江西一次冰雹过程的S波段双偏振雷达回波特征分析

Analysis on characteristics of S-band dual-polarization radar echo of a hail event in Jiangxi Province

2020年3月21—23日江西省出现一次冰雹伴随短时强降水过程,3月21日夜间出现的冰雹天气是在西南气流暖湿强迫背景下产生的,22日中午前后的降雹属于显著冷暖平流导致的斜压锋生类强对流天气。利用常规气象观测资料以及南昌双偏振雷达资料,重点对此次过程强对流云系中水凝物相态演变进行了分析。结果表明:冰雹特征(反射率因子Z_(H)>60 dBz,差分反射率Z_(DR)<0 dB)首先出现在0℃层高度区域,后向上向下发展,由于22日当天干球、湿球0℃层高度均在3.8 km左右,融化层厚度小、高度低,且地面温度>20℃,冰雹在下降过程中Z_(DR)和差分相位率K_(DP)由负值逐渐转为正值,零阶滞后相关系数C_(C)逐渐加大至0.9以上,但某些时刻仍具有大冰雹特征(Z_(DR)<0,CC<0.9),表明冰雹在下落过程中出现融化形成外包水膜的小冰雹或大雨滴,但仍有部分大冰雹落地。三体散射现象持续时间长,对应Z_(DR)具有较大坡度,显示其正值到负值突变,CC呈现非气象回波特征。冰雹粒子特征主要为Z_(H)>60 dBz、Z_(DR)<0dB、CC<0及K_(DP)趋近于零,当存在大冰雹时Z_(DR)<0、CC<0.9;对比短时强降水时发现Z_(DR)≥1 dB、KDP≥1°·km^(-1)并呈现出随ZH增大而增大的特征,CC主要在0.95~1之间,表明强降水主要由大雨滴造成,因此通过双偏振特征变化可判断冰雹向强降水过程的转换。

A hail event accompanied by the short-time several rainfall occurred in Jiangxi from March 21 to 23 in 2020.During this event,the hail at the night of 21 March occurred under the forcing effect of the southwest warm and wet airflow,and the hailfall around the noon of 22 March belongs to a baroclinic frontogenetic type of severe convective weather caused by significantly cold and warm advection.Using the conventional meteorological observational data and the S-band dual polarization radar(SPOL)data at the Nanchang station,we conducted a key analysis of the evolution of phase state of hydrocondensate in the strong convective cloud system in the event.The results show that re⁃flectivity factor(Z_(H))is greater than 60 dBz and differential reflectivity(Z_(DR))is less than 0 dB.These characteristics of hail firstly appears in the height of 0℃level,and develops upward and downward.Because the height of dry and wet bulb temperature reaching zero is about 3.8 km,the melting layer is small in thickness and its height is low.Meanwhile,the ground temperature is higher than 20℃.During the fall⁃ing process of hail,Z_(DR)and KDP(specific differential phase)change gradually from negative value to positive value,CC(zero order lag correla⁃tion coefficient)increases gradually to more than 0.9,but Z_(DR)<0 dB and CC<0.9,at some times.It is shown that the small hailstones or big raindrops with water film formed by melting appear in the falling process of hail,but some large hailstones still fall to the ground.Three body scatter spider(TBSS)lasts for a long time,which is corresponding with the Z_(DR)having a large slope i.e.abrupt change from positive value to negative value and the CC showing the characteristics of non meteorological echo.The hail particles during the event is mainly characterized by Z_(H)greater than 60 dBz,Z_(DR)being less than 0 dB,CC being less than zero and KDP approaching 0°·km^(-1),while Z_(DR)<0 and CC<0.9 when there is big hail.It is found by comprison that the short-term severe precipitation is characterized by Z_(DR)≥1 dB,KDP≥1°·km^(-1)and its in⁃creasing with the increase of Z_(H),and CC being concentrated in the range from 0.95 to 1,which shows that the severe precipitation particles are mainly composed of big raindrops.Therefore,the dual polarization radar echo characteristics mentioned above can be used to decide the tran⁃sition from hail to severe precipitation.