Simulation of Impact of Steep Terrain on East Side of Qinghai-Xizang Plateau on Mesoscale Vortex and Rain Storm over the Basin

The MM5 mesoscalemodel was used to simulate a heavy rain process taking place in northwestern Sichuan Basin on 8~10 August 2003 and some numerical experiments with different kinds of changed topographies were run.The results of experiment show that the mesoscale vortex is favorable for the transportation of positive vorticity and the maintenance of convergent and ascending motion,as a result,the precipitation maintenance and heavy precipitation occuring;The influence of topography on mesoscale vortex mainly shows that the steep and complex topography is of advantage for the generation of mesoscale vortex.In the control experiments with the real plateau topography,the vortexes are more and active over the Plateau and western Sichuan Basin;when the topography is altered in both height and slope,the numbers of vortexes reduce,the intensity of vortex weakenes,and the sources of vortex and their moving paths shift;when the topography is removed,vortexes are less,moreover,vortexes move faster than usual;if the topography is cut down only in height,vortex activities change little;when only the Plateau exists,vortexes are still active in the Plateau area,but in no-topography area vortexes deceased apparently.The results also reveal that the activity of vortex has something to do with the distribution of rainfall.