A numerical study on a mesoscale vortex producing a heavy rainfall event

A heavy rainfall event,which was mainly caused by typhoon Haiou,subtropical high and weak cold air,occurred in Shandong Province on 17—19 August 2008. A vortex in the low-troposphere is the maker of the heavy rainfall event. The structure and mechanism of the mesoscale vortex were studied by using routine observational data and the simulated data from mesoscale model WRF( Weather Research and Forecasting). The results showthat the eastward movement of the mesoscale vortex was accompanied by a process that a newvortex center came into being and combined with the old vortex instead of a simple movement along the shear line. The mesoscale vortex formed in the convection area with obvious temperature rising,deep saturated layer and rising motion. After the formation of the mesoscale vortex,downdraft dominated the center of the vortex,convection area moved to its east part and the rainfall area lay in its northeast and southeast quadrants. The cold pool near boundary layer,dry intrusion between 600 hPa and 400 hPa,weak wind area between 900 hPa and 850 hPa as well as the coupling between lowlevel jets( LLJ) and upper level jets are important characteristics of the mesoscale vortex during its generating and developing phases. During the fading stage of the mesoscale vortex,downdraft became strong and deep, and LLJ and the outflowof upper level weakened obviously. The analysis of vorticity equation suggests that divergence term and tilting term in the low-troposphere,horizontal advection term and convection term in the mid-troposphere are the main contributors for the positive vorticity. The horizontal convergence in lowlevels and the transportation of vorticity from lowlevels to high levels are beneficial for the formation and development of mesoscale vortex. The increase of positive vorticity in lowlevels is partly caused by the transformation from horizontal vorticity to vertical vorticity. The tilting term is very important for the formation of mesoscale vortex. In the later phase of its formation,the weakening of the lowlevel convergence, high level divergence and the vertical transportation led to the weakness of positive vorticity,and thus the weakness of the mesoscale vortex.