Orographic precipitation in the tropics: The Dominica Experiment

Abstract: The Dominica Experiment (DOMEX) took place in the eastern Caribbean from April 4 to May 10, 2011 with 21 research flights of the Wyoming King Air and several other observing systems. The goal was an improved understanding of the physics of convective orographic precipitation in the tropics. Two types of convection were found. During a period of weak trade winds, diurnal thermal convection was seen over Dominica. This convection caused little precipitation but carried aloft air with island- derived aerosol and depleted CO2 from forest uptake. During periods of strong trades, mechanically forced convection over the windward slopes brought heavy rain to the high terrain. This convection was “seeded” by trade wind cumuli or neutrally buoyant cool wet patches of air. In this mechanically-forced convection, air parcels did not touch the island surface to gain buoyancy so no island derived tracers were lofted. With fewer aerosols, the mean cloud droplet diameter increased from 15 to 25microns. Plunging airflow and a wake were found in the lee of Dominica. The DOMEX data set will advance our understanding and test our theories of cumulus triggering and aerosol influence on precipitation.