Measuring ocean total surface current velocity with the KuROS and KaRADOC airborne near-nadir Doppler radars: a multi-scale analysis in preparation for the SKIM mission

Type Article
Date 2020-11
Language English
Author(s) Marié Louis1, Collard Fabrice2, Nouguier FredericORCID1, Pineau-Guillou Lucia1, Hauser Danièle3, Boy François4, Méric Stéphane5, Sutherland Peter1, Peureux Charles10, Monnier Goulven6, Chapron Bertrand1, Martin Adrien7, Dubois Pierre8, Donlon Craig9, Casal Tania9, Ardhuin FabriceORCID10
Affiliation(s) 1 : Laboratoire d'Océanographie Physique et Spatiale (LOPS), UMR 6523, Univ Brest, CNRS, Ifremer, IRD, 29280 Plouzané, France
2 : OceanDataLab, 29280 Locmaria-Plouzané, France
3 : Université Paris-Saclay, UVSQ, Sorbonne Université, CNRS, LATMOS, 78280 Guyancourt, France
4 : Centre Nationale d'Etudes Spatiales, 31400 Toulouse, France
5 : Institut d'Électronique et des Technologies du numéRique (IETR), UMR CNRS 6164, 35700 Rennes, France
6 : SCALIAN, 35700 Rennes, France
7 : National Oceanography Centre, Southampton SO14 3ZH, UK
8 : Collecte Localisation Satellites, 31520 Ramonville-St-Agne, France
9 : European Space Agency, ESTEC, Noordwijk, The Netherlands
10 : Laboratoire d'Océanographie Physique et Spatiale (LOPS), UMR 6523, Univ Brest, CNRS, Ifremer, IRD, 29280 Plouzané, France
Source Ocean Science (1812-0784) (Copernicus GmbH), 2020-11 , Vol. 16 , N. 6 , P. 1399-1429
DOI 10.5194/os-16-1399-2020
WOS© Times Cited 4
Abstract

Surface currents are poorly known over most of the world's oceans. Satellite-borne Doppler wave and current scatterometers (DWaCSs) are among the proposed techniques to fill this observation gap. The Sea surface KInematics Multiscale (SKIM) proposal is the first satellite concept built on a DWaCS design at near-nadir angles and was demonstrated to be technically feasible as part of the European Space Agency Earth Explorer program. This article describes preliminary results from a field experiment performed in November 2018 off the French Atlantic coast, with sea states representative of the open ocean and a well-known tide-dominated current regime, as part of the detailed design and feasibility studies for SKIM. This experiment comprised airborne measurements performed using Ku-band and 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, as well as an extensive set of in situ instruments. The Ku-band Radar for Observation of Surfaces (KuROS) airborne radar provided simultaneous measurements of the radar backscatter and Doppler velocity in a side-looking configuration, with a horizontal resolution of about 5 to 10 m along the line of sight and integrated in the perpendicular direction over the real-aperture 3 dB footprint diameter (about 580 m). The Ka-band RADar for Ocean Current (KaRADOC) system, also operating in the side-looking configuration, had a much narrower beam, with a circular footprint only 45 m in diameter. Results are reported for two days with contrasting conditions, a strong breeze on 22 November 2018 (wind speed 11.5 m s−1, Hs 2.6 m) and gentle breeze on 24 November 2018 (wind speed 5.5 m s−1, Hs 1.7 m). The measured line-of-sight velocity signal is analyzed to separate a non-geophysical contribution linked to the aircraft velocity, a geophysical contribution due to the intrinsic motion of surface waves and the desired surface current contribution. The surface wave contribution is found to be well predicted by Kirchhoff scattering theory using as input parameters in situ measurements of the directional spectrum of long waves, complemented by the short wave spectrum of Elfouhaily et al. (1997). It is found to be closely aligned with the wind direction, with small corrections due to the presence of swell. Its norm is found to be weakly variable with wind speed and sea state, quite stable and close to C0=2.0ms−1 at the Ka band, and more variable and close to C0=2.4ms−1 at the Ku band. These values are 10 %–20 % smaller than previous theoretical estimates. The directional spread of the short gravity waves is found to have a marked influence on this surface wave contribution. Overall, the results of this study support the feasibility of near-nadir radar Doppler remote sensing of the ocean total surface current velocity (TSCV).

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Marié Louis, Collard Fabrice, Nouguier Frederic, Pineau-Guillou Lucia, Hauser Danièle, Boy François, Méric Stéphane, Sutherland Peter, Peureux Charles, Monnier Goulven, Chapron Bertrand, Martin Adrien, Dubois Pierre, Donlon Craig, Casal Tania, Ardhuin Fabrice (2020). Measuring ocean total surface current velocity with the KuROS and KaRADOC airborne near-nadir Doppler radars: a multi-scale analysis in preparation for the SKIM mission. Ocean Science, 16(6), 1399-1429. Publisher's official version : https://doi.org/10.5194/os-16-1399-2020 , Open Access version : https://archimer.ifremer.fr/doc/00658/77056/