Project

SKIM : the Sea surface KInematics Multiscale monitoring satellite mission

Goal: SKIM is an ESA satellite mission (now in phase A, possible launch in 2025) that will measure surface currents and ice drift (within 10 cm/s at 40 km resolution) waves spectra (down to 20 m wavelength) over the whole globe from the Antarctic to 82°N. SKIM is one of the 2 missions pre-selected to become Earth Explorer 9. Final selection in 2019.

On November 15, 2017, ESA announced that we have passed the first round of selections. There are now 2 choices for EE9: FORUM (Far Infrared Outgoing radiation) and SKIM. Final decision in 2019.

Please help by supporting our proposal: send a message to Fabrice with

- your occupation (research, industry, other)
- your main interest and field of work (e.g. coastal engineering, physical oceanography ... )
- affiliation
- and the type of activity for which that SKIM could be useful.

Learn more at
ftp://ftp.ifremer.fr/ifremer/ww3/TEAM/EE9/

http://www.umr-lops.fr/Projets/Projets-actifs/SKIM

and follow updates at
https://www.facebook.com/SKIM4EE9 (somehow more user-friendly than RG)

Messages of interest and support letters can arrive after the proposal is submitted, we will store them all on our ftp:
ftp://ftp.ifremer.fr/ifremer/ww3/TEAM/EE9/SUPPORT_LETTERS/

Methods: Ka-band radar nadir and off-nadir altimeter

Date: 4 January 2016 - 31 December 2021

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Project log

Fabrice Ardhuin
added a research item
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.
Fabrice Ardhuin
added an update
As anounced by ESA in September 2019, the 9th Earth Explorer mission will not be SKIM but FORUM. Congratulations to the FORUM team. The Advisory Committee on Earth Observations (ACEO) also advised ESA to find other ways or means to implement SKIM.
 
Fabrice Ardhuin
added a research item
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.
Fabrice Ardhuin
added a research item
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.
Fabrice Ardhuin
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This reports summarizes the main research objectives of the SKIM candidate mission for ESA Earth Explorer 9
Fabrice Ardhuin
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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.
Fabrice Ardhuin
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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/
Fabrice Ardhuin
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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.
Fabrice Ardhuin
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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/
Fabrice Ardhuin
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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.
Fabrice Ardhuin
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Fabrice Ardhuin
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Hi there,
As we are looking at the impact of different orbits and beam parameters, I'm updating the animations of the SKIM acquistion
Here is one of those animations:
 
Fabrice Ardhuin
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We already have all the radar experts lined up... but we could really benefit from more oceanographers: How can such measurement be used to best constrain the ocean circulation? I expect that in the same way Ducet et al. opened new opportunties by combining drifters & altimeters to get a better grasp of the mean ocean circulation we will be able to do it at a time scale of only a few days... So, beyond the tropical oceans there are new opportunities for ocean research (dynamics, transport ...) all around the globe.
Also CFOSAT pre-lauch workshop, October 8 & 9
 
Fabrice Ardhuin
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Here it is:
not too much there yet, but this will fill up in the coming weeks.
 
Fabrice Ardhuin
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Hi there, we will have a full discussion at the "Doppler Oceanography From Space" meeting ( register here : http://dofs.sciencesconf.org/ ) about what is it that we are really measuring ... vertical current shear effects? Which part of the wave spectrum ... and we really need to go back to sea and measure these things. This is one of the objectives of the DRIFT4SKIM campaign, that we are now discussing with ESA.
One of the possible tools is wave-enabled drifters ... that need to be small enough to follow short wave motion ... and big enough to be picked up in rough seas ... here we have a "SKIB" paper is out and open for reviews...
 
Fabrice Ardhuin
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What motions can be measured from space by Doppler radars? What are we measuring exactly? How can such measurement be used to best constrain the ocean circulation? Here are some of the questions that we will discuss. For registrations please go to :
See you soon in Brest!
 
Fabrice Ardhuin
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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.
Fabrice Ardhuin
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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
Fabrice Ardhuin
added an update
After several discussions, members of the Mission Advisory Group are proposing to use an orbit identical to Metop SGB with a shift of 6.67 degrees to the East.
This would be critical for having good wind vector measurements across the entire swath using the SCA instrument (C-band X-pol scatterometer with a 525 nadir gap and 2 swaths of 660 km wide on either side of the ground track).
Another instrument on Metop SGB is MWI (Microwave Imaging Radiometer) with channels providing precipitation monitoring as well as sea ice extent information. There are key benefits for ice edge detection, and using the Metop radiometer for nadir range corrections or data flags.
Another satellite mission (CIMR) that could be launched around the same time as SKIM is also planned to fly in tandem with Metop SGB: that would give us SST and SSS in the SKIM swath... an oceanographer's dream.
Last and not least: flying higher with Metop means a wider swath (320 km instead of 270 km if we followed Sentinel 1 as initially envisaged), hence a shorter revisit time.
Feasibility for this higher altitude orbit has to be confirmed:
- backscattered power goes down with altitude
- the orbit crosses the equator at 9:03 local time, which may not be optimal for solar power.
 
