SMOS Level 2 Retrieval Algorithm Over Forests: Description and Generation of Global Maps

This paper shows global maps of optical depth and soil moisture over land, obtained using the last prototype of SMOS Level 2 retrieval algorithm, which will be implemented in V600 version of Level 2 product made available by European Space Agency (ESA). The focus is on forested areas, where the approach adopted to develop the algorithm can be subdivided into different steps. First a theoretical model, which was previously developed and tested using ground based and airborne measurements, generated parametric outputs. By fitting this output data set, the albedo and the optical depth of a simple first order radiative transfer model were estimated. Then, this simplified forest model was included in the general ESA Level 2 retrieval algorithm over land, described in the Algorithm Theroretical Baseline Document (ATBD). The paper describes the details of this procedure and shows some retrieval results. First, the prototype algorithm was run with three free parameters: Soil moisture, optical depth, and albedo. The retrieved albedo resulted to be close to the initial estimate (0.08) for Boreal forests, while it was lower for Tropical forests. Running again the algorithm with the albedo fixed, a global map of optical depth was generated. The spatial features of the map follow the global information about forest biomass and forest height available in the literature. Finally it was found that, on average, the influence of seasonal effects on optical depth is moderate.