Convective Cold Pool Associated with Offshore Propagation of Convection System over the East Coast of Southern Sumatra, Indonesia

Abstract

The cold pool outflow has been previously shown to be generated by decaying Mesoscale Convective Complexes (MCCs) in the Maritime Continent. The cold pool also has a main role in the development processes of oceanic convective systems inducing heavy rainfall. This study investigated a cold pool event (January 1-2, 2021) related to a heavy rainfall system over the coastal region of Lampung, Southern Sumatra, within a high-resolution model simulation using a regional numerical weather prediction of the Weather Research and Forecasting (WRF) with convection permitting of 1 km spatial resolution, which was validated by satellite and radar data observations. It is important to note that the intensity, duration, timing, and structure of heavy rainfall simulated were in good agreement with satellite-observed rainfall. The results also showed that a cold pool (CP) plays an important role in inducing Mesoscale Convective Complex (MCC) and was responsible for the development of an offshore propagation of land-based convective systems due to the late afternoon rainfall over inland. This study also suggests that the propagation speed of the CP 8.8 m·s−1 occurring over the seaside of the coastal region, the so-called CP-coastal, is a plausible mechanism for the speed of the offshore-propagating convection, which is dependent on both the background prevailing wind and outflow. These conditions help to maintain the near-surface low temperatures and inhibit cold pool dissipation, which has implications for the development of consecutive convection.

Improvement of Heavy Rainfall Simulated with SST Adjustment Associated with Mesoscale Convective Complexes Related to Severe Flash Flood in Luwu, Sulawesi, Indonesia

“…This precise rapid rainfall onset helped improve the forcing input for the real-time precipitation-runoff forecasting models over the catchment area [25][26][27]. Interestingly, the distinction time of maximum intensity between the SST scenario and GSMaP satellite is also two hours, consistent with the previous study conducted under the same model configurations [18].…”

“…The back-building mechanism was identified as quasi-stationary, southeast-northwest double convective cells, which were initiated on 13 July from 22:00 LST and became mature the following day at 07:00 LST, further dissipating until 10:00 LST (see Figures S5-S7). The back-building mechanism was previously found as a common characteristic of MCSs in the midlatitudes [30][31][32][33], related to extreme rainfall production [34] that can be regulated by large-scale ascent and maintained by the cold pools from the outflow [18,34]. However, Bone Bay also played a role in providing warm, moist airflow near the surface, ultimately producing a warm front where the easterly airflow was met with cold, dry air (Figure 10).…”

“…We used Betts-Miller-Janjić as a cumulus scheme in the first and second domains, and a no-cumulus scheme for the third domain. These parameterization schemes, following Fonseca et al [17], best predict the diurnal rainfall over the Maritime Continent and were proven by Yulihastin et al [18] to be able to capture thunderstorm events in terms of timing, maximum intensity, duration, and location of precipitation. The initial and lateral boundary conditions are obtained from the global National Centers for Environmental Predictions (NCEP) Final Analysis (FNL) Operational Global Analysis data [19] and forecast grids, which have a horizontal resolution of 0.25 • × 0.25 • and a temporal resolution of 6 h. In this case study, we simulated selected episodes of heavy rainfall events from 13 to 14 July 2020 associated with a flash flood in the Luwu region, Sulawesi, Indonesia.…”

“…The cold pool further developed, deepened, and extended over Luwu, helping to maintain the precipitation systems over both the eastern and western parts of Luwu. The cold pool associated with the MCCs and the convection system propagating landward [29] as well as offshore [18] may also have triggered the heavy rainfall over the coastal region of Sumatra, Indonesia. In this case, when a cold pool developed in a large area over the Luwu region (Figure 8b) at 22:00-00:00 LST, deep moist convection (>7 km) evolved over the ocean (B; Eq, 126 • E) under a back-building mechanism.…”

Atmosphere Condition Analysis on Hail Event (Case Study : Pelalawan on September 23rd, 2019)

“…Channel IR1 (Band 13) data, was used to display cloud top temperature time-series graph every 10 minutes [7]. IR1 data is also visualised using GrADS application to show the cloud top temperature images and hovmöller [20] in order to evidence hail cloud stage. The significant decrease of cloud top temperature indicates the development stage while its increase indicates the mature stage until the dissipation stage in the cumulonimbus cloud [5,6].…”