The Impact of Diurnal Gravity Waves on MCS over the Bay of Bengal: Observations, Linear Theory, and Data Assimilation Experiments

Previous observational studies have indicated that mesoscale convective systems (MCSs) contribute the majority of precipitation over the Bay of Bengal (BoB) during the summer monsoon season, but the physical mechanisms responsible for their initiation and propagation remain incompletely understood. To fill this knowledge gap, we conducted a comprehensive study using a combination of satellite observations, reanalysis data, linear theory, and ensemble-based data assimilation. Satellite observations reveal clear diurnal propagation signals of MCS initiation frequency and precipitation from the west coast of the BoB towards the central BoB. Reanalysis data indicate a strong association between the offshore-propagating MCS initiation frequency and rainfall with the diurnal perturbations of low-level winds. This implies the potential role of diurnal gravity waves emitted from the coastlines.

A linear model is then developed to study the control of the large-scale environment on the characteristics of the diurnal gravity waves. By using a numerical method to solve the atmospheric governing equations, the linear model can be driven by the realistic atmospheric profiles from reanalysis data. It is shown that the low-level diurnal wind perturbations, as well as the offshore propagation signals of MCS initiation frequency and precipitation, are associated with diurnal gravity waves caused by the diurnal land-sea thermal contrast. In addition, the propagation, amplitude, and structure of the diurnal gravity waves can be strongly modulated by the background monsoonal wind speed and vertical wind shear. These changes in the gravity wave characteristics can further influence the initiation and precipitation of MCSs over the offshore region. To better understand the physical processes associated with the gravity wave-triggered offshore MCS, we further perform ensemble-based data assimilation experiments on a typical MCS case. By assimilating all-sky satellite radiances and other observations, the ensemble can realistically capture the environment and initiation of the offshore MCS. Linear ensemble sensitivity analysis and nonlinear sensitivity experiments will also be conducted to investigate how the perturbations of the dynamical and thermodynamic fields induced by the diurnal gravity waves can affect the initiation and evolution of the MCS.