The influence of mesoscale airflow dynamics on orographic convection and precipitation in the tropics

The Caribbean Island of Dominica lies in the easterly Atlantic Trade winds (15°N, 61°W). The island is a north-south ridge about 50 km long in the cross-wind, and 20 km wide in the along-wind direction. High terrain on the northern and southern end of the island rises to about 1.4 km, separated by a saddle only about 500m high. Much of the Dominica's interior lies above the lifting condensation level but below the trade-wind inversion. Observations reveal that the lifting of moist trade winds over the island's windward slopes strongly invigorates convection and enhances precipitation, while in the lee convective clouds and rainfall are quickly suppressed.

Previous work using analytic theories and quasi-2D numerical models has illuminated how bulk lifting of the trade winds, with dry and moist/cloudy regions, can lead to large buoyancy contrasts and rapid development of convective rainfall over the island. As part of the DOMEX project, the present study expands on previous work by examining how mesoscale dynamics of 3-dimensional airflow over and around Dominica control the life cycle of convective clouds embedded in this flow (and the rainfall they produce). This is accomplished by synthesizing cloud-resolving numerical simulations using the WRF model with data from DOMEX's intensive observation periods including: Meteo France operational radar scans, time-lapse photography, surface rainfall measurements, and in situ and cloud-radar measurements from University of Wyoming King Air flights.

Specific issues addressed include: 1. How deceleration, lateral-deflection, and lifting of low-level flow upwind of Dominica modify ascent and moisture fluxes over the island, and hence the development of convection and rainfall; 2. How confluence of flow into the Dominica's saddle modifies along-ridge variations in convection and rainfall; 3. How plunging flow, mountain wave breaking, and vertical mixing act to stabilize and dry the air in the Dominica's lee, suppressing convection and rainfall.