Models for estimating solar irradiance at the earth’s surface from satellite data : an initial assessment

The performance of three models which use satellite data to estimate solar irradiance at the Earth's surface is assessed using measured radiation data from a mid-latitude location. In addition the mesoscale spatial variability in the global solar irradiance resolvable by the three models is also evaluated. The data are drawn from a twelve station pyranometric network and represent a variety of sky conditions at different times of the year. Assessment of the models depends upon the accurate Earth location of the satellite imagery and the merging of satellite and pyranometric data. The resolution of the satellite data for the study area is determined to be 0.82 km in the East-West direction and 1.67 km in the North-South direction. Based on this resolution the satellite imagery can be Earth located to within 1.25 km in an East-West direction and to within 1.71 km in a North-South direction. Merging of the satellite and pyranometric data results in the use of as many as three images to represent the radiation characteristics of a given hour. Initial applications of the three models reveal that the original regression coefficients for both the Tarpley (1979) and Hay and Hanson (1978) models are inappropriate for the study area because of the bias introduced. Subsequent revision of these coefficients leads to significant improvements under most conditions. Seasonal assessments of the three models demonstrate that on an hourly basis the overall performance of the Gautier et al. (1980) model under partly cloudy and overcast conditions is superior to that of the other two models. 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 (1978) model. Changes in temporal and spatial averaging configurations reveal that temporal averaging could have an important influence on the radiation estimates under partly cloudy and overcast conditions. Spatial averaging in the context of the Gautier et al. (1980) model does not support the use of an 8 x 8 pixel array to improve the temporal representation of the satellite data. In terms of the mesoscale spatial variability in the global solar irradiance, the best resolution is provided by the Gautier et al. (1980) model. For hourly values an average RMS error of ±17.1% limits the spatial resolution to approximately 15 km; for daily values an average RMS error of 8.2% limits the spatial resolution to approximately 12 km. Suggestions for improvements in the three models include; 1) a more accurate and explicit treatment of cloud absorption; 2) the consideration of the effects of changing Sun-satellite azimuth angle under overcast conditions and 3) in the context of both the Gautier et al . (1980) and Tarpley (1979) models there is the need for the inclusion of the effects of aerosols under clear skies and the accurate and objective specification of a cloud threshold.Arts, Faculty ofGeography, Department ofGraduat