ACCURACY OF AN EMPIRICAL-MODEL FOR ESTIMATING DAILY IRRADIANCE IN SOUTH-AFRICA FROM METEOSAT IMAGERY

An empirical model developed for use on data from the Japanese Geostationary Meteorological Satellite was applied to METEOSAT visible band data, in order to compute daily global irradiance at the Earth's surface (R(s)). The model uses sun-angle corrected pixel brightness values to estimate daily atmospheric transmissivity. The model was calibrated using METEOSAT data captured in December 1991. The regression coefficients obtained were used to determine R(s) from data received in 1992 and 1993. Comparisons were made between satellite-based estimates and ground measurements of irradiance at 16 weather stations. The RMSE of the satellite-based estimates compared with individual ground stations varied between months. The root mean square error (RMSE) was lowest during, winter with values ranging between 1.14 and 1.32 MJ m-1 day-1. The RMSE for summer months varied between 2.02 and 2.57 MJ m-1 day-1. Over 90% of the results obtained from the model were within 20% of the measured value. The model tended to overestimate irradiance on cloudy days. In general however, the technique offers a practical method of estimating spatially distributed irradiance in South Africa.