Improved estimates of vegetation biophysical variables from MERIS TOA images.
Claire Lauvernet(1) , Frédéric Baret(1)
, David Beal(1)
, Marie Weiss(2)
, Kathy Pavageau(1)
, Béatrice Berthelot(2)
, and Peter Regner(3)
(2) Noveltis, Parc technologique du Canal, Toulouse, France
(3) ESA-ESRIN, Via Galileo, Frascati, Italy
The inversion of radiative transfer models is a severely ill-posed problem that may lead to significant uncertainties in the biophysical variables estimates when limiting the information used to the single pixel reflectance values. The improvement of the performances of the inversion process requires more information to be exploited including better radiative transfer models, exploitation of proper prior information on the distribution of the canopy and atmosphere variables, knowledge of uncertainties in MERIS measurements, as well as possible spatial and temporal constraints. In this study we focused on the use of coupled atmosphere-surface radiative transfer models. It is inverted thanks to a variational method based on iterative optimization techniques based on a quasi Newton algorithm (M1QN3) and the adjoint model. The inversion is achieved over an ensemble of pixels belonging to a 10x10 km² window where the aerosol properties are supposed steady, and over a temporal window of 20 days where the vegetation variables are assumed to vary linearly with time. The interest of the method was first tested using twin experiments made of simulations of the direct models. It is eventually applied to actual MERIS observations. Performances are compared to ground measurements of LAI, and to more classical approaches and products. Preliminary results are discussed with due attention to their applicability within operational algorithms.