Case Studies of the Microphysical and Kinematic Structure of Summer Mesoscale Precipitation Clouds over the Eastern Tibetan Plateau

Shuo JIA ,Jiefan YANG ,and Hengchi LEI

1Key Laboratory of Cloud-Precipitation Physics and Severe Storms, Institute of Atmospheric Physics,Chinese Academy of Sciences, Beijing 100029, China

2University of Chinese Academy of Sciences, Beijing 100049, China

ABSTRACT Three cases of microphysical characteristics and kinematic structures in the negative temperature region of summer mesoscale cloud systems over the eastern Tibetan Plateau (TP) were investigated using X-band dual-polarization radar.The time–height series of radar physical variables and mesoscale horizontal divergence derived by quasi-vertical profiles(QVPs) indicated that the dendritic growth layer (DGL,-20°C to -10°C) was ubiquitous,with large-value zones of KDP(specific differential phase),ZDR (differential reflectivity),or both,and corresponded to various dynamic fields (ascent or descent).Ascents in the DGL of cloud systems with vigorous vertical development were coincident with large-value zones of ZDR,signifying ice crystals with a large axis ratio,but with no obvious large values of KDP,which differs from previous findings.It is speculated that ascent in the DGL promoted ice crystals to undergo further growth before sinking.If there was descent in the DGL,a high echo top corresponded to large values of KDP,denoting a large number concentration of ice crystals;but with the echo top descending,small values of KDP formed.This is similar to previous results and reveals that a high echo top is conducive to the generation of ice crystals.When ice particles fall to low levels (-10°C to 0°C),they grow through riming,aggregation,or deposition,and may not be related to the kinematic structure.It is important to note that this study was only based on a limited number of cases and that further research is therefore needed.

Key words: Tibetan Plateau,polarimetric variables,microphysics,dendritic growth layer,kinematic structure,aggregation,riming

1.Introduction

As the highest and largest plateau in the world,the topography of the Tibetan Plateau (TP) has an important impact on atmospheric circulation and water vapor transportation(Yeh and Chang,1974;Xu et al.,2014).Its dynamic and thermal effects determine the monsoon climate characteristics of East Asia (Wu and Zhang,1998;Li et al.,2001;Xu et al.,2010).With the largest total radiation in summer and extending to the middle troposphere,the TP also has a significant impact on the cloud precipitation processes of local and downstream regions (Xu et al.,2019).

In recent decades,a series of studies on clouds over the TP have been performed,most of which focused on convective clouds and precipitation in summer,and the results suggest some unique characteristics.For instance,convective clouds over the plateau are usually characterized by their cold cloud base,low water vapor content,few ice nuclei,thin cloud thickness,and high ice-crystallization temperature(Wang et al.,2002;Fujinami et al.,2005;Dai et al.,2011;Yue et al.,2018).Recently,using the data obtained during the Third Tibetan Plateau Atmospheric Scientific Experiment(Zhao et al.,2018),Chang et al.(2019) found that plateau convective clouds with limited vertical velocity and few cloud condensation nuclei tend to produce wide cloud droplet spectra.Analysis based on multi-source observation data from satellites and ground-based vertically pointing radars revealed that deep convective clouds are mainly ice clouds dominated by mixed-phase and glaciation processes (Wang and Guo,2018).With abundant supercooled water at -20°C to 0°C,snow and graupel particles have the characteristics of high content and deep vertical distribution,and graupel grows mainly via aggregation and riming (Tang et al.,2018).Furthermore,research found that deep or shallow convective clouds correspond to different vertical motion characteristics(Ruan et al.,2018;Pang et al.,2019),and the strength of ascent affects microphysical processes in the clouds (Zhang et al.,2019).

InterestingZDR(differential reflectivity) orKDP(specific differential phase) signatures,observed in the -20°C to-10°C layer of US winter storms and European stratiform clouds,are thought to be indications of dendritic crystals(DCs) and plate-like ice crystals,and are known as the dendritic growth layer (DGL) (Ryzhkov et al.,1998;Bechini et al.,2013;Griffin et al.,2014;Schrom et al.,2015).Kennedy and Rutledge (2011) reported an elevated layer of localKDPmaxima near the -15°C isotherm,and the passage of this region was associated with an increase in surface precipitation (Trömel et al.,2019).Later,Andrić et al.(2013)found that enhancement inZDRandKDPregions was coincident with a large vertical gradient of radar reflectivity (ZH).Regions of ascent were consistently associated with polarimetric signatures,implying planar crystal growth when near-15°C (Bailey and Hallett,2009;Kumjian and Lombardo,2017).Several repetitive signatures for the correlation betweenZDRorKDPin the DGL and the cloud tops have been observed (Griffin et al.,2018).Williams et al.(2015)identified two kinds of features of enhancedZDRand large or smallZHin winter and summer stratiform systems coincident with water-saturated or insufficient amounts of supercooled water environments.

To sum up,the above studies mainly focused on the DGL in winter storms and its characteristics influenced by ascent and its effect on precipitation.Considering that glaciation processes are predominant in summer clouds and precipitation,which may be affected by vertical motion over the TP,we aim to investigate whether there are also DGLs in summer mesoscale precipitation clouds over the TP.If there are,corresponding microphysical characteristics and possible effects of vertical motion will be analyzed,and the similarities and differences compared with previous studies will be discussed.This is of great significance for improving the parameterization schemes of numerical models and the forecasting skill for precipitation in this region.

The remainder of the paper is structured as follows: We first introduce the field campaign and datasets in section 2.The methods of radar data preprocessing and reconstruction are presented in section 3.In section 4,time–height series of physical variables are applied to analyze the macroscopic and microphysical features,and kinematic structure,during different precipitation processes,with particular attention paid to investigating the microphysical characteristics in the negative temperature region,especially the DGL and the influence of dynamic fields on it.Finally,a summary,conclusions and some further discussion are provided in section 5.

