A numerical has been developed for the simulation of the wind, virtual potential air temperature and humidity fields in the
urban area. A space and ensemble averaging technique was applied for the derivation of three-dimensional Reynolds average equations for three components of wind velocity in three directions, virtual potential air temperature, water vapor mixing ratio, turbulent energy and turbulent dissipation rate. The space averaging technique allows computations inside the urban canopy layer with accounting for the volume of buildings and urban structures. This helps understanding of the thermal environment in the urban canopy layer, the most important part of the urban boundary layer.
Feedback of the ground surface and buildings to the atmospheric boundary layer is modeled by a building canopy model. Momentum exchange between buildings and urban structures and the wind field is modeled by a form drag force, which is a function of building wall density. Heat exchange between ground surface and building walls, roof and the air is modeled by a model for the heating of ground surface,
walls and roof. Net direct and diffused solar radiation to a surface facet in the urban canopy is modeled by a computation of the sun position, the view factor from the surface facet for the sky and reflectivity of the surface facet. Incoming longwave radiation to the surface facet is computed based on the view factor from the surface for the sky and other surface facets in the canopy. First reflective solar radiation in the building
canopy is computed by the view factor from the surface facet to other surface facets in the building canopy. Temperatures of the ground surface, walls and roof are computed by solving one-dimensional heat diffusion equation under the ground surface, and inside the walls and roof, respectively with an
assumption of constant temperature at large depth under the ground surface and in the room. The anthropogenic heat release to the atmospheric boundary layer by building air conditioning is evaluated based on a building air conditioner model, which computes the electricity needed for air cooling based on the conducted heat to the room through walls and roof, and human activity in the room.
Evapo-transpiration at the ground surface and plant canopy is evaluated based on the beta method and a model for the plant canopy microclimate.
Verification of the model by observational data at Nagahama city, Japan revealed that the model can successfully compute wind velocity, air temperature and humidity field in the urban area. The model can be very useful for the investigation of the turbulent structure above urban area and for the study
of the effects of urbanization on the climate of cities and surrounding areas. Furthermore, the model can be used for the study of effects of vegetation on the urban climate, and different alternatives for the improvement of urban climate.