Impact of the spacing between tree crowns on the mitigation of daytime heat stress for pedestrians inside E-W urban street canyons under Central European conditions

Abstract

Concerning Central European conditions the mitigation of daytime heat stress for pedestrians by evenly positioned plane trees on both sidewalks of a shallow and a deep E-W street canyon is quantified by scenario simulations. Using the ENVI-met model v4.0 BETA, they were performed for a previous heat wave day in Freiburg (Southwest Germany). Related to both sidewalks, the simulations are focused on the impact of the spacing between two adjacent tree crowns, which in turn depends on the size and number of trees. It determines the coverage of both sidewalks by tree crowns. The simulation results each averaged over 10–16 CET also refer to (i) the aspect ratios (H/W) of both street canyons and (ii) two values of the inflow speed (v) into the simulation domain. Human thermal comfort is determined by the mean radiant temperature (Tmrt) and physiologically equivalent temperature (PET). Due to the maximum solar elevation (59.3°) on the simulation day, spatially high-resolution PET results as well as spatiotemporal averaged Tmrt and PET results clearly show the dominant shading effect of the S-bordering building for pedestrians on the N-facing sidewalk. It masks the mitigation of their daytime heat stress only due to the trees. On the S-facing sidewalk, however, the spatiotemporal averaged Tmrt and PET results are distinctly lowered by the tree shading as compared to tree-free conditions. The reduction of the Tmrt and PET values increases as the spacing between their crowns decreases, i.e. the crown coverage gets larger. This human-biometeorological heat stress mitigation is more pronounced inside the shallow street canyon (H/W = 0.5) than inside the deep one (H/W = 2.0). Regardless of crown coverage, orientation of the sidewalks, H/W ratio and v, the reduction of the spatiotemporal averaged air temperature due to tree shading is almost marginal.