the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Jan 2024
A new airborne system for simultaneous high-resolution ocean vector current and wind mapping: first demonstration of the SeaSTAR mission concept in the macrotidal Iroise Sea
Abstract. Coastal seas, shelf seas and marginal ice zones are dominated by small-scale ocean surface dynamic processes that play a vital role in the transport and exchange of climate-relevant properties like carbon, heat, water and nutrients between land, ocean, ice and atmosphere. Mounting evidence indicates that ocean scales below 10 km have far-ranging impacts on air-sea interactions, lateral ocean dispersion, vertical stratification, ocean carbon cycling, and marine productivity – governing exchanges across key interfaces of the Earth System, the global ocean and atmosphere circulation and climate. Yet, these processes remain poorly observed at the fine spatial and temporal scales necessary to resolve them. Ocean Surface Current Airborne Radar (OSCAR) is a new airborne instrument with the capacity to inform these questions by mapping vectorial fields of total ocean surface currents and winds at high resolution over a wide swath. Developed for the European Space Agency (ESA), OSCAR is the airborne demonstrator of the satellite mission concept ‘SeaSTAR’, which aims to map total surface current and ocean wind vectors with unprecedented accuracy, spatial resolution and temporal revisit across all coastal seas, shelf seas and marginal ice zones. Like SeaSTAR, OSCAR is an active microwave Synthetic Aperture Radar Along-Track Interferometer (SAR-ATI) with optimal three-azimuth sensing enabled by unique highly-squinted beams. In May 2022, OSCAR was flown over the Iroise Sea, France, in its first scientific campaign as part of the ESA-funded SEASTARex project. The campaign successfully demonstrated the capabilities of OSCAR to produce high-resolution 2D images of total surface current vectors and near-surface ocean vector winds, simultaneously, in a highly dynamic, macrotidal coastal environment. OSCAR current and wind vectors show excellent agreement against ground-based X-band radar derived surface currents, numerical model outputs and NovaSAR-1 satellite SAR imagery, with Root-Mean-Square differences against X-band radar better than 0.2 m s-1 for currents at 200 m resolution. These results are the first demonstration of simultaneous retrieval of total current and wind vectors from a high-squint three-look SAR-ATI instrument, and the first geophysical validation of the OSCAR and SeaSTAR observing principle. OSCAR presents a remarkable new ocean observing capability to support the study of small-scale ocean dynamics and air-sea interactions across the Earth’s coastal, shelf and polar.
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Notice on discussion status
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Preprint
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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- Final revised paper
Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-2995', Anonymous Referee #1, 06 Mar 2024
General comments
This manuscript presents the very first validation from real observations of the concept of squinted 3-look SAR along-track interferometry to retrieve the vectors of surface wind and total surface current velocity mapped at high resolution (200m).
The observations were collected with an airborne SAR system called OSCAR in a coastal environment characterized with high tidal flow. OSCAR is the airborne demonstrator of the Seastar satellite concept funded by ESA and based on the same principle of along-track interferometry from a 3-look squinted SAR.
Although based on a limited number of cases studied (two meteo-oceano situations, with one sampled under different flight geometries), the results are very convincing. The retrieved field of wind and surface current are validated against current fields from HF radar observations and from numerical model outputs.
So, the manuscript represents an important step in the assessment of this novel concept.
The manuscript is well organized and well written. Overall, it is a very good paper.
Specific comments
1- Retrieving the Total Surface Current Velocity is the main goal of the OSCAR and Seastar concepts. However, in the manuscript, the notion of TSCV is not defined: in particular does it include a motion component due to waves, in additional to what is classically defined as “current”? This should be clarified.
If it includes a component due to the waves, then in the validation, the comparison with surface currents from a numerical oceanographic model may not be fully appropriate because this latter does not include the wave component.
So more specifically, I suggest that the authors add some words on the definition on TSCV at the beginning of the manuscript (probably in the introduction) and that they add some discussion in the section 4 and/or 5 on the fact that (probably) wave effects are omitted in the current field from the numerical model whereas it is included in the retrieved TSCV.
2- The cost function defined to retrieved both the surface wind vector and the TSCV (Eq.1) is slightly different from the one presented in Martin et al, 2018 . Indeed in this latter the GMF for sigma0 and for the Doppler anomaly is defined as a function of the wind relative to the current (u10-c ), whereas in the submitted manuscript, the current effect is not taken into account in the wind or current GMF model functions.
