地表能量平衡研究，是耦合冰冻圈，大气圈和水圈过程的基础之一，对了解冰盖表面物质平衡过程、提升极地天气气候预报水平有着极其重要的意义。随着近年来极地观测水平的提升，南极冰盖地表能量平衡研究取得了长足的进步，特别是在辐射参数化和有效热传导系数模拟等方面。但在南极广袤的地域和多样的天气条件下，已有的研究大都适用于局地环境，限制了我们的模拟和预测能力。因此，适用于极地特殊环境的传感器的研发、关键气候区的科学试验的开展和基础的气象站网建设，能加深我们对冰/雪—气相互作用过程的理解，进而改进已有的参数化方案，应用于多圈层模式的耦合。（杨堤益，丁明虎）

2.12 “暖北极-冷欧亚”模态的年代际变化及其与北大西洋海温的联系

本文利用美国航空航天局戈达德空间研究所地表气温，美国国家海洋和大气局—环境科学协作研究所20世纪再分析资料，以及第六次国际耦合模式比较计划的多模式Historical试验结果，去除外强迫影响后，研究1910/1911—2019/2020年冬季（DJF）欧亚中高纬地区 “暖北极—冷欧亚”（WACE）模态的年代际变化特征及其物理原因。结果表明：WACE具有显著的年代际变化，在WACE正位相时期，乌拉尔阻塞发生频率偏高，有利于热量向极区输送使得极区出现异常暖平流，且水汽向极区输送导致极区水汽辐合，向下长波辐射增加，另外对流活动增强导致潜热释放，进而极区温度上升。与此同时，极涡及欧亚大陆西风减弱且乌拉尔阻塞发生频率偏高，有利于冷空气侵袭欧亚大陆造成异常冷平流，且欧亚地区水汽辐散，向下长波辐射减少，对流活动减弱进而潜热释放减少，导致欧亚大陆温度降低。最后利用CAM3.0大气环流模式模拟了北大西洋海温正异常对WACE的影响，模式结果与统计结果相符合，进一步说明了北大西洋海温正异常可以通过强迫低层与高层大气环流异常，导致极区水汽辐合，欧亚大陆水汽辐散，进而影响WACE的年代际变化。（王婧， 吕俊梅）

2.13 冬季北半球大气对秋冬季巴伦支海海冰异常的敏感性研究

基于ERA-Interim再分析资料，借助大气模式CAM4，分析了北半球冬季不同月份的平均大气对巴伦支海不同振幅及不同季节海冰扰动的敏感性，并考察了中高纬度典型大气模态的分布变化情况。结果表明，冬季巴伦支海海冰的减少，会导致湍流热通量异常向上、局地异常变暖及水汽含量的异常升高，且相关异常的强度和范围随着海冰减少幅度的减小而减弱。这种局地响应会通过大气环流调整扩散开来，产生远程影响。具体地，冬季大气环流与欧亚地面温度异常对于不同幅度海冰异常的响应是非线性的，且在不同月份也呈现出不同特征。秋季巴伦支海海冰减少虽未引起局地显著的温度异常，但欧亚大陆温度及环流场异常响应的强度更强、范围更广，这表明秋季海冰可以独立地对冬季中纬度大气产生影响。此外，冬季不同月份西伯利亚高压强度、位置对巴伦支海海冰减少的响应是不同的，北大西洋涛动位相的倾向变化对不同季节、不同振幅海冰减少的响应也不相同。冬季海冰减少时，12月和1月，西伯利亚高压强度更易偏强、位置易偏东，2月则与之相反。与冬季相比，秋季海冰偏少时，西伯利亚高压更易稳定维持在欧亚大陆，晚冬时发生北大西洋涛动负位相的概率增大，但出现极端负位相概率降低。这为了解巴伦支海海冰异常对北半球天气、气候的影响提供了参考。（段升妮，姜智娜）

2.14 过去2000年北极地区气候变化特征及其对AMO的响应

北极地区对全球变暖十分敏感，分析北极地区过去2000年历史气候对揭示全球气候变化极其重要，也是国际过去全球变化计划（PAGES）的重要目标。然而，过去2000年北极不同区域的气候随时间变化是否存在一致性仍有待检验。文章基于北极地区及其3个子扇区（北大西洋扇区、阿拉斯加扇区和西伯利亚扇区）的温度序列，对过去2000年北极的气候变化进行了趋势分析与频谱特征分析，初步探讨了大西洋多年代涛动（AMO）作为驱动因子对北极地区温度的影响。结果表明，公元1—1800年间北极地区存在着普遍的降温过程（-0.47 ℃/ka），但温度变化区域差异显著，其中北大西洋扇区与北极地区整体温度间呈现显著的相关性（0.82）。北大西洋扇区温度呈现“平稳—下降—陡升”的趋势，阿拉斯加扇区温度呈现“下降—缓升—下降—陡升”的趋势，西伯利亚扇区温度呈现“平稳波动—陡升”的趋势。在过去200年间北极地区及其3个子扇区气候均出现了快速变暖。频谱分析表明，北极地区温度存在着准14年、准26年、准62年、准75年和准186年周期，其中北极地区温度的LFV谱在准62年与准75年周期与AMO周期大致吻合。综合交叉谱与小波分析的结果，公元1100年后，AMO以准74年的周期影响北极的气候变化，其中，北大西洋扇区受影响最为明显。阿拉斯加扇区与西伯利亚扇区虽然存在着显著的年代际周期特征，但可能与AMO的关联并不显著，这些区域气候变化的影响机理需要进一步深入研究。(祁威，丁明虎)

