Numerical Simulation and Analysis on a Heavy Snow of Return-Flow Events in Beijing

The typical return-flow events with heavy snow during 2—3 January,2010 by using Beijing local high-resolution rapid update cycle mesoscale model system(BJ-RUC) are simulated.The characteristics of synoptic system in each layer and key snowfall factor are analyzed.The results show that the BJ-RUC model can well simulate the snowfall distribution,position and time change,and the main situations of heavy snowfall which are caused by the cold air return-flow in the low-level,and the 850—700 hPa layer low-vortex system.But the snowfall center has still some deficiency.When the vertical ascending motion is produced in lower atmosphere by the cold return-flow encountering terrain,the snowfall is not high. During the snowfall,the dynamic lifting which is caused by the cold return-flow combining with the ascending motion of the foreside of low vortex,forms a deeply and violent ascending motion,that is the key factor to cause the heavy snow.The durative and cold return-flow with high humidity transports water vapor in the lower atmosphere.The humidity in the lower-mid troposphere is increased by the southwestern air flow.The return-flow increases thickness of wet-layer,reaching above 600 hPa.It plays an important role in producing the heavy snow.The start and intensity of snow fall are related to the moment of the direction change to southern wind and the south wind speed.