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

Mesoscale convective systems (MCSs) are the primary contributors to rainfall over the Bay of Bengal (BoB) during the summer monsoon season. Previous studies have established a strong connection between monsoonal 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 experiment is conducted for this case. The ensemble forecast, initialized from data assimilation analyses, shows that most ensemble members accurately capture both the timing and location of MCS initiation. Analysis of the "successful" members reveals diurnal gravity waves play a significant role in preconditioning the offshore environment by increasing CAPE, reducing CIN, and enhancing lower-tropospheric moisture. Surprisingly, similar gravity waves and preconditioning processes 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 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 preconditioning, surface and boundary-layer processes are crucial for the practical predictability of offshore MCS initiation.