The Analysis On The Maintenance Conditions And Mechanism Of Vortex Evolution During A Rainstorm Over North China

Using NCEP/NCAR GFS?Global Forecast System?data,satellite data,radar data and meteorological observation data,etc.,with the WRF model,diagnostic analysis and simulation studies have been conducted to a severe rainstorm in North China around July 19,2016 to explore the maintenance mechanism of rainstorm and the evolution of mesoscale system.The precipitation process occurred in the situation that was conducive to the maintenance of heavy rains in North China.The deep vortex system and the low-level jet were the important impact systems of the heavy rain.The precipitation process on July 19 can be roughly divided into two stages:the early stage is the weaker concentration of precipitation in the zone along the Taihang Mountains;the later stage is the mixed precipitation in the spiral rainband directly affected by the mesoscale vortex.This paper focuses on the latter stage with higher precipitation efficiency.The environmental factors are analyzed by using simulation results with higher precision.The results show that the region always maintains the dynamic conditions of high-level divergence and low-level convergence.The energy front region formed by the vortex and the southwest jet continuously affects the rainstorm area,so that the region always has a very high false equivalent temperature and basically saturated water vapor,with the very strong water vapor transport efficiency.On the front side of the warm conveyor belt,the unstable level formed by the exchange of cold and warm air facilitates the generation of convection and the maintenance of precipitation.The topography makes the spatial distribution of precipitation more concentrated,and the accumulated rainfall in the extreme center increases,but does not affect the overall precipitation.The observation of the low vortex shows that the center vorticity increases first to 90×10^-5/s and then slowly decreases in the process of moving to the northeast.In the vertical direction,it is an upright positive vorticity column that can extend to the top of the troposphere.The intensity change mainly occurs below 500hPa,and the vorticity center is maintained near850hPa.Its growth and maintenance are mainly affected by stretching the vertical transport of vorticity,where the larger stretching term reflects the good configuration of the center of the low vortex and the center of the convergence,and the vertical transport term corresponds to the dry intrusion caused by the abnormal growth of the high-altitude trough.Further analysis indicates that the convergence center of the low layer which facilitating the maintenance and development of low vortex is a superposition of three scale systems,and the mesoscale system has the greatest influence on the strength and position of the convergence center.On the other hand,the convergence between the large-scale and large-scale wind also makes the convergence range larger and stronger.The positive feedback adjustment is formed between the low-level vortex growth and the wind field convergence enhancement,which is of great significance for the maintenance of the low vortex system and the precipitation process.