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

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 reflectivity factor (ZH) is greater than 60 dBz and differential reflectivity (ZDR) 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 falling process of hail, ZDRand KDP (specific differential phase) change gradually from negative value to positive value, CC(zero order lag correlation coefficient) increases gradually to more than 0.9, but ZDR < 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 ZDR 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 ZH greater than 60 dBz, ZDR being less than 0 dB, CCbeing less than zero and KDPapproaching 0°·km-1, while ZDR < 0 and CC < 0.9 when there is big hail. It is found by comprison that the short-term severe precipitation is characterized by ZDR ≥ 1 dB, KDP ≥ 1°·km-1 and its increasing with the increase of ZH, and CCbeing 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 transition from hail to severe precipitation.