Characteristic analysis on the mesoscale vortex system of an excessive heavy rain event

Analysis of an excessive heavy rain event occurred at Tianjin and the eastern part of North China from July 29 to 30 in 2011 was performed with the conventional observation data, intensive automatic weather station observations, satellite images, Doppler-Radar data, microwave radiometer data and NCEP 6-hourly reanalysis data with resolution 1°×1°. The results indicate that the mesoscale vortex quickly strengthening and moving northwards in favorable circulation background is main weather system led to this event. The enhancement of positive vorticity advection in front of upper-trough results in the reducing of air pressure in the low-level and the development of mesoscale vortex, and the center of vortex is moving to absolute value of negative allobaric maximum gradient. Analysis on vertical structure of mesoscale vortex shows that, with the development of ramp airflow within vortex, convergence in the low-level is reinforced and vortex is a consistent vertical ascending movement inside. The maximum convergence center corresponds with positive vorticity center and both centers are located below 900 hPa. With strengthening of southeast flow in the low-level, the easterly winds inflow in the right front side of the vortex strengthens significantly, which not only provides favorable dynamic conditions but also sufficient water vapor for formation and continuance of heavy rain.Strong echo height of the precipitation is lower, and both hook echoes differed from the typical ones and mesoscale cyclones develop in the low-level causing severe precipitation and short-time gale. In addition, the change of both vapor density and liquid water content corresponds better to the precipitation, and the vapor density and liquid water content in the low-level increase rapidly in 15-30 min before severe precipitation start, which provides a reference for forecasting and early warning of severe precipitation.