Numerical Simulations of Island Effects on Airflow and Weather during the Summer over the Island of Oahu

Abstract

The high-resolution (1.5 km) nonhydrostatic fifth-generation Pennsylvania State University?National Center for Atmospheric Research (PSU?NCAR) Mesoscale Model (MM5) and an advanced land surface model (LSM) are used to study the island-induced airflow and weather for the island of Oahu, Hawaii, under summer trade wind conditions. Despite Oahu?s relatively small area (1536 km2), there are considerable spatial variations in horizontal distribution of thermodynamic fields related to terrain, airflow, rain, cloud, and ground cover. The largest diurnal variations in temperature and moisture occur in the lee sides of mountains, especially along the western leeside coast. The island-scale surface airflow is also significantly affected by terrain and land surface forcing. The downslope winds above the leeside slopes of both the Ko?olau and Waianae Mountains are simulated with significant diurnal variations with the strongest downslope winds just before sunrise. The timing of diurnal rainfall maxima over the Ko?olau Mountains is closely related to vertical motions. The early morning rainfall maximum on the windward side is caused by anomalous rising motion due to significant flow deceleration when the land surface is the coolest. The evening rainfall maximum after sunset is related to anomalous orographic lifting due to stronger winds aloft. In the early afternoon, winds aloft are relatively weak with a relatively high level of free convection (LFC) because of vertical mixing. As a result, the rainfall over the Ko?olau Mountains exhibits an afternoon minimum. The westerly reversed flow off the western leeside coast in the afternoon is mainly thermally driven and related to land surface heating superimposed by latent heat release of persistent orographic precipitation over the Ko?olau Mountains. Rainfall along the western leeside slopes has a late afternoon maximum due to the development of the onshore/upslope flow.