"Tau-Omega"- and Two-Stream Emission Models applied to Close-Range and SMOS Measurements

Abstract

An Emission Models (EM) adequate for a retrieval algorithm requires being simple while still capturing the responses of brightness temperatures $T_{\text{B}}^{p,\theta }$ to the retrieval parameters. The objective of this study is to explore the benefits of the multiple-scattering Two-Stream (2S) EM over the Tau-Omega (TO) EM to retrieve soil Water Content WC and vegetation optical depth τ from L-band $T_{\text{B}}^{p,\theta }$. For sparse and low-scattering vegetation $T_{{\text{B,}}EM}^{p,\theta }$ simulated with EM = TO and EM = 2S converge, which is not the case for dense and strongly scattering vegetation. WC RC and τ RC are retrieved with Retrieval Configurations RC = {TO, 2S} from $T_{\text{B}}^{p,\theta }$ measured: i) from a tower within a deciduous forest, and ii) by the Soil Moisture and Ocean Salinity (SMOS) mission. Using 2S EM instead of TO EM resulted in marginally lower WC RC retrievals while τ RC retrievals are reduced more considerably. With respect to in-situ WC in-situ , retrievals WC 2S derived from tower-based $T_{\text{B}}^{p,\theta }$ performed better than forest soil water-content WC TO retrieved via the inversion of the reference TO EM. Likewise, SMOS based WC 2S retrievals revealed better agreement with ECMWF WC simulations than WC TO achieved with the reference RC = TO. In short, our study provides clear evidence that it is meaningful to replace TO EM used for current SMOS and SMAP land retrieval with 2S EM.Further advantages of the 2S EM over the TO EM are outlined in this study.