Assessing surface solar irradiance and its long-term variations in the northern Africa desert climate using Meteosat images

Abstract

Two databases of solar surface irradiance (SSI) derived from satellites were compared to ground measurements in Algeria, Egypt, Libya and Tunisia. We found that it was possible to accurately derive the SSI from geostationary meteorological satellites, even with a coarse spatial resolution. The two databases HelioClim-1 (HC1) and SSE exhibited similar and good performances. The bias was generally lower for SSE than for HC1; however, HC1 exhibited a smaller scattering of data compared to ground measurements (smaller standard deviation) than the SSE, allowing better performance when mapping the long-term variations in SSI. The long-term variations in SSI from 1985 to 2005 show that these four countries as a whole experienced dimming. Detailed analyses of the range of dimming at sites with long-term records and of its spatial distribution were performed. We found that the analysis of SSI from HC1 supports the findings for the individual sites. Dimming may be explained by: (1) transportation of sand dust northwards from the Sahel, (2) an increase in urbanization, and (3) an increase in cloud cover and aerosol loading.

Acknowledgements

The help of the archives department of Eumetsat is acknowledged for the provision of several years of Meteosat images. We thank the Egyptian Meteorological Authority and the World Radiation Data Centre for the provision of ground measurements. For the French team, this study has been performed partly under the project MESoR (Management and Exploitation of Solar Resource Knowledge) funded by the European Commission and under a grant from the French Agency for Environment ADEME under the auspices of Task SHC 36 of the International Energy Agency.

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