Influence of Coastal Topography on Offshore Diurnal Rainfall Propagation: A Linear Theoretical Model Approach

The diurnal cycle is a fundamental driver of offshore rainfall propagation, with diurnal thermally forced gravity waves playing a primary role in triggering convection that propagates offshore. In this study, we conducted a series of idealized simulations using a simplified mesoscale model with varying terrain to investigate the influence of coastal topography on diurnally forced gravity waves. Our findings indicate that coastal topography increases the vertical length scale of diurnal heating near the coastline, resulting in gravity waves with a larger vertical length scale and amplitude, and a faster phase speed, thereby altering the propagation speed of offshore rainfall. Based on this finding, we extended a two-dimensional linear land-sea breeze analytical model by modifying the vertical heating forcing to account for the effect of coastal mountainous terrain. Our results demonstrate that higher coastal terrain significantly enhances the amplitude and horizontal and vertical wavelengths of inertial-gravity waves, successfully capturing the variations in phase speed associated with coastal topography.