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
retrievals from SEVIRI there was limited time to elaborate a complete and robust algorithm. The
feasibility of MSG/SEVIRI for retrievals of aerosol properties over land could be demonstrated, but
with a lot of assumption that could have been avoided if more time would have been available.
We used a simple cloud mask. The algorithm can then be upgraded and improved by using high
performance cloud mask from the CM-SAF or NWCSAF. For the reference surface reflectance
map, the cloud mask should include the detection of neighboured cloudy pixels (i.e. cloud free pixel
near cloudy pixels) to exclude effects due to cloud borders. In future algorithm developments it is
preferable to use the official cloud mask and to include a morphological mask that removes
shadowed pixels that have lower reflectances than the surface. In appendix A the results for the
northern part of the disc, as observed by MSG, are given using a more restrictive cloud mask. We
only used one aerosol model: a continental model used in MERIS LUT's. It would be interesting to
use appropriate model just based on simple climatology: desert dust over Africa, maritime model
over ocean and burning biomass over Amazonia. As presented in the next section called “future
improvements”, the use of others channels (0.865 µm and 1.650 µm) should be researched to get
information on the spectral dependence of aerosols and an index of the size distribution which will
be an indicator of the aerosol type.
We used the single scattering assumption to simplify the calculations and to avoid use of LUT's,
which could not be prepared due to the limited time available for this “visiting scientist” activity. Of
course it is a source of uncertainty and systematic bias. Neglecting multiple scattering in the
atmosphere is a strong assumption, especially when large amount of aerosols occurs and in the case
of large solar-viewing zenith angles. This point can be easily solved by using Look Up Tables
computed with an accurate radiative transfer model. The limitation is that a lambertian surface has
to be assumed in those computations. However it is possible to correct for bidirectional effects
when one knows the shape of the BRDF (Vermote et al. 1997, Ramon et al. 2001), which can be
partly measured by SEVIRI. So an iterative approach seems possible here.
7 Conclusions and perspectives
SEVIRI is a powerful instrument for monitoring rapidly changing parameters of the atmosphere,
and tropospheric aerosols fall into this category (see Appendix B for forest fires in Portugal in July
2005). The survey of user requirements showed the need for an aerosol product from MSG/SEVIRI
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