The effect of urbanization can be represented in a numerical model by several factors, which are anthropogenic heat , trapping of both short and long wave radiation due to higher buildings, the low water content of urban surface due to decrease of vegetation cover and well established drainage networks, and large heat storage of urban structures. What is the main cause of the heat island? In order to get a clue to the answer of this question, we developed a single layer urban canopy scheme for the Japan Meteorological Agency Non Hydrostatic Model (NHM). The basic concept of this urban canopy scheme is similar to that of Masson(2000), Kusaka(2001) and Kanda(2005), however the present scheme slightly differs from them in representation of bulk resistance circuit. The representation of wind velocities inside the canyon and the setting of bulk coefficients were applied from the vegetation canopy scheme of Deardorff(1978).

A case study has been done for the hottest days of 2007 in the Kanto Plane, Japan. The initial date was August 13, 2007. The integration period was 72 hours. The first 24 hours were for model spin up. The following 48 hours were used for analysis with focus on the effect of anthropogenic heat and radiation trapping. In the experiment of anthropogenic heat impact, simulations with and without anthropogenic heating were made. In the experiment of radiation trapping, the ratio between building height and street width was changed but the total volume and the area fraction of buildings were remained the same, in order to keep the total drag of urban canopy between the simulations.

The simulation results show that the anthropogenic heat gives the temperature rise of about 0.2 degrees on an average of the urban area and about 0.7 degrees at a grid point in central Tokyo. The decrease of sky-view factor due to the increase of the canyon aspect ratio is found to reduce the daily range of the surface air temperature by about 0.5 degrees on an average of the urban area.