2.15 BCC_CSM北极海冰模拟性能的改进对东亚冬季气候模拟的影响

国家气候中心气候系统模式（BCC_CSM）将美国Los Alamos国家实验室发展的海冰模式CICE5.0替代原有的海冰模式SIS，形成一个新版本耦合模式，很好地提高了模式对北极海冰和北极气候的模拟能力。在此基础上，本文评估新耦合模式对1985—2014年东亚冬季气候的模拟性能，检验北极海冰模拟性能的改进对东亚冬季气候模拟性能的影响。结果表明，引入CICE5.0后，新耦合模式能较好地模拟出东亚冬季海平面气压、850 hPa风场以及辐射通量，进而改善东亚气温以及降水的气候态空间分布模拟效果。进一步分析发现，与原有耦合模式相比，新耦合模式更好地抓住了东亚冬季海平面气压、总降水量和气温异常对同期巴伦支海—喀拉海海冰密集度异常的响应，进而提高了模式对东亚冬季中高纬度地区气温以及降水变率的模拟能力。（邓汝漳，储敏，任宏利，刘景鹏，陈权亮）

2.16 第三极地区气温和积雪的季节-年际气候预测研究

第三极地区气候多样、灾害频发，是影响全球和亚洲气候异常的关键区域，针对此地区开展季节—年际气候预测研究对于提高区域预报技巧以及减少灾害造成的影响具有重要的科学和指导意义。基于国家气候中心气候预测业务模式（BCC_CSM1.1m）的历史回报和预测数据，对第三极地区2 m气温和积雪的预测结果进行了确定性技巧评估，并分析了海洋因子对于预测技巧的调制作用。研究表明：该模式对于青藏高原及其周边地区气温和积雪的季节—年际气候预测具有一定的预测能力，对夏季气温的预测效果整体上好于冬季气温和积雪深度预测；预测技巧随着模式起报时间的提前而下降，但是存在技巧回升现象。研究也发现，海温异常因子对第三极地区的气候预测技巧具有不同程度的调制作用，厄尔尼诺等海洋信号能够通过直接和间接作用影响第三极地区的气候预测。（汪芋君，任宏利，王琳）

2.17 长城气象站、中山气象台观测业务

2021年，中国气象科学研究院继续执行极地气象业务化观测任务，陕西省气象局武维刚、安徽省气象局凌新锋和山西省气象局董剑3位同志在南极长城站和中山站进行地面气象、臭氧洞及大气成分等观测，获取合乎WMO规范的气象数据并报送中国气象局国家气象信息中心和WMO。在南极冰盖内陆及边缘地区、格陵兰冰盖等地区维护运行的17个自动气象站继续稳定获取自动气象观测数据。选拔并培训了第38次南极考察队员，已前往南极执行下一年度观测任务。

2.18 南极泰山气象站、昆仑气象站正式业务化

在昆仑站、泰山站稳定准业务运行5年和近9年之后，经过对接收到的数据的分析，两站数据质量稳定，数据到报率分别超过99.58%和99.73%，指标超过国家基本气象站业务运行要求。为体现中国气象全球观测、全球预报和全球服务的战略目标，我们积极向院和局观测司、预报司以及国际司等业务主管部门申请，使两站升级为国家基本气象站（站号：89572、89576），自2021年12月1日（北京时间11月30日20:00）起正式开展业务运行，凸显极地特殊环境观测水平。该工作受到WMO和国际关注。

2.19 受疫情影响，开展北冰洋和黄河站考察

由于疫情原因，2020年度无法派人去黄河站现场工作。目前国内接收到截至2020年2月2日数据，内部存储卡可存储一年数据，其余数据需现场取回。2021年，气科院积极参与极地考察工作，在第12次北极科学考察中继续进行走航路线上大气成分的观测，观测内容包括黑碳气溶胶、CH4、CO2、CO、地面臭氧。

3 其他

3 Others

3.1 Effect of preceding soil moisture-snow cover anomalies around Turan Plain on June precipitation over the southern Yangtze River Valley

This study investigates precursory signals of the June precipitation over the southern Yangtze River Valley(SYRV). It is found that the synergistic anomalies of the Turan Plain soil moisture and northern Iranian Plain snow cover (TPSM-IPSC) during April can modulate the June SYRV precipitation. Through the persistence/memory effect of soil moisture anomalies, lower soil moisture around the Turan Plain-Iranian Plain region can maintain from April to June. Because of drier soil (i.e., lower soil moisture), higher surface air temperature (SAT)appears over the Turan Plain during June. The higher SAT anomaly stimulates anomalous upward motion and associated overlying and downstream atmospheric circulation anomalies through modulating the downstream dispersion of Rossby wave energy. As a part of these atmospheric circulation anomalies, the blocking-like anomaly to the west of the Okhotsk Sea facilitates more June precipitation over the SYRV. Additionally, June SYRV precipitation is significantly correlated with sea surface temperature (SST) anomalies in the tropical eastern Pacific (TEP) during the preceding winter. The TPSM-IPSC can compensate for the defect of prediction using the TEP SST (i.e., ENSO) signal in recent years since the former (latter) shows a strengthened (weakened)relationship with SYRV precipitation recently. Considering jointly the traditional pacific SST and new TPSMIPSC precursors, we establish a physics-based statistical prediction model, which shows a good skill in predicting June SYRV precipitation. (He Kejun, Liu Ge, Wu Renguang, Nan Sulan, Li Jingxin, Yue Xiaoyuan,Wang Huimei, Wei Xinchen, Li Rongrong)

3.2 A tripole pattern of summer surface air temperature anomalies over northern Eurasia and its precursory signals in the tropical Atlantic and northern Asian land

This study investigates the variability of summer surface air temperature (SAT) over northern Eurasia and its precursory signals in the tropical Atlantic and northern Asian land. The leading mode of summer SAT variations features a northern Eurasian SAT tripole (NEST) pattern, with two same-sign SAT anomaly regions over eastern Europe-western Siberia and the Far East region and an opposite-sign SAT anomaly region around the Baikal Lake. It is found that sea surface temperature (SST) or SAT anomalies in the tropical Atlantic and rainfall-soil moisture anomalies around the Baikal Lake during May can modulate the NEST pattern. The SST anomalies in the tropical Atlantic persist from May to summer and induce a downstream zonal wave train across northern Eurasia, consequently causing the variation in the summer NEST pattern. May rainfall anomalies around the Baikal Lake affect the overlying atmospheric circulation during summer through the“memory” effect of soil moisture and the soil moisture-rainfall interaction, correspondingly modulating the downstream wave train and the associated NEST pattern. Based on the above results, a statistical prediction model is established using the two precursory signals, that is, SAT in the tropical Atlantic and rainfall around the Baikal Lake during May. The leave-three-out cross-validation shows that the model has a high skill in predicting the summer NEST pattern, with a correlation coefficient of 0.51 (significant at the 99.8% confidence level) between observation and prediction during the period 1980‒2016. (He Kejun, Liu Ge, Wu Renguang, Li Jingxin, Wang Huimei, Yue Xiaoyuan)

