A numerical model has been developed for the simulation of the thermal climate in the urban area. Three-dimensional time averaged Reynolds averaged Navier-Stokes equations, continuity equation and heat and moisture transfer equations in the atmosphere are solved using a 2.5 level turbulence closure model as that of Mellor and Yamada (1974). Forest and building canopies are incorporated in the momentum computation as sinks of momentum by introducing form drag forces. Temperature in the forest canopy are computed by evaluating the absorption rate of short and longwave radiation by the tree leaves. Temperatures of the roof, walls of the building and ground surface are computed by solving one-dimensional heat conduction equation be¬low each surface with appropriate boundary conditions. Fit back of the surfaces to the atmosphere is evaluated through the exchange of sensible heat, longwave radiation and moisture between the surfaces and the atmosphere. Anthropogenic heat is incorporated in the model as a source of heat.
The model was validated with observations carried out at Tama New Town, a city in the west of the Tokyo Metropolitan Area, Japan. A comparison between computed and quantitative measurements on air temperature and relative humidity at many locations inside the Tama Central Park and the surrounding areas reveals that the model can reasonably reproduce the real heating processes here. Computational results and observational data indicated that vegetation could significantly alter the thermal climate in the town. At noon, the highest temperature of the ground surface at the grass field in the park was 40.3°C, which was 19K lower than that of the asphalt surface or 15K lower than that of the concrete surface in the parking or commercial areas. Consequently, at the same time, air temperature measured at 1.2m above the ground at the grass field inside the park was more than 2K lower than that measured at the same height in the surrounding commercial and parking areas. Soon after sunset, temperature of the ground surface at the grass field in the park became lower than that of the air and the park became a cool island whereas paved asphalt or concrete surfaces in the town remained hotter than the overlying air even at late night hours. The hot ground surface in the town heated the overlying air through upward sensible heat and longwave radiation, which together with anthropogenic heat kept air temperature here higher than that in the park.