2.Field experiment

2.1.Instruments and data

The field campaign was organized by the China Meteorological Administration in associated with the Institute of Atmospheric Physics,Chinese Academy of Sciences.Figure 1a shows the location of the measurement site.One of the main goals of this field campaign was to improve understanding of the macrophysical and microphysical characteristics and vertical structure of summer precipitation clouds over the eastern TP.For this purpose,multiple instruments,including an X-band dual-polarization Doppler radar,K-band micro-rain radar,Ka-band cloud radar,microwave radiometer,and particle size velocity (Parsivel) disdrometer,were deployed in Zeku County,Qinghai Province,between August and September from 2019 to 2021.

An X-band dual-polarization Doppler radar (referred to as ZKXR) is located in Zeku County (35.038°N,101.47°E)(Fig.1a,red five-pointed star).Its main specifications are summarized in Table 1,among which its observation range of 75 km means it can only cover part of the mesoscale cloud system we are concerned about.The observations of ZKXR are set in a volume coverage pattern (VCP) consisting of 12 layers with elevation angles from 1.8° to 19.5°,which takes 6–7 min per volume scan.This provides high spatial and temporal resolution base data,including theZH,radial velocity (VR),ZDR,differential propagation phase shift(ΦDP),and correlation coefficient (ρhv),which have been extensively applied to the analysis of microphysical processes in clouds and precipitation (Schneebeli et al.,2013;Picca et al.,2014;Jensen et al.,2016),hydrometeor classification(Snyder et al.,2010;Dolan et al.,2013),and estimation of quantities such as ice water content,particle diameter,and number concentration (Ryzhkov et al.,2018;Ryzhkov and Zrnić,2019).

Fig.1.(a) Topography in the experimental area and instruments (red star denotes the radar station).(b) Quasivertical profile (QVP) reconstruction based on plane position indicator (PPI) scanning data.

A vertically pointing K-band MRR is located at the same station.It works with an operating frequency of 24 GHz and a beamwidth of 2° (Table 1) and produces spectral Doppler density data,with a range resolution of 200 m and a temporal resolution of 1 min.The vertical profile of the raindrop size distribution (DSD) is provided by the MRR,and parameters such as the rain intensity,echo intensity,and liquid water content can be retrieved from this DSD (Wang et al.,2017).

To match the analysis,vertical profiles of temperature over the observation area will be required,but there are no conventional radiosonde stations within 100 km of the radar station.Therefore,the thermodynamic and dynamic fields from the fifth major global reanalysis produced by ECMWF(ERA5) data,with a 0.25° × 0.25° resolution (Hersbach et al.,2018),are employed for supplementary information,which are available from hourly reanalysis climate datasets made available by ECMWF.

2.2.Synoptic situation

In this paper,we mainly focus on the mesoscale cloud precipitation systems in summer over the eastern TP,which are relatively uniform horizontally with less unstable energy.Referring to existing studies (Chang,2019;Zhang et al.,2021),the main synoptic situation controlling the cloud precipitation systems in the rainy season include a plateau shortwave trough pattern,east-high and west-low pattern,and zonal circulation pattern.We selected three cases from the limited observation period belonging to these kinds of synoptic situations–namely,17–18 September,7–8 August,and 12 August 2021 (Table 2).

Table 2.Cases analyzed in this study and their corresponding synoptic situation.

At 1400 and 2000 LST [except when noted,time is referred to as local standard time (LST) in the following sections,LST=UTC+8] 17 September,a 500-hPa West Asia trough located in the Balkesh Lake area moved eastward,the bottom of which slid down to form a shortwave trough over the plateau (Figs.2a and b).Blocked by a subtropical anticyclone,the shortwave trough moved at a slower speed,leading warm and humid southwest air carrying a large amount of water vapor.Therefore,the cloud system developed deeply and lasted longer than one day.At 2000 LST 7 August,a 500-hPa subtropical anticyclone extended westward and northward (Fig.2c),bringing warm and humid air,accompanied by upper cold air,leading to unstable atmospheric stratification.This process consisted of convective and stratiform clouds.At 0800 LST 12 August,a strong low pressure system was observed in Siberia and eastward on the 500-hPa isobaric surface (Fig.2d).The experimental area was in the flat westerly flow at the bottom of the low pressure,in which a shortwave trough moved eastward,with the southerly wind transporting water vapor at the low level continuously.Therefore,low-level and medium-level clouds formed with stable atmospheric stratification in summer.In general,it can be summarized that the duration of the 17–18 September case was the longest,with a good water vapor condition in the upper part;the unstable energy of the 7–8 August case was relatively large;and the unstable energy of the 12 August case was relatively small,with a good water vapor condition in the low level.

Fig.2.Synoptic situation at 500 hPa at (a) 1400 LST and (b) 2000 LST 17 September 2021;(c) 2000 LST 7 August 2021;and (d) 0800 LST 12 August 2021 (contours: geopotential height;color shading: temperature;black rectangle:area of interest).

After quality control and attenuation correction of radar volume-scanning data (see section 3.1),an overview of the evolution of precipitation during the three cases is presented based on PPI at a 6.4° elevation angle.From 1800 to 2200 LST 17 September,radar echoes developed rapidly (ZH>35 dBZ) (Fig.3a) and maintained in a strong stage.After 2200 LST,radar echoes from west (Fig.3b),northwest and southwest of the radar station indicated a stratus configuration and the reflectivity gradually weakened (<30 dBZ).Radar echoes gradually developed once again from 0500 LST(Fig.3c) and strengthened locally over the radar station after 1100 LST (Fig.3d).From PPIs during the 7–8 August precipitation case (Figs.3e–h),the cloud system successively experienced a wide range of strong echoes (from 2000 to 2200 LST,ZH>40 dBZ),scattered weak echoes (from 2300 to 0100 LST),and relatively strong echoes moving in (from 0200 to 0400 LST,ZH>40 dBZ).During the 12 August precipitation process (Figs.3i–l),the cloud system was in periodic development,with the echo intensity first increasing(ZH>40 dBZ) and then decreasing.

