"Tau-Omega"- and Two-Stream emission models used for passive L-Band retrievals: Application to close-range measurements over a forest

Abstract

Microwave Emission Models (EM) are used in retrieval algorithms to estimate geophysical state parameters such as soil Water Content (WC) and vegetation optical depth (τ), from brightness temperatures T Bp,θ measured at nadir angles θ at Horizontal and Vertical polarizations p = (H,V). An EM adequate for implementation in a retrieval algorithm must capture the responses of T Bp,θ to the retrieval parameters, and the EM parameters must be experimentally accessible and representative of the measurement footprint. The objective of this study is to explore the benefits of the multiple-scattering Two-Stream (2S) EM over the "Tau-Omega" (TO) EM considered as the "reference" to retrieve WC and τ from L-band T Bp,θ . For sparse and low-scattering vegetation T B,EMp,θ simulated with EM = (TO, 2S) converge. This is not the case for dense and strongly scattering vegetation. Two-Parameter (2P) retrievals 2P RC = (WC RC , τ RC ) are computed from elevation scans T Bp,θj = T B TOp,θj , synthesized with TO EM and from T Bp,θj measured from a tower within a deciduous forest. Retrieval Configurations (RC) employ either EM = TO or EM = 2S and assume fixed scattering albedos. WC RC achieved with the 2S RC is marginally lower ( 1 m 3 m -3 ) than if achieved with the "reference" TO RC, while τRC is reduced considerably when using 2S EM instead of TO EM. Our study outlines a number of advantages of the 2S EM over the TO EM currently implemented in the operational SMOS and SMAP retrieval algorithms.