The characterization and modelling of the diffuse radiance distribution under partly cloudy skies

Abstract

Results are presented of a detailed analysis of a large set of sky radiance measurements taken in 1982 by an automated robot system built and operated at the University of Toronto, which takes narrow field-of-view sky radiance measurements with a precision and frequency adequate for most analysis and modelling purposes. The analysis of these data has confirmed the supposition that clouds strongly affect the distribution of the diffuse radiance, and has shown that the spatial distribution of the radiance for skies categorized according to cloud type and amount is statistically influenced in an orderly manner by the presence of the clouds. It was observed that the distribution across the sky of the time mean values of the diffuse radiance exhibits circumsolar and horizon brightening, the degree of which depends primarily on cloud type and amount, and on solar zenith angle. Trends were identified in the changes in the sky radiance distributions corresponding to variations in solar zenith angle, cloud amount and cloud group. The measured data appeared to be generally compatible with a model of the form of the Three-Component Continuous Distribution (TCCD) model introduced by Hooper and Brunger, and it was concluded that successful descriptors of the sky radiance could be developed based on that model, or on similar formulations.