Fig.3.The 6.4° PPI of ZKXR at (a) 1910 and (b) 2230 LST 17 September 2021;(c) 0726 and (d) 1257 LST 18 September 2021;(e)2017,(f) 2214 and (g) 2357 LST 7 August 2021;(h) 0222 LST 8 August 2021;and (i) 0526,(j) 0615,(k) 1204 and (l) 1348 LST 12 August 2021.

Combined with surface hourly precipitation data,the precipitation duration of the 17–18 September case was the longest,and the regional average accumulated precipitation was the largest (12.58 mm),but the regional average hourly precipitation was the smallest (0.52 mm).In the 7–8 August case,the precipitation duration was shortest and the regional average hourly precipitation was largest (0.89 mm).The regional average hourly precipitation of the 12 August case was 0.68 mm.All cases had liquid-phase precipitation on the ground,except the 17–18 September case,which occurred with precipitation phase transformation (liquid–ice–liquid).As a result,the maximumZHduring the first case was smaller than in the other two cases.

3.Methods

3.1.Data preprocessing

First,the ZKXR raw data used in this paper are preprocessed.Data withZHsmaller than 15 dBZ,ρhvsmaller than 0.8,and an absolute value ofVRsmaller than 1.0 m s-1are regarded as non-meteorological echoes and excluded from the analysis (Ryzhkov and Zrnić,1998).To eliminate noise effects,a moving average filter is applied to different variables with different range gates.Then,theZDRbias is corrected using the PPI at low elevation angles in drizzle regions (Bringi et al.,2001;Park et al.,2005).Additionally,ρhvis corrected with the signal-to-noise ratio (SNR) by adopting the method described in Shusse et al.(2009).Lastly,raw data with SNRs lower than 20 dB are excluded while saving the ZKXR VCP data.

Next,the value ofKDPis calculated from ΦDPby piecewise linear regression using least-squares fitting over a range that varies with theZHlevel following the conventional algorithm (Wang and Chandrasekar,2010).OnceKDPhas been determined,theZHandZDRbelow the melting layer are corrected for rainfall attenuation withKDPfollowing Jameson (1992),with the height of 0°C isotherms estimated from ERA5 data.

Furthermore,the temperature of each layer from the hourly ERA5 data is interpolated to obtain the isotherm heights from -20°C to 0°C for assisting in analyzing the microphysical and kinematic vertical structure of clouds in the negative temperature area.

3.2.QVP reconstruction of physical variables

To obtain high-resolution vertical profiles of radar variables to analyze the fine m icrophysical structure of clouds,the quasi-vertical profile (QVP) method (Ryzhkov et al.,2016) is used.Moreover,a method of estimating horizontal divergencebased on QVPs ofVR(Kum jian and Lombardo,2017) is also applied to infer the kinematic structure.

3.2.1.Polarimetric variables

In the method,PPI data at every radial gate are averaged along the full azimuth and projected horizontally to a vertical axis w ithin a conical volume to obtain the QVP (Fig.1b).Theoretically,when the cloud system is horizontally uniform w ithin the conical volume,the high-resolution profiles obtained can represent the vertical structure of cloud well compared to conventional methods such as GridRad (Homeyer and Bowman,2017).Although such averaging may smooth out the small-scale horizontal structure,the noise in the polarimetric variables can also be significantly reduced(Ryzhkov et al.,2016).

The effective vertical resolution of QVPs can be approximately estimated as the larger of the two terms

and

where ∆ris the radial range gate (75 m),α is the antenna elevation angle,his the height above the ground and,∆θ is the beam w idth (0.97°).The vertical resolution is approximately 96 m ath=2 km and 191 m ath=4 km,which is significantly better than other interpolation methods.However,due to the effect of beam broadening,the vertical resolution deteriorates w ith increasing height (Ryzhkov et al.,2016).

Usually,PPI data at high elevation angles are selected to reconstruct QVPs to reduce adverse factors such as beam broadening,horizontal inhomogeneity of the cloud system,and hydrometeor falling speed.Considering that the focus in this paper is a mesoscale cloud system in a lim ited area,the QVPs ofZH,ZDRandKDPare reconstructed by using PPI data at an elevation angle of 19.5°.

3.2.2.Horizontal divergence

The QVP ofVRcan be reconstructed by the above method,which suggests net convergence (divergence) at a given height corresponding to a radial gate (Doviak and Zrnić,2006):

where 〈VR〉 signifies the azimuthally averagedVRover 360°,obtained by the QVP method.

The relation betweenVRand can be obtained assum ing that there is a horizontal linear w ind field in the conical scanning area (Kum jian and Lombardo,2017):

Here,θeis the elevation angle w ith respect to the radar,andris the radial range from the radar.In addition,represents the horizontal divergence,in which the overbar indicates regional averaging.Because the averaging of vertical atmospheric motion over a relatively w ide range can be ignored,the vertical velocitywis thus mainly determined by the hydrometeor falling speed.

However,because the terminal velocities of raindrops are too large to be ignored,thefield below the 0°C isotherm is not considered in the estimation.The falling speed of ice-phase hydrometeors at a 9.9° elevation angle is empirically assumed to be approximately 1 m s-1.Therefore,the second term on the right-hand side can be neglected relative to the divergence term in Eq.(4),and QVPs ofVRcan be used to estimate mesoscale horizontal divergence/convergenceinto the conical scan region.Negative/positive values usually represent net convergence/divergence.

Consequently,as is shown in Fig.4,ascent/descent is implied by the vertical convergence–divergence dipole.Convergence in the lower layer and divergence in the upper layer correspond to ascent (Fig.4a),while divergence in the lower layer and convergence in the upper layer indicate descent (Fig.4b).Therefore,based on the vertical profile ofwe can infer the characteristics of the kinematic structure in the conical region.

Fig.4.The (a) ascent and (b) descent implied by the estimated convergence–divergence dipoles at each layer in the conical scanning volume.

