Simulated and Observed Influence of the Nocturnal Urban Heat Island on the Local Wind Field

Abstract

A three-dimensional primative equation model was used to simulate the low-level wind field, given the urban heat island as the lower temperature boundary condition. The specification of the average heat island bypassed the need to calculate the surface energy budget, considerably simplifying the model. This is especially desirable since temperature is a more accurate and easily obtainable measurement. The influence of the Washington, DC urban area on the local airflow was to enhance the vertical mixing due to the increased low-level instability as the air approached the warmer city center. This resulted in the lower-level winds turning anticyclonically (clockwise) from the upwind value. The anticyclonic turning, on the order of a degree per kilometer, is due to an increase in the downward transport of momentum and is accompanied by a wind speed increase of 17%. The upwind direction is quickly reestablished downwind of the city. The influence of the terrain upon the wind field could not be determined from the calculations since terrain effects were included in the surface temperature boundary condition. Observations from three instrumented towers around the perimeter of the urban center were not sufficient to deduce the complex nature of the flow. However, during southwest flow when two of the towers were downwind of the urban center, the mean wind direction between those towers was significantly different by 20°.