Validation of the Meris Fapar L2 Products against Independent Estimates Derived from the Modis and Misr Surface Albedo Operational Products

We present and evaluate results of a validation study of the MERIS Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) product delivered by the operational MGVI algorithm. Our approach is based on the application of an inversion method conducted using both MODIS and MISR derived broadband visible and nearinfrared surface albedo products available during a full seasonal cycle over a variety of selected FLUXNET stations. This investigation includes complex geophysical scenarios involving snow occurrence in mid and highlatitude evergreen and deciduous forest canopy systems. The occurrence of snow during the winter and spring seasons is based on the analysis of the MODIS snow products which assimilation by our package translates into an adaptation of the prior values, both the maximum likelihood and width of the 2-D probability density functions (PDF), characterizing the background conditions of the forest floor. The inversion package delivers meaningful land vegetation fluxes (such as the FAPAR) and associated model parameters (such as the effective LAI) along the year despite the rather high variability in the input products. Our results illustrate the remarkable agreement between the MERIS FAPAR values delivered by the operational processor and those values retrieved independently from our inversion package using the MODIS and MISR surface albedo products.