Effect of city shape on urban wind patterns and convective heat transfer in calm and stable background conditions

Extreme heatwaves and air pollution in urban areas often occur in calm and stable background conditions. In such situations, urban heat dome flows are induced by the urban heat island (UHI) effect, which has a strong influence on the urban thermal and wind environment. Cities have various shapes, but most studies of urban heat island circulation have focused on ideal circular shape and ignored the potential effects of shape. In this study, the effects of the various ideal urban shapes on the natural convective flow over urban areas (urban dome flow) are investigated experimentally. The urban dome flow over an equilateral polygonal city is characterised as convergent inflow along the interior angle bisector at the lower level, upward flow near the urban centre and divergent side outflow perpendicular to the side at the upper level. The characteristics of the urban dome flow over a rectangular urban area depend greatly upon the aspect ratio of the long side to the short side. As the aspect ratio increases, the divergent flows of the short sides disappear, and those of the two long sides dominate. Moreover, recirculation occurs between the divergent outflow perpendicular to the long edges and the convergent inflow near the short edges. Our results also show that the Nusselt (Nu) number (Eq. (1)), which quantifies the heat transfer efficiency, is significantly lower in a stable environment than in a neutral environment. A significant correlation between the shape and Nu was not found. It indicates that strong UHIs prefer stable background conditions.