Environmental Conditions for Nighttime Offshore Migration of Precipitation Area as Revealed by In Situ Observation off Sumatra Island

Abstract

The diurnal cycle over tropical coastal waters is characterized by offshore migration of precipitation area during nighttime. This study analyzes in situ observational data collected during the YMC-Sumatra 2017 field campaign around the western coast of Sumatra Island, Indonesia, to examine the offshore migration phenomenon during 5–31 December 2017, when the Research Vessel Mirai was deployed about 90 km off the coast to perform observation. The offshore migration is observed in only less than a half of the 27 days. A comparison of radiosonde data at the vessel between days with and without the offshore migration reveals that vertical wind shear in the lower troposphere is a key environmental condition. In late afternoon of the days with the offshore migration, offshore (northeasterly) wind shear with height with considerable magnitude is observed, which is due to weaker daily mean southwesterly wind in the lower free troposphere, stronger southwesterly wind in the boundary layer, and sea breeze. As this condition is considered favorable for regeneration of convective cells to the offshore side of old ones, these results support an idea that the regeneration process is critical for the offshore migration. The Madden–Julian oscillation and cold surges play some roles in the weakening of the free-tropospheric wind. The migration speed is estimated at 2–3 m s−1, which is lower than that observed in another field campaign conducted in 2015 (Pre-YMC 2015). This difference is partly due to the difference in the environmental wind in the lower to midtroposphere.

Impact of the Boreal Summer Intraseasonal Oscillation on the Diurnal Cycle of Precipitation near and over the Island of Luzon

“…Precipitation then redeveloped offshore at 0300 LT and persisted through the early morning. This nighttime/early morning precipitation maximum over the ocean has been widely observed and studied in the tropics (Kraus 1963;Ruppert et al 2013;Yang and Smith 2006), and is thought to be more pronounced during convectively active periods (Sakaeda et al 2018). During active BSISO phases, the diurnal cycle over land was once again very clear.…”

“…These gravity waves lead to destabilization over the adjacent waters thereby supporting convection. This was also the mechanism offered by Yokoi et al (2017) to explain the offshore movement of convection observed near the island of Sumatra during the Years of the Maritime Continent (YMC) pilot field campaign. In this field project, a cooling (and subsequent destabilization) of potential temperature was noted in the lower free troposphere on days when offshore precipitation was greatest.…”

“…Very tall afternoon storms were more frequent during inactive phases than active phases over the mountains (57% more MDE heights over 10 km from 1200-2000 LT, 1208-1228E) corresponding with the greater instability shown previously. The out-of-phase relationship between afternoon land-based and overnight ocean-based precipitation (Kraus 1963;Ruppert et al 2013;Yang and Smith 2006) was clearly noted during inactive phases. During active phases, westward offshore movement of the afternoon convection was once again apparent, with a strong precipitation signal moving from 1218E at 1400 LT to 1188E by 0000 LT, as well as a general persistence of precipitation over the ocean around the clock.…”

“…This lower tropospheric cooling was attributed to anomalous upward motion caused by gravity waves, which themselves originated from earlier convection over the land. However, in the subsequent YMC field campaign in 2017, Yokoi et al (2019) noted that although the same low-level cooling was observed in the operations area, it was not always associated with offshore convection, leading to a suggestion that gravity wave propagation may not be a sufficient condition for offshore rainfall. The authors further hypothesized that vertical wind shear was required to allow afternoon land-based convection to migrate offshore.…”

Diurnal Cycle of Precipitation Over the Maritime Continent Under Modulation of MJO: Perspectives From Cloud‐Permitting Scale Simulations

“…(2) same functions of GWs but excited by the evaporative cooling of land precipitation (e.g., Hassim et al, 2016;Love et al, 2011;Vincent & Lane, 2016;Yokoi et al, 2017); (3) gravity (or density) currents, such as land breezes and cold outflows, which trigger new convection via convergence with ambient winds or cold outflows from other islands (e.g., Fujita et al, 2010;Houze et al, 1981;Mori et al, 2004;Wapler & Lane, 2012;Wu et al, 2009); and (4) influences of ambient winds, such as the advection effect (e.g., Ichikawa & Yasunari, 2006Yanase et al, 2017) and the vertical wind shear mechanism (e.g., LeMone et al, 1998;Yokoi et al, 2019). With these proposed mechanisms, we still do not know which one is the dominant process and how they vary with different islands in the MC.…”