Improving cloud information over deserts from SCIAMACHY Oxygen A-band measurements

The retrieval of column densities and concen- tration profiles of atmospheric trace gas species from satel- lites is sensitive to light scattered by clouds. The SCanning Imaging Absorption SpectroMeter for Atmospheric CHar- tographY (SCIAMACHY) instrument on the Envisat satel- lite, principally designed to retrieve trace gases in the atmo- sphere, is also capable of detecting clouds. 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 pro- vides a consistent set of cloud products by retrieving simul- taneously effective cloud fraction and cloud top pressure. The FRESCO retrieved values are compared with the SCIA- MACHY Level 2 operational cloud fraction of OCRA (Opti- cal Cloud Recognition Algorithm) but, also, with cloud infor- mation from HICRU (Heidelberg Iterative Cloud Retrieval Utilities), SACURA (SemiAnalytical CloUd Retrieval Algo- rithm) and the MODIS (Moderate Resolution Imaging Spec- troradiometer) 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 the GOME Absorbing Aerosol Index. It is shown that this approach succeeds well in producing more accurate cloud information over the Sahara.

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