Retrieval of land surface parameters in the Sahel from ERS wind scatterometer data: a "brute force" method

The retrieval of surface parameters, namely, the soil moisture content and the herbaceous above-ground biomass, from European Remote Sensing (ERS) wind-scatterometer data is investigated for a Sahelian study site during the period 1993-1994. Thanks to the low dimension of the unknown parameter vector, a systematic exploration of the parameter space could be carried out. This method allows the recovery of the optimal parameter set as well as an exhaustive description of the subdomain of acceptable solutions. The mapping of this subdomain points out the lack of constraints brought by the ERS dataset on the determination of the surface parameters. Particularly, additional constraints should be found on the rapid and short-scale variation of the soil moisture content. Moreover, it is shown that the distributions of the retrieved parameters are not normal nor log normal, as could be expected from random variables. As a consequence, the optimal parameter set is neither the average nor the maximum likelihood, and the computation of an a posteriori standard deviation of the parameters is meaningless.