Urban albedo as a function of the urban structure ? A two-dimensional numerical simulation

Abstract

Urban albedo change as a function of urban geometrical structure has been examined by using a two-dimensional urban block-canyon array model. The complex multiple reflections of incident photons in the urban canyon are simulated by using a Monte-Carlo method. The photons are tracked until they leave the canyon or are completely absorbed. In the model, the direct and diffuse components of incident solar radiation are introduced and the specular and isotropic reflection characteristics are considered for the relevant urban surfaces. The result shows that the urban albedo decreases as the urban irregularity increases as indicated by the model experiment of Aida (1982). The dependence of albedo on the incident solar zenith angle observed in the experiment is also confirmed for various urban models.As an application, some actual urban structures in the Marunouchi area in Tokyo are examined. Urban planning for absorption and reflection of solar radiation in urban areas is also discussed based on the analysis of the change in albedo with canyon dimensions and solar zenith angle.

High-albedo materials for reducing building cooling energy use

“…In two-dimensional simulations, the effects of canyon geometry on effective urban albedo were demonstrated by Aida and Gotoh [1982] based on a Monte Carlo model developed by Craig and Lowry [1972]. Results from these two-dimensional simulations support the experimental findings reported above: geometry plays an important role in determining the effective urban albedo, and as irregularity increases, the effective albedo decreases (Figure 2).…”

“…In the above findings, it was assumed that all surfaces were reflecting at 50% isotropic and 50% specular. The difference between albedo of specular and diffuse surfaces is smaller at smaller zenith angle and larger at large zenith angles [Aida and Gotoh 1982]. Also, the effective albedo depends on the radiant intensity from reflecting surfaces which, in turn, vary as a function of building density.…”

“…Other simulations by Aida and Gotoh (1982) attempted to identify the urban density that would produce the lowest albedo values. They found that for a fixed building height, an urban area with canyon width about twice the width of buildings produced the lowest albedo at all zenith angles, and that as the streets get narrower (larger roof area per unit urban area) the effective albedo increases because of the reduced effect of the internal reflections within the canyons.…”

The Albedo of Pervious Cement Concrete Linearly Decreases with Porosity

“…Considering a typical summer day with a zenith solar radiation of 1000 W/m 2 , the pervious concrete can absorb 50∼ 150 W/m 2 additional solar radiation. At midday in summer, the evaporation of the dry pervious concrete contributes about 33-47 W/m 2 or even lower to the heat loss of the pervious surface [3,16,20]. Therefore, pervious pavements tend to absorb additional heat compared to dense pavements.…”

Measures at Buildings for Mitigating the Microclimate

“…However, any built-up area represents a composition of separate areas, being aligned in different angles, and entailing an average albedo-value which depends on the Zenith angle [21]. Moreover, in such models [21][22][23] the diurnal warming-up, the intermediate heat storage, and the subsequent cooling-down of the different areas are not taken into account. Hence they are of no practical use.…”