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

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. Plain Language Summary: Climate models have problems modeling the precipitation pattern, timing, and atmospheric circulation associated with tropical island regions. This impacts the prediction of planetary scale events such as the Madden‐Julian Oscillation. Part of this problem originates from the poor representation of islands and their topography, which affects the diurnal cycle of convection over land and the surrounding oceans. This diurnal cycle generates a sea breeze during daytime, which triggers convection over the island in the afternoon, early evening. At night, the breeze reverses and becomes a land breeze that propagates over the ocean. In order to understand what controls this offshore propagation, we study the movement of convection around an idealized tropical island. We show that a land breeze is systematically present even though its distance of propagation depends on the presence of offshore convection triggered by gravity waves earlier and on the humidity and temperature between 1 and 2 km of altitude close to coast. Properly modeling these individual elements may improve the precipitation pattern and atmospheric circulation around tropical islands and be beneficial for improving the representation of atmospheric phenomenons impacted by these regions. Key Points: Systematic offshore propagation of convection is observed at night, with varying distance of propagationOffshore convection triggered by gravity waves modulates the propagation speed of the land breezeLarge‐scale environment controls propagation with a dry and warm anomaly at the top of the boundary layer [ABSTRACT FROM AUTHOR]