4.Results

4.1.The 17–18 September 2021 case

4.1.1.Time–height series of ZH andreconstructed using the QVP method

To verify whether the vertical profiles of ZKXR physical variables reconstructed by QVP show the characteristics of the radar echo appropriately,the reconstructed time–height series ofZHis compared with the vertical pointing observations of the nearby MRR.Meanwhile,the ERA5 divergence data at grid points in every layer that are closest to ZKXR are chosen to compare with theestimated by theVR-based QVP.

The time–height series ofZHprovided by continuous QVPs (Fig.5a) is consistent with the fluctuation shown by the MRR (Fig.5b) in terms of the cloud top and precipitation core.It should be noted that the echo top height displayed by the MRR is slightly lower than that reconstructed by the QVP method based on ZKXR data,which can be attributed to the shorter wavelength of the MRR and the attenuation in echo intensity caused by near-surface precipitation.As a result,the overall echo intensity of the MRR is also weaker than that of the ZKXR.

Fig.5.Comparison of the ZH time–height series between (a) that reconstructed by the QVP method and (b)that of the MRR,and of the time–height series between (c) that estimated by VR QVPs and (d) that of the ERA5 reanalysis data,during the 17–18 September 2021 case.From low to high,the black lines are the 0°C,-5°C,-10°C and -20°C isotherms,and the red line is the -15°C isotherm.The black dashed circles denote the DGL indicated by the characteristics of polarimetric variables,and the white dashed boxes represent the ascent/descent layers indicated by the convergence–divergence dipoles.

The time–height series of (Fig.5c) established based on QVPs ofVRshows that its vertical distribution varied with fluctuation in the echo top and intensity.Prominent convergence appeared at approximately 4 km in the rapid development phase of clouds (from 1800 to 2100 LST;white dashed boxes in Fig.5c),with divergences above and below.A similar convergence–divergence dipole was also indicated by the ERA5 data (white dashed boxes in Fig.5d),but the vertical range was slightly different because of their different temporal and spatial resolutions.From 2200 LST,the radar echo stratified and gradually weakened.There was a strong divergence in the vicinity of 4 km,with weak convergence above.The ERA5 data showed some discrepancies in the conversion of the convergence or divergence layer over time.After 0400 LST 18 September,the convergence dropped below 3 km and divergence appeared above it,with the echo in the lower layer strengthening.The vertical ranges of the convergence and divergence layers from the ERA5 data were different to some extent.The convergence rose above 3 km after 1200 LST,and divergence was observed above it,and the ERA5 data suggested the same distribution as the former.In general,compared with the ERA5 divergence data,the time–height series ofindicated a more detailed vertical distribution of horizontal divergence,which is coincident with the evolutionary characteristics of echo reflectivity.

4.1.2.Microphysical processes and kinematic structure

Figure 6 shows the time–height series of polarimetric variables (ZDR,KDP) with ERA5 hourly temperature data overlaid.Before 1930 LST,large-value zones ofZDRandKDPin the negative temperature zone were simultaneously observed near the -20°C to -10°C layer (Fig.6,black dashed circle) with a weak radar echo (Fig.5a,black dashed circle),consistent with the DGL characteristics demonstrated in previous studies (Ryzhkov et al.,1998;Bechini et al.,2013;Schrom et al.,2015).Furthermore,the time–height series ofindicated that ascent implied by the convergence–divergence dipole appeared above -15°C (Fig.5c,black dashed circle).Similar large-value features also presented from 0730 to 1000 LST,with the echo top height and strong echo decreasing significantly (Fig.5a).The characteristics of the large-value zone were slightly different,with a largevalue zone ofKDPpresented in the -15°C to -10°C layer (a lower height) andZHandZDRincreasing rapidly,coincident with descent implied by the convergence–divergence dipole above the -15°C isotherm but ascent below it.From 1330 to 1500 LST,with the echo top height and strong echo increasing again,a large-value zone ofKDProse above the-15°C isotherm,while enhancements inZHandZDRpresented in the -20°C to -10°C layer.Different from the previous two periods,only weak ascent straddled the -15°C isotherm.

Fig.6.Time–height series of (a) ZDR and (b) KDP reconstructed by the QVP method during 17–18 September 2021.From low to high,the black lines are the 0°C,-5°C,-10°C and -20°C isotherms,and the red line is the -15°C isotherm.The black circles denote the DGL indicated by the characteristics of polarimetric variables.

In summary,it can be concluded that a DGL appeared in the -20°C to -10°C layer during the periods of 1800–1930 LST 17 September and 0730–1000 and 1330–1500 LST 18 September,which we refer to as “typical periods”.However,w ith the decrease in echo top height,the DGL was coincident w ith descent,and its characteristics of polarimetric variables were different.An analysis of the occurrence frequency and variations of the radar physical variables w ith height during each typical period w ill be made in the follow ing,focusing on investigating the microphysical characteristics in the negative temperature region and the influence of kinematic structure on it.

Based on the reconstructed and established time–height serie sof radarphysicalvari ables,the frequenciesofZH,ZDR,KDPandoccurrence with height during the typical periods(1800–1930,0730–1000 and 1330–1500 LST) are presented in Fig.7 w ith vertical profiles of medians overlapped (solid red line).

Fig.7.Frequency of occurrence (color shading) and median (solid red line) of physical variables in the period (a–d) 1800–1930,(e–h) 0730–0900 and (i–l) 1330–1500 LST 17–18 September 2021.Horizontal dashed lines denote the -20°C,-15°C,-10°C,-5°C and 0°C isotherms,from top to bottom.

