Special Issue « New Outstanding Results over Land from the SMOS Mission »

Category : Data

from Amen Al-Yaari and Arnaud Mialon

Call for publications

Dear Colleagues,

Surface soil moisture (the water content in the first centimeters of soil) is an essential climate variable that plays an important role in land–atmosphere interactions. Soil moisture is widely used in improving climate model predictions/projections, weather forecasting, drought monitoring, rainfall estimations, etc.

Monitoring surface soil moisture at a global scale has recently become possible thanks to microwave remote sensing. SMOS (Soil Moisture and Ocean Salinity) was the first dedicated soil moisture mission that has been in orbit for eight years. The SMOS satellite was launched by the European Space Agency (ESA) in 2009, carrying on board a radiometer in the L-band frequency with a native spatial resolution of ~43 km. Since then, soil moisture and vegetation optical depth (VOD) have been retrieved from multi-angular brightness temperature observations relying mainly on a radiative transfer model.

This is a dedicated Special Issue on SMOS. We welcome studies on all subjects that are related to the SMOS satellite and its products.

Potential topics include, but are not limited to, the following:

  • the improvements in the soil moisture/VOD retrieval algorithms;
  • the evaluation/validation of the SMOS soil moisture and VOD products;
  • SMOS synergy with other remote sensing observations or models simulations;
  • SMOS soil moisture/VOD applications for agriculture, hydrology, etc.

Dr. Amen Al-Yaari
Dr. Arnaud Mialon
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Remote Sensing is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1800 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI’s English editing service prior to publication or during author revisions.


Potential of SMOS at measuring SSS with a precision better than 0.2 (psu!)

Category : CATDS, L2, Ocean

By J., Boutin, N. Martin, N. Kolodziejczyk and G. Reverdin from LOCEAN/IPSL, Paris

It has been shown by Durand et al. (2013), Reul et al. (2013), Hasson et al. (2014) that SMOS detects large scale interrannual variability of SSS.

The LOCEAN group check this again over 2010-2014. The monthly anomalies of SMOS SSS with respect to a SMOS SSS monthly climatology very well agree with SSS monthly anomalies derived from in situ SSS using the In Situ Analysis System (ISAS) optimal interpolation by F. Gaillard (LPO) and the Coriolis Center.


Animation (click to start) : Top: SST anomaly in Niño 3 box from http://www.cpc.ncep.noaa.gov/data/indices/sstoi.indices and corresponding Indian Ocean Dipole (IOD) Index (SST difference between eastern and western equatorial Indian Ocean) from the Australian bureau of Meteorology (BOM). Bottom: SSS monthly anomalies with respect to a monthly climatology (July 2010-June2014): Left)SMOS SSS anomalies; Right)ISAS SSS anomalies

By computing anomalies with respect to 4-year monthly means, SMOS SSS systematic biases are removed. This leads to rms differences between SMOS SSS monthly anomalies and ISAS SSS monthly anomalies the order of 0.2 over large regions, while rms difference of SMOS SSS minus ISAS SSS are on the order of 0.4 over large regions.

image boutin

Figure: Bias (top right) and standard deviation (bottom right) of the differences between SMOS and ISAS monthly SSS (red) and between SMOS and ISAS SSSanomalies (Blue). 6 regions are considered (from left to right): 60°S-60°N; 45°S-45°N; 30°S-30°N; (45°S-5°S, 140°W-95°W); (15°N-30°N,45°W-30°W); (5°N-15°N,110°W-180°W)

Part of this rms difference is due to spatial structures at shorter scale than 300km not resolved by ISAS (Hernandez et al. 2014). Hence this result strongly suggests that SMOS has the potential of measuring SSS at monthly and 100×100km2 scale with a precision better than 0.2 (Hernandez et al. 2014 found 0.15 in the subtropical north Atlantic) provided that systematic biases are removed.

This study has been performed with CATDS CEC-LOCEAN maps built using ESA version 5 reprocessed SSS. Systematic latitudinal biases present in version 5 are expected to decrease in version 6.

