The roles of island size and orography on tropical convection and aerosol transport

Abstract. This study investigates the impact of island diameter size and orographic height on precipitation, convective organization and aerosol transport on and around tropical islands. Twenty-four model simulations are set up and run, in which the island diameter (50 km, 100 km, and 200 km) and peak orographic height (flat, 500 m, 1 km, and 2 km) are independently and simultaneously varied under weak and strong zonal wind regimes. In these simulations, unlike many of the previous island flow investigations, the full three-dimensional flow over and around islands is resolved. Analysis of these numerical experiments demonstrates that island orographic height is a stronger control than island size on precipitation, convective organization, and aerosol redistribution. The wind regime is found to modulate these results. Under the strong zonal wind regime, increasing orographic height induces changes to the flow around the island, leading to lee-vortex formation, reverse flow, and the earlier development of deep convection than in the flat simulation. In the weaker zonal wind experiment, increasing orographic height enhances the role of radiational heating, leading to enhanced upslope flow and stronger convergence, which produces earlier deep convection than in the flat simulation. The timing and location of the sea breeze/mountain breeze convergence determines the location of the initial vertical aerosol mixing with cloud formation and entrainment, whereas the timing of deep convection formation dictates how quickly the aerosols are mixed out of the boundary layer. Finally, it is demonstrated that cold pools play an important role in the propagation of convection towards the shore in the simulations with orography.