Roles of terrain, surface roughness, and cold pool outflows in an extreme rainfall event over the monsoon coast

An extreme rainfall event with maximum 12-h accumulated rainfall of 464.8 mm associated with a quasi-stationary mesoscale convective system occurred over the coastal region of South China (a monsoon coastal area) in June 2017. An observational analysis shows that early convective storms were initiated near the mountains and moved northeastwards. A convergence line between cold northerly winds and warm southerly winds in the lower levels was formed, which favored the development of the quasi-stationary system. Cold northerly winds were associated with land breeze, downslope winds as well as previous rainfall during earlier period, and cold pool outflows during later period. High rain rate was mainly contributed by abundant small- and medium-sized raindrops. Cloud-resolving simulations were performed to examine the roles of terrain, land-sea surface roughness contrast, and cold pool outflows in the formation of heavy rainfall. Results demonstrate that orographic lifting and lee side convergence near the coastal hills helped initiate convection. Land-sea surface roughness contrast and mountains in the middle of Yangjiang and Jiangmen affected the formation of convergence line during earlier period, and cold pool outflows sustained it during later period. Mountains in the middle of Yangjiang hindered the movement of the convective system, without which the associated heavy rainfall would shift farther north. Mt. Tianlu with westward concave morphology was vital for the formation of local convergence and the concentration of heavy rainfall. This study suggests the importance of representing processes associated with complex underlying surface in models for the prediction of heavy rainfall in coastal areas.