Abstract
It is very important for urban climatologists to consider the heat balance at the urban area surface in order to understand the mechanisms of the urban climate. One of the questions is whether the surface albedo of urban areas is higher or lower compared with that of the suburbs. A model is developed for an urban area with a grid pattern network of roads and cubic buildings, the surface microstructure of which can be expressed in terms of both the building-land ratio in the total model area and the sky view-factor at the center of the intersection of the model. A technique is also developed to simulate their upward reflected radiance or albedo as a function of the surface structure and the solar elevation when the solar illumination direction is parallel to and at an angle of 45 degrees with the street canyon. In the simulation, the incoming solar radiation is divided into two components, i. e., direct and diffuse radiations, and the amount of upward reflected radiance is formed by four components : primary reflection from the roof, and from primary through tertiary reflections from the canyon floor. It is assumed in this simulation that all the facets of urban area surface are Lambertian, and that all the components of shortwave radiation are isotropic, exclusive of direct solar radiation.
Results from simulation show that the upward reflected radiance, i.e., albedo also, depends clearly upon the building-land ratio, the sky view-factor and the solar elevation: The urban surface albedo is relatively high in both extremely high and low building-land ratio regions, although it is significantly decreased in the middle building-land ratio region. In addition, it is more sensitive to the decreases in the sky view-factor and solar elevation in such a region. The above effects are more significant when the solar illumination direction is at an angle of 45 degrees with the street canyon than when the solar illumination is parallel to the street canyon.
A comparison of satellite-derived albedos in the Kanto Plain on November 5, 1990, by Nakagawa and Nakayama (1995) with the simulated cases in this study confirms that the simulated results by the model, in which the solar illumination direction is at an angle of 45 degrees to the street canyon, are in good agreement with the field measurements, instead that the urban structure of the model is not realistic and the albedos of their facets are assumed to be the same. These facts mean that, if the albedos of all elements of the urban area surface are the same, the albedo of the urban area is decreased as the building-land ratio reaches the middle range and the sky view-factor is decreased, and that this results from trapping of direct solar radiation illuminating the sunny walls in the urban canyon, which would illuminate the canyon floor that is shadowed by obstructions.