Durand, F., G. Alory, R. Dussin, and N. Reul (2013), SMOS reveals the signature of Indian Ocean Dipole events, Ocean Dynamics, 63(11-12), 1203-1212.

Hasson, A., T. Delcroix, J. Boutin, R. Dussin, and J. Ballabrera-Poy (2014), Analyzing the 2010–2011 La Niña signature in the tropical Pacific sea surface salinity using in situ data, SMOS observations, and a numerical simulation, Journal of Geophysical Research: Oceans, 119(6), 3855-3867, doi:10.1002/2013JC009388.

Hernandez, O., J. Boutin, N. Kolodziejczyk, G. Reverdin, N. Martin, F. Gaillard, N. Reul, and J. L. Vergely (2014), SMOS salinity in the subtropical north Atlantic salinity maximum: 1. Comparison with Aquarius and in situ salinity, Journal of Geophysical Research: Oceans, in press.

Reul, N., et al. (2013), Sea Surface Salinity Observations from Space with the SMOS Satellite: A New Means to Monitor the Marine Branch of the Water Cycle, Surv Geophys, 1-42.

Summary of the SMOS Aquarius Workshop

Category : Cal/Val, Data, L1, L2, L3


BREST April 15-17 2013


The SMOS-AQUARIUS science workshop took place on 15-17 April at IFREMER (France). The workshop was jointly organised between ESA, NASA and IFREMER, with support from CNES and the EC’s COST action SMOS-MODE. Even though it was the first event of this kind, it presents a natural extension of a long-standing cooperation between the SMOS and AQUARIUS satellite teams, which started already in the respective development phases. On a working level, members of both teams are regularly attending relevant science and advisory boards on either side. More than 120 scientists from research institutes worldwide attended the workshop, with more than 80 contributions being submitted.

1. Objectives of the meeting

The main objectives of this workshop were to:

• Provide the SMOS and Aquarius science communities with

  • an overview on the present quality of the provided data, and

  • an outlook on future algorithm developments.

• Explore common L-band sensor and algorithm issues for sea surface salinity and soil moisture retrievals derived by SMOS and Aquarius data.

• Present broader science achievements of both missions, including modelling results.

• Explore synergistic use of and validation approaches for SMOS, Aquarius and other sensors’ data over land and ocean.

• Provide a forum for discussion on specific topics related to improving sea surface salinity and soil moisture retrievals through dedicated working groups.

2. The workshop programme

The above objectives were addressed in five sessions, with associated poster sessions. For the detailed programme and all presentations and posters see –>  www.smosaquarius2013.org.

Session 1 – Opening session: This session included programmatic overviews from ESA, NASA and CNES (Mecklenburg, Lindstrom, Lambin), the overall mission status for SMOS and Aquarius (Kerr and Lagerloef) as well as from the ocean user perspective (Reul) and an introduction to the scope of the two working groups on stratification and inter-calibration (Boutin and LeVine), that were established at the workshop.

Session 2 -Instruments’ performance and inter-calibration, algorithm development: the first set of presentations in this session focussed on the quality and expected processor improvements for SMOS level 1 data (Martin-Neira), inter-calibration and inter-comparison between SMOS and AQUARIUS (Cabot, Bindlish, Macelloni and Skou), and detailed aspects of the processor/retrieval evolutions for level 1 brightness temperatures for both missions (Misra, Colliander, Kainulainen, Kao, Turiel). The second set focussed on retrieval algorithm issues for sea surface salinity data products at level 2 for both missions, including dielectric constant modelling, Galactic noise and sea surface roughness corrections etc (Dinnat, Yueh, Meissner, Spurgeon, Tenerelli and Gourrion) and impacts of ascending/descending biases in Level 1 data on the quality of the retrieved sea surface salinity (Banks). This session also provided the input for the discussion in the working group on inter-calibration.

Session 3 -Product validation & stratification: the first part of the session focussed on problems encountered when comparing/validating satellite derived sea surface salinity with in-situ measurements from ARGO and drifters (Drucker, Ward, Reverdin, Boutin, Chao, Asher), these presentations provided the input for the discussion in the working group on stratification. The second part included presentations focussing on the validation of both SMOS and Aquarius derived sea surface salinity in different regions (Lagerloef, Abe, Button, Bulusu) and an error budget estimation due to small-scale horizontal and vertical variability in salinity fields (Vinogradova).

