On the forcing of flow reversal along the windward slopes of Hawaii

In summer 1990 the Hawaiian Rainband Project was conducted in the vicinity of Hilo to examine topographically forced circulations and the forcing of rainbands. This paper examines the magnitudes of dynamical and thermal forcing near a confluence line which usually separates a westerly down-slope flow (DSF) from the incoming easterly tradewinds at night. Before sunrise the DSF typically extends off shore by 10-25 km. Interaction of the westerly flow with the tradewind often leads to the initiation or intensification of convection as well as to its landward propagation. The combination of wind, static stability, and topography yields a dimensionless Froude number (Fr) as given by Smolarkiewicz et. al. (1988). In examining seven DSF events we find ample evidence of thermal forcing and no obvious signatures of forcing due to blocked flow.