Mesoscale convective systems (MCSs) contribute the majority of precipitation over the Bay of Bengal (BoB) during the South Asian Summer Monsoon Season. However, their initiation and propagation are not fully understood, and numerical models struggle to accurately capture these processes. To address this knowledge gap, we conducted a comprehensive study using satellite observations, linear theory, and ensemble-based satellite data assimilation.

Satellite observations reveal clear diurnal propagation signals of MCS initiation frequency and rainfall from the west coast of the BoB toward the central BoB. By applying linear theory, we demonstrate that these propagation signals are linked to diurnal gravity waves generated by the thermal contrast between land and sea. The ambient wind speed and vertical wind shear significantly influence the timing, propagation, and amplitude of these waves, which in turn reduce environmental convective inhibition (CIN), enhance lower tropospheric moisture, and create favorable conditions for MCS initiation offshore.

This raises a critical question: given that diurnal gravity waves have low frequency and long wavelength, which should be realistically captured by numerical models, why do both coarse-resolution global models and high-resolution regional models struggle to accurately capture offshore MCS initiation during the monsoon season? Using advanced satellite all-sky radiances data assimilation and ensemble forecasting, we further show that while diurnal gravity waves play a significant role in preconditioning the environment for MCS initiation, boundary-layer processes are crucial in determining the initiation of offshore MCSs. The practical predictability of MCS initiation hinges on a model's ability to accurately capture convective activity prior to MCS initiation and the associated boundary-layer processes. Satellite all-sky radiances data assimilation can significantly improve the prediction of offshore MCS initiation during the monsoon season.