Satellite-derived surface radiation budget over the African continent. Part I: Estimation of downward solar irradiance and Albedo

Downward surface solar irradiance and albedo of the African continent are e stimated from Meteosat B2 data at 30-km spatial resolution. The algorithm, based on Dedieu et al.'s approach, is verified against other satellite esti mates and ground-based measurements. In the computations, the International Satellite Cloud Climatology Project's (ISCCP) radiometric calibration is a djusted using the Libyan desert as a reference target of constant reflectan ce properties. Surface albedo is corrected for sun zenith angle effects, al lowing for better detection of seasonal changes due to the vegetation cycle . The estimates obtained with Meteosat B2 data agree generally well with ot her satellite estimates, although biases of 20 W m(-2) (downward surface so lar irradiance) and 0.15 (surface albedo) are obtained in some cases. There is evidence, from comparisons with surface measurements, that the clear-sk y downward surface solar irradiance is overestimated over semiarid regions of Africa because of uncertainties in aerosol characteristics. In the Sahel region, where spatial albedo gradients are high, it is advantageous to use 30-km Meteosat B2 products instead of the current, coarser 280-km-resoluti on ISCCP products.