A Study of Structure of Mesoscale Systems Producing a Heavy Rainfall Event in North China

By using routine observation, FY2C TBB and NCEP reanalysis data, a diagnostic analysis and simulation study have been performed to study the mesoscale systems producing the heavy rainfall event in North China during 22－24 July 2005. The results show that two mesoscale convective systems which developed within the inverted typhoon trough directly resulted in the heavy rainfall event. The mesoscale vortexes were firstly formed in the middle troposphere when the mesoscale convective system matured, and then developed to the lower troposphere. Water vapor convergent centers are mainly concentrated near the surface boundary layer. Easterly inflow pushed water vapor to the rainfall area and upward branches of the secondary circulation transported the water vapor to the middle and upper troposphere. Although the water vapor convergence in the surface boundary layer is very important, the rainfall amount depends on the vertically integrated moisture flux convergence. Dry and cold air apparently intruded into the rainfall area from the high troposphere in the development of heavy rainfall and it was located between 600 and 300 hPa. The dry intrusion in upper levels overruns that in low levels. The dry intrusion enhanced moist instability and promoted the development of convection and the formation of mesoscale vortexes, and then the enhancement of precipitation. In the development of the heavy rainfall, the boundary layer jet was induced due to the local heating of terrain in North China in the night. Precipitation and dry intrusion further enhanced the boundary layer jet and strong wind in upper levels emerged accordingly. Jets and heavy rainfalls have positive feedback through the secondary circulation. The coupling among the boundary layer jet, upper and lower tropospheric jet streaks played an important role in this heavy rainfall event.