So, I suggest the authors explain and justify this evolution or comment on this intertwined relation between wind and current in the GMFs.
3- The empirical model (GMF) used to express the Doppler anomaly due to the wave effects (WASV) is derived from the work of Mouche et al (2012). However, in the paper of Mouche et al, it seems that the model is limited to incidence angles less than about 40°, whereas the observations of OSCAR extend up to about 69° . So, for the inversion of OSCAR data, how is the GMF for the Doppler anomaly extended to the largest incidence angles (from 40 to 69°) ? This should be discussed.
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RC2: 'Comment on egusphere-2023-2995', Alejandro Orfila, 12 Jun 2024
The Manuscript presents the results from an airborne radar experiment (OSCAR, a demonstrator of the Seastar concept) on the Irionian Sea, a macrotidal environment in Brittany. The data obtained came from the results of a 6 days experiment where the surface velocities and wind speeds were retrieved over an area west to the coast of the island of Ushant from a 3-look SAR along-track interferometry. Measured data are validated against a ground X-band radar, model outputs from operational ocean and atmospheric models as well with satellite data, showing an excellent agreement.
The paper is very well written and the results show a promising technology to measure ocean surface velocity at very high resolution over relatively large areas in a synoptic way. The only concern that I have is if Ocean Sciences is the correct place to publish this work since it presents a technological development rather than providing insights on ocean processes (however this is an opinion and I agree that this is far away from the role of a referee). The authors claim that more experiments are required to fully demonstrate the capabilities of the system and they prepare a more detailed work with additional tests.
Minor comments:
- Ln 235: Has this better agreement in the y-direction something to do with the preferential direction of the flow?.-
- Please define TSCV
- Is the measured surface ocean velocity the geostrophic component plus the ekman and stokes components?.
- minor typos: ln's # 45, 67 , 240
Citation: https://doi.org/10.5194/egusphere-2023-2995-RC2 -
AC2: 'Reply on RC2', David L. McCann, 12 Jun 2024
Thankyou for your review and comments. We are preparing a revised manuscript that addresses the points you have raised, however here are some short answers to your specific questions for discussion (numbered as in your comment):
- Yes this is the case - throughout a majority of the comparison field (i.e., imaged area with X-band crossover) the vectors are broadly North-South orientated so the V velocity component shows a better comparison. We felt that along with figure 3b that a vector component comparison would be a better way of showing where our results are good and where they are less good - specifically in the high flow region that happens to coincide with low incidence angles. We will add some more discussion of this in Section 5.
- This comment chimes with Reviewer #1, so we will be providing more detail in the Introduction and Section 5, however briefly here:
TSCV corresponds to the effective mass transport. It does include the Stokes drift, but does not include waves artifacts (Wind-wave Artifact Surface Velocity, WASV) in the direct Doppler measurements.
- We measure the total surface current, which in the case here is dominated by the tidal current. The total surface motion measured by the Doppler shift of the SAR imagery is comprised of the tidal current, wave orbital motion, stokes drift, wind drift and any other effect that is moving that parcel of fluid. In order to get TSCV, these effects must be accounted for in the WASV and removed from the total surface motion sensed by the Doppler. With regard to Stokes drift - its possible, but would need more work. Its a complicated issue, but briefly the effect will be dwarfed by other contributions to the total surface motion and will be difficult to unpick. This is an interesting thought and we will address this in Section 5.
On your comment concerning the fit with OS, we felt that as this manuscript contains the first results of a new system for oceanographic remote sensing that a broader oceanographic audience would be interested rather than restricting these first results to a more specialist remote sensing publication. Previous papers similar to this have been published in OS (e.g., https://os.copernicus.org/articles/16/1399/2020/), which gave us confidence that it is indeed a good fit.
Citation: https://doi.org/10.5194/egusphere-2023-2995-AC2
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AC1: 'Reply on RC1', David L. McCann, 12 Jun 2024
Many thanks for your review and comments. We are preparing a revised manuscript that will address the points you have raised, but here are some short responses (numbered after your specific comments):
- TSCV corresponds to the effective mass transport. It does include the Stokes drift, but does not include waves artifacts (Wind-wave Artifact Surface Velocity, WASV) in the direct Doppler measurements. This will be clarified in the Introduction and discussed more fully in Section 5
-
In Martin et al., 2018, u10 was considered as being the Earth Relative Wind (same as for Normalised Wind Product, NWP). Due to confusion in the community, where u10 is considered as the Ocean Surface Wind (Earth Relative Wind minus Current), we used this later definition in the submitted manuscript. Thank you for raising it, we will highlight it and make reference to the difference with Martin et al., 2018.
