Hurricane Dean and the Mountains of Dominica

On August 17, 2007, the center of Hurricane Dean passed within 92km of the mountainous island of Dominica in the West Indies. Four rain gauges, a MODIS image and 5-minute radar scans from Guadeloupe are used to determine the storm structure and the mountain's effect on precipitation. The encounter is best described in three phases: 1) ENE dry flow with three isolated drifting cells, 2) a brief passage of the narrow outer rain-band, 3) An eight-hour period with SSE airflow and heavy rain in a trailing spiral band. All three phases show local orographic doubling of rainfall but no evidence for direct orographic triggering of new convection. A simple 3-D linear model captures the orographic enhancement and lee-side suppression of the precipitation. We speculate that, unlike in fair-weather conditions, the faster hurricane winds and the super-critical Froude Number prevented orographic cell generation by lack of upwind wave propagation and the rapid advection of incipient convection to the lee side. Instead, the enhancement was due to a modified seeder-feeder mechanism that creates a “dipole” pattern of enhanced and suppressed low level cloud water accretion.