3.3 ROx budgets and O3 formation during summertime at Xianghe suburban site in the North China Plain

Photochemical smog characterized by high concentrations of ozone (O3) is a serious air pollution issue in the North China Plain (NCP) region, especially in summer and autumn. For this study, measurements of O3,nitrogen oxides (NOx), volatile organic compounds (VOCs), carbon monoxide (CO), nitrous acid (HONO),and a number of key physical parameters were taken at a suburban site, Xianghe, in the NCP region during the summer of 2018 in order to better understand the photochemical processes leading to O3formation and find an optimal way to control O3pollution. Here, the radical chemistry and O3photochemical budget based on measurement data during 1–23 July using a chemical box model is investigated. The daytime (06:00–18:00 LST) average production rate of the primary radicals referred to as ROx(OH + HO2+ RO2) is 3.9×10−9(V) h−1.HONO photolysis is the largest primary ROxsource (41%). Reaction of NO2+ OH is the largest contributor to radical termination (41%), followed by reactions of RO2+ NO2(26%). The average diurnal maximum O3production and loss rates are 32.9×10−9(V) h−1and 4.3×10−9(V) h−1, respectively. Sensitivity tests without the HONO constraint lead to decreases in daytime average primary ROxproduction by 55% and O3photochemical production by 42%, highlighting the importance of accurate HONO measurements when quantifying the ROxbudget and O3photochemical production. Considering heterogeneous reactions of trace gases and radicals on aerosols, aerosol uptake of HO2contributes 11% to ROxsink, and the daytime average O3photochemical production decreases by 14%. The O3-NOx-VOCs sensitivity shows that the O3production at Xianghe during the investigation period is mainly controlled by VOCs. (Xue min, Ma Jianzhong)

3.4 Reduction in extreme climate events and potential impacts by the use of technological advances

Technological advances have the potential to balance climate change mitigation and economic development. However, it remains unclear how much technological advances alone can mitigate climate change and the associated economic losses in the future. Through designing a suite of technological advances scenarios and using an earth system model with an integrated assessment model, we illustrate that rapid technological progress without production control might achieve the 2 global warming target in the 2100s.Relative to a world of stagnant technology, the frequency (intensity) of extreme warm events at the end of the 21st century (2081‒2100) would be reduced by about 21% (5.5 ) via rapid technological advances, with a reduction in extreme precipitation (droughts) by about 41% (10 times). Furthermore, fast technological advances may reduce the global economic losses linked with climate change at 2081‒2100 by about 21% and those in China related to floods (droughts) by 86% (67%). Our results highlight the potential of technological advances to fill the emission gap between the Paris Agreement and unconditional Nationally Determined Contributions and hence to efficiently mitigate global warming. (Wei Ting, Liu Changxin, Dong Wenjie)

3.5 Effects of organic soil in the Noah-MP land-surface model on simulated skin and soil temperature profiles and surface energy exchanges for China

Inclusion of the thermal and hydraulic effect of soil organic matter plays an important role in land-surface models (LSMs) for simulating soil temperature and surface energy exchanges. The current officially released Noah with multiparameterization (Noah-MP) LSM, implemented in the community weather research and forecasting (WRF) model, does not include a parameterization for soil organic matter. In this study, the thermal and hydraulic effect of soil organic matter was parameterized into Noah-MP LSM. Based on the profiled soil organic matter data for China and the observations of skin temperature and soil temperature profiles from more than 2000 surface meteorological stations, we investigated the effects of organic soil using the Noah-MP LSM with and without the profiled soil organic matter at the regional scales. Compared with the simulation without the inclusion of organic soil parameterization, the Noah-MP LSM simulation with the profiled soil organic matter improved the skin temperature and soil temperature profiles, especially soil temperature in deep soils under cold and arid regions. The realistic representation of snow depth and the snow insulation dependency on snow depth were confirmed to be a pre-requisite in Noah-MP in the high latitudes. By incorporating the profiled soil organic matter, the Noah-MP LSM enlarged the regional mean sensible heat flux (SH) and lowered the regional mean latent heat flux (LH). In the warm and humid regions, a relatively smaller effect of organic soil on soil temperature could lead to a larger effect on SH and LH, especially LH. (Zhang Guo, Chen Yueli, Li Jianduo)

3.6 Asymmetry and pathways of inter-hemispheric transport in the upper troposphere and lower stratosphere

Inter-hemispheric transport may strongly affect the trace gas composition of the atmosphere, especially in relation to anthropogenic emissions, which originate mainly in the Northern Hemisphere. This study investigates the transport from the boundary surface layer of the Northern Hemispheric (NH) extratropics(30º–90º N), Southern Hemispheric (SH) extratropics (30º–90º S), and tropics (30º S–30º N) into the global upper troposphere and lower stratosphere (UTLS) using simulations with the chemical Lagrangian model of the stratosphere (CLaMS). In particular, we diagnose inter-hemispheric transport in terms of the air mass fractions (AMFs), age spectra, and the mean age of air (AoA) calculated for these three source regions. We find that the AMFs from the NH extratropics to the UTLS are about 5 times larger than the corresponding contributions from the SH extratropics and almost 20 times smaller than those from the tropics. The amplitude of the AMF seasonal variability originating from the NH extratropics is comparable to that from the tropics.The NH and SH extratropical age spectra show much stronger seasonality compared to the seasonality of the tropical age spectra. The transit time of NH-extratropical-origin air to the SH extratropics is longer than vice versa. The asymmetry of the inter-hemispheric transport is mainly driven by the Asian summer monsoon (ASM).We confirm the important role of ASM and westerly ducts in the inter-hemispheric transport from the NH extratropics to the SH. Furthermore, we find that it is an interplay between the ASM and westerly ducts which triggers such cross-equator transport from boreal summer to fall in the UTLS between 350 and 370 K. (Yan Xiaolu, Paul Konopka, Marius Hauck, Aurélien Podglajen, Felix Ploeger)

