The nonlinear MCS generated in a surface vortex circulation near Meiyu front.A strong water vapor passage which surrounds with subtropical high ridge comes from the South China Sea. It lands at the Beibu Gulf, goes through the east of Yunnan-Guizhou Plateau, and arrives at the lower reaches of Yangtze River. At the same time, there is a strong water vapor convergence belt exits at the north of water vapor passage, it is the concrete form of warm wet flow of East Asian monsoon, and the level of 850hPa which represent lower troposphere is decisive. The strong water vapor convergence belt is the product of Meiyu dynamic mechanism, where one of the centers locates over Xianning. The cold air invasion and the front uplift are the main triggers of convection. The unstable warm wet flow in the south of the shear line concentrates in the middle reaches of the Yangtze River, the convergence of local ground airflow, and the wind shear in the boundary layer, all of them are good conditions for the development and maintain of non-linear MCS at Xianning.
The favorable conditions of MCS evolution and formation mechanism during 13:00-18:00 on 14th have been shown by a variety of observational encryption data, such as the development of small-scale ground vortex, the evolution of vertical wind shear in boundary layer, local change of water vapor, and so on. Based on the boundary layer wind profile data, it is found that the disturbance of boundary layer is advantageous to the heavy rain. Besides, some short-term disturbances can also generate a non-linear convection system without the impact of large-scale system.In general, the high spatial-temporal resolution observation data can be well applied in analyzing the MCS evolution process.