Orographic precipitation in the tropics: Experiments in Dominica

The island of Dominica (15° 20'N) in the Lesser Antilles, with mountains reaching 1400 meters, is a natural laboratory for orographic precipitation in the tropics. It provides an opportunity to see how a steep mountain will disturb the Atlantic trade-wind cumuli regime studied previously in BOMEX. The island also experiences hurricane passages. Recent precipitation data confirms historical evidence that the orographic precipitation enhancement factor in Dominica exceeds four for typical conditions and is nearly two for hurricane passage. Newly installed high time resolution rain gauges, meteorological radars on nearby Guadeloupe and Martinique, vertically pointing radar, time-lapse photography, and stable isotope data allow various hypotheses to be tested. For example, as the diurnal modulation of precipitation is small, we can already rule out a dominant thermal heat island effect.

During the rainy summer and fall seasons, the island precipitation arises in part from discrete non-precipitating or lightly-precipitating cumuli drifting across the windward coast. As these cells make landfall, the combination of sudden convective development and a preexisting cap cloud near the mountain crest enhances precipitation and minimizes sub-cloud evaporation. On the lee slopes, the convective clouds disappear very rapidly, giving a factor-of-four reduction of precipitation over a distance of 10km. Another important contribution to the island precipitation comes from easterly wave events, where the island causes a local amplification of the regional rainfall. During the passage of Hurricane Dean (2007), the radar showed a quasi-stationary region of rain enhancement over the island. The dynamical and cloud-microphysical mechanisms underlying the strong precipitation enhancements over the island are elucidated through large-eddy simulations of trade-wind cumuli impinging on the Dominica terrain.