Why a Large Amount of Rain Falls over the Sea in the Vicinity of Western Sumatra Island during Nighttime

Precipitation measurements from the Tropical Rainfall Measuring Mission satellite indicate that annual rainfall over the sea in the vicinity of western Sumatra Island is among the highest on the earth, and most of this rainfall occurs during nighttime. Surface meteorological observations at Tabing on the western coast of the island show frequent occurrences of sudden offshore winds accompanied by an abrupt drop in surface temperatures in the late afternoon and evening. Model simulations for a 1-month period during the rainy season of the region successfully simulate the satellite-observed regional distribution and diurnal variation of rainfall. The simulation results show that convection develops across a wide area over the mountainous areas of the island at similar times in the afternoon with the development of thermally induced local circulations. At these times of the day, convection over the sea along the western coast of the island is suppressed by the thermally and topographically induced diurnal changes in the boundary layer flow. When convection over the mountains of the island dissipates in the late afternoon and evening, a zone of cold surface outflow along the western coast results from the mountain convection breaking out to the sea. Meanwhile, the convective inhibition offshore is reduced in the evening, and the offshore flow causes regular occurrences of convection over the sea near the coast. The triggered convective systems propagate offshore and westward in multicell cluster storms during nighttime, bringing heavy rainfall over the sea off the western coast of the island. Sensitivity experiments with a flat-topography model demonstrate that the frequent occurrence of offshore flow in the late afternoon and evening on the western coast is caused by the mountainous topography of the island and its induced afternoon convection. The mountains on the island and the resultant thermally and convectively induced local circulations can play an important role in the formation of nocturnal abundant rainfall over the sea west of Sumatra Island.