Journal of the Atmospheric Sciences Factors controlling rain on small tropical islands: diurnal cycle, large-scale wind speed, and topography

Abstract: A set of idealized cloud-permitting simulations is performed to explore the influence of small islands on precipitating convection as a function of large-scale wind speed. The islands are situated in a long, narrow ocean domain that is in radiative-convective equilibrium (RCE) as a whole, constraining the domain-average precipitation. The island occupies a small part of the domain, so that significant precipitation variations over the island can occur, compensated by smaller variations over the larger surrounding area. Over flat islands, as we vary the wind speed, three distinct flow regimes occur. Rainfall is greatly enhanced, and a local symmetric circulation is formed in the time mean around the island, when the prevailing wind speed is small. The rainfall enhancement over the island is much reduced when the wind speed is increased to a moderate value. This difference is characterized by a difference in the mechanisms by which convection is forced in the two regimes. A thermally forced sea breeze due to surface heating dominates when the large-scale wind is weak. Mechanically forced convection, on the other hand, is favored when the large-scale wind is moderately strong, as horizontal advection of temperature reduces the land-sea thermal contrast that drives the sea breeze. Further increases of the wind lead to strong asymmetry between the windward and leeward sides of the island, due to gravity waves which result from the land-sea contrast in surface roughness as well as flow passing over elevated diurnal heating. Small-amplitude topography has a quantitative impact, but does not qualitatively alter the flow regimes or their dependence on wind speed.