湖南冬季2次暴雪过程的对比诊断分析

Diagnostic Analysis of Two Winter Snowstorm Events in Hunan Province

【目的】湖南的暴雪天气较为少见,为了提高暴雪强度和落区的预报准确率。【方法】利用常规地面和高空观测资料及欧洲中心ERA-5再分析数据,对2021年12月25—27日(21·12)、2022年2月21—23日(22·02)发生在湖南的2次暴雪过程进行诊断分析。【结果】2次暴雪过程有强度大、影响范围广,暴雪落区较为重合的特点。2次暴雪过程也表现出明显差异,21·12过程的累计降雪量小于22·02过程,但21·12以干雪为主,22·02以湿雪为主。【结论】分析差异的成因发现:(1)22·02过程水汽辐合强度强、延伸高度高、厚度厚,动力条件由南支槽和低涡提供;21·12过程水汽条件相对弱,动力条件由冷锋强迫抬升提供。(2)21·12过程冷空气从低层南下,云中冰相粒子比例大,温度层结满足干雪条件;22·02过程中层气温下降,云中冰相粒子与水相粒子共存,地面气温在0℃以上,故以湿雪为主。

It is rare to have a snowstorm in Hunan Province.To improve the forecast accuracy rate of the intensity and area of snowstorm,this article diagnostically analyzes two snowstorm processes in Hunan during December 25-27,2021,and February 21-23,2022,by using conventional ground and upper-air data as well as the ERA-5 reanalysis data from European Centre for Medium-Range Weather Forecasts(ECMWF).The results show that both of the two snowstorm processes were featured with heavy intensity,extensive and overlapped impact area.However,obvious difference has been found in the two processes,that is,the accumulated snowfall in the December 2021 process was less than that in the February 2022 process,and the snow in the former process was mainly dry snow while that in the latter process was wet snow.The analysis of the difference causes suggests that:(1)The February 2022 process had stronger water vapor convergence,high extension height and thick thickness,and the dynamic condition was provided by the south branch trough and low vortex.In comparison,the moisture condition for the process in December 2021 was weaker,and the dynamic condition was provided by the forced lifting of the cold front.(2)The cold air of the December 2021 moved south from the lower layer,the specific cloud ice water content was large,and the temperature stratification was advantageous to the formation of dry snow.Then,during the February 2022 process,the air temperature dropped in the middle layer,the specific cloud ice water content was comparable to the specific cloud liquid water content in clouds,and the ground temperature was above 0℃,so wet snow was dominant in this snowstorm event.