Solar irradiation retrieval in Cameroon from Meteosat satellite imagery using the Heliosat_2 method

摘要

The radiation climate of Cameroon is not well known. Measurements of global and diffuse incident solar irradiations were carried out from 1982 to 1987 in ten meteorological stations throughout the country using Eppley PSP pyranometers. The distance between two neighboring stations was about 200 km. This ambitious program of collecting solar radiation data throughout the Cameroon territory was abruptly stopped in 1987 because the maintenance of the pyranometers network together with the monthly field data collection became too cumbersome and costly. It therefore became necessary to look for a valuable tool to retrieve incident solar irradiation from low cost satellite imagery such as the geostationary Meteosat satellite data. In this work we use a method referred to as Heliosat_2, well documented in [12, 13], and developed by Wald and his co-workers Rigollier and Lefèvre. This method, which uses known models of physical processes in atmospheric optics, consists of first calculating the atmospheric cloud index for each pixel from images acquired in the visible band of the Meteosat sensor. The cloud index is then related to the pixel clear sky index defined as the ratio of the observed irradiation to the clear sky irradiation that should be observed at the pixel site at that instant. The computation of the clear sky irradiation is done by solving the radiative transfer equation in a cloud free atmosphere as documented in the European Solar Radiation Atlas (ESRA 2000). We have performed calculations of incident solar irradiations over the Cameroon territory using 5 km resolution Meteosat imagery of the year 1997 available from the Modeling and Remote Sensing Group of the Ecole des Mines de Paris based at Sophia Antipolis. A real advantage with the Heliosat method is that we can calculate the incident solar radiation for contiguous pixels. To assess the performances of the method for African sites we have compared the solar irradiation values derived from Meteosat data using the Heliosat_2 method to ground measurements available in Yaoundé (3°52’ N ; 11°32’ E) and Garoua (9°20’ N ; 13°23’ E). We observed that the accuracy of the Heliosat method is very satisfactory for the monthly variations of the daily solar irradiations in sahelian sites, while in forest sites the variations of the retrieved irradiations do not fit the measurements. The accuracy of the Heliosat_2 method could be greatly improved if the method was implemented using all the “high resolution” Meteosat data received every half an hour in a day; the computations of the daily irradiations might therefore be more accurate.