Abstract:By using conventional observation data, automatic meteorological station data and NCEP reanalysis data, as well as the data of CINRAD and meteorological satellite, the mesoscale weather system development models in the boundary layer of extreme rainstorm under three different weather types are summarized, based on the systematic analysis about the dynamic and thermal characteristics of the boundary layer mesoscale weather system in 39 cases of extreme rainstorms. The results show that the mesoscale weather systems in extreme rainstorm share the same regularity under the same weather system. The occurrence of extreme rainstorm is closely related to the strong development of mesoscale weather system in boundary layer. The special topography in the middle reaches of the Yangtze River plays an important role in the occurrence and development of mesoscale weather systems. By the trigger and organization of strong developing mesoscale weather system in boundary layer under the background of favorable circulation, the consolidation strengthening, stagnation, backward propagation of mesoscale convective system (MCS) and convective cell train effect are important reasons for extreme heavy rainfalls. To be specific, there are three paterns of surface mesoscale synoptic systems related to frontal cyclone type, which are all developed from the intersection of cold shear line, warm mesoscale convergence line and the gust front of MCS in the west of Dabie Mountains. The low vortex shear-type mesoscale weather system is mainly a local mesoscale vortex, newly born in boundary layer in Jianghan-Dongting Lake Plain in the eastern Wuling Mountains of Hunan and Hubei and the scale is about 150-300 km. Its formation is accompanied by the strong convergence of multiple flows in the boundary layer and the development of local baroclinicity. The strengthening of the southwest vortex and the formation of heavy precipitation in front of the second-order regional vortex in the west of Hunan and Hubei are the main inducements for the development of local mesoscale vortex in the boundary layer of the eastern plain. The horseshoe-shaped terrain in the middle reaches of the Yangtze River, which is conducive to the convergence of multiple air streams, is the key factor for the occurrence of local mesoscale vortex in the eastern plain. The formation and development of the extreme rainstorm mesoscale weather system in the warm sector of the weakly forced Meiyu front is mainly related to the enhanced convergence and maintenance of the thunderstorm cold pool countercurrent on the westside of the Dabie Mountains and the nighttime boundary layer ultra-low-level jet.