Revealing the dominant synoptic effects leading to spring precipitation over the foot and slope of the southeastern Tibetan Plateau

Spring precipitation over South Asia during 2001 to 2010 was concentrated in the foot and slope of the southeastern Tibetan Plateau with two significant precipitation centers. Rainy day accounted for 99.3% and the average daily precipitation can exceed 11 mm, which was distinct from its surrounding areas. Given the spatial variability of spring precipitation, self-organizing map method (SOM) was firstly applied to cluster the precipitation types, and then the reanalysis product from ERA5 was used to explore the synoptic characteristics of different precipitation types. The results show that the westerly wind-dominated precipitation type occurred 92.2% of the frequency, but the cumulative precipitation only contributed to 73% and the maximum average precipitation was lower than 9.5 mm/day. While other precipitation types were classified to heavy rainfall, which were characterized by the development of a low-level trough, abundant water vapor from the Bay of Bengal, and upward movement. However, the difference of synoptic conditions caused the spatial variability of heavy rainfall: the dynamic-dominated precipitation was distributed at the junction of the Himalayas and the Hengduan Mountains, and the deep boundary layer jet to the north caused strong local convergence and uplift motion, which induced strong precipitation reaching 111.32 mm/day. The moisture-dominated precipitation was close to the northern coastline of the Bay of Bengal far away from the topography, and was feathered by abundant water vapor provided by strong southerly, leading to widespread precipitation up to 79.06 mm/day. We further investigate the reasons for the different circulation conditions. The surface sensible heat provided the necessary conditions for the development of a large-scale low-pressure trough, and apparent heat source generated by heavy precipitation further enhanced the local positive vorticity. This work reveals the dominant synoptic effects leading to spring precipitation over the foot and slope of the southeastern Tibetan Plateau, and deepens the understanding of hydrological cycle in South Asia with the joint influence of complex terrains.