A Multi-Layer Urban Canopy Meteorological Model with Trees (BEP-Tree): Street Tree Impacts on Pedestrian-Level Climate

Urban greening is a key design strategy for moderation of local urban climate, and many cities already boast extensive vegetation. Unlike shorter vegetation, urban trees impact local climate via shade and shelter. Urban canopy models (UCMs) are coupled with mesoscale or regional climate models for assessment of neighbourhood-scale climate, but their representation of urban trees is currently limited. We present BEP-Tree, a multi-layer UCM that integrates trees in place of the ‘tile’ approach. BEP-Tree resolves the microclimate underneath trees where outdoor human thermal exposure occurs; these conditions are largely inaccessible to current mesoscale modelling and remote sensing approaches. Moreover, BEP-Tree allows trees to protrude above buildings, enabling assessment of low-rise neighbourhoods. The new model combines several existing models with detailed radiative and hydrodynamic models that assess built-tree interactions and include new parameterizations for impacts of tree foliage distribution. BEP-Tree is evaluated against datasets from three cities enabling assessment of modelled radiation, energy exchanges and road and air temperatures at multiple scales and across the diurnal cycle. Urban trees redirect sensible heat into latent heat and reduce pedestrian-level solar radiation, wind, and temperatures during daytime. The model suggests that thermal climate and energy exchanges are more sensitive to street tree density than height. Finally, BEP-Tree is applied to assess the effects of urban trees on multiple urban canopy layer processes and associated impacts on pedestrian-level temperatures across the diurnal cycle.