Comparative analysis of two rainstorm processes under the influence of topography at the southern foot of Daba Mountain

Two heavy rainfall processes (referred to as "9.27" process and "6.18" process respectively) at the southern foot of Daba Mountain in northeast Chongqing on September 27, 2017 and June 18, 2018 were selected to compare the similarities and differences of the two processes in circulation background, mesoscale environmental conditions, topographic action mechanism and the evolution of mesoscale convective systems by using multi-source observation data. The results were as follows: (1) "9.27" was frontal rainstorm, and "6.18" was similar to a warm rainstorm. Both were affected by the eastward movement of the 500hPa trough, low-level shear, low-level jet and surface convergence line, etc. The difference in location and intensity of the surface convergence line leads to the difference in the fall area and rain shapes of the two rainstorms. (2) Both processes began with weak convective instability and weak uplift conditions. The convective instability improved significantly in the afternoon of the "6.18" process. Both processes established water vapor transport channels in the South China Sea. The "9.27" process had stronger water vapor transport and convergence intensity. (3) The mechanisms of topographic effects during the two rainstorms are different from each other. During the "9.27" process, the topographic uplift of Guanmian Mountain intensified the thermal contrast near the front, enhanced baroclinic instability. Simultaneously, it blocked and lifted the warm and wet air. Under the triggering of the surface convergence line, four convective cells developed and continuously passed through the heavy rain area in the north of Kaizhou, forming the "train effect". During the "6.18" process, the dynamic uplift and thermal forcing of the Yangtze River valley and the mountains on both sides caused a large wind velocity area near the surface of the river valley, enhanced surface convergence and triggered several convective cells to move eastward along the Yangtze River Valley. Subsequently, under the action of the topographic effects of Longwangshan Mountain in the east of Fengjie, the development of convective cells was enhanced and the movement was stable, resulting in sustained heavy rainfall along and north of the Yangtze River.