Monsoonal MCS Initiation and Gravity Wave Propagation over the Bay of Bengal: Observation and a Linear Theory

Previous observational studies have indicated that mesoscale convective systems (MCSs) are responsible for the majority of summer monsoonal precipitation over the Bay of Bengal (BoB), yet their initiation processes remain poorly understood. To fill this knowledge gap, we conducted a comprehensive study using a combination of 20-yr satellite observations, MCS tracking, reanalysis data, and a theoretical linear model. By applying these methods, we aimed to uncover the physical processes that trigger the initiation of monsoonal MCSs over the BoB. Satellite observations and MCS tracking reveal that the majority of MCSs responsible for monsoonal precipitation over the BoB are initiated from the coastal or open ocean regions, rather than inland areas. Diurnal MCSs frequently initiated near the coastlines are due to land-sea breezes, while early morning MCSs initiated over the open ocean are strongly influenced by diurnal radiative forcings. Our findings also highlight clear propagating signals of diurnal MCS initiation from the west and north coastlines of the BoB towards the central BoB region. Reanalysis data indicates a strong association between the propagating signals and the offshore propagating diurnal wind and temperature perturbations, implying the critical role of diurnal gravity waves in this process. To better understand these mechanisms, we developed a theoretical linear model that can realistically reproduce the primary characteristics of these offshore propagating diurnal perturbations. The results suggest that these perturbations, as well as the propagating signals of diurnal MCS initiation, are generated by diurnal gravity waves emitted from coastal regions, which are in turn caused by diurnal variations in the land-sea sensible-heat difference and latent heating of coastal convective systems. Using both the linear model and observations, we also investigate the effects of subseasonal variations in the prevailing monsoonal wind speed, vertical wind shear, and coastal convective heating profile on the intensity, structure, and propagation characteristics of the diurnal gravity waves.