Investigating the Effects of Orography and Ambient Wind on Deep Convection over Tropical Islands

Abstract Using an observation–model synthesis, we investigate variations of cumulus convection over islands with varying characteristics and under varying environmental conditions to clarify controls on convective vigor. In this paper, we define the intensity of deep convection as the average 40-dB Z -radar echo-top height or 85-GHz brightness temperature among raining precipitation features having 40-dB Z echoes (RPF40s) and the frequency of deep convection as the fraction of RPF40s to the total number of precipitation features. Examination of the Tropical Rainfall Measuring Mission (TRMM) satellite database (1994–2015) of 272 tropical and subtropical islands reveals a modest weakening of convective intensity with increased terrain height h or ambient wind U (for a given island area A ) and a strengthening with increasing A . Quasi-idealized, convection-permitting simulations broadly reproduce these sensitivities to h and A , but not that to U . In both observations and simulations, intensity increases with the island-averaged convective available potential energy (CAPE). Because CAPE generally decreases over taller islands that protrude deeper into the free troposphere, convective intensity varies inversely with h . The frequency increases with the total island area over which both large CAPE and strong near-surface horizontal convergence coincide. This trend favors higher frequencies over larger islands with complex (but shallow) terrain. The model’s inability to reproduce the observed decrease of convective intensity with U stems from a negative observed correlation between CAPE and U that was neglected in the simulations. Thus, as with h , the negative observed trend between intensity and U ultimately stems from the impacts of CAPE on convective intensity.