Application of an Advanced Energy Balance Model for Urban Canopies with Trees during Heat Wave Conditions

Trees have been shown to effectively mitigate heat stress in urban areas by providing shade and converting absorbed solar radiation to latent heat through transpiration. Trees located in urban canyons also interact with built elements in terms of radiation exchange and flow dynamics. With most of the ground covered by impervious materials, tree transpiration can be the main source of latent heat flux in the built environment. Tree transpiration is controlled mainly be the vapour pressure deficit, net radiation, soil water supply and stomatal control of water loss, which differs among species. During heat waves events, those conditions may experience extreme values, altering the tree transpiration and as a consequence the canyon energy balance.

The Building Effect Parameterization with Trees (BEP-Tree) is used here to model the impact of trees on the energy balance of a street canyon during heat wave conditions. BEP-Tree is a neighbourhood-scale multi-layer urban canopy model (UCM) that explicitly includes the interactions between vegetation and built elements in a street canyon. We have implemented an advanced stomatal conductance model in BEP-Tree in order to consider the effects of environmental conditions (air temperature and humidity), soil moisture and vegetation species on transpiration.

The sensitivity of the canyon energy balance to tree transpiration controlling factors is estimated using observations from recent heat wave events in Switzerland. Flux measurements are used to assess the performance of the improved model. The results of this study show the importance of using an advanced stomata model in urban canyon energy balance modelling for street canyons with trees.