Non-uniform ground-level wind patterns in a heat dome over a uniformly heated non-circular city

Abstract

Urban heat island-induced circulation, or urban heat dome flow, significantly influences urban thermal environments and air quality in calm wind conditions with stable stratification. An urban heat dome is characterized by convergent inflow at the ground level, divergent outflow at the upper level and upward flow over the city center in calm conditions. We report a new city-shape effect on heat dome flow patterns in a laboratory modeling experiment. For a circular city, both the inflow at the lower level and the outflow at the upper level are axisymmetric. For a square urban area, a non-uniform flow pattern was observed with four dominant diagonal inflows at the ground level and four dominant side outflows (perpendicular to city edges) at the upper level, indicating that the inflow changes direction as it rises over the urban area. The experiments were carried out in two water tank models with stable stratification, using thermal image velocimetry and particle image velocimetry. “Cell-like” and “stripe-like” eddy structures were identified over the modeled urban area, depending on the mean flow speed. To the best of our knowledge, this study reveals for the first time that in calm conditions the shape of an urban area may significantly affect the winds within a city, and thus the local heat transfer coefficients, urban air temperature and urban haze distribution will not be uniform under such conditions. Results on the eddy structures and mean flow fields can provide insights for theoretical analysis on heat transfer models in future studies.