A Mechanism for the Southward Propagation of Mesoscale Convective Systems Over the Bay of Bengal

Equatorward propagating precipitation episodes over the Bay of Bengal have been documented in many previous observational studies. Proposed propagation mechanisms include mean surface to midtropospheric wind shear driving the convection orthogonal to the lower tropospheric winds and the gravity currents generated by outflow from convection initiated by the diurnally varying land-ocean circulations dispersing south. In this study, we perform high-resolution simulations using the Weather Research and Forecast model capable of resolving mesoscale convective systems during the South Asian summer monsoon season. This mesoscale system is shown to have squall line-like structure with leading line/trailing stratiform. The rear inflow due to saturated downdraft and jump updraft indicates a gravity current-like structure. The rear inflow jet produces horizontal momentum tendencies in the direction of propagation. The center of convection is shown to move faster than the midtropospheric winds and at the same speed as that of the rear inflow jet near the surface. These systems are also shown to be tightly coupled to the diurnal land surface heating cycle. We perform additional model simulations with varying horizontal resolution and with the inclusion of cumulus parameterization. A model with cumulus parameterization is unable to simulate the updraft-downdraft pair and the gravity current structure of this southward propagating mesoscale system. We find that high model resolution is needed to resolve the updraft-downdraft pair and cumulus parameterization assumptions break down at such high resolutions. Using cloud microphysics exclusively becomes essential in simulating these mesoscale systems.