2020年内蒙古东南部一次特大暴雪伴冻雨灾害天气特征分析

Analysis of a Disastrous Heavy Snow with Freezing Rainfall in 2020 in Southeast of Inner Mongolian

利用常规气象观测资料、NCEP逐6 h和逐日再分析资料、FY-2G卫星云图资料,从环流形势、水汽条件、动力条件及层结特征方面对2020年11月18—19日内蒙古自治区东南部罕见的特大暴雪伴冻雨过程进行分析。结果表明:(1)大气环流异常是造成此次灾害性天气的主要环流背景,500 hPa西风槽和南支槽、700 hPa西南低空急流及切变线、850 hPa以下东北急流、地面气旋是此次过程的主要影响系统,具有回流暴雪的天气特征;(2)低空偏南和偏东急流两支水汽输送路径及水汽强烈辐合为极端暴雪提供了充足的水汽,850 hPa和700 hPa比湿最大分别为5g·kg^(-1)和4 g·kg^(-1);(3)低空偏南急流代表的暖空气与北方的冷空气剧烈交汇,暖湿空气在低层“冷垫”上爬升,加剧上升运动的发展,导致该区域降雪迅速加强;(4)非绝热加热项F对锋面生消作用最小,水平运动项F是锋生函数变化的主要贡献项,锋生函数F变化与降水强度变化一致,在F>10的较强锋生区都出现暴雪,因此锋面的强迫抬升对暴雪的增幅作用不容忽视;(5)锋区的维持使得低层维持低温天气,锋上逆温,暖湿空气沿锋面抬升,900 hPa以下“冷垫”与之上的“暖盖”长时间存在,导致暴雪发生并持续;(6)锋上逆温且逆温区存在融化层,这种垂直结构变化有利于降水相态转化,高层和低层为低于0℃的冷层,中间形成温度高于0℃的暖层,符合融化类冻雨的层结特征。

Based on routine observational data,NCEP reanalysis data by every 6 hours and daily FY-2 G satellite data,a heavy snow with freezing rain happened on 18-19 November 2020 in the southeast of Inner Mongolian is analyzed,including the circulation situation,water vapour condition,dynamic condition and stratification characteristics.The results show that:The atmospheric circulation anomaly was the main atmospheric circulation background of the disastrous weather;The westerly trough at 500 hPa,southwest jet at 700 hPa and northeast jet below 850 hPa,and the cyclone were mainly influencing systems,with the characteristics of a return-flow heavy snow.The water vapour transportation by the southerly and easterly and the moisture convergence provided sufficient water vapour supply,with the maximum specific humidity of 5 g·kg^(-1)at 850 hPa and 4 g·kg^(-1)at 700 hPa.The warm air represented by the southerly jet converged with the cold air of the north.The warm and humid air lifted on the low-level“cold pad”,which intensified the development of ascending motions and led to the rapid enhancement of precipitation.The diabatic heating had little effect on the frontogenesis,and the horizontal movement term primarily contributed to frontogenesis.The change of frontogenesis corresponded to the intensity of snow very well,so the front lifting force couldn’t be ignored in the increase of heavy snow;the maintenance of the frontal zone made the low temperature in the low level,and there was temperature inversion on the front.The warm and humid air rised along the front,and the“cold pad”below 900 hPa and“warm cover”above existed for a long time,which led to the occurrence and persistence of heavy snow.The temperature inversion on the front and melting level was conducive to the precipitation phase transformation.The temperature of the middle layer was higher than 0℃,and the upper and lower layer temperature was below 0℃,which corresponded to the stratification characteristic of melting freezing rain.