Meso-scale Convective Characteristics of "7·22" Extreme Rain in the West Mountainous Area of Fujian

An extreme severe rain occurred in the west mountainous area of Fujian on 22 July 2015, with the precipitation of 254.9 mm for 6 hours and the maximum total precipitation of 295.5 mm. Using conventional observations, automatic weather station data, satellite data, wind-profiling radar and CINRAD-SA Doppler radar data, this extreme severe precipitation is analyzed focusing on the environmental conditions and structure characteristics of the meso-scale convective system (MCS). Results show that predominate influencing systems are the low-level shear line and the upper trough between subtropical high and the anticyclone over Yunnan and Guangxi. The reinforcement of unsteady convective stratification, decrease in the level of lifting condensation and free convection, high atmospheric precipitable water and weak vertical wind shear over the rainstorm-hit area are all favorable to the development of MCS. The initial convective cloud develops on the edge of Guangdong and Fujian. Convective cells develop strongly with the favorable meso-scale environmental conditions. At the stage of northeast development, convective cells are born in the north of MCS and move southeast. Many northwest and southeast short echo bands come into being one after another. MCS moves northeastward under actions of cell advection and storm propagation. At the stage of quasi-stationary, the strong temperature gradient area locates in the middle of MCS. The north and south cloud clusters weaken rapidly. The structure of MCS changes from training line and adjoining stratiform MCS to back-building and quasi-stationary MCS in the stage of development. Back-building and quasi-stationary MCS results in the extreme severe rain. The northwest airflow on high level increases and extends to low level. The inflow in front of MCS on low level is strengthened and extends to high level. The cold air intrusion at high level and reinforcement of southwest jet with wind velocity convergence at low level over the rainstorm-hit area lead to the development of MCS. The effect of the trumpet-shaped topography strengthens the southwest airflow on the boundary layer. A small cyclonic eddy generates in the north of Liancheng and is closely related to the southwest airflow within the boundary layer blocked by mountain. When the northwest convective cell moves in, the convective cell strongly develops. The northeast-southwest back-propagating MCS rarely moves because directions of cell advection and storm propagation are in confrontation. The back-propagating MCS causes obvious train effect, which brings about extreme severe rain.