河北省南部回流暴雪天气结构特征

摘要

Using the conventional observational meteorological data from weather stations and snowfall data from intensive automatic weather stations as well as the NCEP reanalysis data with resolution of 1 °×1 °,two return-flow snowstorm processes on November 10-12,2009 and November 29-30,201 1 in the southern Hebei province were compared.The results show that spatial and temporal distributions of the two processes have mesoscale features. Low trough moves eastward at 500 hPa in Hetao area and it has shear line at 700 hPa;the Mongolian cold high pressure in the ground moves to east and arrives in the northeastern China,and inverted trough develops in Hetao area;surface pressure is higher in east and lower in west of the north China.All the mentioned-above are the typi-cal weather patterns of the two return-flow snowstorms.Upper-level and low-level jets play a very important role during the return-flow snowstorm.Cold air returns to the southern Hebei province with the northeast stream under 850 hPa and forms cold cushion.Moist air is transported into the southern Hebei province with southwest jet and convergence uplifts on cold cushion at 700 hPa.Upward movement is strengthened by divergence pumping action on the right rear side of 200 hPa upper-level jet.There is a vertical circulation in the North China plain.The north-east wind of boundary layer arrives in the east side of Taihang Mountain then lifts to mid-upper troposphere,and it becomes southwest wind and flows to northeastern China,where it becomes downward flow and finally forms a complete vertical circulation with low-level northeast wind.The intensive area ofθse stretches to 700 hPa from the ground,and frontal structure is obvious.Front advances from north to south.θse lines are upright from the ground to 850 hPa near the front.Atmospheric stratification is neutral convection stability.Return-flow snowfall happens in cold air mass after ground front.%利用常规气象观测资料、降雪加密观测资料和NCEP的1°×1°再分析资料，对2009年11月10—12日和2011年11月29—30日河北省南部两次回流暴雪天气过程进行对比分析。结果表明：河北省南部两次回流暴雪的时空分布具有中尺度特征。两次回流暴雪的典型天气形势为500 hPa高空河套地区有低压槽东移，700 hPa有切变线影响，地面蒙古冷高压东移至东北地区南下，河套倒槽发展，华北地面为东高西低形势。回流暴雪过程中高低空急流有非常重要的作用，冷空气自850 hPa以下随强劲的东北风回流至河北省南部形成冷垫，700 hPa暖湿气流随西南急流输送至河北省南部叠加在冷垫上辐合抬升，高空200 hPa急流右后侧的辐散抽吸作用使上升运动加强。华北平原高空存在一支垂直环流，边界层东北风到达太行山东麓，在迎风坡抬升至对流层中高层转为西南风，到达东北地区转为下沉气流，再与低层东北风构成一个完整的垂直环流。θse密集区由地面向上向北伸展至700 hPa，锋面结构特征明显，锋面的前沿从北向南推进，地面锋面附近850 hPa以下等θse线与地面垂直，具有对流中性层结。回流强降雪发生在地面锋后冷气团中。