Orographic Influences on an Oahu Flood

Abstract

Abstract On 2 April 2006, Oahu’s Ko‘olau Mountain Range endured more than 6 h of heavy rain with accompanying flash flooding along its northeast-facing slopes. The storm responsible for the event left a pattern of precipitation characteristic of orographic anchoring of convection with extreme rainfall gradients along the slopes and maxima along the crest of the mountain range. In fact, this was the third flash-flood event to impact the Ko‘olau Mountains in just over 1 month, with each event occurring under conditions of moist southeasterly flow at low levels and moderate conditional instability. Under these conditions persistent convection and localized heavy rainfall often occur over the Ko‘olau Mountain Range. Mesoscale analyses of the thunderstorm complex responsible for the 2 April 2006 heavy rain event and the results of a high-resolution numerical simulation employing the Weather Research and Forecasting (WRF) model are described in this study. Key features of the convection that contributed to the longevity of the event include repeat formation of convective cells along the eastern side of the central Ko‘olaus, minimal horizontal cloud motion, and strong updrafts that sloped toward the northwest in the lower levels. The westerly shear of the low-level flow determined the pattern of accumulated precipitation by aligning the slope of the convective updrafts nearly parallel to the southeast-to-northwest-orientated Ko‘olau Mountain Range. The microphysical structure of the convection was complex, with the vertical advection of hydrometeors originating below the freezing level facilitating high concentrations of ice particles and an environment conducive to charge separation and lightning.