-
Thank you for raising this point that we overlooked. Indeed Mouche et al., 2012 has only been developed up to 44° of incidence angle. The model implemented in our methodology has been extended to higher incidence angles. From Yurosky et al's 2019 results (with incidence angles up to 65 degrees), this is sensible. This extension to higher incidence angle will be highlighted in the Methodology (Section 2.2) and will be discussed more fully in Section 5.
Citation: https://doi.org/10.5194/egusphere-2023-2995-AC1
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RC2: 'Comment on egusphere-2023-2995', Alejandro Orfila, 12 Jun 2024
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AC3: 'Comment on egusphere-2023-2995', David L. McCann, 22 Jul 2024
We thank the reviewers for their helpful comments and suggestions. We have uploaded a revised manuscript and changedoc detailing the changes made with regard to reviewers comments.
Citation: https://doi.org/10.5194/egusphere-2023-2995-AC3
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2023-2995', Anonymous Referee #1, 06 Mar 2024
General comments
This manuscript presents the very first validation from real observations of the concept of squinted 3-look SAR along-track interferometry to retrieve the vectors of surface wind and total surface current velocity mapped at high resolution (200m).
The observations were collected with an airborne SAR system called OSCAR in a coastal environment characterized with high tidal flow. OSCAR is the airborne demonstrator of the Seastar satellite concept funded by ESA and based on the same principle of along-track interferometry from a 3-look squinted SAR.
Although based on a limited number of cases studied (two meteo-oceano situations, with one sampled under different flight geometries), the results are very convincing. The retrieved field of wind and surface current are validated against current fields from HF radar observations and from numerical model outputs.
So, the manuscript represents an important step in the assessment of this novel concept.
The manuscript is well organized and well written. Overall, it is a very good paper.
Specific comments
1- Retrieving the Total Surface Current Velocity is the main goal of the OSCAR and Seastar concepts. However, in the manuscript, the notion of TSCV is not defined: in particular does it include a motion component due to waves, in additional to what is classically defined as “current”? This should be clarified.
If it includes a component due to the waves, then in the validation, the comparison with surface currents from a numerical oceanographic model may not be fully appropriate because this latter does not include the wave component.
So more specifically, I suggest that the authors add some words on the definition on TSCV at the beginning of the manuscript (probably in the introduction) and that they add some discussion in the section 4 and/or 5 on the fact that (probably) wave effects are omitted in the current field from the numerical model whereas it is included in the retrieved TSCV.
2- The cost function defined to retrieved both the surface wind vector and the TSCV (Eq.1) is slightly different from the one presented in Martin et al, 2018 . Indeed in this latter the GMF for sigma0 and for the Doppler anomaly is defined as a function of the wind relative to the current (u10-c ), whereas in the submitted manuscript, the current effect is not taken into account in the wind or current GMF model functions.
So, I suggest the authors explain and justify this evolution or comment on this intertwined relation between wind and current in the GMFs.
3- The empirical model (GMF) used to express the Doppler anomaly due to the wave effects (WASV) is derived from the work of Mouche et al (2012). However, in the paper of Mouche et al, it seems that the model is limited to incidence angles less than about 40°, whereas the observations of OSCAR extend up to about 69° . So, for the inversion of OSCAR data, how is the GMF for the Doppler anomaly extended to the largest incidence angles (from 40 to 69°) ? This should be discussed.
-
RC2: 'Comment on egusphere-2023-2995', Alejandro Orfila, 12 Jun 2024
The Manuscript presents the results from an airborne radar experiment (OSCAR, a demonstrator of the Seastar concept) on the Irionian Sea, a macrotidal environment in Brittany. The data obtained came from the results of a 6 days experiment where the surface velocities and wind speeds were retrieved over an area west to the coast of the island of Ushant from a 3-look SAR along-track interferometry. Measured data are validated against a ground X-band radar, model outputs from operational ocean and atmospheric models as well with satellite data, showing an excellent agreement.
The paper is very well written and the results show a promising technology to measure ocean surface velocity at very high resolution over relatively large areas in a synoptic way. The only concern that I have is if Ocean Sciences is the correct place to publish this work since it presents a technological development rather than providing insights on ocean processes (however this is an opinion and I agree that this is far away from the role of a referee). The authors claim that more experiments are required to fully demonstrate the capabilities of the system and they prepare a more detailed work with additional tests.
