An Approach to the Estimation of the Surface Radiative Properties and Radiation Budgets of Cities

The principles of radiation geometry and the Lambertian assumption are employed to construct a numerical model of the multiple reflection effects within an urban canyon of variable geometry and surface materials. The canyon model is used to estimate the reflection coefficients for the direct and diffuse short-wave and incoming longwave radiative fluxes and the longwave emissivity of an urban surface. The procedures described are applied to various land-use zones in Columbus, Ohio, for the solstices and equinoxes. The diurnal variation of shortwave reflection coefficients is illustrated; daily values generally increase from the CBD to the residential suburbs. Longwave radiative properties differ little between zones. Radiation budgets are synthesized for each land-use type, assuming negligible atmospheric pollution, and are compared with those for appropriate rural surfaces. Net longwave radiation varies little between zones but net shortwave and net radiation decrease from the CBD to the residential areas at the city periphery. Net radiation gradients in cities are shown to depend critically on the relative effects of urban atmospheric pollution on the incident fluxes of longwave and shortwave radiation.