Mehmet Kurum, ESSIC, University of Maryland

Abstract: The tau-omega model is a zero-order radiative transfer solution. It links terrain geophysical variables to the observed brightness temperature through reflectivity and two vegetation parameters, the optical depth and the single scattering albedo. It has extensive heritage and has been effectively used in soil moisture campaigns that cover grasslands, agricultural crops, and generally light to moderate vegetation. The current baseline soil moisture retrieval algorithms for SMOS and the candidate algorithms for SMAP are based on this model. The main assumption behind the tau-omega model is that the scattering is largely ignored. This essentially places a limit on the density of the vegetation through which soil moisture can be accurately retrieved. In this talk, we will re-derive the tau-omega model from the basic radiative transfer theory without low scattering assumption. The modified tau-omega model that will be presented here explicitly incorporates multiple-scattering effects into the effective albedo and differs from the original model only in terms of the form of the albedo used. This model is applicable to dense vegetation (with large scatterers) such as mature corn and forest canopy to retrieve SM. In addition, the modified model has the same vegetation opacity as defined in the original model. This implies that the tau-omega model can be used to retrieve vegetation opacity as well, as long as effective albedo is parameterized as described in this talk. The difference between the single-scattering albedo and the effective albedo will be pointed out. Finally, the effective albedo is shown to be SM dependent while single-scattering albedo represents single scattering properties of vegetation elements only.