基于S波段双偏振雷达观测的雹暴偏振特征分析

Analysis of the polarimetric characteristics of hail storm from S band dual polarization radar observations

为了研究雹暴的偏振特征及其在实际业务中的应用,使用S波段双偏振雷达所观测到的46例冰雹数据,对其中反复出现的3种偏振特征:冰雹在各高度层的偏振参数特征、差分反射率因子柱、三体散射偏振特征进行了分析,重点分析了3种偏振特征大、小冰雹事件的差异。结果表明:(1)大冰雹的水平反射率因子中位数要高于小冰雹,二者的水平反射率因子分布存在较大重合区域。(2)冰雹偏振参数的变化主要集中在融化层之下,相较于小冰雹,大冰雹具有更低的差分反射率因子和相关系数。(3)所有雹暴的差分反射率因子柱最大伸展高度均超过−10℃层,有83%的大冰雹事件其差分反射率因子柱最大伸展高度可超过−20℃层;在小冰雹事件中这一比例仅为46%。(4)差分反射率因子柱高度的演变对于雹暴的发展具有预示性,特别是在持续降雹过程中,差分反射率因子柱的再度发展预示着雹暴的再次增强。差分反射率因子柱高度的极值相对于降雹具有时间提前量,在大冰雹事件中这一提前量的中位数为24 min,而在小冰雹事件中这一数值为11 min。(5)三体散射的偏振特征有助于识别高空的冰雹,尤其是当冰雹核的后侧存在其他降水回波时。在所统计的19例大冰雹事件中均发现三体散射偏振特征。在小冰雹事件中同样可发现三体散射偏振特征,出现比例为52%。

This study aims to investigate the polarimetric radar characteristics of hail storm and application of these characteristics for operational work.A total of 46 hail storm reports that have been observed with the S-band dual-polarization radar are used for this study.Three polarimetric signatures,i.e.,the polarimetric variables of hail at various height,Zdr column,the polarimetric TBSS,are repeatedly found in these hail storms and analyzed with the focus on the differences between polarimetric signatures of large and small hails.The study yields the following results:(1)The median value of Zh for large hails is higher than the value for small hails.A significant overlap of the Zh distributions for large and small hails is also present.(2)The variation of polarimetric variables of hail is concentrated below the melting level.Compared with small hails,large hails have lower Zdr and correlation coefficient(CC)values.(3)The maximum height of Zdr column can reach the−10℃level in all of the hail storms,while 83%of large hail reports and 46%of small hail reports show that the Zdr column could penetrate the−20℃level.(4)The evolution of the Zdr column height is an appropriate index that can reflect the development of the hail storm.During a continuous hail process,the re-development of the Zdr column height occurs earlier than that of the hail storm.It is also found that the peak of Zdr column height usually occurs about 24 min(median value)prior to a large hail report and about 11 min(median value)prior to a small hail report.(5)The polarimetric TBSS is a good indicator for identifying hails aloft,especially when there are other precipitation echoes located radially behind the hail core.In this study,the polarimetric TBSS can be found in all of the large hail reports and in 52%of the small hail reports.