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Ann. Geophys., 24, 2065-2073, 2006
www.ann-geophys.net/24/2065/2006/
© European Geosciences Union 2006
Improvement of the detection of desert dust over the Sahel using METEOSAT IR imagery
G. Vergé-Dépré1, M. Legrand1, C. Moulin2, A. Alias3, and P. François1
1Laboratoire d'Optique Atmosphérique, CNRS-USTL, Villeneuve d'Ascq, France
2LSCE/IPSL, CEA-CNRS-UVSQ, Gif-sur-Yvette, France
3Météo France CNRM/GMGEC-EAC, Toulouse, France
Abstract. Desert dust over the arid regions of Africa is detected using the Infrared
Difference Dust Index (IDDI) derived from the thermal infrared (TIR) channel
of METEOSAT. However, the comparison with photometric aerosol optical
thickness (AOT) of this dust index reveals some discrepancies. Using an
instrumented site in Sahel where aerosol properties and meteorological
conditions were monitored daily during the dry season, we performed
radiative transfer computations with the MODTRAN 4.1 code to develop a
method to improve the IDDI usefulness. We found that discrepancies between
AOT and IDDI variations mostly come from changes in the surface temperature
(Ts), which is an important parameter for radiative transfer
computations in the TIR. We show that this temperature varies from day to
day with the surface wind speed and during the course of the season with the
solar elevation, and that it is possible, for the site considered, to
correct Ts from these combined effect using a simple parameterization.
We also observe that the dust layer itself has an impact on Ts by
reducing the amount of solar radiation at the surface, and that this
phenomenon can also be accounted for by adding an AOT-dependence to the
above parameterization of Ts. We show that this parameterization allows
improving the agreement between the IDDI and the photometric AOT.
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