2009年深秋河北省特大暴雪天气成因分析

Diagnoses of Heavy Snowstorm in Hebei Province in Late Autumn of 2009

利用多种资料对2009年11月10—11日河北省特大暴雪天气过程进行了分析。结果表明:冷锋系统长时间影响河北,并产生正、逆双向次级环流,其强上升运动与中层强辐合及正涡度柱耦合,为暴雪提供了很好的动力条件;由于锋面附近强温度梯度及风的垂直切变作用,在暴雪区形成条件性对称不稳定和对流不稳定;另外西南暖湿气流为暴雪提供了充沛的水汽条件,强上升运动可将水汽输送到高空冷区凝华、冻结为雪及冰晶;向东开口的喇叭口地形使得干冷空气在低层堆积,增强了回流降雪的低层冷垫作用;在低层由于地形作用产生的弱上升运动与锋面次级环流上升运动叠加,有利于强对流天气发展和增强。

Based on the observed data and output data from MM5,the causes for the blizzard process in Hebei Province form November 10 to 11 in 2009 are analyzed.The results show that a cold front system influences Hebei Province for a long time and brings about positive and contrary bi-directional secondary circulations with strong upward flows and convergence at the middle levels coupled with positive vorticity column,which are the main dynamic factors for snowstorm.On the other hand the results show that the conditional symmetric instability and convective instability are formed in the blizzard area due to the stronger temperature gradient and wind vertical shear.Moreover,plentiful water vapor is transported to Hebei through the southwest warm and wet airflows,which change into snow and ice by deposition and freezing when being carried to the higher cold levels.The northeast dry and cold airflows acting as a cold wedge of backflow snowfall are accumulated and lifted in the lower level because of the eastward bell-mouthed topography. It favors development of severe convective weather when the two kinds of upward flows caused by topography and front-surface secondary circulation are superimposed.