An Assessment of Models which use Satellite Data to Estimate Solar Irradiance at the Earth's Surface

Abstract

The performances of three models which use satellite data to estimate solar irradiance at the Earth's surface are assessed using measured radiation data from a midlatitude location. Assessment of the models is made possible through the accurate Earth location of the satellite imagery (to within±2 pixels). Evaluations of the models for a variety of conditions reveal the need for revised coefficients for the Hay and Hanson model and Tarpley model and demonstrate the superior performance of the physically-based Gautier et al. model on an hourly basis for partly cloudy and overcast conditions. However, compared to the clear sky case all three models give poor results under partly cloudy and overcast conditions. An increase in the averaging period leads to marked decreases in the rms errors observed for the three models under all conditions, with the greatest improvement occurring for the Hay and Hanson model. Suggestions for improvements in the three models include 1) a more accurate and explicit treatment of cloud absorption and 2) in the Gautier et al. model and Tarpley model, the inclusion of the effects of aerosols under clear skies and the accurate and objective specification of a cloud threshold separating clear from partly cloudy and partly cloudy from overcast conditions.