Estimating diffuse irradiance on organisms: A comparison of results from isotropic and anisotropic sky radiance models

Abstract

In calculating the diffuse irradiances of both solar and longwave radiation on geometric solids chosen to simulate organisms or parts of organisms, it is conventional to employ an isotropic sky model. This simplifies computations but may be unrealistic for radiative fluxes of atmospheric origin. The purpose of this investigation is to evaluate the significance of error brought about by the adoption of the isotropic assumption to compute the diffuse irradiance on the sphere, the vertical circular cylinder, the horizontal circular cylinder, the cone, the inclined plane, the parallelipiped, the vertical elliptical cylinder, the horizontal cylinder with hemispheric ends and the vertical cylinder with an upfacing hemispheric end. Theory is presented for the diffuse irradiance on an infinitessimal element, and for that on the solid forms. Results using the isotropic assumption are compared to those obtained using empirically-derived radiance distributions for atmospheric longwave irradiance and for solar diffuse irradiance under both overcast and cloud-free skies. These experiments suggest that, for the longwave irradiance, the isotropic sky model performs well. For solar diffuse radiation, however, errors are larger. Data are presented which permit estimation of the magnitude and direction of the error under a variety of conditions. Ultimately, the decision to employ an isotropic or anisotropic sky model will depend on the nature of the application. The methods described provide a means of evaluating the likely error resulting from the adoption of the more convenient isotropic sky model.