How relevant is urban planning for the thermal comfort of pedestrians? Numerical case studies in two districts of the City of Dresden (Saxony/Germany)
Goldberg, Valeri; Kurbjuhn, Cornelia; Bernhofer, Christian
This study investigates the effects of different urban planning scenarios on the thermal comfort of pedestrians in two districts of the city of Dresden. A total of three alternatives to reconstruct these districts were created by considering realistic approaches for urban planning projections in the next ten years. These alternatives include the development and densification of existing buildings as well as the greening of undeveloped areas or the reduction of existing green areas. Based on these planning alternatives, micro-meteorological conditions as well as bioclimatic indices within the chosen city districts of Dresden for a hot summer day were simulated with a combination of the 3D microclimatic model ENVI-met and the bioclimatic model Rayman. Using both, potential air temperature (T) and the bioclimatic index UTCI (Universal Thermal Comfort Index), spatial distribution at different times of the day, the cumulative thermal stress conditions at both exposed "receptor" points and on a virtual course of pedestrians through the inner city of Dresden, were analysed. Simulation results clearly show the local reduction of T and UTCI due to the shading effect of denser vegetation or building structures during the daytime. For example, a greening of an industrial wasteland locally reduces T and UTCI in the early afternoon of a hot and sunny summer day by up to 2 and 14 K, respectively. The same increase in T and UTCI can be found in non-shaded partitions of this area if it is sparsely rebuilt. At night, areas with widespread building placement show a higher T and UTCI (up to 1 and 4 K, respectively), and areas with dense vegetation are cooler (0.1 to 0.4 K) than open areas. Due to the small extension of modified urban structures, the mean effect over the complete area amounts to only a fraction of a degree at most. The simulation results demonstrate that a greening of urban wastelands has primarily positive effects (that means cooling) on thermal extremes during hot summer periods. A dense rebuilding causes similar positive effects during the daytime due to the generation of new shading zones as well as negative effects (warming) by night. For citizens occupying these city areas, the local effect of a modified urban structure could be more relevant than the mean effect over the whole city district. Based on these findings, modern approaches of city development should optimally combine green areas, shadow zones of large buildings and the maintenance of "street canyon ventilation".