Aerosol retrievals from METEOSAT-8 - CM SAF

SAF on Climate Monitoring Visiting Scientists Report Doc. No: 1.0
Issue : 1.0
Date : 4 October 2006
Appendix C
Clear sky observations and contribution of the temporal
resolution of MSG/SEVIRI
Dominique Jolivet and Didier Ramon (HYGEOS)
&
Jérôme Riedi and Jean-Marc Nicolas (LOA)
The algorithm to separate cloud free from cloud contaminated or cloud filled pixels has been
developed with the intention of being applicable globally without requiring ancillary data such as
surface temperature or atmospheric profiles. Each test implemented within the algorithm has been
designed based on fundamental physical differences (emissivity, spectral behaviour) between
clouds and surface.
Many lessons learned from the Moderate Resolution Imaging Spectroradiometer (MODIS),
regarding discrimination between clouds and heavy aerosols events or sunglint for instance (spatial
variability, spectral behaviour), have been taken into account in the design of this cloud detection
scheme. However, the present algorithm shall not be considered as just a simplified MODIS-like
cloud mask because : (i) some tests have been adapted and modified to account for the differences
in spectral channels, calibration and/or spatial resolution and make them applicable to SEVIRI, (ii)
the number of tests used is currently much smaller than the one used in the operational MODIS
algorithm (Ackerman et al. 1998; Platnick et al. 2003) and (iii) the decision logic differs
significantly from the one used for MODIS (http://www-loa.univ-lille1.fr/~riedi/).
The input to the SEVIRI cloud detection algorithm consists of normalized reflectances from the
visible (0.6 and 0.8 µm) and near-infrared (1.6 µm) channels, whereas brightness temperatures are
used from the thermal infrared channels (3.8, 8.7, 10.8 and 12.0 µm). Additionally, the algorithm
uses ancillary data on solar and viewing geometry and a land/sea map but none is required from
model reanalysis. There are spectral threshold and spatial coherence cloud detection tests that are
different for land and ocean surfaces. The various tests implemented are grouped together in such a
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