FORCING OF FLOW REVERSAL ALONG THE WINDWARD SLOPES OF HAWAII

Hawaii is an island approximately 4 km high, the lower portion of whic h is immersed in an easterly trade-wind layer that is typically 2 km d eep. Blockage of the trade wind combined with diurnal, thermally drive n circulations, leads to a general flow stagnation along the windward slopes and to a reversal of Bow at night. This westerly downslope Bow is confluent with the incoming trade wind and usually extends offshore . From observations in the Hawaiian Rainband Project, this flow is exa mined to determine if it is primarily driven by blocking dynamics or t hermal forcing. It is determined that the westerly downslope flow is p rincipally a thermally driven circulation and that it closely resemble s a density drainage current. This flow is normally initiated by evapo rative cooling, principally from orographic rainfall, but perhaps also from evapotranspiration. Under less cloudy and precipitation-free con ditions, downslope flow can be initiated and maintained by radiative c ooling of the land surface, but such conditions are unusual at the ini tiation stage. Once the downslope flow is initiated, the ratio of cool ing to moistening indicates that radiative cooling also contributes si gnificantly, and sometimes strongly, to the production of negative buo yancy. Offshore, the pressure gradient created by blocked flow maintai ns the westerly flow, such that the low-level current sometimes extend s 20-30 km upwind of the shoreline. These findings should be generally applicable to windward mountain locations where the upstream air has small conditional instability and low Froude number. Rainbands often f orm at the convergence line between this offshore flow and the incomin g trade wind, where there is usually a density discontinuity of order 1%. The organization and propagation of those rainbands will be influe nced by the low-level cold pool and by the pressure gradient that resu lts from blocked flow.