Simulated Mitigation of Heat Stress for Pedestrians in an Urban District of a Central European City by Different Green Scenarios

Simulations of regional climate change in Central Europe project not only an increase of the near-surface temperature, but also an intensification of embedded heat waves. Combined with the demographic change, which leads to an enhanced portion of the risk group "elderly people", the future atmospheric situation presents urban planning with the challenge, to develop and apply methods in a preventive way in order to mitigate the regionally predetermined heat stress on the local urban scale for pedestrians. Several investigations have shown that the promotion of urban green turns out to be a particularly effective method to achieve mitigation aims under the point of view of urban human-biometeorology. Against this background specific simulations by use of the ENVI-met model, version 4.0, were conducted in an urban district in Freiburg, a mid-size city in Southwest Germany. Their general aim was to quantify the potential of three different green scenarios for the maintaining of thermal comfort for pedestrians during severe heat. The current situation in this district characterised by regularly arranged three-storey buildings in E-W and N-S orientation, street canyons in the same directions, grassland and trees represents scenario A. In scenario B, only all trees were removed. Thus, buildings, streets and grassland remained. In scenario C, the complete urban green was removed, i.e. the urban district consisted of only buildings and asphalt surfaces. To describe thermal comfort in a human-biometeorologically significant way, near-surface air temperature T, mean radiant temperature MRT and physiologically equivalent temperature PET were used. For each of the three scenarios, the simulations were performed on a clear-sky day (27 July 2009), which is characteristic for summer in Central Europe, and a day (4 August 2003) within the severe heat wave in August 2003. It exemplarily represents the thermal conditions in the future due to regional climate change. On 27 July 2009, additional experimental investigations on human thermal comfort were carried out at five sites within this urban district. Their results enabled the validation of the T and MRT simulation results for the scenario A. The correlations between simulated and experimentally determined T and MRT values were distinctly stronger than it was reported in the literature for previous versions of ENVI-met. For each of the three scenarios, the simulations provide T, MRT and PET results in different spatial scales. They were averaged over the period 10-16 CET as it represents the typical time scale of outdoor heat for Central European citizens on clear-sky summer days. The spatially detailed results are analysed on the micro-scale as well as for selected sectors, e.g. both sidewalks of an E-W oriented street canyon. In addition, mean values including standard deviations of T, MRT and PET are calculated for the complete simulation area. Thus, the influences of the three green scenarios on human thermal comfort can be quantified in a detailed way.