The Mesoscale Dynamics of Ascent-Forced Orographic Convection: Results from DOMEX

The mountainous Caribbean island of Dominica lies in the easterly Atlantic Trade winds (15°N, 61°W). 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.

These mesoscale processes controlling the life cycle of convective clouds over the island (and the rainfall they produce) are examined in the context of a prototypical "strong wind" case from the DOMEX 2011 field campaign. This is accomplished using data from rain gauges, Meteo France operational radars, and both in situ and cloud radar observations from a flight of the University of Wyoming King Air. In addition, output from cloud-resolving numerical simulations with the Weather Research and Forecasting model are also examined.

Minimal deceleration or deflection of the flow occurs upwind of Dominica, so most low-level air lifts over the terrain, providing moisture for convective rainfall. Over the crest of the terrain, an abrupt transition occurs, from moist convection to a turbulent dry and plunging flow that quickly dissipates convection but allows for spillover rainfall. A pronounced wake is found in the lee, where low-level air that has passed over Dominica is decelerated, warmed, dried, and deficient in Bernoulli function relative to the upwind. Gravity wave dynamics, latent heat release, and turbulent dissipation are used to explain the dynamical origin of these mesoscale features.