3.7 Frequent locally absent rings indicate increased threats of extreme droughts to semi-arid Pinus tabuliformis in North China

Global climatic change indicates that some climate systems have passed tipping points, e.g. the inner East Asia; however it’s still uncertain when the tipping point of forest mortality could be triggered. Absent rings are early warnings of irreversible mortality. They widely exist in Northern Hemisphere forests, but they’re locally rare and under-discussed in previous studies. We reported a tree ring network, including 17 collections among the extensive distribution of Pinus tabuliformis in North China. Locally absent rings (LAR) were counted and logistic models were built to determine biotic and abiotic driving forcings of LAR. Pinus tabuliformis suffered increasing LAR frequency in recent decades, and the highest LAR frequency occurred at the driest distributional margin. At the semi-arid region, LAR frequency increased along with increasing age, decreasing previous September moisture, and decreasing May moisture. LAR risk would gradually increase, and older trees would suffer quite high LAR risk in the future. We highlight the connection between frequent LAR and extreme droughts in semi-arid pine forests, and propose that LAR can be a proxy of the potential tipping points of forest systems. (Zhao Shoudong, Jiang Yuan, Wen Yan, Jiao Liang, Li Wenqing, Xu Hui, Ding Minghu)

3.8 Spatiotemporal variation of precipitation on a global scale from 1960 to 2016 in a new normalized daily precipitation dataset

Increases in total and extreme precipitation have been investigated worldwide using limited spatiotemporal data based on surface observations and satellite remote sensing during recent decades.However, it remains unclear whether rain gauge daily precipitation records also show a significant increase in global precipitation. In this study, we first apply the transformed version of the penalized maximum F test(“transPMFred”) to homogenize daily precipitation (P) records from 10629 stations around the world during 1960–2016 from a dataset (GHCN-CMA) that combines the daily precipitation data from the Global Historical Climatology Network (GHCN-Daily) dataset and the China Meteorological Administration (CMA), and then utilize the homogenized data to study the frequency of daily precipitation on a global scale. The results indicate that precipitation frequency exhibited significant increasing trends from 1960 to 2016 across most of the globe,except for East Asia. Moreover, the variability of global precipitation frequency was more pronounced in wet regions than dry regions, especially in North America, Europe and East Asia based multiple daily precipitation datasets. The regional average frequency for moderate (30th < P ≤ 60th percentile), heavy (60th < P≤ 90th percentile) and very heavy (P > 90th percentile) precipitation increased in North America and Europe. Changes in precipitation frequency decreased for light (P ≤ 30th percentile) and moderate precipitation but increased for heavy and very heavy precipitation, which mainly occurs in dry regions of Australia. Moreover, precipitation frequency in wet and dry regions of East Asia showed opposite trends, with a decrease in wet regions and an increase in dry regions. (Liu Jun, Wu Dongyou, Li Yue, Ren Hongli, Zhao Yongtao, Sun Xiaoyu, Zhang Haitao,Ji Mingxia)

3.9 The UK-China climate science to service partnership

We present results from the first 6 years of this major UK government funded project to accelerate and enhance collaborative research and development in climate science, forge a strong strategic partnership between UK and Chinese climate scientists, and demonstrate new climate services developed in partnership.The development of novel climate services is described in the context of new modeling and prediction capability, enhanced understanding of climate variability and change, and improved observational datasets.Selected highlights are presented from over 300 peer reviewed studies generated jointly by UK and Chinese scientists within this project. We illustrate new observational datasets for Asia and enhanced capability through training workshops on the attribution of climate extremes to anthropogenic forcing. Joint studies on the dynamics and predictability of climate have identified new opportunities for skillful predictions of important aspects of Chinese climate such as East Asian summer monsoon rainfall. In addition, the development of improved modeling capability has led to profound changes in model computer codes and climate model configurations, with demonstrable increases in performance. We also describe the successes and difficulties in bridging the gap between fundamental climate research and the development of novel real-time climate services. Participation of dozens of institutes through subprojects in this program, which is governed by the Met Office Hadley Centre, the China Meteorological Administration, and the Institute of Atmospheric Physics,is creating an important legacy for future collaboration in climate science and services. (Scaife Adam A., Good Elizabeth, Sun Ying, Yan Zhongwei, Dunstone Nick, Ren Hongli)

3.10 Change of East-Asian summer precipitation associated with strong El Niño under the future emission scenarios

Strong Eastern-Pacific type El Niño (EP-El Niño) events have significant impacts on the decaying-summer precipitation over East Asia (EA). It has been demonstrated that frequency of strong EP-El Niños will increase and associated precipitation will become more severe and complex under future high emission scenarios. In this study, using simulations of CMIP5 and CMIP6, changes of the summer precipitation pattern related to strong EP-El Niño during its decay phase and the possible mechanism as responding to high emission scenarios are examined. Precipitation anomaly patterns over EA of strong EP events show a large inter-model spread in historical simulations between the CMIP models where CMIP6 is not superior to CMIP5. Under high emission scenarios, changes of summer precipitation anomalies related to strong EP events tend to increase over the southern EA and decrease around the northern EA from CMIP5, while there is an overall increase in the whole EA from CMIP6. The common change is featured by the increase of precipitation over southeastern China under high emission scenarios. This could be mainly attributed to the anticyclonic circulation from the South China Sea to the western North Pacific as a delayed response to more frequent strong EP-El Niños, which favors an increase in water vapor fluxes converging into the southeastern China. (Huang Yu, Ren Hongli, Liu Minghong)