During the period between 1800 and 1930 LST,KDPabove -20°C gradually decreased from the maximum(median of 0.5° km-1) (Fig.7c),indicating that pristine ice crystals w ith a certain number concentration were formed(Kennedy and Rutledge,2011).ZDRincreased rapidly in the-20°C to -15°C layer (Fig.7b),w ith a median of up to 1.5 dB,corresponding to gradually increasingZHw ith a median of approximately 20 dBZ(Fig.7a).These are associated w ith vigorous dendritic grow th and the onset of aggregation between -20°C and -15°C isotherms (W illiams et al.,2015),coincident w ith ascent implied by thefield(Fig.7d).A slight enhancement inZHand a reduction inZDRand KDPin the -15°C to -7°C layer are commonly attributed to aggregation of ice-phase particles,as particles decrease in effective density and their axis ratio becomes close to unity (Bechini et al.,2013;Schneebeli et al.,2013).As the height continued to drop to the 0°C isotherm,ZDRandKDPdecreased significantly (close to 0° km-1),whileZHincreased significantly.It is speculated that ice-phase particles rime to generate graupel.The partial melting of graupel near the 0°C isotherm leads to a significant increase inZHandZDR,which decrease w ith graupel melting into the complete liquid phase.

The height of isotherms decreased during the period 0730–0900 LST.Descent implied by the convergence–divergence dipole was presented above the -15°C isotherm(Fig.7h),w ith a weak echo and lack of polarimetric measurements.KDPreached its maximum (median of 1.1° km-1) in the -15°C to -10°C layer (Fig.7g),which is thought to signify a large number concentration of ice crystals,implying highly efficient aggregation (Moisseev et al.,2015;Schrom et al.,2015).Noticeable increases inZHandZDR(medians of 15 dBZand 1.4 dB,respectively) were presented as well(Figs.7e and f),w ith weak ascent below,indicating the generation of DCs.The significant reduction ofKDPand the moderate enhancement inZHandZDRin the -10°C to 0°C layer could therefore be ascribed to the aggregation and deposition of ice-phase particles.From 1330 to 1500 LST,there was descent implied bythe field (Fig.7l) above -20°C w ith polarimetric variables not measured as well.The thickness of isotherms between 0°C and -20°C decreased,suggesting cold advection consistent w ith the w ind field from the ERA5 data.W ith weak ascent near the -15°C isotherm,the polarimetric characteristics in the -20°C to -10°C layer were sim ilar to those of the first typical period,w ith the maximumKDP(Fig.7k,median of 0.7° km-1) higher than that ofZDR(Fig.7j,median 1.6°km-1),indicating the generation of DCs,but the heights of large-value zones were generally lower than those of the 1800–1930 LST period.In the lower layer of -10°C to 0°C,ZH(Fig.7i) andZDRwere almost unchanged,andKDPdecreased rapidly,which may be related to weak aggregation and deposition of ice-phase particles,like in the second typical period.

In summary,w ith different vertical development of echoes,there are some sim ilarities and differences in the kinematic structures and m icrophysical characteristics of the DGLs (-20°C to -10°C) among the three typical periods.The characteristics of polarimetric variables in the DGL during the three typical periods are sim ilar in that the largevalue zones ofKDPappear above the ascent,and then the l arge-value zones of ZDR appear below the ascent.The ascent implied by thefieldin the firstand third typical periods appeared higher (above -20°C,near -15°C) than that in the second period (-10°C).The differences are as follows:Ascent and sim ilar features of polarimetric variables were presented in the DGLs during the first and third periods,w ithKDPgradually decreasing from the maximum,ZDRgradually increasing to the maximum,andZHgradually increasing.This is consistent w ith the DGL characteristics suggested by Schrom et al.(2015),indicating generation of DCs w ith a big axis ratio.However,during the second typical period,there was descent in the DGL,and the maximumKDPwas significantly large,while the large-value zone ofZDRwas not obvious,indicating a large number concentration of DCs generating,but w ith an axis ratio close to 1.Therefore,it is concluded that when the DGL at -20°C to -10°C corresponds to ascent,the correspondingZDRandZHare large;but w ith descent,the correspondingZDRandZHare small,andKDPis large.

Previous studies have shown large ice supersaturation over the TP.Thus,many ice crystals could be formed in the-20°C to -10°C layer via deposition.A small air density over the plateau results in ice crystals w ith an axis ratio close to 1 (corresponding to a smallZDR) tending to fall,and ice crystals could undergo further grow th w ith ascent in the DGL (the first and third periods) before sinking (Sulia and Harrington,2011),leading to a largeZDR(big axis ratio)andZH.However,w ithout ascent presented in the DGL during the second period,a large number concentration of ice crystals is conducive to form ing w ith a smallerZDR,a largerKDP,and a smallerZH.

The DCs grow through aggregation and riming in the first typical period but through aggregation and deposition in the other two typical periods when they fall into the-10°C to 0°C layer.Combined w ith the vertical distribution of thefield,it is considered that m icrophysical processes in the -10°C to 0°C layer have nothing to do w ith the kinematic structure.Only a large-value layer ofZHandZDRpresented near the 0°C isotherm during the first typical period,indicating a transformation in the precipitation phase,and there was ice-phase precipitation near the surface during the last two typical periods,due to the isotherms descending obviously.

4.2.The 7–8 August 2021 case

4.2.1.Time–height series ofreconstructed using the QVP method

The same as with the first case,the ERA5 divergence data at grid points closest to ZKXR are chosen to compare w ith theestimated based on QVPs ofVR.The time–height series ofshows that divergence or convergence alternately appeared between 4 km and 5 km before 2200 LST,and divergence occurred mainly below it.The vertical distribution of divergence or convergence revealed by the ERA5 data was different from the former,as well as its change over time.After 0200 LST,descent implied by the convergence–divergence dipole was observed near 3 km,and the vertical structure of the divergence field shown by the ERA5 data was consistent w ith that.In conclusion,when the vertical development of radar echoes is strong w ith a high echo top,the ERA5 data cannot show the vertical structure of the divergence field accurately compared w ith theestimated by QVPs ofVR.The characteristics of the two are consistent in the weak vertical development stage.

