Numerical Simulations and Observations of Airflow through the ‘Alenuihāhā Channel, Hawaii

During the summer, sustained winds in the ‘Alenuihāhā Channel, Hawaii, may exceed 20 m s−1 with higher gusts. The Advanced Research Weather Research and Forecasting model is used to diagnose airflow in the Hawaiian coastal waters. High-resolution (2 km) runs are performed for July 2005 covering the ‘Alenuihāhā Channel and nested in a 6-km state domain. Under normal trade wind conditions (7–8 m s−1), winds at the channel entrance are 1–2 m s−1 faster than upstream due to the convergence of the deflected airflows by the islands of Maui and Hawaii, and accelerate through the channel due to along-gap pressure gradients and lower pressure in the wakes of both islands. The acceleration is accompanied by descending airflow (>9 cm s−1) in the exit region with lowering of the trade wind inversion. Deceleration occurs downstream of the channel exit with a rapid change from sinking motion to rising motion (>3 cm s−1). Under normal or strong trade wind conditions, the flow is subcritical [Froude number (Fr) < 1] upstream of the channel, supercritical (Fr > 1) in the exit region, and subcritical again (Fr < 1) downstream with a weak hydraulic jump. The localized sinking motion on the lee side of bordering ridgelines (>1 m s−1) is most significant in the afternoon hours and results in warming and lowering of surface pressure on the lee side, into the channel, and farther downstream. As a result, the channel winds and the wind speed maximum along the southeastern coast of Maui exhibit an afternoon maximum.