This report describes the principle and implementation of a radar remote sensing simulator that includes an ocean surface generation (defined from a wave spectrum) and computes radar back-scatter power and Doppler shifts.
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.
Surface currents are poorly known over most of the oceans, and this observation gap can be filled by satellite-borne Doppler Wave and Current Scatterometers (DWCS). The Sea surface KInematics Multiscale (SKIM) proposal, is the first satellite concept built on a DWCS design at near-5 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-10 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 con-15 figuration, 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 experiment took place in November 2018 out of the French Atlantic coasts with sea states characteristic 20 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 mea-25 sured Doppler velocity contains a characteristic wave phase speed, called here C 0 that is analogous to the Bragg phase speed of coastal High Frequency radars that use a grazing measurement geometry, with little variations ∆ C associated to changes in sea state. 30 Our Ka-band measurements at an incidence of 12 • are 10% lower than the theoretical estimate C 0 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. ∆ C 35 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. Copyright statement. The article is distributed under the Creative Commons Attribution 4.0 License.
Sea state information is needed for many applications, ranging from safety at sea and on the coast, for which real time data are essential, to planning and design needs for infrastructure that require long time series. The definition of the wave climate and its possible evolution requires high resolution data, and knowledge on possible drift in the observing system. Sea state is also an important climate variable that enters in air-sea fluxes parameterizations. Finally, sea state patterns can reveal the intensity of storms and associated climate patterns at large scales, and the intensity of currents at small scales. A synthesis of user requirements leads to requests for spatial resolution at kilometer scales, and estimations of trends of a few centimeters per decade. Such requirements cannot be met by observations alone in the foreseeable future, and numerical wave models can be combined with in situ and remote sensing data to achieve the required resolution. As today's models are far from perfect, observations are critical in providing forcing data, namely winds, currents and ice, and validation data, in particular for frequency and direction information, and extreme wave heights. In situ and satellite observations are particularly critical for the correction and calibration of significant wave heights to ensure the stability of model time series. A number of developments are underway for extending the capabilities of satellites and in situ observing systems. These include the generalization of directional measurements, an easier exchange of moored buoy data, the measurement of waves on drifting buoys, the evolution of satellite altimeter technology, and the measurement of directional wave spectra from satellite radar instruments. For each of these observing systems, the stability of the data is a very important issue. The combination of the different data sources, including numerical models, can help better fulfill the needs of users.
This technical note explain the forward and inverse models used for the SKIM mission proposal
The "Workshop on Doppler Oceanography from Space" brought together oceanographers and radar experts to discuss how new radar technology can be used in existing and future satellite missions to directly measure the motions at the ocean surface, namely currents and waves, and their relation to ocean vector winds, for a wide range of applications from sub-kilometer scales to the global ocean. Satellite remote sensing has revolutionized oceanography, starting from sea surface temperature, ocean color, sea level, winds, waves, and the recent addition of sea surface salinity, providing a global view of upper ocean processes. The possible addition of a direct measurement of surface velocities related to currents, winds and waves opens great opportunities for research and applications. Velocity can be measured using Doppler radar, using along-track interferometry with two synthetic aperture radars (InSAR) or the Doppler centroid (DC) from a single radar. Both techniques measure the same surface motions (Romeiser et al. 2014), with different resolving and revisit capabilities, summarized in Figure 1. See the workshop website: https://dofs.sciencesconf.org/
The Sea surface KInematics Multiscale monitoring (SKIM) satellite mission is designed to explore ocean surface current and waves. This includes tropical currents, notably the unknown patterns of divergence and their impact on the ocean heat budget near the Equator, monitoring of the emerging Arctic up to 82.5°N. SKIM will also make unprecedented direct measurements of strong currents, from boundary currents to the Antarctic circumpolar current, and their interaction with ocean waves with expected impacts on air-sea fluxes and extreme waves. For the first time, SKIM will directly measure the ocean surface current vector from space. The main instrument on SKIM is a Ka-band conically scanning, multi-beam Doppler radar altimeter/wave scatterometer that includes a state-of-the-art nadir beam comparable to the Poseidon-4 instrument on Sentinel 6. The well proven Doppler pulse-pair technique will give a surface drift velocity representative of the top two meters of the ocean, after subtracting a large wave-induced contribution. Horizontal velocity components will be obtained with an accuracy better than 7 cm/s for horizontal wavelengths larger than 80~km and time resolutions larger than 15 days, with a mean revisit time of 4 days for of 99% of the global oceans. This will provide unique and innovative measurements that will further our understanding of the transports in the upper ocean layer, permanently distributing heat, carbon, plankton, and plastics. SKIM will also benefit from co-located measurements of water vapor, rain rate, sea ice concentration, and wind vectors provided by the European operational satellite MetOp-SG(B), allowing many joint analyses. SKIM is one of the two candidate satellite missions under development for ESA Earth Explorer 9. The other candidate is the Far infrared Radiation Understanding and Monitoring (FORUM). The final selection will be announced by September 2019, for a launch in the coming decade.