Fabrice Ardhuin
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After a veerrryyy long review process ... and my discovery of handling discussion papers. It has a brand new appendix B on attitude corrections and a revised introduction... enjoy!
 
Fabrice Ardhuin
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Nothing new in the contents but a few nicer plots. For movies, see the presentation online:
 
Fabrice Ardhuin
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Hi to all,
after circulating this on the Coastal List, we got some really good candidates. Hope to interview them this Monday and Tuesday... so there is still a chance to apply now:
Numerical wave modeller Lab. of Ocean Physics & remote Sensing (LOPS) Univ. Brest/CNRS/Ifremer/IRD
Applications are invited for a full time 12-month position with the Satellite & Air-Sea interaction to work on the SKIM satellite project. SKIM has been preselected by ESA for a possible launch in 2025. It will carry a Ka-band Doppler radar that will measure currents and wave spectra across a 300-km wide swath, from the Equator to 82° latitude.  The position is available until filled and could start as early as January 2018. The 12 month duration is likely to be extended based on later fund availability.
Job description During the detailed investigation of SKIM feasibility (phase A), the numerical simulation of current and waves and how they are recorded by a Ka-band radar will play a prominent role in the choice of algorithms and processing. This will be combined with detailed experiments on current and waves using platform-based and airborne measurements. 
Duties: the successful candidate is expected to perform, post-process and analyse high resolution numerical simulations of waves over realistic currents, using the WAVEWATCH III model (see e.g. Ardhuin et al. JGR 2017). This will include management and dissemination of large volumes of data (~100 To) and use and evolution of a python-based SKIM simulator developed by other partners. The candidate may also contribute to the processing and analysis of stereo-video and polarimetric measurements of short waves (1 cm to 10 m wavelength). Writing internal reports and contributing to scientific publications will also be part of the project.
Qualifications/Skills required:  - M. Sc. in oceanography, ocean engineering, applied maths or related field - demonstrable experience with numerical wave modelling, possibly using WAVEWATCH III - good writing skills - experience of work in LINUX environment
Salary will depend on experience following CNRS guidelines
To Apply: Applications to include a covering letter, CV, and the contact details of three referees should be sent, via e-mail (in word or PDF only) to  both fabrice.ardhuin@ifremer.fr  and mickael.accensi@ifremer.fr
More information:  www.facebook.com/SKIM4EE9/      and     http://tinyurl.com/SKIMonRG
 
Fabrice Ardhuin
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These slides describe the general context and concept of the SKIM mission
Fabrice Ardhuin
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Calling Mission Experts! the Mission Advisory Group (MAG) call for the ESA Earth Explorer 9 Sea surface KInematics Multiscale monitoring (SKIM) Mission is now open at http://missionadvice.esa.int/index.php
 
Fabrice Ardhuin
added an update
Basically ESA will choose either FORUM (Far Infrared Outgoing Radiation) or SKIM, based on a detailed feasibility study involving lots of simulations and validation with real data. That final choice will be mid-2019. The race is on: the oceans need more data, and SKIM can deliver these.
 
Fabrice Ardhuin
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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.
Fabrice Ardhuin
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... this is when the 2 missions selected for phase A should be announced. Let's get ready to go back to work ...
 
Fabrice Ardhuin
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Fabrice Ardhuin
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Dear colleagues,
The work performed in preparation of the EE9 proposal is now summarized in 3 papers, the latest (which was the first but took some time to finalize) is the theory and validation of Doppler signals from near-nadir radar by Nouguier et al., using in particular data from the AirSWOT Ka-band system.
Ongoing work is related to the simulation of measured wave spectra (Doppler and backscatter) and associated noise. Also, ESA will issue soon a new call for Earth Explorer 10, with a larger budget... this would allow a SKAR + SAR combination or different antenna design, both allowing a wider swath.
 
Fabrice Ardhuin
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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.
Fabrice Ardhuin
added a research item
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 [1] 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.
Fabrice Ardhuin
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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.
Fabrice Ardhuin
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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.
Fabrice Ardhuin
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You can help get SKIM off the ground by contributing to the science of Doppler oceanography and making comments / corrections at :
 
Fabrice Ardhuin
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There will also be a conference proceeding by Caubet et al. for ESA's Ka-band workshop in September... Now is time for holidays! More news at the end of August.
 
Fabrice Ardhuin
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Et voila. I've just uploaded the FINAL proposal as will be evaluated by ESA (with some confidential pages from Thales Alenia Space cut out).
Deleted publication
We will know in November how good we were (science + technology + budget constraints). I heard there are 12 proposals submitted. 2 will make it to a "Phase A" (detailed review in 2018) ... so we can celebrate the achievement of putting everything together from theory all the way to Observation System Simulation Experiments (OSSE) and validation with in situ and airborne data, and simulation tools... just amazing!.. but we have to get ready for next year, an the final Earth Explorer 9 selection. THANKS to all... and my apologies to those who provided bits and pieces that I could not fit in the proposal in the last crazy days.
ESA news should appear on that website: http://explorercall.esa.int/index.php/15-mission-ee9
 
