Precipitation over tropical islands

Precipitation over mountainous islands is strongly regulated by their sizes and elevations, and by the large scale prevailing wind. Yet the relative roles of thermal and mechanical forcing are not understood, nor is there any general theory for orographic precipitation that includes free moist convection. To address these issues, we perform several sets of highly idealized simulations with a cloud resolving model to explore precipitation and flow regimes around an idealized small island with diurnally-varying surface temperature. 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.

We first examine the effect of variations in large scale wind speed and island elevation around an island 120 km wide. Over flat islands, as we vary the 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. The rainfall enhancement is much reduced when the wind speed is increased to a moderate value (e.g., 5 m/s). We interpret these two regimes as due to a shift in the triggering 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 over low level diurnally-varying heating or terrain is moderately strong. Further increase of the prevailing wind speed to large values (e.g., 15 m/s) excites large-amplitude gravity waves, inducing strong asymmetry in the windward and leeward side of island and its coastal area, and suppressing diurnal cycles in rain. We vary topography to several hundred meters, which has a quantitative impact on the results, but does not alter the qualitative shift of flow regimes as a function of wind speed .

We further test the effect of island size for flat islands with zero mean wind, varying the size from 10 - 200 km. Island precipitation enhancement occurs over the parameter space we examined. The enhancement is greatest for an island of 30-40 km wide, in broad agreement with observations. The roles of moisture convergence feedback, sea breeze, and gravity waves in this set of simulations are studied to explain this size optimum.