Atmospheric correction over coastal and inland waters: Doing better with MERIS?
Université du Littoral,
32 avenue Foch,
Over coastal areas, the atmospheric correction algorithms of satellite sensors dedicated to the ocean colour observation are more challenging than over open oceans because of the larger complexity of the atmosphere-ocean system. So far, classical algorithms often derive negative water leaving radiances at short wavelengths. Proper atmospheric corrections require:
(i) A good pixel classification. The major issue here is the ability for MERIS to detect thin clouds, mainly cirrus clouds. We will first illustrate the potential use for cirrus cloud detection of the two band ratio 761 nm/ 765 nm which describes the oxygen absorption.
(ii) A good formulation of the signal. We will illustrate here the influence of the land with the classical adjacency effect (photons reflected by the land and then scattered towards the sensor when viewing water) and a so called land mask on the Fresnel reflection (the reduction of the coupling between direct Fresnel reflection and atmospheric scattering).
(iii) Good aerosol models. Standard aerosol models are used in the atmospheric correction scheme. Thanks to ground based measurements of the solar extinction and of the sky radiances, we can state on the ability of the standard aerosol model to describe the major optical properties required for atmospheric corrections.
On MERIS imagery, classical validations of the aerosol product are conducted only from measurements of the aerosol optical thicknesses. We will illustrate the usefulness of sky radiance measurements in the validation process.