Workshop on Doppler Oceanography from Space. What: This workshop brought together oceanographers and radar experts to discuss how new radar technology can be used in existing and future satellite missions to measure directly the motions at the ocean surface, namely cur- rents and waves, and their relation to ocean vector winds, for a wide range of applications from sub-kilometer scales to the global ocean. When: 10-12 October 2018 Where: Brest, France website: https://dofs.sciencesconf.org/
This paper is a mini-review contributing to the Oceanobs'19 conference, giving a very short summary and update on the progress of the SKIM mission.
We propose a satellite mission that uses a near-nadir Ka-band Doppler radar to measure surface currents, ice drift and ocean waves at spatial scales of 40 km and more, with snapshots at least every day for latitudes 75 to 82°, and every few days for other latitudes. The use of incidence angles of 6 and 12° allows for measurement of the directional wave spectrum, which yields accurate corrections of the wave-induced bias in the current measurements. The instrument's design, an algorithm for current vector retrieval and the expected mission performance are presented here. The instrument proposed can reveal features of tropical ocean and marginal ice zone (MIZ) dynamics that are inaccessible to other measurement systems, and providing global monitoring of the ocean mesoscale that surpasses the capability of today's nadir altimeters. Measuring ocean wave properties has many applications, including examining wave–current interactions, air–sea fluxes, the transport and convergence of marine plastic debris and assessment of marine and coastal hazards.
This presentation was the occasion to jointly present and discuss the objectives of 3 missions based on Doppler radar concept that are proposing to map ocean surface currents. For the slides about SEASTAR please go to C. Gommengiger or the SEASTAR page
Strong winds may be biased in atmospheric models. Here the ECMWF coupled wave-atmosphere model is used (1) to evaluate strong winds against observations, (2) to test how alternative wind stress parameterizations could lead to a more accurate model. For the period of storms Kaat and Lilli (23 to 27 January 2014), we compared simulated winds with in-situ-moored buoys and platforms-and satellite observations available from the North Atlantic. Five wind stress parameterizations were evaluated. The first result is that moderate simulated winds (5-20 m s-1) match with all observations. However, for strong winds (above 20 m s-1), mean differences appear, as much as-7 m s-1 at 30 m s-1. Significant differences also exist between observations, with buoys and ASCAT-KNMI generally showing lower wind speeds than the platforms and other remote sensing data used in this study (AMSR2, ASCAT-RSS, WindSat, SMOS and JASON-2). It is difficult to conclude which dataset should be used as a reference. Even so, buoy and ASCAT-KNMI winds are likely to underestimate the real wind speed. The second result is that common wave-age dependent parameterizations produce unrealistic drags and are not appropriate for coupling, whereas a newly empirically-adjusted Charnock parameterization leads to higher winds compared to the default ECMWF parameterization. This proposed new parameterization may lead to more accurate results in an operational context.
General presentation of SKIM and current status on error budget and ogoing work.
Doppler radars at all incidence angles measure mean velocities and spread that have complex relations to oceanic motions, with opportunities to measure winds, waves and currents. Here we extend previous theoretical models of backscatter and Doppler using a Kirchhoff approximation and physical optics model. We show that in Ka-band, around 12 • incidence , range-resolved measurements of Doppler and backscatter provide unambiguous estimations of the wave spectrum and surface current. This property is illustrated with numerical examples and airborne data from the AirSWOT instrument. The same measurement conditions can be exploited for global ocean mapping from low Earth orbit sensor satellite configuration.
