Analysis of Vorticity during Vortex Producing Snowstorm in North China in Winter

The snowstorm occurred in North China on 2~3 January 2010 was simulated using WRF model.And potential vorticity and vorticity equation were used to diagnose the developing mechanism of the vortex producing heavy snowfall process in winter.The results showed that,firstly,the weak trough in middle troposphere leaded to lower and surface systems moving from west to east.Secondly,potential vorticity from upper-level intensified development of vortex in middle-lower troposphere.Thirdly,the mean flux and stretching term of the integrated vorticity equation was the main source of vorticity,but eddy flux and like-tilt term was relatively weak.During the mature period of vortex,the vorticity decreased because of the divergence of environmental circulation to vortex area while the vortex moved faster than surrounding air,and to the contrary,vorticity increased for the convergence between environmental circulation and vortex area.During the dissipating period,the vorticity decreased since the loss of the absolute vorticity flux.On the basis of all diagnosis,the PV forcing in upper-troposphere,convergence in lower-troposphere and the vorticity produced by environmental circulation are main mechanisms of vorticity developing.