Offshore propagation of coastal precipitation

This work focuses on the seaward propagation of coastal precipitation with or without mountainous terrain nearby. Offshore of India, diurnal propagation of precipitation is observed in the Bay of Bengal. On the east side of the Bay a diurnal but non-propagating signal is observed near the west coast of Burma. This asymmetry suggests that the propagating mechanism is inertio-gravity waves. Perturbations generated by the diurnal heating over the coastal mountains of India propagate offshore, amplify in the upwind direction and dissipate in the downwind direction owing to critical level consideration.

A linear model is applied to study these gravity waves. Analytical analysis is performed for various heating depths, mountain widths, friction, Coriolis effect, background mean wind, and stability. We assess how these affect the amplitude, dissipation, initiation phase, and propagation speed of the diurnal disturbance. Idealized WRF simulations exhibit the propagation of precipitation initially triggered by sea breeze, and distinguished from a mountain-plain circulation. Convection from mountain heating starts earlier; propagates slower; and damps faster than that of the sea breeze; the nonlinear combination of which depends on proximity to mountains. Over the open ocean, the dominant signal propagates as that of the sea breeze but with stronger convection. For mountains near the coast, there is slower propagation and stronger convection for several dynamical reasons to be discussed.