The Influence of Land–Sea Contrast and Topography on the Diurnal Cycle of Precipitation in the Bay of Bengal

An offshore propagating diurnal signal of rainfall is observed from the eastern coast of India to the eastern Bay of Bengal from satellite data. A series of numerical model experiments reveal that this is closely related to the diurnal land-sea thermal contrast between India and the Bay of Bengal. The experiments show that the inertia-gravity waves driven by the land-sea thermal contrast play a significant role in the initiation and offshore propagation of mesoscale convective systems (MCSs) in the bay, and their phase speed well match the propagation speed of rainfall. The offshore propagating signal remains even after removing the topography in India or turning off latent heating from MCSs over the land of India. This implies that terrain and latent heating have secondary but not dominant effects on the offshore propagation of rainfall. The diurnal boundary layer heating depth is modulated by topography and diurnal MCS’s latent heating over land areas, and thus affect the amplitude, phase and speed of the inertia-gravity waves. Higher mountains or stronger latent heating in convection over land result in stronger and faster propagating gravity waves, leading to faster rainfall propagation.