3.11 Impacts of late-spring North Eurasian soil moisture variation on summer rainfall anomalies in Northern East Asia

This paper reports findings from a diagnostic and modeling analysis that investigates the impacts of the boreal late-spring soil moisture anomalies over North Eurasia on the summer rainfall over the northern East Asia (NEA). The soil moisture in May from the Kara-Laptev Sea coasts to Central Siberian Plateau is found to be negatively correlated with the summer rainfall from Mongolia to Northeast China. The atmospheric circulation anomalies associated with the anomalously dry soil are characterized by a pressure dipole with the high-pressure center located over North Eurasia and low-pressure center over NEA, where an anomalous water vapor convergence occurs, favoring rainfall formation. Diagnoses and modeling experiments demonstrate that the effects of the spring lower soil moisture over North Eurasia may persist into the following summer,then increase the low-level air temperature at higher latitudes through modulating local surface latent and sensible heat fluxes, and effectively reduce the meridional temperature gradient north of NEA. The weakened temperature gradient could induce the decreased vertical shear of zonal wind and generate an anomalously cyclonic center over NEA by affecting the baroclinicity around 60° N, associated with a favorable condition of local synoptic activity to increase rainfall. The above relationships and mechanisms are vice versa for the prior wetter North Eurasian soil and decreased NEA rainfall. These findings suggest that the soil moisture anomalies at higher latitudes may act as a new precursor providing an additional predictability source for better predicting the summer rainfall in NEA. (Sang Yinghan, Ren Hongli, Deng Yi, Xu Xiaofeng, Shi Xueli, Zhao Shuo)

3.12 Climatic factors contributing to interannual and interdecadal variations in the meridional displacement of the East Asian jet stream in boreal winter

We employ the fifth generation of the European Centre for medium-range weather forecasts reanalysis to investigate the leading mode of the East Asian zonal wind and the contribution of associated climatic factors in boreal winter during 1979‒2020. The dominant mode, accounting for 46% of the explained variance,indicates that the East Asian subtropical jet (EASJ) and East Asian polar frontal jet (EAPJ) are meridionally displaced in opposite directions. The corresponding first principal component (PC1) shows clear interannual and interdecadal variations, with a regime shift in the year 1997. The dominant mode is significantly related to the Niño3.4 sea surface temperature (SST), the Barents-Kara Seas (BKS) sea ice concentration (SIC),the Arctic oscillation (AO) and the Tibetan Plateau (TP) surface air temperature (SAT) on interannual and interdecadal timescales. Multiple linear regression with these variables reproduces PC1 well. The interdecadal variation of PC1 is dominated by the BKS SIC and Niño3.4 SST. The Niño3.4 SST explains more than half of the variance of PC1 on an interannual time scale, which is more than all of the other factors combined.The El Niño phase and BKS SIC are associated with the distance between the EASJ axis (EASJA) and the EAPJ axis (EAPJA). The AO and TP SAT may influence the meridional migration of the EASJA and EAPJA,respectively. The distance between the EASJA and EAPJA also depends on the configuration of the southern Asian cyclone and Siberian anticyclone. (Dong Xiao, Zhao Ping, Ren Hongli)

3.13 Diagnosing SST error growth during ENSO developing phase in BCC_CSM1.1m prediction system

In this study, the predictability of the El Niño-south oscillation (ENSO) in an operational prediction model from the perspective of initial errors is diagnosed using the seasonal hindcasts of the Beijing Climate Center system model, BCC_CSM1.1m. Forecast skills during the different ENSO phases are analyzed and it is shown that the ENSO forecasts appear to be more challenging during the developing phase, compared to the decay phase. During ENSO development, the SST prediction errors are significantly negative and cover a large area in the central and eastern tropical Pacific, thus limiting the model skill in predicting the intensity of El Niño. The large-scale SST errors, at their early stage, are generated gradually in terms of negative anomalies in the subsurface ocean temperature over the central-western equatorial Pacific, featuring an error evolutionary process similar to that of El Niño decay and the transition to the La Niña growth phase. Meanwhile, for short lead-time ENSO predictions, the initial wind errors begin to play an increasing role, particularly in linking with the subsurface heat content errors in the central-western Pacific. By comparing the multiple samples of initial fields in the model, it is clearly found that poor SST predictions of the Niño3.4 region are largely due to contributions of the initial errors in certain specific locations in the tropical Pacific. This demonstrates that those sensitive areas for initial fields in ENSO prediction are fairly consistent in both previous ideal experiments and our operational predictions, indicating the need for targeted observations to further improve operational forecasts of ENSO. (Tian Ben, Ren Hongli )

3.14 Understanding the causes of rapidly declining prediction skill of the East Asian summer monsoon rainfall with lead time in BCC_CSM1.1m

Dynamical prediction of monsoon rainfall has been an important topic and a long-standing issue in both research and operational community. This paper provides a comprehensive evaluation of the subseasonal-toseasonal (S2S) prediction skill of the East Asian summer monsoon (EASM) rainfall using the hindcast record from the Beijing Climate Center climate system model, BCC CSM1.1m, during the period 1983–2019. The model exhibits reasonable skills for predicting the EASM rainfall at all lead times with the skill dropping dramatically from the shortest lead time of about 2 weeks (LM0) to 1-month lead (LM1), and then fluctuating remarkably throughout 2-month to 12-month lead times. Over the EASM domain, the rapid decline of the S2S rainfall prediction skill from LM0 to LM1 is mainly caused by the inferior skills over Central China in July and over Northeast China in August. Composite analysis based on hindcast records suggests that these inferior skills are directly tied to the model’s difficulties in capturing above-normal precipitation over the eastern Central China and Northeast China in the respective months, which are further shown to be associated with anomalous weakening and meridional movement of the Northwestern Pacific subtropical high and the activity of large-scale teleconnection pattern hard to be predicted over the northeastern Asia in summer, respectively.These findings inform the intrinsic limits of the S2S predictability of the EASM rainfall by a dynamical model,and strongly suggest that the level of confidence placed upon S2S forecasts should be stratified by large-scale circulation anomalies known to significantly affect the prediction skill, e.g., the subtropical high and highlatitude teleconnection patterns for summer monsoon rainfall prediction in this region. (Wang Na, Ren Hongli,Deng Yi, Zhao Siyu)

