Studying the distribution of convection during the warm season over Fujian Province has important significance for forecasting and warning of convective weather. Based on four weather radars data of Jianyan, Longyan, Changle and Xiamen in Fujian from 2008 to 2017, convection in warm season (from April to August) was identified, and the spatial-temporal and vertical structure of convection, as well as spatial and diurnal variation characteristics of convective systems with different areas and extension heights were analyzed. The results show that the peak period of convective activity is from May to June and August during the warm season in Fujian. The convection in warm season has obvious regional distribution characteristics, the high incidence area is located in the inland from April to June, and from July to August it is in the coastal mountains. From June to August, it is dominated by medium-deep convection, and it is dominated by large area convection in April, and from May to June and from July to August it is dominated by medium and large area convection, medium and small area convection, respectively. The monthly distribution of vertical structure of convection is different. The echo intensity of moderate convection is the maximum from July to August. The echo extension height in northwest Fujian is the highest. There is obvious diurnal variation of convective frequency. Convection often occurs from 15:00 to16:00 (BST), and it is the mainly single peak in the afternoon from July to August, while from April to May there are bimodal or multi-peak. The frequency peaks of convective systems with larger area appear later, and the mid-deep and deep convective peaks are obvious in the afternoon. From May to June, the high incidence of inland convection from night to morning was caused by the diurnal variation of the wind field formed by the inertia oscillation of the boundary layer and the interaction between the disturbing wind and terrain. In August, the high incidence of convection in coastal mountains was caused by the fact that the coastal mountains are located in the near ground heating center, the convergence of wind field and the high energy value area in the afternoon.

Based on the conventional observation data, automatic weather station data, wind-profiling radar and CINRAD-SA Doppler radar data as well as ERA-Interim 0.125°×0.125° 6 h reanalysis data, the extreme severe rain occurring in the west area of Fujian on 19 May 2015 was analyzed. The results show that the extreme severe rain process included two stages: warm area precipitation and frontal precipitation. The heavy rain area located in the left of south-west jet. Sufficient water vapor, the confluence of cold and warm air, the strengthening of unsteady convective stratification, decrease of lifting condensation and free convection height and moderate vertical wind shear over the rainstorm-hit area were all favorable to the development of MCS. During the warm area precipitation, southwest airflow was converged on the southwest side of rainfall area. The air with sufficient water vapor and high energy was lifted by the cold outflow air near the mesoscale boundary. New convective cells generated continuously. The convective cells moved to the east-northeast and formed a short rain belt. Several NE-SW short oriented rainbands established on the north side of the outflow boundary, moving eastward slowly and affecting the rainfall area repeatedly. Convergence line and wind speed convergence over the rainstorm area provided a good dynamic lifting condition for precipitation. The convective cells were strengthened by the valley topography opening to the southwest. The continuous back-building of convective cells and several NE-SW oriented rainbands were two major developing modes of MCS. During the frontal precipitation, with the shear of low-level vortex moving eastward and southward, new convective cells were born in the region of wind speed convergence, water vapor and high energy and moved southeast. Northwest wind of middle and high level showed forward tilting feature. The vertical wind shear increased. The confluence of cold and warm air caused rainfall strengthening and maintenance. Backward prorogation and eastward moving of convective cells forming train effect was the main developing mode of MCS.