4.2.2.Microphysical processes and kinematic structure

Figure 8 show s the time–height series of radar variables(ZH,ZDR,KDP) andw ith ERA5 hourly temperature data overlaid.Combined w ith PPIs of ZKXR,two strong echoes passed through the radar station from 2000 to 2130 LST sequentially,w ith echo tops close to 8 km (Fig.8a).From 2020 LST,there was descent implied by the convergence–divergence dipole near the -15°C isotherm (Fig.8d),with the echo top height and strong echo descending and then increasing again.A large-value zone ofKDP(0.6°–1.0° km-1)in the negative temperature region,with smallZH(<20 dBZ)appearing in the -20°C to -10°C layer (Figs.8c and 8a).From 0210 LST,the radar echo developed again,descent appeared near the -10°C isotherm,along with a large-value zone ofKDP(0.5°–0.8° km-1) and largeZDR(1.6–2.2 dB)(Fig.8b) above it,whileZHwas still smaller than 20 dBZ.

Fig.8.Time–height series of the (a) ZH,(b) ZDR,(c) KDP,and (d) reconstructed by the QVP method for the 7–8 August 2021 case.From low to high,the black lines are the 0°C,-5°C,-10°C and -20°C isotherms,and the red line is the -15°C isotherm.The black dashed circles denote the DGL indicated by the characteristics of polarimetric variables.

It can be concluded that the typical periods with DGL characteristics in the -20°C to -10°C layer were 2020–2130 LST 7 August and 0210–0315 LST 8 August.Compared with the process on 17 and 18 September,the DGLs were coincident with descent,with smallZH(<20 dBZ),and a significant increase in echo intensity was concentrated in the-5°C to 0°C layer.An analysis of the occurrence frequency and variations of radar variables (ZH,ZDR,KDP)andwith height during each typical period will be made in the following,focusing on investigating the microphysical characteristics in the negative temperature region and the influence of the kinematic structure on it.

The frequencies ofZH,ZDR,KDPandoccurrence with height during the typical periods (2020–2130 and 0210–0315 LST) are presented in Fig.9 with vertical profiles of medians overlaid (solid red line).

Fig.9.Frequency of occurrence (color shading) and median (solid red line) of physical variables in the periods (a–d) 2020–2130 LST and (e–h) 0210–0315 LST 7–8 August 2021.Horizontal dashed lines denote the -20°C,-15°C,-10°C,-5°C and 0°C isotherms,from top to bottom.

During the period from 2020 to 2130 LST,the maximumKDP(median of 1° km-1) appeared near the -20°C isotherm(Fig.9c),coincident with a small value ofZDR(Fig.9b,median of 0.9 dB),suggesting the generation of pristine ice crystals (including DCs).Then,KDPgradually decreased andZDRremained unchanged in the -20°C to -10°C layer,with a small enhancement inZH(Fig.9a,median of 16 dBZ),corresponding to the descent implied by thefield near the -15°C isotherm (Fig.9d) and indicating deposition and aggregation of ice crystals (Andrić et al.,2013;Schneebeli et al.,2013).The scattered frequency distributions ofZDRandKDPimplied that the size and density of generated ice-phase particles varied greatly.In the -10°C to 0°C layer,KDPdecreased (median close to 0° km-1) andZDRgradually increased (median of 1.2 dB),coincident withZHincreasing significantly (median of 25 dBZ).It is speculated that icephase particles grew through riming with supercooled water.

From 0210 to 0315 LST,the height of isotherms descended.The maximumKDP(median of 0.4° km-1)(Fig.9g) at -15°C to -10°C was smaller than that in the first period,coincident with a slight enhancement inZDR(Fig.9f,median of 1.5 dB) andZH(Fig.9e,median of 13 dBZ),coincident with descent near the -10°C isotherm(Fig.9h).Combined with the relatively scattered frequency distribution ofZDR(0.8–2.0 dB),it is thought that various types of ice crystals (including DCs) were formed,with a small number concentration and large deviation in the axis ratio.ZDRandKDPgradually increased to their maximums and then decreased in the -10°C to 0°C layer,withZHfirst invariant and then increasing significantly (median close to 20 dBZ),possibly indicating that ice-phase particles underwent deposition and riming,accompanied by secondary ice crystal production (Griffin et al.,2018).

Large-value layers ofZHandZDR[namely,the melting layer (ML)] all presented near the 0°C isotherm (Figs.9b and f) in the two typical periods.Through comparison between the height of the ML and 0°C isotherm,it is speculated that the height of the 0°C isotherm from the ERA5 data was lower than the actual value in the first typical period.Graupel becomes flat and the dielectric constant increases after melting,resulting in the increasing axis ratio and reflectivity,soZHandZDRincrease considerably in the ML.In addition,it takes a longer path to complete melting for graupel than aggregation (Giangrande et al.,2016),so the thickness of the ML is larger compared with the 17–18 September process.

In general,there are large-value zones ofKDPin the DGLs during the two typical periods of the 7–8 August process,coincident with descent implied by the convergence–divergence dipole.When the echo top reached a high height between 2020 and 2130 LST,largeKDPdenoting a large number concentration of DCs generated appeared near the-20°C isotherm.However,with the echo top descending during the period from 0210 to 0315 LST,smallKDPand largeZDRdenoted that various types of ice crystals (including DCs) were formed at -15°C to -10°C,with a large axis ratio and small number concentration.This reveals that if there is descent in the DGL,a high echo top is conducive to the generation of DCs,as indicated by largeKDP,which is similar to the results demonstrated by Griffin et al.(2018)for the ice-phase microphysical processes of snowfall.Ice crystals formed in the DGLs went through aggregation and deposition,then grew mainly by riming in the falling process under the DGL,which may have nothing to do with the kinematic structure implied by thefield.Comparing the growth rates ofZHfrom echo tops to 0°C isotherms,it is found that the growth rate ofZHin the first typical period is larger than that in the second period.So,it is speculated that DCs with a large number concentration are conducive to rime with supercooled water,which leads to the development and strengthening of a low-level echo in the negative temperature region.

