Abstract:Using automatic station data, ERA5 reanalysis data, and radar detection data, this paper analyzes the synoptic situation, mesoscale convective system and propagation characteristics, as well as the combined effects of the parallel ridge-valley in East Sichuan Region and the Daba Mountain on a heavy rainfall event in 17-18 June 2021. The results show that the heavy rainfall occurred in the junction area of the Daba Mountain and the parallel ridge-valley region, and it went through three stages of evolution of mesoscale convective systems. Multiple northeast-southwest oriented strip-shaped convective systems were organized developing along the mountains of the parallel ridge-valley, with convective cell train effects and rainband stacking effects intensifying the precipitation. The cold front over the basin and the surface convergence line played an important role in triggering and organizing the mesoscale convective systems. Additionally, the cold front over the basin interacted with the eastward-moving cold air invading along the Wushan Mountain and the Yangtze River Valley, leading to the strengthening and maintenance of the convective systems. The role of the Daba Mountain in the heavy rainfall was reflected in the blocking effect and thermodynamic uplift. The windward slope of the terrain had a blocking and converging effect on the transport of water vapor and energy, with east-west oriented water vapor convergence belts and high pseudoequivalent potential temperature areas maintaining on the southern slope of the Daba Mountain. The boundary layer frontogenesis, positive vorticity areas, low-level convergence areas, and upward motion areas developed along the slope of the Daba Mountain. The role of the parallel ridge-valley terrain in precipitation was reflected in the aggregation of boundary layer water vapor, the quasi-stationary maintenance of the surface convergence line, and the formation of mesoscale occlusion fronts, which caused the mesoscale convective systems to move northward to the southern slope of the Daba Mountain. The mesoscale convective systems stagnated there and developed vigorously, thus triggering this heavy rainfall event in the mountainous area.