Idealized simulations of precipitation regimes over a small tropical island

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Monday, 5 January 2015
Shuguang Wang, Columbia Univ., New York, NY; and A. H. Sobel

Tropical islands are global focal points of convection. Observational studies have shown that precipitation over islands is strongly regulated by their size and elevation, and by the large scale prevailing wind. To further develop physical understanding of convection over tropical islands, we perform a set of highly idealized simulations with uniform sea surface temperature to explore flow regimes around an idealized small island (~120 km wide) as a function of large scale wind speed and island elevation. In our simulations the domain as a whole is in radiative-convective equilibrium (RCE), which constrains 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 anomalies over the larger surrounding area.

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.