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 b oundary layer above urban areas. The model is of k-epsilon type. The ensemb le-spatial averaged three-dimensional Reynolds equations, equation of conti nuity, turbulent kinetic energy equation (k-equation), and equation for dis sipation rate of turbulent energy (epsilon-equation) ars solved together wi th equations of heat and moisture transfer in the air. Inside the urban can opy layer, volumes of buildings and other urban structures are accounted fo r by a spatial averaging procedure. With given average building height and building width for each grid mesh, effects of buildings on the momentum tra nsfer are modelled by introducing a form drag force. Temperatures of the gr ound surface, building walls or roof are computed by the solution of the he at conduction equation in the ground or walls, roof. Evaporation at the gro und surface is evaluated using a Bowen ratio. The exhausted heat by buildin g air conditioning is evaluated by employing a building air conditioning mo del. 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 i nvestigate 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 mas s transfer in the urban boundary layer. Especially, the model can compute a ir temperature, humidity and wind velocity at the street level, which canno t be computed by a general above city atmospheric circulation model. (C) 19 99 Elsevier Science Ltd. All rights reserved.