冷垫背景下冻雨和极端大暴雪成因机制分析

Analysis on the Genesis Mechanism of Freezing Rain and Extreme Heavy Snowstorm under the Background of Cold Cushion

利用常规气象资料、通辽市和赤峰市多普勒雷达资料、气候极端降雪以及NCEP的FNL(1°×1°)逐6 h再分析资料,对2020年11月17-19日内蒙古中东部极端回流大暴雪天气进行分析。研究表明:500 hPa东移高空槽前暖湿气流、700 hPa西南急流以及暖式切变线为降雪提供了丰富的水汽和动力辐合抬升机制,地面至850 hPa均为偏东风冷垫,中高空西南暖湿空气沿低层冷垫爬升产生锋生,是造成此次大暴雪的主要原因。降雪最强时段,从低层到高层均为上升运动,中低层水汽几乎接近饱和状态,深厚湿层有利于产生高效率的强降雪;通辽探空图有冰相层、逆温层、融化层、中性层等多种特殊层结,并有明显表征冻雨的“象鼻”层结曲线;低层东北风急流与中高层西南急流形成强的垂直风切变和温度差,动力锋生在降雪期间一直维持,动力锋生最强阶段和降雪最强时刻相对应。雷达反射率有0℃层亮带,50~55 dBz带状强回波;基本径向速度低层长时间维持东北急流构成的冷垫,并有一对正负速度中心的风速核,形成“牛眼”结构,“牛眼”结构代表边界层出现急流核;雷达基本径向速度图低层东北风,中高层西南急流,很好地反映了西南暖湿急流在冷垫上爬升的天气学结构特征,通过多普勒雷达实时监测弥补了普通降雪短时观测的不足,这对暴雪短时预报预警具有非常好的指示意义。

Based on meteorological observations,Doppler radar observation data of Tongliao station and Chifeng station,climate extreme snowfall and FNL(1°×1°)reanalysis data of NECP every 6h,this paper analyzes the disastrous extreme backflow heavy snowstorm in early winter in the central and eastern Inner Mongolia from November 17 to 19,2020.The results show that 500 hPa eastward moving upper-level trough,700 hPa southwest jet and warm shear line provide abundant moisture and dynamic convergence lifting mechanism for heavy snowfall.The northeast cold air cushion is located from the surface to 850 hPa,and the warm and humid air in the middle and high-altitude southwest climbs along the low-level cold cushion to generate frontogenesis,which is the main reason for this heavy snowstorm.During the heavy snowfall stage,the whole layer moves upward from the lower layer to the upper layer,and the moisture in the middle and lower layers is almost saturated.The deep wet layer is conducive to the production of high-efficiency heavy snowfall.The T-lnP in Tongliao has a variety of special stratifications such as ice facies,temperature inversion,melting,and neutral layers.It also has a"trunk"stratification curve that clearly characterizes freezing rain.The low-level northeast wind jet and the middle and high-level southwest jet are formed strong vertical wind shear and temperature difference.The dynamic frontogenesis is maintained during the snowfall and the strongest stage of the dynamic frontogenesis corresponds to the moment when the snowfall is the strongest;The radar reflectivity has a bright zone of 0℃ and a 50~55 dBz band echo;the low layer of basic radial velocity maintains the cold pad formed by the northeast jet stream for a long time,and there is a pair of wind velocity cores at the center of positive and negative velocity,forming a"bull's eye".The radar's basic radial velocity map shows the low-level northeasterly wind and the middle-level and high-level southwest jet,which can reflect the synoptic structure of the southwest warm and humid jet climbing on the cold cushion.It plays a very effective indication for the short-term forecast and warning of snowstorm.