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SEVIRI - AEROSOL RETRIEVAL
SEVIRI - AEROSOL RETRIEVAL
Observations of Saharan dust
ORAC
This work is based on the ORAC aerosol retrieval algorithm, developed at Oxford University and Rutherford Appleton Laboratory
for the visible and near infrared channels of ATSR and SEVIRI.
We are working to extend the algorithm to include the two SEVIRI infrared
window channels centered at 10.8 and 12.1 microns.
The aerosol parameters we retrieve are optical depth (at 550nm) and effective radius.
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SEVIRI
Spinning Enhanced Visible and Infra-Red Imager (SEVIRI)
On board of Meteosat Second Generation (MSG) geostationary satellite
Spatial resolution 3 Km. 15 min time resolution.
SEVIRI has 12 channels in the 0.6-14mm range. We use
3 VIS-NIR + 2 IR channles centered at 0.640,0.809,1.64, 10.78, 11.94 [um]
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FORWARD MODEL
The radiative transfer model used for the vis/nir channels takes into account atmospheric scattering and absorption as
obtained by DISORT (DIScrete Ordinate Radiative Transfer) radiative transfer code.
For the IR channels DISORT is used to parameterize the aerosol scattering, absorption and emission terms which are
combined with the clear-sky and surface contributions
(themselves based on ECMWF data and RTTOV optical depth computations).
The forward model uses an aerosol database of macrophysical
optical properties computed from different sets of published aerosol micophysical properties.
Details:
- aerosol optical properties from OPAC or user defined components (updated for two IR channels)
- gas absorbing optical depth profile from MODTRAN computation (VIS) and from ECMWF data + RTTOV computation (IR) .
- vertical profile of temperature from ECMWF data.
- radiative transfer solved calling DISORT for absorption, scattering and emission.
- vector of Lambertian spectral surface reflectances for SEVIRI VIS/IR channels from MODIS data.
- geometric conditions (satellite zenith angle, solar zenith angle, relative azimuth angle).
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Really important is the choice of the microphysical properties, and in particolar of aerosol spectral refractive index.
The retrieval is sensitive to the consistency of the assumed IR and vis/nir refractive indices: not all aerosol models
which result in reasonable fits to the measurements in the vis/nir range are capable of simultaneously fitting IR radiances.
The IR channels are sensitive to large (effective radius ~> 1um) aerosol particles (like desert dust),
to surface temperature and spectral emissivity.
The analysis is focused on a desert dust for which the IR channels
are expected to add useful information to the solar channels, by adding sensitivity over the bright desert
surface and information on aerosol layer height.
 Optical properties scheme
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VIS-NIR FORWARD MODEL
Radiances are modelled by combining the spectral surface reflectance with pre-computed LUTs of the atmospheric reflectances and transmittances.
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IR FORWARD MODEL
We need to divide the contributions between atmosphere and aerosol layer components.
Aerosol layer optical properties are precompiled in LUTs.
Other atmospheric parameters (radiances above/below aerosol layer going up/down) are computed with RTTOV using ECMWF atmospheric profiles.
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PRELIMINARY RESULTS: 3-20 March 2006
Sequence of aerosol optical depth retrievals from SEVIRI images at noon using microphysical properties from OPAC database
and Dan refractive index in IR spectral range.
It is possible to observe the desert dust event starting on 6 march 2006
over north west Africa and propagating and spreading to the south in the following days.
Maintained by Elisa Carboni
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