Session 4 – SMOS and Aquarius science application and synergies: Presentations in this session explored the value of both missions’ data for monitoring (multi-annual) oceanographic features/anomalies (Durand, Lee, Reul) and climate change aspects (Hasson, Reul), the link to the global and local features of the water cycle (Gordon, Sato) and synergistic use of both missions’ data (Xie, Liu).

Session 5 -Beyond salinity: soil moisture, storms and cryosphere: contributions in this session were all presented as posters and comprised storm monitoring (Reul, Calla), cryospheric applications of SMOS and Aquarius data for sea ice monitoring and freeze/thaw detection (Maass, Huntemann, de Matthaeis, Miernecki, Tian-Kunze, Xu), land applications (Miernecki) and iceberg tracking (Slominska).

3. Working groups

Further to the science programme two working groups were established, one on surface stratification and the other on inter-calibration/inter-comparison issues. The summary of the respective discussions and a brief indication how to organise future work can be found below.

3.1 Satellite & In Situ Salinity (SISS) Working Group (WG):

Understanding Stratification and Sub-Footprint Processes Co-Chairs: J. Boutin (LOCEAN/CNRS), Yi Chao (RSSI), C.Banks (NOC)

Contact: To join this working group please contact Jaqueline Boutin at jacqueline.boutin@locean-ipsl.upmc.fr or Yi Chao at ychao001@gmail.com;

A common working group mailing list and webpage/webforum, including document sharing and data links is planned.

Scope: The major goal of this WG is to improve our understanding of the link between L- band satellite remotely sensed sea surface salinity (SSS), as provided by SMOS and Aquarius at approximately 1 cm below the sea surface, and in-situ measured salinity, routinely measured at a few meters’ depth by ships and ARGO floats and recently accessible up to a few cm depth by new profilers, surface drifters etc. and to develop methodologies for relating satellite derived salinity to other estimates of sea surface salinity.

This working group will, amongst others, address the following main questions:

o What is the salinity vertical distribution (stratification) in the first 10m below the sea surface and what is the relation with atmospheric forcing including wind and rain? Under which conditions are the new satellite SSS able to provide new reliable information on SSS within the first cm complementary to existing deeper in-situ information for studying air-sea exchange processes? In case of a rain event, what is the order of magnitude for rain-induced vertical gradients and how to best correct for other perturbing effects impacting L-band radiometric measurements (squall, downdraft/updraft, falling droplets induced roughness, atmospheric rain-induced contribution to the mission)?

o What is the magnitude of SSS variability within a satellite footprint (SMOS (40 km) and Aquarius (50-150 km)); how could this variability be taken into account for in- situ/satellite SSS comparisons?

o How can satellite derived SSS be assimilated/used as input to ocean circulation models? Are current ocean circulation models configured in a way to deal with the surface layer processes inherent in satellite derived SSS?

o Which kind of experimental/numerical experience could support investigations related to the questions above (e.g. field campaigns)?

First activities will concentrate on better defining the physical meaning of the satellite and in-situ sampled salinity, and how L-band satellite salinity and ocean salinity measured at a few centimetres/metres depth should be referred to.

The working group findings and results should be summarized in a report and/or a white paper to be published on the working group webpage. Further publishing could be done in scholarly journals (e.g. BAMS).

3.2 Working group on salinity inter-comparison/cross-calibration

Co-Chairs: David LeVine (NASA), Gary Lagerloef (ESR), Yann Kerr (CESBIO),

JordiFont (ICM/CSIC), M.Portabella (UTM-CSIC)

Contact: To join this working group please contact David LeVine at David.M.LeVine@nasa.gov, who will form a working group mailing list and begin an email « conversation“.

