VALIDATION OF THE MERIS ATMOSPHERIC CORRECTION OVER WATER USING GROUND-BASED MEASUREMENTS OF THE SOLAR EXTINCTION AND OF THE SKY RADIANCES

摘要

In order to validate the MERIS (MEdium Resolution Imaging Spectrometer) level-2 products over ocean, a classical approach is to check the aerosol product through solar extinction measurements by comparing the aerosol optical thicknesses (AOT's), and/or to measure the water-leaving radiances at sea level. The limitations of this method are twofold: (i) Actually, what is derived from satellite measurements in the near-infrared (NIR) region is the aerosol path radiance. Therefore, we need to know the aerosol phase function (or the aerosol model) to convert this aerosol path radiance into AOT; (ii) Measurements at sea level are quite difficult to achieve and their relevance with satellite observations is doubtful, mainly for coastal waters. An alternative method is suggested by the authors. The latter is composed of two parts: (i) to check that the MERIS level-2 aerosol model (type and AOT) allows to retrieve in the NIR region the sky radiance measured at the time of MERIS overpass and at the same scattering angle, and (ii) to evaluate the scattering functions (atmospheric path radiance and transmittances) required to perform the atmospheric corrections. This alternative approach has been fully tested on 14 MERIS scenes acquired over five selected sites from AERONET (AErosol RObotic NETwork). The accuracy of the MERIS aerosol products as well as the evaluation of the quality of atmospheric corrections over ocean are then discussed and illustrated.