Diurnal Cycle of Precipitation Around Western Coast of Sumatra Island Observed during Pre-YMC and YMC-Sumatra2017 Field Campaigns

Diurnal cycle of precipitation over tropical coastal waters is generally characterized as offshore migration of convective systems during nighttime. Mechanisms responsible for this migration feature are elusive, partly due to scarcity of observational data of thermodynamic profile of the offshore atmosphere. With one of the aims being to obtain such observations, we conducted the Pre-YMC (Years of the Maritime Continent) field campaign in the western coastal area of Sumatra Island, Indonesia, in November and December of 2015. As a part of the campaign, we performed 3-hourly radiosonde observation and continuous weather radar observation at the research vessel (R/V) Mirai deployed about 50 km off the coast, and successfully observed typical diurnal cycle. Analyses of the observations reveal that the lower free troposphere starts cooling in late afternoon, a couple of hours earlier than the boundary layer does. This cooling seems to contribute to the offshore precipitation via destabilization of the lower atmosphere. This cooling is mainly caused by vertical ascent, which is possibly a component of shallow gravity waves. These results support the idea that gravity waves that are excited by convection over land and have ascending wave front in the lower troposphere play a significant role in the offshore migration. Furthermore, in November 2017 - January 2018, we conducted a field campaign named YMC-Sumatra2017 and deployed the R/V Mirai near the western coast, similar to the Pre-YMC. Preliminary results of the YMC-Sumatra2017 campaign regarding the diurnal cycle will also be presented.