the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
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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Status: open (until 26 May 2024)
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RC1: 'Comment on egusphere-2023-2995', Anonymous Referee #1, 06 Mar 2024
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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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