Forcing of Flow Reversal along the Windward Slopes of Hawaii

Abstract

Hawaii is an island approximately 4 km high, the lower portion of which is immersed in an easterly trade-wind layer that is typically 2 km deep. Blockage of the trade wind combined with diurnal, thermally driven circulations, leads to a general flow stagnation along the windward slopes and to a reversal of flow at night. This westerly downslope flow is confluent with the incoming trade wind and usually extends offshore. From observations in the Hawaiian Rainband Project, this flow is examined to determine if it is primarily driven by blocking dynamics or thermal forcing. It is determined that the westerly downslope flow is principally a thermally driven circulation and that it closely resembles a density drainage current. This flow is normally initiated by evaporative cooling, principally from orographic rainfall, but perhaps also from evapotranspiration. Under less cloudy and precipitation-free conditions, downslope flow can be initiated and maintained by radiative cooling of the land surface, but such conditions are unusual at the initiation stage. Once the downslope flow is initiated, the ratio of cooling to moistening indicates that radiative cooling also contributes significantly, and sometimes strongly, to the production of negative buoyancy. Offshore, the pressure gradient created by blocked flow maintains the westerly flow, such that the low-level current sometimes extends 20?30 km upwind of the shoreline. These findings should he generally applicable to windward mountain locations where the upstream air has small conditional instability and low Froude number. Rainbands often form at the convergence line between this offshore flow and the incoming trade wind, where there is usually a density discontinuity of order 1%. The organization and propagation of those rainbands will be influenced by the low-level cold pool and by the pressure gradient that results from blocked flow.