A Generic First-Order Radiative Transfer Modelling Approach for the Inversion of Soil and Vegetation Parameters from Scatterometer Observations

We present the application of a generic, semi-empirical first-order radiative transfer modelling approach for the retrieval of soil- and vegetation related parameters from coarse-resolution space-borne scatterometer measurements (sigma(0)). It is shown that both angular- and temporal variabilities of ASCAT sigma(0) measurements can be sufficiently represented by modelling the scattering characteristics of the soil-surface and the covering vegetation-layer via linear combinations of idealized distribution-functions. The temporal variations are modelled using only two dynamic variables, the vegetation optical depth (tau) and the nadir hemispherical reflectance (N) of the chosen soil-bidirectional reflectance distribution function (BRDF). The remaining spatial variabilities of the soil- and vegetation composition are accounted for via temporally constant parameters. The model was applied to series of 158 selected test-sites within France. Parameter estimates are obtained by using ASCAT sigma(0) measurements together with auxiliary Leaf Area Index (LAI) and soil-moisture (SM) datasets provided by the Interactions between Soil, Biosphere, and Atmosphere (ISBA) land-surface model within the SURFEX modelling platform for a time-period from 2007-2009. The resulting parametrization was then used used to perform SM and tau retrievals both with and without the incorporation of auxiliary LAI and SM datasets for a subsequent time-period from 2010 to 2012.