Mesoscale Processes Driving Offshore MCS Initiation in the South Asian Summer Monsoon: Insights from an Ensemble-Based Satellite Data Assimilation Experiment

Abstract Mesoscale convective systems (MCSs) are the primary rainfall contributors over the Bay of Bengal (BoB) during the South Asian summer monsoon. Previous studies have established a strong connection between MCS initiation over the BoB and diurnal gravity waves propagating from India. However, the precise role these waves play in triggering offshore MCSs remains unquantified. In this study, we analyze a typical MCS event, representative of the climatological spatiotemporal characteristics of MCS initiation in the region, to investigate the relative roles of diurnal gravity waves and other mesoscale processes in offshore MCS initiation. An ensemble-based satellite data assimilation (DA) experiment is conducted, assimilating all-sky infrared radiances from Meteosat-8 into the Weather Research and Forecasting (WRF) Model. The ensemble forecast, initialized from DA analyses, shows that many ensemble members accurately capture both the timing and location of MCS initiation. The analysis of the “successful” members reveals diurnal gravity waves play a significant role in enhancing lower-tropospheric moisture and destabilizing the offshore environment. Surprisingly, similar gravity waves and destabilization are also present in members that failed to capture MCS initiation. Further analysis indicates that land-breeze front from northern Sri Lanka is a key factor distinguishing successful from “unsuccessful” members, which, in successful members, is strong enough to lift air above the level of free convection (LFC) and lead to MCS initiation. Accurately simulating the land-breeze front depends on the correct representation of pre-MCS clouds and surface winds. This suggests that while diurnal gravity waves contribute to environmental destabilization, surface and boundary layer processes are crucial for the practical predictability of offshore MCS initiation. Significance Statement The majority of summer monsoon rainfall over the Bay of Bengal is attributed to large and long-lasting thunderstorms. Scientists hypothesized atmospheric waves from India play a role in the initiation of these thunderstorms, but the role has not been quantified yet. Therefore, we study a typical thunderstorm using satellite data and computer models. The results show that atmospheric waves from India create an environment that is favorable for thunderstorm formation. However, the favorable environment alone does not guarantee thunderstorm initiation. A key thunderstorm-triggering process is the land breeze from northern Sri Lanka. Accurately simulating the land breeze depends on correctly simulating prestorm clouds and near-surface winds. Assimilation of satellite observations can improve the forecast of thunderstorm initiation.