This paper provides an overview of the Ka-band conical scanning Doppler scatterometer designed for the Sea surface KInematics Multiscale (SKIM) monitoring mission. SKIM will demonstrate the maturity of Doppler oceanography to obtain ocean surface currents at a global scale by direct measurements. This mission has been proposed for ESA Explorer 9, with a launch date in 2025. Mission objectives and concept are also presented. Index terms — Spaceborne Doppler Scatterometer, radar Ka-band, Ocean remote sensing, sea surface waves, currents and waves. INTRODUCTION Ongoing operationnal sea surface current estimations from satellite rely on the combination of geostrophic current anomalies obtained from radar altimetry, mean dynamic topography obtained from gravimetry or drifters  and Ekman currents derived from ocean surface wind scatterometers. This approach misses many features of the real currents whereas in-situ measurements like drifting buoys and HF coastal radar provide insufficient or local data.
Mapping of surface currents using the Doppler information of radar backscatter is now a mature technique using shore-based radars operating at frequencies between 5 and 50 MHz, and new concepts of satellite missions are emerging. A characteristic that is common to these concepts and HF radars is the measurement of radial velocities unevenly distributed on a swath. We propose here a multivariate Optimal Interpolation method to reconstruct the full surface current involving a rotational and divergent Helmotz decomposition. The method should be generally applicable to any Doppler measurement system, and it is tested with an Observing System Simulation Experiment of the SKIM concept. We estimate that SKIM has interesting resolving capabilities for the full surface current under 100km wavelength for different key regions of the Ocean, taking into account a 1 to 5 day revisit time.
Ardhuin et al. (2008) gave a second-order approximation in the wave slope of the exact Generalized Lagrangian Mean (GLM) equations derived by Andrews and McIntyre (1978), and also performed a coordinate transformation, going from a from GLM to a ’GLMz’ set of equations. That latter step removed the wandering of the GLM mean sea level away from the Eulerian-mean sea level, making the GLMz flow non-divergent. That step contained some inaccuarate statements about the coordinate transformation, while the rest of the paper contained an error on the surface dynamic boundary condition for viscous stresses. I am thankful to Mathias Delpey and Hidenori Aiki for pointing out these errors, which are corrected below.
The Sea Surface KInematics Multiscale monitoring (SKIM) mission proposes to use Doppler-based measurements of velocities to provide global estimates of surface currents and ice drift at spatial scales of 40 km and more, with snapshots at least every day for latitudes 75 to 82, and every few days otherwise. Given the contribution of wave motion to Doppler measurements we have chosen to favor near-nadir incidence angles, between 6 and 12 degrees, in order to measure the directional wave spectrum and perform an accurate correction of the wave-induced bias. The resulting instrument design, algorithm for current velocity and mission performance are presented here. We find that a Ka-band near-nadir instrument can reveal features on tropical ocean and marginal ice zone dynamics that are inaccessible to other measurement system, as well as a global monitoring of the ocean mesoscale that surpasses the capability of today's nadir altimeters. Measuring ocean wave properties allows many applications from wave-current interactions and air-sea fluxes to microplastics convergence and coastal hazards.
The directional distribution of the energy of young waves is bimodal for frequencies above twice the peak frequency, and that distribution can be obscured by the presence of bound waves. Here we analyze in detail a typical case measured with a peak frequency fp = 0.18 Hz and a wind speed of 10.7 m/s. The directional distribution for a given wavenumber is nearly symmetric, with the separation of the two lobes of the directional distribution growing with frequency, reaching 150° at 35 times the peak wave number kp and increasing up to 45 kp. When considering only free waves, the lobe ratio, the ratio of oblique peak energy density over energy in the wind direction, increases linearly with the non-dimensional wavenumber k/kp, up to a value of 6 at k/kp = 22, possibly more for shorter components. These observations extend to shorter components previous measurements, and have important consequences for wave properties sensitive to the directional distribution, such as surface slopes, Stokes drift or microseism sources.
Between December 2013 and March 2014, a cluster of about 12 storm events hit the coast of Brittany with an exceptional frequency. It was in February that these storm events were the most frequent and particularly virulent. The significant wave heights measured off Finistère reached respectively 12.3 m and 12.4 m during Petra and Ulla storms on February 5 and 14. However, analysis of hydrodynamic conditions shows that only three episodes promoted extreme morphogenetic conditions because they were combined with high spring tide level. The first one occurred on January 1rst to 4, it was followed by events from February 1rst to 3, and March 2-3. During these three extreme events observed tide levels were above highest astronomical tide level (HAT). Maximum surge level (0.97m) was reached during Ulla storm of February 14-15. For comparison, we must go back in the winter of 1989-90 to find such extreme storm frequency. High frequency topomorphological measurements were achieved on more than ten coastal zones distributed around Brittany peninsula to assess the effects of these storms on shoreline erosion. They show that during the first phase (December-January), meeting it's climax from 1rst to 4th January 2014, shoreline erosion has been limited, with the exception of southern Brittany. This is due to the SW orientation of waves. For all monitoring sites, it has averaged -2.7m, the averaged minimum equal to 0.6m, and the averaged maximum at -6.20m. During the second phase from mid-January to mid-February, reaching it's climax on 1-2 of February storm corresponding to the most morphogenetic event of the winter, the average of shoreline retreat reached -4.2m, the averaged minimum reached approximately -1.5m, the averaged maximum -9.5m. It is essentially the Northern and Western coast of Brittany that experienced largest shoreline retreat due to W-NW storm wave orientation. During the third and last phase, running from mid-February to mid-March, and characterized by the March 2-3 extreme morphogenetic event, shoreline retreat was very low. It reaches -1m on average, for an averaged minimum of -0.6m and an averaged minimum of -1.9m. Considering the whole winter 2013-14 period, shoreline erosion for all monitoring sites reached -6.3m on average, with a minimum of about -0.2 m and a maximum of -30.1m. Depending on the type of environment, it appears that the dunes have retreated the most, followed by gravel or sandy barriers; the lowest erosion rates concern beaches backed by low cliffs cut in highly consistent materials such as periglacial deposits (head). Considering the three morphogenous episodes, the morphological response in terms of shoreline retreat of beaches and barriers was different. Storm occurring at the beginning of February induced the largest erosive rates partly explained by the large morphological sensitivity of beaches and barriers which were weakened by the previous storm events in the beginning of January. Conversely, the storm of March induced very few impacts. These elements show that there is no cumulative of storm effect attested. Over a long period marked by a cluster of storms, beyond a certain threshold in the shoreline retreat process, the erosive action of morphogenesis events is no longer significant, regardless of their intensity.
The poorly understood attenuation of surface waves in sea ice is generally attributed to the combination of scattering and dissipation. Scattering and dissipation have very different effects on the directional and temporal distribution of wave energy, making it possible to better understand their relative importance by analysis of swell directional spreading and arrival times. Here we compare results of a spectral wave model – using adjustable scattering and dissipation attenuation formulations – with wave measurements far inside the ice pack. In this case, scattering plays a negligible role in the attenuation of long swells. Specifically, scattering-dominated attenuation would produce directional wave spectra much broader than the ones recorded, and swell events arriving later and lasting much longer than observed. Details of the dissipation process remain uncertain. Average dissipation rates are consistent with creep effects but are 12 times those expected for a laminar boundary layer under a smooth solid ice plate.
Microseismic activity, recorded everywhere on Earth, is largely due to ocean waves. Recent progress has clearly identified sources of microseisms in the most energetic band, with periods from 3 to 10 s. In contrast, the generation of longer period microseisms has been strongly debated. Two mechanisms have been proposed to explain seismic wave generation: a primary mechanism, by which ocean waves propagating over bottom slopes generate seismic waves, and a secondary mechanism which relies on the non-linear interaction of ocean waves. Here we show that the primary mechanism explains the average power, frequency distribution, and most of the variability in signals recorded by vertical seismometers, for seismic periods ranging from 13 to 300 s. The secondary mechanism only explains seismic motions with periods shorter than 13 s. Our results build on a quantitative numerical model that gives access to time-varying maps of seismic noise sources.