SMOS has now been in operation for 12 years and is still in good shape. Throughout those years, several steps had to be done to ensure scientific successes, as it was both the first L band mission and the first interferometer in space. The focus was first on basic variables (soil moisture and sea surface salinity) but very quickly several other science domain and applications emerged. This very rapid transition can be explained by the simple fact that for the first time a space instrument (L band radiometer) could give access directly and in an absolute fashion to surface soil moisture (i.e., without scaling, change detection or strong assumptions).
Soil moisture and derived quantities
The first direct application was to infer root zone soil moisture from surface soil moisture using simplified approaches. This led to a number of very interesting research topics such as elaboration of reliable drought indices and analysis of interactions between water storage and vegetation stress. Soil moisture was also used was proved to be also a very important variable when assessing flood risks. With soil saturation coupled with heavy rainfall forecasts, it is possible to delineate area where flood risks (or flash floods in another context) are very likely. One limitation of soil moisture products is the current spatial resolution (typically 40 km even when distributed on higher resolution grids. So many efforts have been made to infer higher resolution products for use in agriculture and hydrology. Using different disaggregation approaches relying either on optical data sets or on radar/ SAR measurements, high resolution soil moisture fields were derived. They enabled studies on irrigation for example or for locust prevention, but such approaches have also been used to get finer information on other variables such as biomass, water bodies etc. It is well understood that the quality of the outputs is slightly degraded when compared to what would be obtained by a real radiometer with a higher resolution but it fills a gap.
Having access to either brightness temperatures or soil moisture fields in near real time fostered the use of SMOS in NWP. It was used first for monitoring at ECMWF and then assimilated in the model. The results show improvements but limited as models are not designed to assimilate real soil moisture. So soil moisture was improved but not necessarily the scores. SMOS significantly contributed to establish where models will always have issues and need improvements to be able to assimilate not only SMOS but also other sensors in land data assimilation systems.
It is well known the precipitations derived from satellite are very useful but suffer from inaccuracies in several areas. using an assimilation scheme ingesting SMOS data enabled to significantly improve rainfall estimates . Using such information was also very useful for food security programs as demonstrated as early as 2011 at USDA and then in Europe.
SMOS data proved to be also useful in hydrology. First studies were made to see how assimilating SMOS data in hydrological models could improve model outputs and recent studies should that some improvements were noticeable. Other groups studies how it was possible to monitor water bodies even below dense canopies with a relatively high temporal sampling, quantities not necessarily measurable from space with other sensors. It led to a seasonal monitoring of the Amazon and Congo basins for instance, monitoring which could be enhanced using disaggregation approaches. This offered the first opportunity to understand and monitor the hydrology of these large basins largely covered with dense forests. With now 12 years of data with several El Nino / la Nina events we can better describe and thus understand climate teleconnections. Monitoring water bodies also opens a new research field related to gas exchanges between water bodies and the atmosphere.
Sea Surface salinity
Estimating sea surface salinity (SSS) represented a very significant challenge as the signal is rather weak. Nevertheless, very soon SMOS delivered SSS fields and most of the efforts from then on was to both improve the signal accuracy and to extend the retrievals to very complex areas. Actually, retrieving SSS can be even more challenging either because of land contamination (near the coasts) and it is now possible to retrieve reliable information closer to the coast and or in cold seas (due to reduced sensitivity as temperature decreases). Currently both issues have been tackled and SSS map now get close to coasts and also at high latitudes, opening new studies and climate related analysis. It can be mentioned that, thanks to the high quality of the SMOS data the possibility to infer rainfall over oceans has been demonstrated.
The ability to infer wind speed, especially for strong winds (i.e., hurricanes for instance) was also demonstrated at an early stage and this without any notable saturation effect. After an extensive validation, a wind speed product has been established and is now run operationally.
Now the data is also used in mesoscale ocean circulation, to infer ocean alkalinity, assess river plumes and fresh water “tongues” in ocean. All these results leading to a better understanding of ocean circulation and air sea interactions together with an improved understanding of climate signals such as the IOD, ENSO or NAO.
Using the multi-angular capability of SMOS it is possible to infer both soil moisture and vegetation opacity (often called L-VOD for L band Vegetation Optical Depth). So this quantity is part of the SMOS data since day one but suffered at the beginning the trial and errors of accurate image reconstruction and related calibration. After the second reprocessing though, brightness temperatures became sufficiently good to be used even over dense canopies to infer L-VOD which is linked to low vegetation water content (i.e., grass, crops etc) and branch/ trunk biomass for trees and forested areas. Very early it was shown that the relationship between L-VOD and tree height (and thus Biomass) was clear but since the last reprocessing, significant advances were made leading to a number of very significant results related to biomass monitoring and more generally carbon related issues.
Obviously monitoring both biomass and vegetation water content together with surface soil moisture puts forward a new venue in terms of deforestation and deforestation impact fire risks mapping or fire recovery.
Even though not one of the priorities at launch, SMOS proved to be a very valuable tool to monitor cryosphere. The first application was to infer thin sea ice as the signal is very complementary to that of altimeters such as CryoSat-2 as it measures well thin sea ice while CryoSat measures well thick sea ice. As a consequence, by fusing both measurements, it is possible to monitor globally sea ice with an unprecedented accuracy the artic polar cap and described its shrinking trend since 2010 as well as monitoring sea ice around Antarctica.
Studying Antarctica also enabled to make significant progresses in terms of assessing snow melt periods (which are increasing regularly) and ice sheet internal temperatures. It was also found that SMOS could allow assessing the amount of water in liquid form in either snow or ice, opening new paths on Greenland deep melting and avalanche risks mapping. Such results are leading to the possibility in the mid-term to reach at long last a way to estimate snow water equivalent.
Over land masses, due to the sharp change in dielectric constant when soil freezes, a freeze thaw detection approach was rapidly established at FMI giving way to an operational F/T product. This information is obviously of paramount interest for assessing climate induced changes at high latitudes but also can be related to methane exchanges which are very much linked to the thawed period. To extend this high latitude monitoring further, as L band enables to estimate soil’s temperature below a snow layer, studies to monitor soils temperature throughout the year and thus to monitor permafrost extent variations have been initiated, capitalizing on the 12 years of SMOS data.
After 12 years in space and being the first of its kind SMOS has enable a wealth of science results. The results also cover and very large range of science topics going from smart irrigation to climate trends and from locust mitigation dust transport. The impressive publication record is one of the most traceable indicator and, should the satellite stay healthy for several years, new insights of our changing climate will be at hand.