Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 5 4 2005 10.5194/acpd-5-6013-2005 http://www.atmos-chem-phys-discuss.net/5/6013/2005/ http://www.atmos-chem-phys-discuss.net/5/6013/2005/acpd-5-6013-2005.html http://www.atmos-chem-phys-discuss.net/5/6013/2005/acpd-5-6013-2005.pdf 6013 6039 2005-08-16 Improving cloud information over deserts from SCIAMACHY O₂ A-band N. Fournier P. Stammes M. de Graaf R. van der A A. Piters R. Koelemeijer A. Kokhanovsky KNMI, Climate Research and Seismology Department, De Bilt, The Netherlands RIVM, National Institute of Public Health and the Environment, Bilthoven, The Netherlands Institute of Remote Sensing, University of Bremen, Germany The retrieval of column densities and concentration profiles of atmospheric trace gas species from satellites is sensitive to light scattered by clouds. The SCanning Imaging Absorption SpectroMeter for Atmospheric CHartographY (SCIAMACHY) instrument on the Envisat satellite, principally designed to retrieve trace gases in the atmosphere, is also capable of detecting clouds. <br><br> FRESCO (Fast Retrieval Scheme for Clouds from the Oxygen A-band) is a fast and robust algorithm providing cloud information from the O₂ A-band for cloud correction of ozone. FRESCO provides a consistent set of cloud products by retrieving simultaneously effective cloud fraction and cloud top pressure. The FRESCO retrieved values are compared with both the SCIAMACHY Level&nbsp;2 operational cloud fraction of OCRA (Optical Cloud Recognition Algorithm) and cloud information deduced from the MODIS instrument. The results correlate well, but FRESCO overestimates cloud fraction over deserts. Thus, to improve retrievals at these locations, the FRESCO surface albedo databases are decontaminated from the presence of desert dust aerosols. This is achieved by using a GOME Absorbing Aerosol Index. It is shown that this approach succeeds well in producing more accurate cloud information over the Sahara.