Trade Wind Rainfall near the Windward Coast of Hawaii

Abstract

A dynamically based mesoscale climatology of rainfall and other data are examined for typical trade wind conditions near Hawaii. Relative distributions of rainfall are deduced from radar reflectivity data for a 4000 km2 region over the windward island and the upstream ocean. The cumulative rainfall data suggest three zones of forcing that are characteristic of 1) the regional ocean, 2) upstream island-induced divergence, and 3) flow reversal near and over the island. It is confirmed that most intense rainband amplifications occur over a mesoscale convergence line that separates the easterly trade winds from an island-induced westerly flow. This flow separation line resembles a classical gravity 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 Fr is in the ordinary range (<0.3), the island maximum is five times greater than the oceanic average. When Fr is elevated (>0.3), the rainfall maximum increases to 10 times the oceanic average. Overall, the windward island maximum is seven times the upstream oceanic average. Increased rainfall is more strongly correlated with increased wind speed than with a reduction in dry static stability, both component variables of Fr. Unlike rainfall amount, the position of the island maximum is relatively insensitive to Fr, except in the hours surrounding sunrise. In the upstream divergence zone, 20?45 km offshore, rainfall is double the oceanic background. It is speculated that the cause is blocking related. There are very strong diurnal variations in rainfall amount and distribution, both over the island and the upstream ocean. Windward island rainfall is at a maximum in the hours surrounding midnight when breeze and blocking forcings are fully cooperative in the coastal region. Seemingly unrelated, oceanic rainfall exhibits a strong nocturnal maximum, the temporal phase of which may be influenced by wind speed. A deep minimum in domain-scale rainfall occurs near noon.