Interaction Between Downslope Flow and Trade-Wind Showers Over The Island ofHawaii During 7-8 August, 1990

摘要

Data from the Hawaiian Rainband Project (HaRP) are used to study the interactionbetween downslope flow and trade-wind showers over windward sections of the island of Hawaii on 7-8 August 1990. Large-scale conditions produce strong upstream trade-winds of 11 m s-1 which are among the strongest observed during HaRP. In addition, frequent trade-wind showers are responsible for the largest precipitation accumulation between 0300-0700 HST over windward Hawaii during HaRP. The evolution of downslope flow and its interaction with the trade-wind showers under these conditions will be studied in detail. The initiation of downslope flow occurs under dry conditions on the upper slopes by radiative cooling, and then progresses downslope until it arrives over windward lowlands. Here the downslope flow transition is held up by the strong trade winds occurring at the coast. Downslope flow is finally observed over the coast only after a band of rainshowers produces evaporative cooling allowing downslope winds to extend offshore. The evolution of downslope flow afterwards is affected by rainshowers. Vertical mixing associated with rainshowers transports trade-wind momentum into the weak downslope flow layer, replacing westerly winds with strong easterly winds. At the surface, a clear increase in temperature and moist static energy is observed as potentially warmer air aloft mixes with the surface layer. This temporary destruction of downslope flow produces an oscillation of the drainage front from 5-8 km offshore to 10 km inland.