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Monte Carlo Code as a Reference Radiative Tranfer Code to Compute Atmospheric Scattering and Correction

Pierre-Yves Deschamps(1), Dominique Jolivet(2) and Patrice Henry(3)

(1) Université de Lille 1-CNRS, Université de Lille 1, F-59655 Villeneuve d'Ascq Cedex, France
(2) HYGEOS Company, 5 Rue Héloïse, F-59655 Villeneuve d'Ascq Cedex, France
(3) CNES, 18 Avenue Edouard Belin, F-31401 TOULOUSE CEDEX 4, France

Abstract

Remote sensing of ocean colour from space requires performing an atmospheric scattering correction with an accuracy better than 1% and a goal of 0.1 %. We have developed a Monte Carlo (MC) Radiative Transfer (RT) code to accurately compute the scattering by a spherical Earth’s atmosphere bounded an agitated ocean surface. The paramount advantage of a MC code is its reliable and easy representation of physical processes, at the expense of a very large computing time. The MC code has been used in a variety of cases to check the accuracy of the present computations of atmospheric scattering correction in the ocean colour processing:

(i) MC code has been used to validate computations by the popular SOS (Successive Orders of Scattering) in the case of a plane-parallel atmosphere bounded by a flat ocean: results agree with an accuracy better than 0.1 % when the SOS code is properly tuned.

(ii) For the case of a wavy ocean surface, the (TOA radiance computed by the) SOS code underestimates the reference MC Code by up to 2% at large view zenith angles, and 0.5 % at small view zenith angles.

(iii) The plane parallel approximation used by the RT codes becomes inaccurate at view zenith angles larger than 75 °. Furthermore, the plane-parallel approximation wrongly overestimates the TOA radiance by about 0.5 % at small zenith angles.

We strongly recommend that LUTs (Look Up Tables) for atmospheric correction be computed with a reference to an accurate MC RT code working for a spherical atmosphere.

 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry