Compared performances of SMOS-IC soil moisture and vegetation optical depth retrievals based on Tau-Omega and Two-Stream microwave emission models

Abstract

Since 2010, SMOS (Soil Moisture and Ocean Salinity) retrievals of surface soil moisture (SM) and vegetation optical depth (VOD) have been produced through the inversion of the so-called Tau-Omega (TO) vegetation emission model. Tau-Omega is a 0th-order solution of the radiative transfer equations that neglects multiple scattering, conversely to 1st-order solutions as Two-Stream (2S). To date, very little is known about the compared retrieval performances of these emission models. Here, we inter-compared (SM, VOD) retrievals using the SMOS-IC algorithm running with the TO and 2S emission models. Retrieval performances obtained from TO and 2S were found to be relatively similar, except that a larger dry bias and a slightly lower SM unbiased RMSD were obtained with 2S and the VOD values of the two models vary over dense vegetation areas, both in terms of magnitude and seasonal variations. Considering this and the enhanced physical background of 2S that allows its implementation as a unified emission model for different applications, our study reveals the high interest of using Two-Stream in global retrieval algorithms at L-band.