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Discussion papers
https://doi.org/10.5194/os-2019-77
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-2019-77
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 27 Aug 2019

Submitted as: research article | 27 Aug 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).

Measuring ocean surface velocities with the KuROS and KaRADOC airborne near-nadir Doppler radars: a multi-scale analysis in preparation of the SKIM mission

Louis Marié1, Fabrice Collard2, Frédéric Nouguier1, Lucia Pineau-Guillou1, Danièle Hauser3, François Boy4, Stéphane Méric5, Charles Peureux1, Goulven Monnier6, Bertrand Chapron1, Adrien Martin7, Pierre Dubois8, Craig Donlon9, Tania Casal9, and Fabrice Ardhuin1 Louis Marié et al.
  • 1Laboratoire d’Océanographie Physique et Spatiale (LOPS), UMR 6523, Univ. Brest, CNRS, Ifremer, IRD, Brest, France
  • 22OceanDataLab, Locmaria Plouzané, France
  • 3CNRS, Univ. Versailles St Quentin, Sorbonne Université, LATMOS, France
  • 4CNES, Toulouse, France
  • 5Institut d’Électronique et de Télécommunication de Rennes (IETR), UMR CNRS 6164, Rennes, France
  • 6SCALIAN, Rennes, France
  • 7NOC, Southampton, UK
  • 8CLS, Ramonville St Agne, France
  • 9ESA, Nordwijk, The Netherlands

Abstract. Surface currents are poorly known over most of the oceans. Satellite-borne Doppler Waves and Current Scatterometers (DWCS) can be used to fill this observation gap. The Sea surface KInematics Multiscale (SKIM) proposal, is the first satellite concept built on a DWCS design at near-nadir angles, and now one of the two candidates to become the 9th mission of the European Space Agency Earth Explorer program. As part of the detailed design and feasibility studies (phase A) funded by ESA, airborne measurements were carried out with both a Ku-Band and a Ka-Band Doppler radars looking at the sea surface at near nadir-incidence in a real-aperture mode, i.e. in a geometry and mode similar to that of SKIM. The airborne radar KuROS was deployed to provide simultaneous measurements of the radar backscatter and Doppler velocity, in a side-looking configuration, with an horizontal resolution of about 5 to 10 m along the line of sight and integrated in the perpendicular direction over the real-aperture 1-way 3-dB footprint diameter (about 580 m). The KaRADOC system has a much narrower beam and footprint that only about 45 m in diameter.

The experiment took place in November 2018 off the French Atlantic coast, with sea states representative of the open ocean and a well known tide-dominated current regime. The data set is analyzed to explore the contribution of non-geophysical velocities to the measurement and how the geophysical part of the measured velocity combines wave-resolved and wave-averaged scales. We find that the measured Doppler velocity contains a characteristic wave phase speed, called here C0 that is analogous to the Bragg phase speed of coastal High Frequency radars that use a grazing measurement geometry, with little variations ΔC associated to changes in sea state.

The Ka-band measurements at an incidence of 12° are 10 % lower than the theoretical estimate C0 ~ 2.4 m/s for typical oceanic conditions defined by a wind speed of 7 m/s and a significant wave height of 2 m. For Ku-band the measured data is 30 % lower than the theoretical estimate 2.8 m/s. ΔC is of the order of 0.2 m/s for a 1 m change in wave height, and cannot be confused with a 1 m/s change in tidal current. The actual measurement of the current velocity from an aircraft at 4 to 18° incidence angle is, however, made difficult by uncertainties on the measurement geometry, which are much reduced in satellite measurements.

Louis Marié et al.
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Short summary
With present-day techniques, ocean surface currents are poorly known near the Equator and globally for spatial scales under 200 km and time scales under 30 days. Alternative wide-swath radar pulse-pair Doppler measurements can be used. This direct velocity measurement contains radar motions, waves and currents. These contributions are measured and simulated for the ocean conditions recorded during the DRIFT4SKIM experiment, demonstrating the capability to measure currents from space globally.
With present-day techniques, ocean surface currents are poorly known near the Equator and...
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