华北南部一次回流暴雪天气的诊断分析

Diagnostic analysis of a heavy snow of return-flow event in North China

利用常规观测资料和NCEP再分析资料,对发生在华北南部的一次回流暴雪天气过程进行了动力、热力等诊断分析。结果表明:该回流暴雪天气属于华北回流中的两槽一脊型,导致这次强降雪的影响系统是高空急流、西来槽、低涡切变和低空急流,东北冷空气起到了触发作用。最大降水出现在南北风转换阶段,当东北风完全控制低层,降水结束。高空辐散和低层辐合相叠置及高空正涡度的下传,有强降水的产生,但上升运动中心较低。降雪前的增暖增湿与低层冷空气的楔入使华北南部位于θse能量锋区和水汽辐合区内,有利于强降雪的产生。回流天气的水汽主要来自于南方,低层东北冷空气也有间接输送水汽作用。

Based on the conventional observational data and the NCEP reanalysis data,the dynamic and thermodynamic characteristics of a heavy snow of return-flow event in the southern part of North China were analyzed.The results indicate that synoptic situation of this process is two troughs and one ridge.The influencing system leading to the heavy snow of return-flow event includes high level jet,westerly trough,low vortex shear and low level jet,and cold air in Northeast China triggers this event.The maximum precipitation occurs just at the changing period of southerly wind and northerly wind.Precipitation ends when northeasterly wind controls entirely the low level.The stacking of high level divergence and low level convergence,subsidence of positive vorticity in the high level causes strong precipitation,while the ascending movement center is fairly low.Warming and humidifying before snowfall and insertion of cold air in the low level make the south of North China locating in energy front area of θse and water vapor convergence area,which is benefit to the generation of heavy snow.Water vapor of return-flow event is mainly from the south,and it is transported indirectly by cold air from Northeast China in low level.