A Model for Predicting the Depth of the Mixing Layer Over an Urban Heat Island with Applications to New York City

Abstract

Over a city within a mixing layer, pollutants tend to be homogeneously distributed in the vertical. A means of predicting the depth of the mixing layer is therefore important for air pollution predictions. In this study a simple advective thermodynamic model was used to predict the depth of this layer over the New York urban area for five early morning periods which were characterized by synoptic-scale inversions. In the model, the wind was assumed constant in time and space and the lapse rate within the mixing layer assumed to be adiabatic. The model is essentially the same as the one proposed by Summers except for the addition of two “heat-sink” terms. These terms reduce the artificial heat available to the air and allow one to consider the effects of cooling downwind of an urban area. In this study their magnitude was about 30% of the magnitude of the artificial heat release in the New York City region. Correlation coefficients between observed and predicted mixing depths for the five non-summer mornings were about 0.86. Experience showed that the correlation was not sensitive to small variations in wind velocity or to the inclusion of the heat-sink terms. The results of the model indicate that the intensity of inversions upwind of a city greatly influences the depth of the mixing layer. More knowledge is needed of the relation between air stability and synoptic-scale heat exchange processes before reliable forecasts can be made of urban heat island effects.