Physical Mechanisms Controlling the Offshore Propagation of Convection in the Tropics: 1. Flat Island

Abstract

An idealized convection-permitting simulation is performed to investigate the physical mechanisms responsible for the nighttime offshore propagation of convection around tropical islands. An idealized island is placed in the middle of a long, nonrotating channel oceanic domain with constant sea surface temperature. A strong diurnal cycle typical of a tropical island is simulated, with a thermally forced sea breeze in daytime and the associated inland propagation of precipitation. Offshore propagation of a land breeze and its associated convection is simulated every night but with varying extent. Gravity waves of first and second baroclinic modes trigger convection far from the coast if the offshore conditions are favorable. This accelerates the propagation speed of the land breeze as it reduces the onshore wind associated with the lower branch of the overturning large-scale circulation. Higher-order modes may trigger convection or reinforce existing convection but less systematically. The distance of propagation is particularly sensitive to humidity and temperature at the top of the boundary layer, with occasional incursions of a dry anomaly at the top of the boundary layer near the island preventing convection from developing far from the island.