Evolution of Downslope Flow under Strong Opposing Trade Winds and Frequent Trade-Wind Rainshowers over the Island of Hawaii

Abstract

Abstract The evolution of downslope flow on the windward side of the island of Hawaii during 7–8 August 1990 is investigated. This period is characterized by atypical strong (∼11 m s−1) upstream trade winds and frequent nocturnal rainshowers. In the late afternoon, the onset of downslope flow first occurs on the upper slope, which is frequently dominated by orographic clouds. With mean weak surface winds on the windward slopes because of island blocking, the downslope flow onset occurs after the slope surface becomes negatively buoyant. Along the Hilo coastal areas that are well exposed to decelerating trade-wind flow, the offshore flow onset there occurs much later (∼2 h) than the Hawaiian Rainband Project (HaRP) mean as the drainage front moves slowly from the windward lowlands toward the coast against the atypical strong incoming trade-wind flow. The downslope flow onset along the coast occurs after a band of trade-wind rainshowers has moved over the coast. These rainshowers produce evaporatively coole...