3.15 A dynamical and machine learning hybrid seasonal prediction of summer rainfall in China

Seasonal prediction of summer rainfall is crucial to reduction of regional disasters, but currently it has a low prediction skill. We developed a dynamical and machine learning hybrid (MLD) seasonal prediction method for summer rainfall in China based on circulation fields from the Chinese Academy of Sciences(CAS) flexible global ocean-atmosphere-land system model finite volume version 2 (FGOALS-f2) operational dynamical prediction model. Through selecting optimum hyperparameters for three machine learning methods to obtain the best fit and least overfitting, an ensemble mean of the random forest and gradient boosting regression tree methods was shown to have the highest prediction skill measured by the anomalous correlation coefficient. The skill has an average value of 0.34 in the historical cross-validation period (1981‒2010) and 0.20 in the 10-year period (2011‒2020) of independent prediction, which significantly improves the dynamical prediction skill by 400%. Both reducing overfitting and using the best dynamical prediction are important in applications of the MLD method and in-depth analysis of these warrants a further investigation. (Wang Jialin,Yang Jing, Ren Hongli, Li Jinxiao, Bao Qing, Gao Miaoni)

3.16 Interdecadal changes in synoptic transient eddy activity over the Northeast Pacific and their role in tropospheric Arctic amplification

Arctic amplification refers to the greater surface warming of the Arctic than of other regions during recent decades. A similar phenomenon occurs in the troposphere and is termed “tropospheric Arctic amplification”(TAA). The poleward eddy heat flux and eddy moisture flux are critical to Arctic warming. In this study, we investigate the synoptic transient eddy activity over the North Pacific associated with TAA and its relationship with the subtropical jet stream, and propose the following mechanism. A poleward shift of the subtropical jet axis results in anomalies of the meridional gradient of zonal wind over the North Pacific, which drive a meridional dipole pattern of synoptic transient wave intensity over the North Pacific, referred to as the North Pacific synoptic transient wave intensity dipole (NPSTD). The NPSTD index underwent an interdecadal shift in the late 1990s accompanying that of the subtropical jet stream. During the positive phase of the NPSTD index, synoptic eddy heat flux transports more heat to the Arctic circle, and the eddy heat flux diverges,increasing Arctic temperature. This mechanism highlights the need to consider synoptic transient eddy activity over the North Pacific as the link between the mean state of the North Pacific subtropical upper jet and TAA.(Dong Xiao, Ren Hongli)

3.17 ENSO phase-locking behavior in climate models: From CMIP5 to CMIP6

The phase-locking behavior of El Niño-southern oscillation (ENSO) in models from Coupled Model Intercomparison Project (CMIP) phase 5 to phase 6 is assessed in terms of the locking-month of ENSO peak and the sharpness of locking tendency. Overall, a robust improvement exists in CMIP6. Compared to CMIP5,more CMIP6 models truly reproduce the locking-month in November-January. Meanwhile, the sharpness of phase-locking in CMIP6 models also improves, though most of them are still far from the observations.The locking-month is verified to be highly corresponding to the phase of seasonal modulation of ENSO’s instabilities. The sharpness is mainly controlled by the intensity of this modulation and noise. Compared to CMIP5, CMIP6 models generally simulate these affecting factors better. Besides, models displaying an exaggerated semi-annual variation of ENSO’s instabilities simulate the ENSO phase-locking relative-poorly,and these models show no reduction from CMIP5 to CMIP6. (Liu Minghong, Ren Hongli, Zhang Renhe, Sarah Ineson, Wang Run)

3.18 Improving long-lead seasonal forecasts of precipitation over southern China based on statistical downscaling using BCC_CSM1.1m

Long-lead precipitation forecasts for 1‒4 seasons ahead are usually difficult in dynamical climate models due to the model deficiencies and the limited persistence of initial signals. But, these forecasts could be empirically improved by statistical approaches. In this study, to improve the seasonal precipitation forecast over the southern China (SC), the statistical downscaling (SD) models are built by using the predictors of atmospheric circulation and sea surface temperature (SST) simulated by the Beijing Climate Center climate system model version 1.1 m (BCC_CSM1.1 m). The different predictors involved in each SD model is selected based on both its close relationship with the target seasonal precipitation and its reasonable prediction skill in the BCC_CSM1.1 m. Cross and independent validations show the superior performance of the SD models,relative to the BCC_CSM1.1 m. The temporal correlation coefficient of SD models could reach over 0.4,exceeding the 95% confidence level. The SC precipitation index can be much better forecasted by the SD models than by the BCC_CSM1.1 m in terms of the interannual variability. In addition, the errors of the precipitation forecast in all four seasons are significantly reduced over most of SC in the SD models. For the 2015/2016 strong El Niño event, the SD models outperform the dynamical BCC_CSM1.1 m model on the spatial and regional-average precipitation anomalies, mostly due to the effective SST predictor in the SD models and the weak response of the SC precipitation to El Niño-related SST anomalies in the BCC_CSM1.1 m. (Liu Ying, Ren Hongli, N. P. Klingaman, Liu Jingpeng, Zhang Peiqun)

3.19 Intercomparison of MJO column moist static energy and water vapor budget among six modern reanalysis products

This study conducts an intercomparison of the column-integrated moist static energy (MSE) and water vapor budget of the Madden-Julian oscillation (MJO) among six modern global reanalysis products (RAs).Inter-RA differences in the mean MSE, MJO MSE anomalies, individual MSE budget terms, and their relative contributions to the propagation and maintenance of MJO MSE anomalies are examined. Also investigated is the relationship between the MJO column water vapor (CWV) budget residuals with the other CWV budget terms as well as with the two parameters that characterize cloud-radiation feedback and moisture–convection coupling. Results show a noticeable inter-RA spread in the mean-state MSE, especially its vertical structure. In all RAs, horizontal MSE advection dominates the propagation of the MJO MSE while columnintegrated longwave radiative heating and vertical MSE advection are found to be the key processes for MJO maintenance. The MSE budget terms directly affected by the model parameterization schemes exhibit high uncertainty. The differences in anomalous vertical velocity mainly contribute to the large differences in vertical MSE advection among the RAs. The budget residuals show large inter-RA differences and have nonnegligible contributions to MJO maintenance and propagation in most RAs. RAs that underestimate (overestimate) the strength of cloud-radiation feedback and the convective moisture adjustment time scale tend to have positive(negative) MJO CWV budget residual, indicating the critical role of these processes in the maintenance of MJO CWV anomalies. Our results emphasize that a correct representation of the interactions among moisture,convection, cloud, and radiation is the key for an accurate depiction of the MJO MSE and CWV budget in RAs. (Ren Pengfei, Daehyun Kim, Min-Seop Ahn, Daehyun Kang, Ren Hongli )

