Numerical simulations and observations of airflow through the Alenuihaha Channel

Abstract

The characteristics of airflow in the coastal waters of the Hawaiian archipelago, with special emphasis on the Alenuihaha Channel, are examined using a combination of observations and model simulations from the Advanced Research Weather and Forecasting Model (WRF-ARW). General features of airflow related to interaction with the archipelago, including seasonal and diurnal changes, are presented using historical data from the QuikSCAT satellite, various buoys and sounding data gathered in the Alenuihaha Channel. Verification of the WRF model's performance is achieved through comparisons of several years of buoy data with the respective years of a WRF model hindcast. Special attention is paid to the Alenuihaha Channel, the site of two historical buoys, where a notable acceleration of winds occurs in conjunction with a sinking of the trade wind inversion. Model support for the existence of accelerated winds, a lowering of the inversion and a hydraulic jump within the channel are demonstrated. A WRF model case study for July 2005 clarifies the location of the fastest channel winds, their magnitude, degree of deflection and diurnal variations while elucidating the mechanics by which these features develop. Topographical influences of Maui on the airflow through the Alenuihaha Channel are explored by comparing WRF runs with Maui topography with those in which it has been removed. These effects amount to a combination of island blocking/orographic lifting on the windward side and adiabatic descent on the leeward side, which augment the pressure gradient through the channel. Model simulations represent the sinking inversion and hydraulic jump, delineating their respective amplitude and placement.