Scope: The focus of this working group is on identifying an approach for inter- comparison/cross-calibration of SMOS and AQUARIUS data. This will include an agreement on the techniques and reference sites being used for this purpose. Before entering the actual inter-comparison, some fundamental issues in the understanding of L-Band remote sensing need to be addressed, specific to both instruments. In the long run this working group aims toward a merged and validated data product using commonly agreed standards. It will need to be confirmed on what level of data the merged product should occur, i.e. level 1 brightness temperatures (i.e. Fundamental Climate Data Records) or level 2 sea surface salinity (i.e. Thematic Climate Data Records).

This working group will, amongst others, address the following main questions:

o Even though both instruments measure in the same spectral region, both data sets are based on

o different technology (SMOS microwave interferometric radiometer versus AQUARIUS conventional pushbroom radiometer and scatterometer);

o a different approach to calibration (SMOS uses cold sky and warm load and AQUARIUS uses ocean and cold sky);

o differences introduced through the instrument hardware itself (orbital and long-term drifts, uncertainties in antenna pattern, pointing, etc);

o differences in the retrieval algorithms for the Level 2 sea surface salinity data product ( e.g. in the choice of auxiliary data and the models used).

Hence it will be necessary to first address these differences and their impact on the results of an inter-comparison. Initial work will concentrate on solving several key common geophysical modelling issues related to galaxy, roughness, emissivity, ascending-descending biases, RFI. This will lead to inter-comparable data sets, even though it is recognised that residual uncertainties will remain related to the above differences.

o The next step will concentrate on agreeing on the approach for inter-comparison (e.g. using a double difference or a tuned-location approach).

4 Areas of research that we should encourage more

o Assimilation of SSS data into ocean circulation models.

o Real synergistic use of the SMOS and AQUARIUS data to exploit their complementarity.

5 Recommendations

o Long-term data continuity required to establish merged/synergistic data products and provide answers in climate change research; how will continuity be ensured beyond SMOS, AQUARIUS and SMAP? What could be possible implementation routes for future concepts such as SMOS Ops or SMOS Next?

o Funding agencies should work together in considering long-term plan to provide L-Band observations continuity.

o Common problems should be pursued on international level: RFI!

o Establish a nomenclature and agreed standard for inter-comparisons between satellite derived and in-situ salinity observations, which will be a pre-requisite for merged products between SMOS and AQUARIUS, also in sight of SMAP.

o Sea surface salinity has already been recognised as an Essential Climate Variable (ECV) but is presently not yet pursued as part of the ESA Climate Change Initiative (CCI) studies. SSS CCI study should be initiated as part of the second phase of ESA’s CCI studies.

o Recognising the benefit of this event, SMOS-Aquarius workshops could be continued in the future, possibly every 1-2 years.

Large scale SSS contrasts detected by SMOS and in situ SSS analysis in the tropical Pacific and Indian Oceans

Category : CATDS, Cal/Val, L2, Ocean

Recent results from LOCEAN  team

Variability in the tropical Pacific and Indian Oceans between 2010 and 2011 is very large linked to the ending 2009-2010 El Nino, 2010 strong La Niña and 2011 weak La Niña events, and to negative Indian Ocean Dipole (IOD) in 2010 (see Figure below).


figure1 Time series of SST anomalies in the four Niño regions from http://www.cpc.ncep.noaa.gov/data/indices/sstoi.indices in 2010-2011 and corresponding Indian Ocean Dipole (IOD) Index (SST difference between eastern and western equatorial Indian Ocean) from the Australian bureau of Meteorology.


Large scale SSS variability is very consistently observed on SMOS SSS maps (1) compared with ISAS SSS maps (2) derived from in situ (ARGO floats and ship data) measurements, as seen on the film. SMOS anomalies are often more contrasted than ISAS anomalies due to fresher SMOS SSS in fresh regions associated with rain (Boutin et al., 2013). Signal is much noisier from January to May due to worse SMOS SSS quality during commissioning phase in January-May 2010, but nevertheless spatial structures of anomalies remain very consistent with ISAS maps.

(1) SMOS SSS maps are generated using ESA reprocessing version 5 as described in J. Boutin, N. Martin, G. Reverdin, X. Yin and F. Gaillard, Sea surface freshening inferred from
SMOS and ARGO salinity: Impact of rain, Ocean Sci., 9, 183-192, doi:10.5194/os-9-183-2013, 2013.

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