The Influence of Topography on the Diurnal Rainfall Propagation in the Bay of Bengal

Abstract A significant diurnal offshore propagation of rainfall is observed extending from the eastern coast of India to the central Bay of Bengal. This study focuses on understanding the influence of topography over the Indian subcontinent on this rainfall propagation through a series of semi-idealized mesoscale numerical simulations. These simulations with varying topography highlight the crucial role of inertia–gravity waves, driven by diurnal mountain–land–sea thermal contrast between India and the Bay of Bengal, in initiating and promoting the offshore propagation of convective systems in the Bay. These waves’ phase speed of around 14.8 m s −1 aligns well with the speed of diurnal rainfall propagation. Even after eliminating the impact of Indian topography, the offshore propagating signal persists, suggesting a secondary rather than dominant effect of terrain on offshore rainfall propagation. Furthermore, the topography affects the depth of diurnal heating within the land’s boundary layer, which thus influences the amplitude, phase, and speed of the inertia–gravity waves. Specifically, the presence of higher mountains along the coastal area drives faster waves by increasing heating depth, resulting in faster rainfall propagation. Significance Statement This study advances our comprehension of the fundamental driver behind diurnal offshore rainfall propagation and the manner in which coastal terrain influences the rainfall pattern. We demonstrate that the diurnal offshore propagation of rainfall is closely related to inertia–gravity waves generated by thermal contrasts, and we successfully distinguish these waves in our study. Furthermore, our findings indicate that elevated coastal topography contributes to a greater heating depth near the coastline, which plays a crucial role in driving faster gravity waves and, consequently, leading to faster rainfall propagation. These outcomes provide a deeper insight into the mechanism governing offshore rainfall propagation and underscore the impact of real-world topography.