The structure of cumulus clouds above Dominica and its response to cloud-layer humidity variations

The Dominica Experiment (DOMEX) was carried out in the eastern Caribbean in the spring of 2011. It included 21 research flights of the Wyoming King Air over and around the mountainous island of Dominica. During the six-week field campaign, higher amounts of precipitation were observed on Dominica under strong trade wind conditions while precipitation was suppressed under weak trade wind conditions. A number of hypotheses exist as to what controls precipitation on the steep slopes of Dominica. In this presentation, we focus on 1) the influence of thermally- versus mechanically-driven orographic convection on cloud structure above the island; and 2) how observed day-to-day differences in the cloud-layer humidity influence cloud development. We use aircraft observations above the island, a simple entraining-parcel model, large-eddy simulations of a thermally buoyant bubble and a climatology of balloon soundings from the neighboring island of Guadeloupe to examine the influence of these two important factors. The results paint a complicated picture of the ingredients that shape convection and cloud development over Dominica. On the one hand, the speed of the incoming flow controls the convective regime and the vigor of mechanically-driven convection. On the other hand, the drier cloud layer on low wind days and the nature of the convection (e.g., the dominant width and frequency of convection) appears to have an important influence on cloud and precipitation development.