Assessing surface solar irradiance in Northern Africa desert climate and its long-term variations from Meteosat images

Two databases of solar surface irradiance (SSI) derived from satellites are compared to ground measurements for Algeria, Egypt, Libya and Tunisia. It is found that it is possible to accurately derive the SSI from geostationary meteorological satellites, even with a coarse spatial resolution. The two databases HelioClim-1 and SSE exhibit similar and good performances. The bias is lower for SSE than for HelioClim-1, as a whole; inversely, HelioClim-1exhibits a smaller scattering of data compared to ground measurements (smaller standard-deviation) than the SSE, allowing better performances when mapping the long-term variations in the SSI. These long-term variations from 1985 to 2005 show that these four nations experience dimming as a whole. Detailed analyses of the range of dimming at sites with long-term records and of its spatial distribution have been performed. It has been found that the analysis of SSI from HelioClim-1 supports the findings for the individual sites. Several phenomena may explain the dimming. One is the transportation of sand dust northwards from the Sahel; another ha l-0 03 56 12 2, v er si on 2 2 Ja n 20 10 Author manuscript, published in "International Journal of Remote Sensing 31, 01 (2009) 261-280" DOI : 10.1080/01431160902882645 Assessing solar irradiance in Northern Africa from Meteosat images 2 _________________________________________________________________________________________ one is the increase in urbanization and a third one is the increase in cloud cover and aerosol loading.