Idealized simulations of precipitation regimes over a small tropical island
Over flat islands, as we vary prevailing wind speed, the island experiences three distinct flow regimes: rainfall is greatly enhanced and a local symmetric circulation is formed in the time mean around the island when the prevailing large scale wind speed is small; but the enhancement is much reduced when the wind speed is increased to a moderate value. We interpret these two regimes as the shift of triggering mechanism of convection: thermally forced convection due to surface solar heating dominates when large scale wind is very weak, while mechanically forced convection is favored when the impinging wind is moderately strong. Further increase of the prevailing wind speed to large values leads to strong asymmetry in the windward and leeward side of island and its coastal area, likely due to gravity waves from flow passing over elevated diurnal heating. Small amplitude topography has a quantitative impact, but does not alter the qualitative shift of flow regimes as a function of wind speed.