3.20 Decomposition of projected summer rainfall change over East Asia based on timeslice experiments

The summer rainfall change over East Asia in response to CO2forcing and the associated processes are investigated via a set of pilot timeslice piSST experiments from the Cloud Feedback Model Intercomparison Project phase 3 (CFMIP-3). The total response of rainfall to 4 × CO2in coupled models is decomposed into components associated with uniform SST warming, SST pattern change, the direct radiative effect of increased CO2, and the plant physiological response. In general, the contributions of the individual responses of summer rainfall to different forcings are subject to a regional dependence. The uniform SST warming reduces rainfall in many land regions in East Asia, but increases rainfall over the northern East Asia. The spatial patterns of the rainfall change as the result of SST pattern change are nearly opposite to those of the uniform SST warming,which account for most inter-model uncertainty in the simulations. The direct radiative effect of increasing CO2is largely responsible for an increase of rainfall across the East Asian continent, especially over the central China and southern Tibetan Plateau, and the plant physiological effect appears to increase rainfall over the eastern and southern China. Also discussed are the atmospheric circulation changes that are driven by the distinct aspects of CO2forcing and directly tied to the summer rainfall changes. (Huang Yu, Ren Hongli, Robin Chadwick, Deng Yi)

3.21 Improvement of soil moisture simulation in Eurasia by the Beijing Climate Center Climate System Model (BCC-CSM) from CMIP5 to CMIP6

This study provides a comprehensive evaluation of historical surface soil moisture simulation (1979–2012) over Eurasia at annual and seasonal time scales between two medium-resolution versions of the Beijing Climate Center climate system model (BCC-CSM)—one that is currently participating in the Coupled Model Intercomparison Project phase 6 (CMIP6), i.e., BCC-CSM2-MR, and the other, BCC-CSM1.1m, which participated in CMIP5. We show that BCC-CSM2-MR is more skillful in reproducing the climate mean states and standard deviations of soil moisture, with pattern correlations increased and biases reduced significantly.BCC-CSM2-MR performs better in capturing the first two primary patterns of soil moisture anomalies, where the period of the corresponding time series is closer to that of reference data. Comparisons show that BCCCSM2-MR performs at a high level among multiple models of CMIP6 in terms of centered pattern correlation and “amplitude of variation” (relative standard deviation). In general, the centered pattern correlation of BCC-CSM2-MR, ranging from 0.61 to 0.87, is higher than the multi-model mean of CMIP6, and the relative standard deviation is 0.75, which surmounts the overestimations in most of the CMIP6 models. Due to the vital role played by precipitation in land-atmosphere interaction, possible causes of the improvement of soil moisture simulation are further related to precipitation in BCC-CSM2-MR. The results indicate that a better description of the relationship between soil moisture and precipitation and a better reproduction of the climate mean precipitation by the model may result in the improved performance of soil moisture simulation. (Sang Yinghan, Ren Hongli, Shi Xueli, Xu Xiaofeng, Chen Haishan)

3.22 Leading modes of Asian-Australian monsoon interannual variability as represented in CMIP5 models

The Asian-Australian monsoon (AAM) exerts a strong impact on the regional climate. The ability of climate models to simulate the AAM is of great importance. This study evaluates the performance of 26 models, which participate in the Coupled Model Intercomparison Project phase 5 (CMIP5), in representing features of the leading modes of the AAM interannual variability. This is achieved by diagnosing quantitative metrics derived from a season-reliant empirical orthogonal function (S-EOF) analysis, and plausible external forcing contributing to the simulation results are presented. Our results show that the CMIP5 models can generally capture the spatial patterns of the leading modes of AAM interannual variability during El Niño development periods because of strong air‒sea interactions. The variance percentage explained by the first two S-EOF modes is found to be closely related to the amplitude of the El Niño-southern oscillation (ENSO) in the models. In contrast, simulation of the periodicity remains challenging, and the models show deficiencies in the accurate capture of periodic characteristics. Models with realistic ENSO simulations typically perform well in simulating S-EOF1, and the key to improving the simulation of S-EOF2 is the greater intensity of the simulated field of background heating over the Northwest Pacific warm pool. The multimodel ensemble (MME) performs better than most of the 26 individual models. Models in the MME with better ENSO simulation and stronger warm pool heating are found to be better able to represent the relationships between the first two leading S-EOF modes and ENSO, although biases remained. (Zhou Fang, Ren Hongli, Liu Minghong, Wang Run, Huang Kai)

3.23 Evaluation of simulation capability for multiple tropical cyclone events in the western North Pacific of the UH_HCM model

The intraseasonal variability of multiple tropical cyclone (MTC) events in the western North Pacific (WNP)during 1979–2015 is analyzed using the best-track dataset archived at the Joint Typhoon Warning Center. MTC events are divided into three phases according to the time intervals of the tropical cyclone (TC) genesis, that is, active, normal, and inactive phases. Composite analysis results indicate that MTC events tend to occur in the active phase when the monsoon trough is stronger and located farther north than at other times. Initialized by the data from a 10-year stable running result, a 12-year control experiment is carried out using the hybrid atmosphere-ocean coupled model developed at the University of Hawaii (UH_HCM model) to evaluate its simulation capability. Compared with the climate observations, the model shows good skill in simulating the large-scale environmental conditions in the WNP, especially the subtropical high and the monsoon trough. In addition, the model can well simulate the climate characteristics of TCs in the WNP, as well as the differences in each MTC phase. However, the simulated frequency of TCs is less and their locations are more northeast,compared with the observations. The vorticity and moisture in the model appear to be the two main factors affecting MTC activity based on analyses of the genesis potential index. (Li Tianhang, Ren Hongli, Wu Yujie,Gao Jianyun)

