Characteristics of Summer Trade Wind Rainfall over Oahu

In this study, characteristics of summer trade wind rainfall over Oahu, Hawaii, are analyzed. In the early morning before sunrise, flow deceleration on the windward coastal area is the greatest when the island land surface is the coldest. Furthermore, relatively calm winds are found over central Oahu between the Ko'olau Mountains and the Waianae Mountains, with weak westerly katabatic winds on the windward side of the Waianae Mountains. Most windward stations have an early morning rainfall maximum with a secondary rainfall maximum in the early evening. Morning (afternoon) land (sea) breezes dominate under variable winds, and are more pronounced over leeward Oahu. Precipitation on the western leeside coast has a slight peak in the afternoon due to an increase of cloud development from the afternoon sea breezes. Daily oro-graphic rainfall over Oahu is greater under the strong trade wind regime, and less under weak trade wind and variable-wind flow regimes. However, the maximum correlation between daily trade wind rainfall and trade wind speed is <0.3. Days with high rainfall generally occur under strong trades, but not all strong trade wind days produce significant rainfall. With its relatively low terrain height compared to lifted condensation level (LCL) and relatively small size, orographic lifting alone is inadequate to initiate precipitation through warm rain processes. The existence of trade wind cumuli upstream is necessary. In addition, a deeper moist layer and higher moisture content upstream are two conditions that are favorable for higher orographic precipitation over Oahu under undisturbed summertime trade wind weather.