Ocean Salinity Estimated by SMOS: An Overview after more than 3 Years in Operation
Font, Jordi¹; Boutin, Jacqueline²; Reul, Nicolas³; Spurgeon, Paul⁴; and SMOS, L2OS Team^5
1Institut de Ciencies del Mar - CSIC, SPAIN; ²LOCEAN, FRANCE; ³IFREMER, FRANCE; ⁴ARGANS, UNITED KINGDOM; ⁵ESA-ESRIN, ITALY

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, and increase the present knowledge on: Large-scale ocean circulation, Water cycle exchange rates quantitative estimation, Occurrence of natural catastrophic events, Management of water resources, Role of the ocean in the climate system. 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].

Even the SMOS frequency, 1413 MHz within the microwave L-band, is close to the maximum sensitivity of brightness temperature (TB) to salinity variations, this sensitivity is much smaller than the one for soil moisture. The total range of ocean SSS spans a TB range of 5K, while for soil moisture is 100 K. This implies that the SSS retrieval by SMOS requires a higher 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 modelisation 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.

The main issues to reach the requirements of SMOS SSS retrieval are linked to instrument and level 1 performance [4]. Even with the best MIRAS calibrations, after several improvements of antenna loss models and other instrumental aspects, there are still some drifts at short (i.e. differences in ascending and descending orbits behaviour) and long term that require correction. The interferometric nature of the measurement implies an image reconstruction process to build the TB snapshots that are then used in the salinity retrieval. This is a complex process that introduces additional inaccuracies as the propagation of ripples from bright sources, like the Sun even if it is located outside the SMOS field-of-view. Or the impact of Radio Frequency Interferences (RFI) from illegal man-made emissions far away from the antenna boresight.

At level 2 the adequate modelisation of the several above mentioned geophysical processes, like the effect of surface roughness in the ocean emission or the scattering of incoming galactic radiation, is also affecting the quality of the SMOS salinity products. The pioneer character of the mission, with an interferometric bidimensional radiometer being put for the first time on a satellite for Earth observation, and lack of previous experience on salinity remote sensing from space, resulted in continuous improvements of the data processing algorithms even three years after the end of SMOS commissioning phase in May 2010.

Due to the TB sensitivity to SSS and the instrument and image reconstruction limitations, the accuracy of the salinity determination by SMOS at each satellite overpass in the L2OS products (15 km grid size) cannot be better than 1 salinity unit. To reduce noise and meet the mission requirements it is necessary to average in space and time, then building gridded level 3 SSS maps.

The left panel of the figure displays a L2OS product corresponding to an ascending pass on February 10, 2013. Grid points that do not meet an established quality threshold have been filtered out. The map on the right panel has been built by objective analysis of 10 days of L2OS products from January 2012. Areas where it is not possible to retrieve SSS (mainly due to RFI contamination) have been shadowed.

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], 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, impact of land or RFI contamination) or still poor modelisation of variable conditions like the presence of intense galactic noise.

Some examples will be presented of the use of SMOS salinity data in different oceanographic applications where the available in situ data have too coarse spatial resolution or do not provide information on salinity in the very top ocean layer. These include interannual and seasonal SSS variability signals, tracking river plumes, detection of rain lenses, etc. as well as the result of applying fusion techniques to increase the spatial resolution of SMOS-derived SSS maps. Details for some of these results are the subject of other presentations to the Living Planet Symposium 2013.

References:

[1] Font J., Camps A., Borges A., Martín-Neira M., Boutin J., Reul N., Kerr Y.H., Hahne A., Mecklenburg S. SMOS: The challenging measurement of sea surface salinity from space, P. IEEE, 98, 649-665, 2010.

[2] McMullan K., Brown M., Martin-Neira M., Rits W., Ekholm S., Marti J., Lemanzyk J. SMOS: The payload, IEEE T. Geosci. Remote, 46, 594-605, 2008.

[3] Zine S., Boutin J., Font J., Reul N., Waldteufel P., Gabarró C., Tenerelli J., Petitcolin F., Vergely J.L., Talone M., Delwart S. Overview of the SMOS sea surface salinity prototype processor, IEEE T. Geosci. Remote, 46, 621-645, 2008.

[4] Font J., Boutin J., Reul N., Spurgeon P., Ballabrera-Poy J., Chuprin A., Gabarró C., Gourrion J., Hénocq C., Lavender S., Martin N., Martinez J., McCulloch M., Meirold-Mautner I., Mugérin C., Petitcolin F., Portabella M., Sabia R., Talone M., Tenerelli J., Turiel A., Vergely J.L., Waldteufel P., Yin X., Zine S., Delwart S. SMOS first data analysis for sea surface salinity determination, Int. J. Rem. Sens., 34 (9-10), published online Oct. 2012, DOI: 10.1080/01431161.2012.716541.

[5] Boutin J., Martin N., Yin X., Font J., Reul N., Spurgeon P. First assessment of SMOS measurements over open ocean: Part II sea surface salinity. IEEE T. Geosci. Remote, 50, 1662-1675, 2012.