Heavy Rain Events over the South-Facing Slopes of Hawaii: Attendant Conditions

Abstract

Heavy rain events (>100 mm day?1) over the southeast flank of Mauna Loa volcano on the island of Hawaii are examined using surface, rawinsonde, rain gauge, and satellite data. The events occur in the presence of four types of synoptic-scale disturbances that include Kona storms, cold fronts, upper-tropospheric troughs, and tropical systems. The heaviest rainfall occurs at elevations above 0.5 km over the volcano slopes facing the prevailing low-level flow with the leeside slopes experiencing considerably smaller amounts. Rainfall duration and patterns demonstrate that the rains and flash floods are the result of more than one convective cell. Soundings prior to the heavy rain events show only modest instability, with only the K index serving as a useful predictor for heavy rain among the standard stability indices. There is an increase in both the moisture content of the midlevels (750?450 mb) and the onshore flow normal to the terrain gradient prior to the heavy rain. The lack of the trade winds and the attendant inversion is a significant signal considering the persistence of these features over the Hawaiian Islands throughout the year. Possible roles for midlevel moisture in lessening the detrimental effects of entrainment on updraft buoyancy and suppressing cold outflow formation and thus storm movement are also discussed.