TRADE-WIND RAINFALL NEAR THE WINDWARD COAST OF HAWAII

A dynamically based mesoscale climatology of rainfall and other data a re examined for typical trade wind conditions near Hawaii. Relative di stributions of rainfall are deduced from radar reflectivity data for a 4000 km(2) region over the windward island and the upstream ocean. Th e cumulative rainfall data suggest three zones of forcing that are cha racteristic of 1) the regional ocean, 2) upstream island-induced diver gence, and 3) flow reversal near and over the island. It is confirmed that most intense rainband amplifications occur over a mesoscale conve rgence line that separates the easterly trade winds from an island-ind uced westerly flow. This flow separation line resembles a classical gr avity current when positioned over the windward island and near shore. The domain-scale cumulative rainfall is statistically associated with the strength of island blocking, as defined by a Froude number (Fr). When Pr is in the ordinary range (<0.3), the island maximum is five ti mes greater than the oceanic average. When Fr is elevated (>0.3), the rainfall maximum increases to 10 times the oceanic average. Overall, t he windward island maximum is seven times the upstream oceanic average . Increased rainfall is more strongly correlated with increased wind s peed than with a reduction in dry static stability, both component var iables of Fr. Unlike rainfall amount, the position of the island maxim um is relatively insensitive to Fr, except in the hours surrounding su nrise. In the upstream divergence zone, 20-45 km offshore, rainfall is double the oceanic background. It is speculated that the cause is blo cking related. There are very strong diurnal variations in rainfall am ount and distribution, both over the island and the upstream ocean. Wi ndward island rainfall is at a maximum in the hours surrounding midnig ht when breeze and blocking forcings are fully cooperative in the coas tal region. Seemingly unrelated, oceanic rainfall exhibits a strong no cturnal maximum, the temporal phase of which may be influenced by wind speed. A deep minimum in domain-scale rainfall occurs near noon.