Characteristics and Mechanisms of Heavy Precipitation in Spring of Eastern Himalayas

Abstract In spring, eastern Himalayas (EH) receives the largest amount of rainfall in South Asia. However, the characteristics and formation mechanisms of spring precipitation, especially heavy precipitation (HP), in this region remain poorly understood. Based on 10-yr IMERG precipitation and the clustering approach of the self-organizing map, two types of HP are revealed with the rainfall center located at the northern coast of the Bay of Bengal (BOB) and the Gangetic Plain near the foot of EH, respectively. Despite low occurrence frequency, HP contributes importantly to the total rainfall, especially after the onset of South Asian summer monsoon. The coastal HP and inland HP are of opposite diurnal cycles, which peak in the early afternoon and midnight, respectively. Composite analyses are conducted for the synoptic circulation patterns using ERA5 reanalysis. The occurrence of HP is promoted by the moisture transport and uplifting of southwesterly boundary layer jet (BLJ) associated with an anomalous lower-tropospheric cyclonic circulation over BOB, which is dynamically induced by the excessive surface sensible heating (SH) over the Indian subcontinent. When the surface SH is enhanced in northwestern-central India, BLJ terminates near the northern coast of BOB, resulting in coastal-type HP. By contrast, when the enhanced surface SH mainly occurs in northwestern India, BLJ penetrates further northward to the foot of EH and thus produces inland HP. In the latter case, the interaction between BLJ and EH topography helps increase the water vapor convergence and dynamical lifting, leading to heavier precipitation than the former.