Spectral surface albedo over Morocco and its impact on radiative forcing of Saharan dust

In May-June 2006, airborne and ground-based solar (0.3-2.2 mu m) and thermal infrared (4-42 mu m) radiation measurements have been performed in Morocco within the Saharan Mineral Dust Experiment (SAMUM). Upwelling and downwelling solar irradiances have been measured using the Spectral Modular Airborne Radiation Measurement System (SMART)-Albedometer. With these data, the areal spectral surface albedo for typical surface types in southeastern Morocco was derived from airborne measurements for the first time. The results are compared to the surface albedo retrieved from collocated satellite measurements, and partly considerable deviations are observed. Using measured surface and atmospheric properties, the spectral and broad-band dust radiative forcing at top-of-atmosphere (TOA) and at the surface has been estimated. The impact of the surface albedo on the solar radiative forcing of Saharan dust is quantified. In the SAMUM case of 19 May 2006, TOA solar radiative forcing varies by 12 W m(-2) per 0.1 surface-albedo change. For the thermal infrared component, values of up to +22 W m(-2) were derived. The net (solar plus thermal infrared) TOA radiative forcing varies between -19 and +24 W m(-2) for a broad-band solar surface albedo of 0.0 and 0.32, respectively. Over the bright surface of southeastern Morocco, the Saharan dust always has a net warming effect.