Minor comments:
- Ln 235: Has this better agreement in the y-direction something to do with the preferential direction of the flow?.-
- Please define TSCV
- Is the measured surface ocean velocity the geostrophic component plus the ekman and stokes components?.
- minor typos: ln's # 45, 67 , 240
Citation: https://doi.org/10.5194/egusphere-2023-2995-RC2 -
AC2: 'Reply on RC2', David L. McCann, 12 Jun 2024
Thankyou for your review and comments. We are preparing a revised manuscript that addresses the points you have raised, however here are some short answers to your specific questions for discussion (numbered as in your comment):
- Yes this is the case - throughout a majority of the comparison field (i.e., imaged area with X-band crossover) the vectors are broadly North-South orientated so the V velocity component shows a better comparison. We felt that along with figure 3b that a vector component comparison would be a better way of showing where our results are good and where they are less good - specifically in the high flow region that happens to coincide with low incidence angles. We will add some more discussion of this in Section 5.
- This comment chimes with Reviewer #1, so we will be providing more detail in the Introduction and Section 5, however briefly here:
TSCV corresponds to the effective mass transport. It does include the Stokes drift, but does not include waves artifacts (Wind-wave Artifact Surface Velocity, WASV) in the direct Doppler measurements.
- We measure the total surface current, which in the case here is dominated by the tidal current. The total surface motion measured by the Doppler shift of the SAR imagery is comprised of the tidal current, wave orbital motion, stokes drift, wind drift and any other effect that is moving that parcel of fluid. In order to get TSCV, these effects must be accounted for in the WASV and removed from the total surface motion sensed by the Doppler. With regard to Stokes drift - its possible, but would need more work. Its a complicated issue, but briefly the effect will be dwarfed by other contributions to the total surface motion and will be difficult to unpick. This is an interesting thought and we will address this in Section 5.
On your comment concerning the fit with OS, we felt that as this manuscript contains the first results of a new system for oceanographic remote sensing that a broader oceanographic audience would be interested rather than restricting these first results to a more specialist remote sensing publication. Previous papers similar to this have been published in OS (e.g., https://os.copernicus.org/articles/16/1399/2020/), which gave us confidence that it is indeed a good fit.
Citation: https://doi.org/10.5194/egusphere-2023-2995-AC2
-
AC1: 'Reply on RC1', David L. McCann, 12 Jun 2024
Many thanks for your review and comments. We are preparing a revised manuscript that will address the points you have raised, but here are some short responses (numbered after your specific comments):
- TSCV corresponds to the effective mass transport. It does include the Stokes drift, but does not include waves artifacts (Wind-wave Artifact Surface Velocity, WASV) in the direct Doppler measurements. This will be clarified in the Introduction and discussed more fully in Section 5
-
In Martin et al., 2018, u10 was considered as being the Earth Relative Wind (same as for Normalised Wind Product, NWP). Due to confusion in the community, where u10 is considered as the Ocean Surface Wind (Earth Relative Wind minus Current), we used this later definition in the submitted manuscript. Thank you for raising it, we will highlight it and make reference to the difference with Martin et al., 2018.
-
Thank you for raising this point that we overlooked. Indeed Mouche et al., 2012 has only been developed up to 44° of incidence angle. The model implemented in our methodology has been extended to higher incidence angles. From Yurosky et al's 2019 results (with incidence angles up to 65 degrees), this is sensible. This extension to higher incidence angle will be highlighted in the Methodology (Section 2.2) and will be discussed more fully in Section 5.
Citation: https://doi.org/10.5194/egusphere-2023-2995-AC1
-
RC2: 'Comment on egusphere-2023-2995', Alejandro Orfila, 12 Jun 2024
-
AC3: 'Comment on egusphere-2023-2995', David L. McCann, 22 Jul 2024
We thank the reviewers for their helpful comments and suggestions. We have uploaded a revised manuscript and changedoc detailing the changes made with regard to reviewers comments.
Citation: https://doi.org/10.5194/egusphere-2023-2995-AC3
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David Lewis McCann
Adrien C. H. Martin
Karlus Macedo
Ruben Carrasco Alvarez
Jochen Horstmann
Louis Marié
José Márquez-Martínez
Marcos Portabella
Adriano Meta
Christine Gommenginger
Petronilo Martin-Iglesias
Tania Casal
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
- Preprint
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