The Structure of Orographic Cumulus Clouds and Precipitation over Dominica: Results from DOMEX

The Dominica Experiment (DOMEX) was carried out in the eastern Carribean from April 4 to May 10, 2011, and included 21 research flights of the Wyoming King Air over and around the mountainous island. Two convective regimes have been identified over Dominica from the field campaign: when the trade winds are weak, diurnal thermal convection occurs over the island, island-derived aerosols are carried aloft and precipitation is weak; when the trade winds are strong, mechanically forced convection over the windward slopes brings heavy precipitation (with typical rates of 40 mm/hr) to the high terrain.

A primary unresolved issue is the structure of the clouds and precipitation over the island during these high and low wind events. The typical flight level over the island was 1780 m above sea level, which is around 1100 m above the cloud base and 380 m above the highest mountain peak. The aircraft made numerous penetrations of shallow cumulus clouds at varying stages of maturity, and the onboard instruments measured a range of variables at up to 25 Hz including cloud and rain droplets, aerosols and the three-dimensional wind field. A cloud radar, lidar and forward-pointing camera mounted on the aircraft also provide information on the spatial structure of the clouds. Data obtained from several research flights are hereby examined to provide in situ measurements of the convection over Dominica.

Clouds of varying sizes are found to develop over Dominica. They can extend beyond 3 km elevation and have undiluted turbulent cores that span 300-600 m in diameter. The updrafts associated with these turbulent cores often exceed 5 m/s and are typically associated with a low pressure perturbation and divergent updrafts. In cases where cloud extends uninterrupted for several kilometers across the windward slope, multiple turbulent cores with a scale of 1 km are observed within the cloud system, separated by regions of weaker, compensating downdrafts. Consistent with past studies of shallow marine cumulus, a shell of descending air is present outside the cloud edge.

An expansive region of detrained air is detected over the mountain on low wind days by out-of-cloud turbulence and aerosol measurements. However, on high wind days, the out-of-cloud turbulent region contracts and exists only within close proximity to the cloud edge; this is probably due to the stronger trade winds rapidly advecting detrained air away from its source.