A Numerical Model of Trade Wind Weather on Oahu

Abstract

A single-layer mesoscale model, originally developed for the study of wintertime take-effect storms, is applied to the airflow over the Island of Oahu, Hawaii. Terrain effects, land roughness, island heating, and surface evaporation are all included. Convective precipitation and latent heating are parameterized in terms of larger-scale motions resolved on the 3-km × 3-km grid. Experiments were conducted with an atmospheric structure consistent with typical trade conditions as well as with variations from normal wind speed and inversion height. Satisfactory agreement was achieved with observed patterns of inversion height, cloud base height, temperature, relative humidity, wind flow, and precipitation under typical conditions. Mixing of dry air through the inversion from above was deduced to be very important. In most experiments the model generated a hydraulic jump to the lee of the mountains. Sea breeze effects were only partially simulated. Under some conditions the blocking effect of the Island in the model resulted in less than 40% of the upstream air surmounting the central part of the windward mountain ridge, so that vertical cross-section models are contraindicated.