Development of a numerical model for the evaluation of the urban thermal environment

A numerical model was developed for the computation of the wind field, air temperature and humidity in the urban canopy layer and in the atmospheric boundary layer above urban areas. The model is of k-epsilon type. The ensemble-spatial averaged three-dimensional Reynolds equations, equation of continuity, turbulent kinetic energy equation (k-equation), and equation for dissipation rate of turbulent energy (epsilon-equation) ars solved together with equations of heat and moisture transfer in the air. Inside the urban canopy layer, volumes of buildings and other urban structures are accounted for by a spatial averaging procedure. With given average building height and building width for each grid mesh, effects of buildings on the momentum transfer are modelled by introducing a form drag force. Temperatures of the ground surface, building walls or roof are computed by the solution of the heat conduction equation in the ground or walls, roof. Evaporation at the ground surface is evaluated using a Bowen ratio. The exhausted heat by building air conditioning is evaluated by employing a building air conditioning model. This heat together with traffic-induced artificial heat are accounted for in the model as heal sources. A numerical model for the momentum, heat and moisture transfer in the plant canopy is also coupled to the model to investigate the effects of vegetation on the urban climate. Verification of the model against observational data in the Tokyo Metropolitan area, Japan, reveals that the model is capable of simulating the momentum, heat and mass transfer in the urban boundary layer. Especially, the model can compute air temperature, humidity and wind velocity at the street level, which cannot be computed by a general above city atmospheric circulation model. (C) 1999 Elsevier Science Ltd. All rights reserved.