Solar elevation impact on the heat stress mitigation of pedestrians on tree-lined sidewalks of E-W street canyons – Analysis under Central European heat wave conditions

Abstract

For both tree-lined sidewalks of a shallow and deep E-W street canyon located in the city of Freiburg (Southwest Germany), the solar elevation impact on the magnitude of the daytime human heat stress mitigation (hhsm) is analysed in dependence of different tree scenarios. Identic ENVI-met simulations are carried out on the summer solstice day 21 June 2003 and heat wave day 4 August 2003. All simulation scenarios indicate an almost negligible solar elevation impact on hhsm in terms of spatiotemporal averaged air temperature. The results achieved on both simulation days for the spatiotemporal averaged mean radiation temperature (Tmrt) and physiologically equivalent temperature (PET) as well as spatially high-resolution PET reflect that the north-facing sidewalk in both street canyons is entirely shaded by the south-bounding building. Secondarily it is influenced by lower radiant flux densities from the trees near the curb edges of both sidewalks. On both simulation days, the south-facing sidewalk in the shallow street canyon is only shaded by the tree crowns. In the deep street canyon, however, the south-facing sidewalk is completely shaded on 4 August by the south-bounding building, while on 21 June this shade is limited to its southern half, i.e. its northern half is directly influenced by the shade of trees. Due to these shading conditions, the results focused on pedestrians on both sidewalks show different patterns of the solar elevation impact on Tmrt and PET as well as hhsm in terms of Tmrt and PET. While increasing tree crown projection areas lead to a lower solar elevation impact on Tmrt and PET, they cause a more distinct hhsm in terms of Tmrt and PET for higher solar elevations. The non-negligible magnitude of the solar elevation impact in all scenarios leads to the recommendation to carry out ensemble simulations in order to achieve Tmrt, PET, hhsm-Tmrt and hhsm-PET results, which are reliable for planning applications.