Impact of biomass burning in equatorial Africa on the downward surface shortwave irradiance: Observations versus calculations

Long-term ground-based measurements of downward surface shortwave irradiance are compared with the satellite-derived downward surface shortwave irradiance for equatorial Africa. Good agreement between satellite-derived and measured values are shown in the spring and fall (seasons relative to the northern hemisphere). Large differences between satellite-derived and measured values are found in regions where extensive savanna fires take place during the dry season. Discrepancies are found in regions north of the equator during the winter months, and the differences shift to the southern hemisphere during summer. Significant differences of 40-80 W m(-2) are found for extended areas, corresponding to 25-40% of the measured monthly mean downward surface shortwave irradiance. Savanna fires release large amounts of particles into the atmosphere, increasing the aerosol optical depth. The increased optical depth leads to an overestimation of the downward surface shortwave irradiance by the satellite reduction algorithms which do not account for the fire-produced aerosol.