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
5.1.1 Algorithms using angular information
The POLDER aerosol retrieval over land is based on a Bidirectional Reflectance Distribution
Function (BRDF) model deduced from polarized and multidirectional radiances measurements in
the red and the NIR channels. This model has empirical coefficients adjusted for different classes of
land surfaces (Deuzé et al. 2001). Because of the geometry dependence of the surface reflectance,
multidirectional measurements are a promising method for the aerosol property retrievals over land.
Veefkind et al. (2000) developed a method based on the dual-view image radiometer of the Along
Tracking Scanning Radiometer 2 (ATSR-2). The method is based on the wavelength-independent
ratio between forward and nadir view surface reflection that depends only on the sun/satellite
geometry. The value of the ratio is first estimated in the Mid-IR channel of ATSR-2 where the
atmospheric contribution is neglected. It can be combined with another sensor with high spectral
resolution to obtain information about the aerosol model to use. For example an algorithm
combining AASTR and SCHIAMACHY aboard ENVISAT or ATSR-2 and GOME aboard ERS-2
has been developed and provides AOT and aerosol types (Holzer-Popp et al. 2002). Another
multidirectional measurements algorithm has been developed for MISR. The algorithm uses the
presence of spatial contrast to derive empirical function representation of the angular variation of
the scene reflectance, which is then used to estimate the path radiance (Martonchik et al. 1998).
Figure 1 and 2 present examples of AASTR/SCHIAMACHY and POLDER-3/PARASOL
retrievals, respectively.
5.1.2 Algorithms using spectral indices
MODIS uses a classical approach to predict the surface reflectance, which relates the TOA
radiances in the IR at 2.13 µm to the visible surface reflectance in the blue (0.47 µm) and in the red
(0.66 µm). Aerosol retrieval (spectral optical thickness in two channels) is performed for targets as
bright as 0.25 in reflectance unit in the IR and an average is performed in a 20x20 box after
rejecting some outliers, thus resulting in a 10x10km spatial resolution for the aerosol product
(Kaufman and Tanré, 1998). After intensive validation with AERONET data, Remer et al. (2005)
show that the AOTs accuracy is within ∆τ = ±0.05 ±0.15τ. The surface coverage is good as can be
seen in Fig. 3. This product is available twice a day (Terra, ~10:30 p.m. and Aqua, ~1:30 p.m.).
MERIS aerosol property retrievals over land are based on the use of pixels covered by vegetation.
Dense Dark Vegetation pixels are selected using the Atmospheric Resistant Vegetation Index
(ARVI) as spectral index, which uses Rayleigh corrected reflectances at 443, 670 and 865 nm.
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