Flow over Small Heat Islands: A Numerical Sensitivity Study

Abstract

A two-dimensional nonlinear model with physical parameterizations was applied to simulate the observed diurnal variation on the 5-km-wide flat tropical island of Nauru in the trade wind zone. Both the model and Atmospheric Radiation Measurement (ARM) campaign aircraft observations indicate vigorous mixing in the typical sunny daytime conditions, leading to a warm plume downstream of the island. The model's afternoon wind field displayed rising motion downstream and downwash ahead of the island with gravity wave structure, in accordance with linear models of steady flow over a heated island. The roughness difference between sea and land added local rising motion above the windward coast and sinking motion above the lee. Without large-scale wind U, a weakish sea-breeze (SB) pattern develops during the day in this model over the 5-km-wide island/peninsula. This pure SB circulation intensifies with increasing island width up to 40 km. When large-scale wind is present, the morning leeside SB cell is advected out to sea and disappears while the windward coast SB cell tilts over the island and is transformed into the steady heat island?type perturbation during the day. During the night, a reversed heat island?type weak and shallow perturbation develops for nonzero U. ForU = 0, the sea breeze dies in the evening and no land breeze appears. If a 200-m-high central mountain is added to a 20-km-wide island/peninsula in calm daytime conditions, the SB circulation is enhanced by upslope winds followed by weak katabatic flow down the cool slopes during the night. When any large-scale flow is present, the forced flow up and down the slopes appears to dominate the wind perturbation patterns day and night.