SMOS ocean salinity: an overview after 3 years in operation

13th URSI Commission F Triennial Open Symposium, on Radiowave Propagation & Remote Sensing, April 30 - May 3, 2013, Ottawa, Canada
The SMOS (Soil Moisture and Ocean Salinity) objectives for sea surface salinity (SSS) are to provide global coverage with repetition rate and accuracy adequate for oceanographic, climatological and hydrological studies. To this end the mission requirements were set to determine SSS values with an accuracy of 0.1, in boxes of 100-200 km and temporal averages of 10-30 days [1]. The SMOS frequency, 1413 MHz within the microwave L-band, has been chosen as a tradeoff between sensitivity of brightness temperature (TB) to salinity variations, spatial resolution, influence of ionosphere and because it is assumed to be a protected band. The sensitivity of Tb to salinity is much smaller than the one for soil moisture, on the order of 0.5K per salinity unit. This implies that the SSS retrieval by SMOS requires a high performance of the MIRAS interferometric radiometer, the single payload on board [2]. The ESA SMOS Ocean Salinity Level 2 (L2OS) Processor has been designed from 2004 by the team that co-authors this abstract, and is being now improved to increase its performance. It relies on a minimisation of the comparison between the TB at different incidence angles measured by SMOS when overflying a single ocean spot, and a modeling of the sea surface L-band emission that takes into account the actual environmental conditions and all the processes that impact or modify this emission [3]. In this paper we present the status of the L2 ocean products as they are operationally generated by mid 2013, more than three years after the mission being in operations phase. A detailed analysis of the SSS fields retrieved by SMOS, and comparison to other data sources like the Argo array of profiling floats, evidences that the mission requirements are almost reached in some regions [5, 6], while in others the results are degraded either due to geographical unfavourable conditions (lower TB sensitivity in cold waters, difficult roughness correction under high winds, difficult galactic signal scattering modelling under high galactic signal, impact of land or RFI contamination). In rainy regions, systematic low biases are observed possibly linked to vertical stratification of salinity between L-band skin depth (1cm) and ARGO measurement (~5m) [6]