Effects of Trade Wind Strength on Airflow and Cloudiness over Oahu

Satellite observations and high-resolution modeling during July-August 2013 are used to study the effects of trade wind strength on island wake circulations and cloudiness over Oahu, Hawaii. Oahu comprises two northwest-southeast- orientated mountain ranges: the Waianae Range (similar to 1227m) along the western leeside coast and the Koolau Range (similar to 944m) along the eastern windward coast. At night, the flow deceleration of the incoming northeasterly trade winds on the eastern windward side is more significant when trades are stronger. In the afternoon hours, effective albedo and simulated cloud water are greater over the Koolau Range when trades are stronger, and clouds are advected downstream by the trade winds aloft. Over the Waianae Range, orographic clouds are more significant when trades are weaker due to less moisture removal by orographic precipitation over the Koolau Range and the development of both upslope flow on the eastern slope and upslope/sea-breeze flow along the western coast, the latter of which brings in warm, moist air from the ocean. When trades are weaker, cloudiness off the western leeside coast is more extensive and originates from orographic cloud development over the Waianae Range, which drifts downstream due to a combination of trade winds and the easterly return flow aloft. The latter is associated with the low-level sea-breeze/upslope flow.