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Technique to retrieve the spectral aerosol optical thickness using fast radiative transfer code and its application to MERIS data

Eleonora Zege(1), Iosif Katsev(1), Alexander Prikhach(1), Arkady Ivanov(1) and Alexander Kokhanovsky(2)

(1) B. I. Stepanov Institute of Physics, National Acad, Pr Nezavisimosti 68, 220072, Minsk, Belarus
(2) Institute of Environmental Physics,, O. Hahn Allee 1, D-28359 Bremen, Germany

Abstract

We will present two versions of the new technique to retrieve aerosol optical thickness (AOT), Angstrom parameter, and land reflectance from satellite data. The most important difference of the proposed techniques from MODIS, MERIS and some other well-known AOT retrieval codes is that our retrievals do not use the Look-Up-Table (LUT) technique but instead they are based on our earlier developed extremely fast code RAY for radiative transfer (RT) computations. The first version, Aerosol Retrieval Technique (ART), is completely based on the RT computations. The second one, FAR (Fast Aerosol Retrieval), includes the approximate analytical solutions for the multiply scattered part of radiation instead of RT computations. The accuracy of these approximate solutions is thoroughly checked with accurate RT computations. The main advantage of FAR is the considerable time saving. Using the RT computations in the course of the AOT retrieval allows one to include any available local models of molecular atmosphere and of aerosol in upper and middle atmosphere layers for the treated area. Any set of wavelengths from any satellite optical instruments can be processed. Moreover, we use the method of least squares in the retrieval of optical parameters of aerosol because the RAY code provides the derivatives of the radiation characteristics with respect to the searching for system parameters. This technique allows the optimal use on multi-spectral information if it is available. The retrieval methods are flexible and can be used in synergetic algorithms, which couple data of two or more satellite receivers. These features may be considered as definite merits in comparison with the LUT technique. The crucial question in the case is speed of calculations because of need to process a large amount of pixels (e.g., for operational processing). At present, the retrieval of the AOT, Angstrom parameter, and land reflectance for the area with 1.0E+05 MERIS pixels is processed for about 30 minutes with ART and less than 1 minute with FAR at usual PC. The successful comparison of data retrieved using ART and FAR algorithms with results of some other well known algorithms (MERIS ESA, MERIS BAER, MODIS NASA, etc.) and with Aeronet measurements will be demonstrated.