Online coupling of an urban canopy model with trees and a mesoscale atmospheric model to assess the cooling effects of trees

Urban trees play a crucial role in mitigating heat stress. Accurately representing the physical processes associated with trees in mesoscale models is essential for reliable simulations of urban environments across city to regional scales. We implement an urban canopy model incorporating trees into the Weather Research and Forecasting model to develop a fully coupled modeling system. The model is evaluated against pedestrian-level temperature, flux tower, and soil moisture measurements from the Seoul metropolitan area. Two heatwave episodes are simulated, and the model demonstrates reasonable performance in reproducing intra-city temperature differences, canyon air temperatures, and sensible and latent heat fluxes. A stronger cooling effect of trees is observed in commercial/industrial areas, with a daily mean temperature reduction of 1.1°C compared to 0.64°C in residential areas. The cooling effect is pronounced at night in narrower and deeper canyons, attributed to the greater longwave cooling of leaves. As soil moisture decreases, the cooling effects of trees diminish; however, significant cooling persists under very dry conditions due to tree shading, which is more prominent in commercial/industrial areas than in residential areas. Our findings indicate that comprehensive studies encompassing various tree and urban configurations are necessary to optimize the role of trees in sustainable cities.