石家庄一次秋季罕见暴雪天气过程物理诊断

Diagnosis of a rare snowstorm process in autumn over Shijiazhuang,Hebei province

利用NCEP再分析资料,采用天气学诊断方法,对2009年11月10—12日石家庄地区出现的一次历史同期罕见区域性暴雪天气过程的环流特征和物理量场进行了探讨。结果表明:此次暴雪天气过程属典型的东北回流型降雪,地面从贝加尔湖南下冷高压与中国河套低压倒槽、700 hPa暖式切变线和500 hPa高空槽是主要影响系统。低空西南急流与超低空东北急流耦合,在为暴雪提供水汽和热量输送的同时加强了抬升运动。水汽的垂直输送导致局地比湿显著增大,深厚的湿层和强烈的水汽辐合为暴雪提供了充沛的水汽条件。"高空辐散、低空辐合"以及强劲的上升运动是暴雪的动力条件,降雪强度最大时段对应上升运动的强盛发展阶段。暴雪开始阶段云水含量的时空演变特征,一方面显示了水汽的迅速增加与爬升,另一方面也说明了地形的强迫抬升作用不容忽视。850 hPa温度低于700 hPa,有利于水汽经过此层时被凝华成固态。逆温层提前24 h出现,而且暴雪最强时段内两层温差均为5℃以上,这对暴雪预报具有指示意义。

Based on reanalysis data from NCEP,the circulation characteristics and physical fields of the rare snowstorm process on November 10-12,2009 in Shijiazhuang of Hebei province were analyzed by synoptic diagnosis method.The results show that the snowstorm belongs to the typical northeast backflow type.The main influencing systems include the southward cold high pressure from Lake Baikal,the low pressure inverted trough in Hetao regions,the warm shear line in 700 hPa and the upper level trough in 500 hPa.The southwest low level jet is coupled with the northeast low level jet,which not only affords water vapor and energy for the snowstorm but also strengthens uplift motion.Vertical allocation of water vapor leads to the increase of the local specific humidity.The deep humidity layer and strong water vapor convergence provide the abundant water vapor conditions for the snowstorm.The upper level divergence and low level convergence,the strong uplift motion are the dynamic conditions of this process.The period of large snowfall is in accord with the strong development stage of uplift motion.The temporal and spatial characteristics show that water vapor increases rapidly and ascends at initial stage of the snowstorm.At the same time,it indicates that forced uplift action of topography is also important.Temperature is higher in 850 hPa than in 700 hPa,which is favorable to water vapor desublimation.The inversion layer appears ahead of 24 hours and temperature difference between layers at the strongest stage of snowstorm is more than 5 ℃,which is indicative to snowstorm forecast.