4.3.The 12 August 2021 case

4.3.1.Time–height series ofreconstructed using the QVP method

4.3.2.Microphysical processes and kinematic structure

Figure 10 shows the QVPs of radar variables (ZH,ZDR,KDP) and estimatedin a time versus height format on 12 August 2021 with ERA5 hourly temperature data overlaid.The time–height series ofZH(Fig.10a) indicates that the echo top maintained at approximately 5 km (-20°C) as a whole (Fig.10a),while the strong echo (>20 dBZ) top height and strong echo center fluctuated.Polarimetric signatures and the dynamic field of the cloud system are then analyzed.

Fig.10.Time–height series of the (a) ZH,(b) ZDR,(c) KDP and (d) reconstructed by the QVP method on 12 August 2021.From low to high,the black lines are the 0°C,-5°C,-10°C and 20°C isotherms,and the red line is the -15°C isotherm.The black dashed circles denote the DGL indicated by the characteristics of polarimetric variables.

Descent implied by the convergence–divergence dipole occurred above the -15°C or -10°C isotherm before 0900 LST (Fig.10d).From 0500 to 0600 LST,KDPshowed maximums (0.6°–1.0° km-1) in the negative temperature region near the -15°C isotherm (Fig.10c),corresponding to a weak echo (Fig.10a,ZH<15 dB),which is consistent with the characteristics of the DGL investigated in the 7–8 August process.With further echo development,large-value zones ofKDP(0.8°–1.2° km-1) andZDR(1.6–2.4 dB)appeared in the -20°C to -10°C layer between 0800 and 0900 (Figs.10c and 10b),coincident with descent near the-10°C isotherm andZH<20 dBZ.After 1200 LST,the echo top descended,and large-value zones ofKDP(0.6°–0.8° km-1) appeared at -15°C to -10°C,coincident with descent in this layer.

In general,it can be concluded that characteristics of DGLs appeared in the -20°C to -10°C layer during the periods of 0500–0600,0800–0900 and 1220–1420 LST 12 August,which we refer to as “typical periods”.There was descent in the DGLs,which were all located near the echo top in this process.An analysis focusing on investigating the microphysical characteristics in the negative temperature region and the influence of the kinematic structure on it will be made in the following.

Based on the reconstructed and established time–height series of radar physical variables,the frequencies ofZH,ZDR,KDPandoccurrence with height during the typical periods(0500–0600,0800–0900 and 1220–1420 LST) are presented in Fig.11,with vertical profiles of medians overlapped(solid red line).

Fig.11.Frequency of occurrence (color shading) and median (solid red line) of physical variables during (a–d) 0500–0600,(e–h)0800–0900 and (i–l) 1220–1420 LST 12 August 2021.Horizontal dashed lines denote the -20°C,-15°C,-10°C,-5°C and 0°C isotherms,from top to bottom.

During the period from 0500 to 0600 LST,there are maximums ofKDP(median of 0.8° km-1) andZDR(median of 1.4 dB) in the negative temperature region near the -15°C isotherm (Figs.11c and 11b),with small echo reflectivity(Fig.11a,median of 10 dBZ),coincident with descent below it (Fig.11d),which indicates vigorous dendritic growth in the DGL.In the -15°C to -10°C layer,withZHgradually increasing,almost invariantZDR,andKDPdecreasing (median of 0.3° km-1),it is speculated that ice crystals grow by aggregation (mainly) and deposition.The polarimetric signatures in the -10°C to 0°C layer were similar to those in the period of 0210–0315 LST 8 August,which suggests that ice-phase particles deposited to accumulate rime,accompanied by secondary ice crystal production.

Between 0800 and 0900 LST,KDP(median of 0.9° km-1)andZDR(median of 2.6 dB) decreased from the maximums of the negative temperature zone in the -20°C to -15°C layer (Fig.11g,f) with a relatively scattered frequency distribution,whileZHincreased slightly (Fig.11e),coincident with descent near the -10°C isotherm (Fig.11h).It is inferred that there are many types of ice crystals (including DCs) generated,and they grow by aggregation (mainly) and deposition,indicated by invariantZDR,prominently decreasingKDP,and slightly increasingZHin the -15°C to -5°C layer.Dropping to the -5°C to 0°C layer,KDPdecreased(close to 0° km-1) and values ofZDRwere more concentrated and unchanged,coincident with gradually increasingZH,indicating that ice-phase particles rime with supercooled water.

From 1220 to 1420 LST,descent implied by the convergence–divergence dipole presented in the -15°C to-10°C layer (Fig.11l).KDPgradually decreased from the maximum of the negative temperature region (Fig.11k,median of 0.5° km-1) in this layer,corresponding to a median ofZDRclose to 1.2 dB (Fig.11j) and a large enhancement inZH(Fig.11i,median of 17 dBZ),which suggests that DCs formed and aggregated but with a small number concentration.In the -10°C to -5°C layer,ZHincreased slightly,ZDRfirst increased and then decreased,andKDPdecreased gradually,suggesting that ice-phase particles grew by deposition and riming.The scattered frequency distribution ofZDRandKDPimplies that the size and density of ice-phase particles varied greatly.

Furthermore,there were large-value layers ofZHandZDRbelow the 0°C isotherms during the three typical periods,resulting from the generated graupel melting into a liquid state gradually,with a higher dielectric constant and flatter particle shape compared to ice-phase graupel.After melting into water droplets completely,ZHandZDRdecreased from large values owing to the volume reduction and close to round shape.