Fabrice Ardhuin
added an update
Hello, here is a draft of the proposal due tomorrow (confidential pages have been cut out):
Deleted publication
Comments are welcome. For those of you who are co-I, please use the google-doc I'm working on inserting the latest edits in the stand-alone document. Thanks,
As part of the latest additions are results from our Bergen / NERSC team members with an OSSE using simulated SKIM data: it would clearly have a great impact on the Gulf Stream and tropical Atlantic regions ... we still have to figure out exactly the interpretation of impacts around the ice edge. Here is the figure caption:
Degrees of Freedom for Signal of all assimilated observations during one week in May 2015. From Top line from left to right: OSTIA Sea Surface Temperature, CMEMS delayed-mode altimeter tracks, SKIM sea surface. Bottom line: OSI SAF Sea ice concentrations, in situ (incl. Argo) Temperature and Salinity profiles. The figure shows relative DFS as fraction of the total. Note than SKIM surface currents are the main source of information in the Gulf Stream and in the Equatorial Counter-Current.
 
Fabrice Ardhuin
added 2 research items
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.
Fabrice Ardhuin
added an update
Measuring currents is better done with measuring surface velocity instead of sea level ... this is the conclusion of performance analysis of SKIM and SWOT measurement systems. Indeed, small scale sea surface height (SSH) patterns, caused by internal waves or other, are misinterpreted as geostrophic currents, creating noise in the SWOT retrieved map. In the end, the effective resolution of SKIM-derived currents around the Gulf Stream is at a wavelength of 70 km instead of 115 km for SWOT, thanks to the shorter revisit time that comes with the wider swath. These numbers vary with the type of current measured.
5 days ahead of the ESA deadline the proposal gaps are rapidly filling up. You should have a nearly final version posted here on June 13, with a short opportunity for feedback and adjustments.
 
Fabrice Ardhuin
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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.
Fabrice Ardhuin
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Two weeks to go for the full proposal submission to ESA. We are finally getting maps of simulated currents, up to divergence showing nicely the equatorial upwelling. I'll post a first draft of the proposal tomorrow.
 
Fabrice Ardhuin
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Tomorrow, March 9, ESA is hosting a workshop for all EE9 proposers. In the meantime we are advancing on refining the surface current error model, which is why we need wave information. Backscatter from a rough sea surface gives a mean Doppler shift that is proportional to the surface Stokes drift Uss, which is mostly a function of wind speed and wave height... This wave-induced bias is Uwb = G Uss . For any Doppler system this bias is of the order of 1 m/s (at large incidence angles) or more (in the case of SKIM). Correcting for this bias requires a detailed knowledge of the wave kinematics at the ocean surface, and their spatial variability. Previous work on Envisat ASAR used a wind-only parameterization of G... this is not enough for an accurate current estimate: this is why Envisat current fields had to be averaged over looonnng times (few months) to provide reliable estimates.
Another issue could be the poorly known satellite motion which is corrupting the Doppler estimate: this is the problem today on Sentinel . With a rotating antenna beam, measuring over 360° in azimuth will take care of most of this problem.
 
Fabrice Ardhuin
added 8 research items
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.
Fabrice Ardhuin
added a research item
Presentation of SKIM for team members
Fabrice Ardhuin
added an update
Dear colleagues,
first of all, I'd like to thank again all the people that helped put the proposal together and the very good feedback that we have had from everybody. Second, there is no official information from ESA but it sounds that even if the EE9 call may not be the right one, there could be some real opportunity in the near future. I will thus continue to advocate for the SKIM mission, possibly broadening a little the goals and doing some more detailed simulations of mission performance. In that context I will probably propose a paper to Eos or a similar "broad audience" journal (we have something already lined up for BAMS on wave-current interactions).
 
Fabrice Ardhuin
added an update
Thanks to all for the suggestions, corrections and support... as time is running out, they may not all be included in the proposal, but we are trying. Support letters can arrive after the deadline, no problem. It was in general very good to see reactions on issues of space and time sampling or on the basic understading of how the measurement works, that will help us clarify things.
 
Fabrice Ardhuin
added a project goal
SKIM is an ESA satellite mission (now in phase A, possible launch in 2025) that will measure surface currents and ice drift (within 10 cm/s at 40 km resolution) waves spectra (down to 20 m wavelength) over the whole globe from the Antarctic to 82°N. SKIM is one of the 2 missions pre-selected to become Earth Explorer 9. Final selection in 2019.
On November 15, 2017, ESA announced that we have passed the first round of selections. There are now 2 choices for EE9: FORUM (Far Infrared Outgoing radiation) and SKIM. Final decision in 2019.
Please help by supporting our proposal: send a message to Fabrice with
- your occupation (research, industry, other)
- your main interest and field of work (e.g. coastal engineering, physical oceanography ... )
- affiliation
- and the type of activity for which that SKIM could be useful.
Learn more at
and follow updates at
https://www.facebook.com/SKIM4EE9 (somehow more user-friendly than RG)
Messages of interest and support letters can arrive after the proposal is submitted, we will store them all on our ftp: