The SUMOS campaign contributes to the CFOSAT Cal/Val effort and strengthens the case for SKIM.

Sea surface waves are the "clutch" that modulates the transfer of momentum, heat, and climate-impacting gases between the atmosphere and oceans.

Our understanding of the interactions of these two major components of the climate system is thus directly impacted by any progress in our comprehension of the processes controlling the wave generation, propagation and dissipation.

To address these needs, CFOSAT, the China-France Oceanography SATellite, was launched in 2018 by the CNES and the CNSA to provide simultaneous global observations of waves and wind at the sea surface. In particular, it carries the SWIM instrument, a conically-scanning wave spectrometer radar with the potential to provide global-scale observations of the magnitude of ocean waves and their energy distribution as a function of direction and wavelength.

The SKIM mission proposal, which was selected as one of the two final contestants for the ESA EE9 call, aims to extend further the SWIM concept, by adding to the instrument the capability to provide direct Doppler measurements of another critically important quantity, the Total Surface Current Vector (TSCV).

As part of the CFOSAT cal/val effort, CNES funded the SUrface Measurements for Oceanographic Satellite (SUMOS) field campaign (PIs D. Hauser, CNRS/LATMOS, P. Sutherland, IFREMER/LOPS), which took place in February/March 2021 in the Bay of Biscay, and involved participants from CNES, CNRS, HZG, IFREMER, Météo-France, ODL, Université de Bretagne Occidentale, Université de Rennes, Université de Toulon, and Université de Versailles-Saint Quentin.

The massive dataset collected during this campaign will contribute to the understanding and validation of the CFOSAT observations, but the data collection strategy was also designed to maximize its impact on SKIM-related science.

During the month-long campaign, the scientists performed two types of complementary measurements:

• on the one hand, 17 scientific flights were carried out on the ATR-42 plane operated by SAFIRE, the organism in charge of running the French fleet of scientific aircraft. The ATR-42 carried two radar instruments designed to observe the signature of waves on the amplitude and Doppler shift of microwaves reflection by the sea surface. The first one, KuROS, specifically designed and built at LATMOS during the pre-launch phase of CFOSAT, is a rotating fan-beam instrument operating in Ku-band (~2 cm wavelength). This instrument essentially mimics the SWIM instrument onboard CFOSAT, and provides access to the directional spectrum of sea surface waves. The second instrument, KaRADOC, designed and built at IETR, also performs measurements of the backscattering cross section of the ocean surface in Ka-band (~8 mm wavelength), but has a much narrower radar beam. This permits a much easier analysis of the Doppler shift of the reflected radar waves, and makes this instrument very relevant for SKIM.
• on the other hand, other researchers spent the one-month period onboard the R/V Atalante of the French Oceanographic Fleet, deploying a large variety of instruments to collect ground truth observations of sea surface waves, wind, current, air-sea fluxes, atmospheric boundary layer aerosol concentration, oceanic boundary layer turbulence intensity...

The schedule and location of airborne and in-situ measurements were carefully arranged to coincide with CFOSAT overpasses.

The Bay of Biscay is a particularly appropriate place for an experiment of this kind: it is notorious for its energetic and rapidly variable sea state, representative of the mid-latitude open-ocean basins, logistically easy to access, and at a latitude where the satellite passage times are convenient. Indeed, while most points of the satellite orbit are only overflown every 13 days, the crossing points between the ascending and descending passes of the orbit at the latitude of the Bay of Biscay are overflown twice at 13 hours interval, once at 7 PM (UTC) in the evening and once at 8 AM (UTC) on the next morning. This reduces the workload for the ship crew, as the buoys and other in-situ assets provide twice as much ground truth data per deployment.

The R/V Atalante thus spent the period racing between the five crossover points to deploy and recover instruments in time for the CFOSAT overpasses that occurred during the campaign, while the ATR-42 flew out of Quimper to collect KuROS and KaRADOC measurements along the satellite tracks.

At each crossover point, operations started by the deployment at the crossover point of a "FLAME" buoy to collect wind and air/sea fluxes measurements, followed by the deployment over the CFOSAT swath of a dozen wave-measuring "Spotter" buoys. During the satellite / ATR-42 overpass, other measurements were made from the ship, including stereo-video imagery observations of the sea surface, surface current measurements using an autonomous platform carrying an acoustic Doppler current profiler (ADCP), or using a towed catamaran carrying two similar instruments. After the overpass, the hunt was on to recover the buoys and rush to the next deployment point.

Over the duration of the SUMOS campaign, observations were collected during 13 CFOSAT overpasses, and 4 extra flights, similar to the flights performed during the Drift4SKIM experiment, were dedicated to SKIM-specific studies.  A large variety of environmental situations were encountered, from the very energetic sea state and wind of the first days of the campaign to much more pleasant weather towards the end, including unusual cases with easterly short fetch wind sea opposing westerly swell. The instruments collected a comprehensive description of sea state, surface currents, air/sea exchanges and atmospheric boundary layer aerosol properties in all these situations. The surface current observations during the SKIM-dedicated flights will allow a much finer characterization of the bias induced by the sea state in the raw Doppler observations, a significant part of the SKIM signal.

Though the measurements generally confirm the quality of the CFOSAT measurements, there is no doubt that going further and exploiting the full potential of the incredibly rich SUMOS dataset will permit unexpected discoveries for years!