Abstract:Based on the high spatio temporal resolution data of automatic weather stations,Doppler weather radar and wind profiler radar, we analyzed the development of the mesoscale convective systems during the heavy rainfall in Tianjin on 6 July 2017, and discussed the structural characteristics of easterly flow from the Bohai Sea and its influence on the local heavy rainfall. The results are as follows: The local heavy rainfall was caused by two warm zone mesoscale convective systems and a vortex shear line system. Among them, the second mesoscale convective system in warm area dominated the formation of local heavy rainfall, and it was closely related to the easterly flow in the boundary layer. The easterly flow during this heavy rainfall had warm and wet characteristics, which was conductive to the occurrence and development of heavy rainstorms. At the same time, the wind speed from the Bohai Sea to the inland showed obviously disturbance, leading to the convergence of water vapor in the boundary layer and the development of the ascending movement. Furthermore, the mesoscale disturbance of easterly flow below 0.6 km played an important role in the triggering and maintaining mechanism of the heavy rainfall. Under the convergence system caused by the wind speed disturbance, the updraft flow was forced, triggering the occurrence of the meso γ scale convection. On the other hand, the outflow of cold pool caused by the heavy rainfall and the increasing easterly inflow interacted each other and formed the convergence, and the meso scale convective system was maintained stably, resulting in the strong precipitation echo above 40 dBz lasting nearly 3 h, with an average precipitation of 6.8 mm per 6 min. In addition, the change of rain intensity was closely related to the fluctuation of the easterly jet in the boundary layer. The establishment, development, weakening and disappearance of the jet corresponded to the four phases of steep increasing, peak, weakening and steep dropping of precipitation.