Development and implementation of tree processes in an urban canopy model
1Princeton University, United States of America; 2Arizona State University, United States of America
Trees in cities can play an important role in modifying the surface energy balance and thus have great potential for improving the thermal environment in cities. In this study, physical processes associated with trees are implemented in the Princeton Urban Canopy Model, which include radiation interception/exchange in urban canyons, sensible and latent heat exchange with canyon air, transpiration, and root water uptake. A geometrical approach to radiation processes is adopted, considering two symmetric tree crowns in the canyon. This approach takes into account shadows by trees and buildings for direct shortwave radiation and multiple reflection for diffuse/reflected shortwave radiation and longwave radiation. The Monte Carlo method is employed to obtain view factors that are used for multiple reflection computation. Sensible heat exchange is computed using leaf boundary layer resistance, and the transpiration, that is basically equivalent to latent heat release from a leaf, is computed based on the Penman-Monteith equation for a single leaf. The developed model is validated against observations at two urban sites and a suburban site. The model including tree processes clearly and consistently shows an increase in latent heat and decreases in sensible heat and canyon air temperature for all the three sites, and these results are in better agreement with observations compared to the results without trees. It is of interest that the tree effects are larger for the narrower urban canyon with smaller natural area fraction that corresponds to the densely built-up urban site.