Diurnal Cycle of Precipitation Observed in the Western Coastal Area of Sumatra Island: Offshore Preconditioning by Gravity Waves

Abstract

This study analyzes data obtained by intensive observation during a pilot field campaign of the Years of the Maritime Continent Project (Pre-YMC) to investigate the diurnal cycle of precipitation in the western coastal area of Sumatra Island. The diurnal cycle during the campaign period (November–December 2015) is found to have a number of similarities with statistical behavior of the diurnal cycle as revealed by previous studies, such as afternoon precipitation over land, nighttime offshore migration of the precipitation zone, and dependency on Madden–Julian oscillation (MJO) phase. Composite analyses of radiosonde soundings from the Research Vessel (R/V) Mirai, deployed about 50 km off the coast, demonstrate that the lower free troposphere starts cooling in late afternoon (a couple of hours earlier than the cooling in the boundary layer), making the lower troposphere more unstable just before precipitation starts to increase. As the nighttime offshore precipitation tends to be more vigorous on days when the cooling in the lower free troposphere is larger, it is possible that the destabilization due to the cooling contributes to the offshore migration of the precipitation zone via enhancement of convective activity. Comparison of potential temperature and water vapor mixing ratio tendencies suggests that this cooling is substantially due to vertical advection by an ascent motion, which is possibly a component of shallow gravity waves. These results support the idea that gravity waves emanating from convective systems over land play a significant role in the offshore migration of the precipitation zone.

Diurnal Cycle of IMERG V06 Precipitation

“…At the same time, sparse subdaily ground observations limit our ability to understand the diurnal cycles in many regions around the world. The results in this study suggest that IMERG has the potential to advance our understanding in regions of sparse ground measurements and complement ongoing efforts to improve its representation in global models (Covey et al, 2016;Tapiador et al, 2017Tapiador et al, , 2019Xie et al, 2018).…”

Drastic thickening of the barrier layer off the western coast of Sumatra due to the Madden-Julian oscillation passage during the Pre-Years of the Maritime Continent campaign

“…Figure 3 shows the time-longitude cross section of the Tb and the surface westerly wind speed averaged for 5°S-5°N. The large-scale cloud system associated with westerly winds passed over the R/V Mirai after 11 December (Wu et al 2017;Yokoi et al 2017). This MJO was in phase 4 (the western part of the Maritime Continent) during 7-16 December and phase 5 (the western part of the Maritime Continent) during 17-22 December by the Bureau of Meteorology's MJO index (Wheeler and Hendon 2004).…”

“…1; Yokoi et al 2017;Wu et al 2017). The basic surface meteorological observations (e.g., surface pressure, temperature, humidity, surface wind, precipitation, and sea surface temperature) were obtained every minute.…”

Maritime continent coastlines controlling Earth’s climate

“…As a preliminary activity, pre-YMC campaign observations have been carried out in and off Bengkulu (Wu et al , 2018Yokoi et al 2017), which is located about 400 km south of Padang (Sumatera Barat area in Table 2). The Bengkulu area has not yet been observed well and is almost free from small islands distributed off Padang.…”

“…For the case shown in Fig. 9, detailed features including those associated with an MJO landing are also shown elsewhere (Wu et al , 2018Yokoi et al 2017). On land, these processes lead to rapid land/hydrosphere −atmosphere water exchange (Sulistyowati et al 2014) and local air pollutant washout.…”