The effect of urban geometry on mean radiant temperature under future climate change: a study of three European cities

Abstract

Future anthropogenic climate change is likely to increase the air temperature (T _a ) across Europe and increase the frequency, duration and magnitude of severe heat stress events. Heat stress events are generally associated with clear-sky conditions and high T _a , which give rise to high radiant heat load, i.e. mean radiant temperature (T _mrt ). In urban environments, T _mrt is strongly influenced by urban geometry. The present study examines the effect of urban geometry on daytime heat stress in three European cities (Gothenburg in Sweden, Frankfurt in Germany and Porto in Portugal) under present and future climates, using T _mrt as an indicator of heat stress. It is found that severe heat stress occurs in all three cities. Similar maximum daytime T _mrt is found in open areas in all three cities despite of the latitudinal differences in average daytime T _mrt . In contrast, dense urban structures like narrow street canyons are able to mitigate heat stress in the summer, without causing substantial changes in T _mrt in the winter. Although the T _mrt averages are similar for the north–south and east–west street canyons in each city, the number of hours when T _mrt exceeds the threshold values of 55.5 and 59.4 °C—used as indicators of moderate and severe heat stress—in the north–south canyons is much higher than that in the east–west canyons. Using statistically downscaled data from a regional climate model, it is found that the study sites were generally warmer in the future scenario, especially Porto, which would further exacerbate heat stress in urban areas. However, a decrease in solar radiation in Gothenburg and Frankfurt reduces T _mrt in the spring, while the reduction in T _mrt is somewhat offset by increasing T _a in other seasons. It suggests that changes in the T _mrt under the future scenario are dominated by variations in T _a . Nonetheless, the intra-urban differences remain relatively stable in the future. These findings suggest that dense urban structure can reduce daytime heat stress since it reduces the number of hours of high T _mrt in the summer and does not cause substantial changes in average and minimum T _mrt in the winter. In dense urban settings, a more diverse urban thermal environment is also preferred to compensate for reduced solar access in the winter. The extent to which the urban geometry can be optimized for the future climate is also influenced by local urban characteristics.