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
Figure 4: Example of the daily aerosol optical thickness product derived from MISR on 14 th of
July 2005. The colour scale ranges from 0 to 0.5. source: EOS LARC MISR web server
5.1.3 GOES
The GOES Aerosol-Smoke Product (GASP) provides aerosol optical depth retrievals over the U.S.
at 4 km spatial resolution and 30 minute intervals. The retrieval is performed over ocean and land
and is available once per day for the full disk of the Earth from GOES-East (providing information
for South America). The algorithm is described in Knapp et al. (2002, 2005). The main approach is
to build a surface reflectance reference by selecting, for each pixel, the second darkest image in the
composite time period and then to perform aerosol retrieval over land based on Look Up Tables of
gaseous and Rayleigh corrected reflectance for various surface reflectance and aerosol loading (see
an example of the AOT map over the US East coast for the 19 April 2005 in Fig. 5). The second
darkest pixel is chosen to reduce the effect of cloud shadows (shadowed pixel reflectance is
generally darker than surface -alone- reflectance). The reflectance reference is computed for a
composite time period varying along the year from 7 days to 28 days. The length of the time period
depends on the accuracy expected on the reflectance reference. More observations increase the
chance of observing cloud-free and aerosol-free day, but if too many days of observations are used,
the surface reflectance may change. For the eastern US, Knapp et al. found that 14-days period was
a good compromise. The main limitation is that it requires that the TOA reflectances increase with
increasing aerosol loading, and thus it does not work for a combination of bright surface and
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