Mesoscale convective systems (MCSs) are the primary contributors to precipitation over the Bay of Bengal (BoB) during the South Asian Summer Monsoon. However, the mechanisms underlying their initiation and propagation remain poorly understood, and numerical models often struggle to accurately represent these processes. To address this gap, we conducted a comprehensive study integrating satellite observations, linear theory, and ensemble-based satellite data assimilation.

Satellite observations reveal distinct diurnal propagation patterns in MCS initiation frequency and rainfall, moving from the western coast of the BoB towards its central region. Through the application of linear theory, we demonstrate that these propagation patterns are driven by diurnal gravity waves, which are generated by the thermal contrast between land and sea. The timing, propagation, and amplitude of these waves are strongly influenced by ambient wind speed and vertical wind shear, which subsequently reduce environmental convective inhibition (CIN), enhance lower tropospheric moisture, and create favorable conditions for offshore MCS initiation.

This raises a crucial question: given that diurnal gravity waves have low frequency and long wavelength—characteristics that numerical models should be able to capture—why do models struggle to accurately simulate offshore MCS initiation during the monsoon season? To explore this, we employed satellite all-sky radiance data assimilation and ensemble forecasting. Our findings indicate that while diurnal gravity waves are significant in preconditioning the environment for MCS initiation, boundary-layer processes play a critical role in determining the actual initiation of offshore MCSs. Therefore, the practical predictability of MCS initiation depends heavily on a model's ability to accurately represent pre-MCS convective activity and the associated boundary-layer dynamics. The assimilation of satellite all-sky radiances can substantially enhance the prediction of offshore MCS initiation by improving the initial moisture and cloud fields.