3.24 Improvement in the prediction of summer precipitation in the North China-Hetao region using the tropospheric temperature over the Tibetan Plateau in spring

We investigated the relationship between the spring tropospheric temperature over the Tibetan Plateau(TPT) and summer precipitation in the eastern China on an interannual timescale using the monthly mean ERA-Interim reanalysis dataset, the HadISST dataset and the daily mean precipitation dataset for China.We found that there is a significant positive correlation between the spring TPT and summer precipitation in the North China-Hetao region. The relationship is manifested in the context of the East Asia-Pacific pattern teleconnection. In the high spring TPT index years, the geopotential height anomalies over East Asia and the western North Pacific present a negative phase of the East Asia-Pacific pattern teleconnection in the subsequent summer. This circulation pattern is beneficial for the water vapor transport from the western Pacific to inland,which further transport to the North China-Hetao region from the Yangtze River-Yellow Rivers region.Anomalous upward motion occurs in the North China-Hetao region, which increases precipitation. The East Asian subtropical westerly jet shifts further north and the South Asian high weakens and shrinks westward.These conditions all favor an increase in precipitation over the North China-Hetao region. The spring TPT plays an important part in the prediction of summer precipitation in the North China-Hetao region. The improvement in the use of the spring TPT to predict summer precipitation in the North China-Hetao region is examined by comparing the prediction equations with and without the prediction factor of the spring TPT on the basis of the sea surface temperatures in key regions. After considering the impact of the spring TPT, the explanatory variance of the prediction equation for precipitation in the North China-Hetao region increases by 17.3%. (Chen Dan, Nan Sulan, Liu Ge)

3.25 GRAPES-GEPS环流集合预报的分类释用方法研究与检验

集合预报在数值天气预报体系中具有重要地位，因此如何有效提取集合样本信息以提高集合预报技巧一直是一个重要课题。基于中国全球集合预报业务系统（GRAPES-GEPS）的500 hPa高度场集合资料开展对环流集合预报的分类释用方法研究，并对集合聚类预报结果进行了检验分析。通过在传统Ward聚类法中引入动态聚类的“手肘法”方案，发展了环流集合预报分类释用方法。针对该方法的个例分析表明，对于中国中东部地区环流集合预报的聚类释用方法能够有效地划分出最有可能发生的环流形势类型并提供发生概率。确定性预报综合检验结果显示，集合预报聚类结果中发生概率最高的集合大类相对于集合平均的预报技巧有明显提升，并随着预报时效的延长提升更明显。总体来看，通过集合预报的分类释用方法划分环流形势类型可以为天气预报提供参考依据，具有实际应用价值。（罗月琳，高丽，陈权亮，蔡宏珂，任宏利）

3.26 热带大西洋海温异常季节内演变对中国江南地区夏季持续性高温事件影响的初步研究

中国江南地区是高温热浪灾害的高影响区。以往的一些研究发现了不同海域海温异常在年际或年代际尺度上的变化对中国南方夏季平均温度异常的影响效应。但是,关于这些关键海域海温季节内尺度变化对江南地区高温事件发生和维持影响的研究尚不多见。为此，本文利用中国站点观测、美国气象环境预报中心和美国国家大 气研究中心（NCEP/NCAR）再分析以及美国国家海洋大气管理局（NOAA）海温等资料，首先以 2016 年江南地区夏季 2 次高温事件为例（分别发生在7 月21—31日和8月15—25日），重点探讨了热带大西洋海温季节内变化的可能贡献。在此基础上，基于1981—2016年多高温事件合成结果，进一步分析了热带大西洋海温季节内变化影响江南高温事件的可能链接过程。研究发现，热带西大西洋暖海温异常在季节内尺度上的发展与维持有利于在欧亚大陆激发出较为稳定的Rossby波列结构，使东亚及其沿海地区为深厚的高压系统控制，进而引发江南地区持续性高温事件。这种热带大西洋暖海温的阶段性增强与维持及其相应的稳定Rossby波列结构超前于持续性高温事件：在热带大西洋海温显著升高1个月之内，江南地区可能出现持续性高温事件。在季节内尺度上，热带大西洋显著暖海温异常出现明显的阶段性增强之后10天左右，北印度洋暖海温也出现了阶段性增强。这暗示热带大西洋热力异常除通过直接激发欧亚大陆 Rossby 波列之外，还有可能通过影响热带印度洋海温的阶段性异常，进而对江南地区高温事件的发生和维持产生一定影响。另外，在厄尔尼诺衰减并向拉尼娜转变阶段，热带中东太平洋冷海温异常和北印度洋暖海温异常在季节内的协同阶段性变化可能也对持续性高温事件有贡献。上述关键区海温的季节内变化对中国江南地区高温事件具有一定的前期指示意义，但它们的具体影响过程，特别是在季节内尺度上的协同影响效应和物理过程，尚需未来进一步研究。(王慧美, 刘舸, 彭京备, 纪立人)

3.27 地面观测资料地形高度订正对我国东部暴雨数值模拟的影响

采用中尺度气象模式WRFv3.8.1和GSI同化系统，建立了一套循环同化系统，开展了一个月的地面观测资料、探空资料、雷达资料和卫星资料的同化试验。在试验结果基础上，针对我国西部地区复杂地形，根据模式地形高度与实际测站地形高度的差异对地面观测资料进行订正，以2016年6月30日至7月6日长江中下游地区一次暴雨天气过程为例开展了数值模拟试验，并对比分析地面观测资料的高度订正及同化对我国东部暴雨数值模拟的可能影响。模拟结果表明：（1）批量循环同化试验模拟的地面变量较ERA-Interim再分析资料的均方根误差普遍减小，对地面气象要素的模拟有一定的改善。（2）地面观测资料经地形差异订正后同化到模式中，模拟的降雨落区和量级与观测雨带更为接近，TS评分、散度和垂直速度方面都有一定的改进，表明西部地区地面观测资料经地形高度订正后再同化到数值模式中能改进数值模式对我国东部暴雨的模拟效果。（曹润东，陈军明，赵平）