In summary,the DCLs with large-value zones ofKDPlocated near the echo top were all coincident with descent in the three typical periods of the 12 August process.With the echo top height varying,large-value zones ofKDP(0.8°–0.9°km-1) were located above the descent implied by convergence–divergence dipoles in the first two periods,while that of the third period was located below descent with a median of 0.5° km-1.AsKDPis proportional to the number concentration (e.g.,Ryzhkov et al.,1998),it is speculated that,with descent in the DGL,a high echo top indicates relatively vigorous radar echo development (low temperature,high supersaturation with respect to ice),which results in a large number concentration of ice crystals (including DCs) generating(Chu et al.,2018),indicated by large values ofKDP.The positive correlation between theKDPvalues in the DGLs and echo top heights in the typical periods was similar to that of the 7–8 August process,corresponding to a weak echo (ZH<20 dBZ).As with the microphysical processes,generated ice crystals (including DCs) first usually go through aggregation,and then grow by deposition and riming with the production of secondary ice crystals in the falling process under the DGL,which is thought to have nothing to do with the kinematic structure implied by thefield.We compared theZHgrowth rates from echo tops to 0°C isotherms,and could infer similar results that the formation of DCs with a large number concentration is more conducive to riming and echo enhancement at the lower level.

5.Summary and discussion

This study provides insight into three cases of microphysical processes and corresponding kinematic structures in the negative temperature region of summer mesoscale cloud systems over the eastern TP,based on transportable X-band dual-polarization radar VCP data.The latest QVP reconstruction method and estimation algorithm were applied to establish time–height series of radar physical variables and mesoscale horizontal divergence.

The reconstructed time–height series ofZHwas compared with vertical-pointing observations of a nearby MRR,allowing us to conclude that the two were consistent in terms of the cloud echo top and precipitation core.Meanwhile,ERA5 divergence data at grid points in each layer were used to compare with theestimated byVR-based QVP.The results showed that the estimatedaccurately captured the vertical distribution of the divergence field,and the two were consistent in the weak vertical development stage of the cloud system,which we attributed to the ERA5 data not fully considering the influence of the underlying surface over the TP,and the limited vertical and temporal resolutions.

Next,the time–height series of radar physical variables(ZH,ZDR,KDP) andwith ERA5 hourly temperature data overlaid,were used to investigate the characteristics of polarimetric variables and kinematic structures in the mesoscale cloud systems.We focused on the microphysical characteristics in the DGL of selected typical periods with DGL features and assessed the possible mechanism of dynamic field influence on microphysical processes.The vertical distribution ofwas applied to innovatively analyze the dynamic field in the DGL.

DGL signatures like those found in winter storms over plains also exist in summer mesoscale precipitation clouds over the eastern TP,such as large-value zones of eitherKDPorZDR,or both,in the -20°C to -10°C layer.Three cases of mesoscale cloud systems affected by different kinds of typical synoptic situations (including a plateau shortwave trough pattern,east-high and west-low pattern,and zonal circulation pattern) were analyzed and compared with each other.There were apparent discrepancies of polarimetric characteristics and microphysical processes in the DGLs with various dynamic fields.The main conclusions are as follows:

In the case of 17–18 September 2021,the cloud system developed vigorously and the echo top fluctuated.Various features of polarimetric variables with ascent or descent were implied by the convergence–divergence dipole in the DGLs,so the influence of the dynamic field on microphysical processes in the DGL was analyzed.Large-value zones ofZDRrepresenting a big axis ratio of DCs presented when there was ascent in the DGLs,but with smallKDPvalues denoting a limited number concentration of DCs.While there was descent in the DGL,a large-value zone ofKDPrepresenting a large number concentration of DCs presented,but large values ofZDRwere not obvious.These results are different from previous research (Kumjian et al.,2014;Moisseev et al.,2015).It is speculated to be related to atmospheric background conditions,and ice crystals could undergo further growth with ascent in the DGL before sinking (Sulia and Harrington,2011).Therefore,during the typical periods,largevalue zones ofKDPusually appeared above the ascent,and large-value zones ofZDRappeared below the ascent.

In the two August 2021 cases,the vertical development of the cloud systems was slightly weaker overall.There were large-value zones ofKDPwith no obvious large values ofZDRin the DGLs during the typical periods,all coincident with descent implied by the convergence–divergence dipoles.Obvious differences in vertical development of cloud systems are thought to result in different maximums ofKDP.When the echo top reached a high height,largeKDP,denoting a large number concentration of DCs,generated and appeared in the DGLs;however,with the echo top descending,smallKDPformed.This is similar to the results reported by Griffin et al.(2018) regarding the ice-phase microphysical processes of snowfall,and reveals that when there is descent in the DGL,a high echo top is conducive to the generation of ice crystals,with large-value zones ofKDPabove the descent.

When falling to a low level (-10°C to 0°C),ice crystals formed in the DGLs went through aggregation,deposition,and riming during the different typical periods,which may have nothing to do with the kinematic structure.Ice crystals with a large axis ratio are more likely to grow by aggregation.The formation of ice crystals with large number concentrations is conducive to riming and echo enhancement at the lower level.During the typical periods with liquid-phase precipitation near the surface,there were large-value layers ofZHandZDRpresented near the 0°C isotherm,i.e.,the ML,indicating a transformation of particle phase.

The QVP method is suitable for the reconstruction of cloud systems with uniform horizontal distributions.Certain assumptions should be met when estimatingby QVPs ofVR;that is,the wind field in the conical scanning area changes linearly.Due to a lack of reliable observation data,the effects of water vapor conditions on microphysical processes were not considered in this paper.Special observations,such as those made by aircraft in-situ instruments,need to be added and applied for comparison and verification in future studies.Also,it is important to note that this study only investigated a limited number of cases.Further research is needed to establish general features of the cloud structure over the eastern TP based on statistical analysis.

Acknowledgements.This study was jointly funded by the Northwest Regional Weather Modification Capacity Building Project of the China Meteorological Administration (Grant No.ZQC-R18209) and the National Natural Science Foundation of China (Grant Nos.41875172 and 42075192).We greatly appreciate the strong support of the project team and Weather Modification Office of Qinghai Province in the organization and implementation of the field experiment.Furthermore,we thank Dr.Liang FENG and Dr.Hong WANG for their guidance and suggestions on the data preprocessing.