SMOS L2 OS ATBD - ARGANS

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2. Algorithm overview 25 ICM-CSIC LOCEAN/SA/CETP IFREMER SMOS L2 OS Algorithm Theoretical Baseline Document Doc: SO-TN-ARG-GS-0007 Issue: 3 Rev: 9 Date: 25 January 2013 Page: 25 In the primary algorithm approach (iterative retrieval) a series of physical models (separate modules 4.1 to 4.10 in this ATBD) are applied to auxiliary data (SST, wind, etc.) and a first guess SSS, to compute the brightness temperature that should be measured at a specific polarization and geometric configuration. These values are transported to SMOS antenna level (module 4.12) and then compared to actually measured Tb. An iterative process (considering all measurements/views of a single grid point obtained in consecutive snapshots, module 4.14) allows minimization of the difference between modeled and measured values, until identifying a retrieved SSS for this grid point. Three different models are proposed for the effect of ocean surface roughness in L-band emissivity (modules 4.2 to 4.4) and then three retrieval processes will be run in parallel, and three SSS values provided in the L2 Output Product. A selection of the optimal method is expected to be achieved during SMOS commissioning phase and then implemented in the operational L2 processor. An alternative approach (section 5) uses the neural network technique to retrieve SSS from SMOS measurements and a previous training data set. The SSS retrieval algorithms described in this ATBD will be applied to all ISEA grid points included in a SMOS L1c product (half-orbit swath). 2.1 Sea Surface Salinity retrieval scheme 2.1.1 Iterative approach Per ISEA grid point (once selected as coast/ocean or ocean) on each SMOS half-orbit: • Get preprocessed auxiliary data (SST, wind, …) • For all L1c Tb measurements available – Decision on processing/flagging according to several tests – Forward model for Tb at each angular measurement θ: • Emissivity of flat sea at θ with auxiliary SST and guessed SSS • Add roughness effects (3 model options + foam) • Add external noise (reflected signals: atmosphere, galactic, sun effects) • Add atmospheric atenuation and emision to antenna – Transport modeled Tb from TOA to antenna level • Compare modelled with measured Tb • Iterative convergente to retrieve SSS and adjust other parameters • Output values: SSS, SST, roughness descriptors, TEC (depending on model options) corresponding to this specific gris point As three different options are being studied to describe the effect of surface roughness on Tb, three SSS values will be retrieved at the end of the process. In some submodels, reference values for Tb are needed to compute flags. In this case one of the three roughness models will
26 ICM-CSIC LOCEAN/SA/CETP IFREMER SMOS L2 OS Algorithm Theoretical Baseline Document Doc: SO-TN-ARG-GS-0007 Issue: 3 Rev: 9 Date: 25 January 2013 Page: 26 be used as default for the computation. At Qualification Review 1 (19 July 2006) it was decided that Two-scale will be the default model. 2.1.2 General description
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I(θ_mean)=A1(θx)+A2(θy) where θ
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Uncertainty origin Sea state 4 Sea
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SMOS L2 OS ATBD - ARGANS

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