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

Remote sensing of rainfall events is critical for both operational and scientific needs, including for example weather forecasting, extreme flood mitigation, water cycle monitoring, etc. Ground-based weather radars, such as NOAA's Next-Generation Radar (NEXRAD), provide reflectivity and precipitation estimates of rainfall events. However, their observation range is limited to a few hundred kilometers, prompting the exploration of other remote sensing methods, particularly over the open ocean, that represents large areas not covered by land-based radars. Here we propose a deep learning approach to extract rainfall information from SAR imagery. SAR has the advantage of providing global coverage and a very high resolution. We demonstrate that a convolutional neural network trained on a collocated Sentinel-1/NEXRAD dataset clearly outperforms state-of-the-art filtering schemes such as the Koch's filters, which has been implemented here as a neural network in a machine learning framework. Our results indicate high performance in segmenting precipitation regimes, delineated by thresholds at 24.7, 31.5, and 38.8 dBZ. Compared to current methods that rely on Koch's filters to draw binary rainfall maps, these multi-threshold learning-based models can provide rainfall estimation. They may be of great interest for improving the qualification of SAR-derived wind field data., Comment: 9 pages, 9 figures

The wave spectrum is a representation of the state of the ocean surface from which many parameters can be deduced: significant wave height, peak parameters of dominant waves, directional parameters, etc... For more than 30 years, Synthetic Aperture Radars allow their routine montoring far from the coast in all surface conditions (through clouds and despite night), where buoys cannot be deployed. These measurements benefit from scientific efforts that now make it a reliable measurement technique. Sentinel-1 constellation is one of them and operates since 2016. However known limitations, including wave blurring caused by the azimuth cut-off, limit the performance of wave spectra measurement to long swells. SWIM is a new rotating Radar onboard the Chinese-French CFOSAT satellite dedicated to directional wave spectra measurement. Not suffering from the cut-off limitations, new horizons and perspectives for synergies are opened in terms of spectral limit and directionality. Sentinel-1 wave spectra measurements are limited to Wave Mode acquisitions, only available over deep ocean basins and away from the North-East Atlantic Oean. SWIM measurements offer hundreds of co-located measurements over these regions, but also extends the coverage to closed seas worldwide and to European waters. Other complementarities exist in wave measured wavelengths : Sentinel-1 extends to long swell (up to 800 m wavelength) while SWIM shows greater ability to measure wind sea components (close or below 50 m) and a significant part of the Stokes drift. Those complementarities are assessed at different levels and with different comparisons methodologies. First, partition integral parameters are compared between SWIM Level-2P products or S1 Level-2 products and numerical wave model outputs alone. Second, dynamical co-locations are performed between S1 and SWIM, using cross-overs given by Level-3 spectral products. These measurements, also referred to as Fireworks, allow to dramatically increase of the number of co-located points and better inter-compare. These new performances have applications in data assimilation and prospects for new products such as Stokes drift, a first-estimated spaceborne measurement.

The Stokes drift is the main contribution from ocean surface waves to particles drift in the ocean's upper layer. At the moment, estimates of the Stokes drift at the global scale rely on numerical models, due to the unavailability of such measurements from space. CFOSAT is a new mission concept, carrying SWIM, a rotating scatterometer, operating both at nadir and off-nadir. It aims at measuring the ocean surface waves slopes spectrum, from which the Stokes drift can be estimated. Potential for Stokes drift estimation from SWIM is illustrated using statistical diagnostics on SWIM ocean waves spectra. Although the Stokes drift is carried in majority by waves which are not resolved by SWIM, it is shown that an algorithm based on the extrapolation of SWIM waves spectra could be thought of. Such potential combined with the characteristics of SWIM make the effort to design and qualify a Stokes drift product interesting. Such a product would be of particular interest in areas such as the Southern Ocean, where Stokes drift is significant and poorly known, and where the observational conditions are favorable to SWIM (resolved wind sea, favorable wave direction)

CFOSAT is a new mission concept carrying SWIM, a scatterometer in Ku-band, aimed at measuring ocean waves spectra worldwide. It carries 6 low incidence rotating beams: one at nadir, and the 5 others at 2, 4, 6, 8 and 10 degrees incidence. These characteristics make CFOSAT a good intermediate between altimeters and classical scatterometers operating at higher incidence angles. At nadir, sea-ice reflects more energy than over open water, implying peaky waveforms, that are not well processed using classical retraking algorithms. An adaptative retracking algorithm is applied to CFOSAT nadir data, which provides a so-called pseudo mss parameter over sea-ice, that directly relatesto its surface roughness. Off nadir, sea-ice reflects less energy than open water and exhibits a convex response with incidence. To our knowledge, apart from very specific studies, these characteristics had never been performed in the past to discriminate sea-ice from open water. A bayesian sea-ice flagging algorithm is developped, that is based on the prior knowledge of the average behavior of NRCS over sea-ice and open water, the so-called Geophysical Model Functions. Comparison with SSMI sea-ice concentration data shows better performances than the existing flag based on forecasts, and shows promising potential for more specific sea-ice characterization.

Surface currents are poorly known over most of the oceans. Satellite-borne Doppler Waves and Current Scatterom-eters (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 5 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 3-dB footprint diameter (about 580 m). The KaRADOC system has a much narrower beam, with a circular footprint only 45 m in diameter. 10 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 C 0 that is analogous to the Bragg phase speed of coastal High Frequency radars that use a grazing measurement geometry, with little 15 variations ∆ C associated to changes in sea state. The 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 1 https://doi. 30% lower than the theoretical estimate 2.8 m/s. ∆ C 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.

The Sea surface KInematics Multiscale monitoring (SKIM) satellite mission is designed to explore ocean surface current and waves. This includes tropical currents, notably the poorly known patterns of divergence and their impact on the ocean heat budget, and 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 meter 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. Copyright © 2019 Ardhuin, Brandt, Gaultier, Donlon, Battaglia, Boy, Casal, Chapron, Collard, Cravatte, Delouis, De Witte, Dibarboure, Engen, Johnsen, Lique, Lopez-Dekker, Maes, Martin, Marié, Menemenlis, Nouguier, Peureux, Rampal, Ressler, Rio, Rommen, Shutler, Suess, Tsamados, Ubelmann, van Sebille, van den Oever and Stammer. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY).

Short surface gravity waves are ubiquitous at the ocean surface, with lengths from a few tens of meters to a meter typically.Knowing their propagation directions at sea is important in several respects, especially for the understanding of sea-state dynamics, airsea interactions and particles surface drift.Their directional distributions are here investigated in the light of the recent progress made in instrumentation techniques. The analysis of ocean bottom seismo-acoustic noise records allows for the extraction of a quasi-universal behavior which indirectly depends on this distribution through the socalled overlap integral. It is coherent with direct observations of the wave field obtained from tri-dimensional reconstructions of the ocean surface elevation field. While the propagation direction of long waves aligns with the wind direction, short waves progressively detach from it towards small scales (bimodality).Comparing those measurements with the predictions of a spectral numerical wave model, based on WAVEWATCH®III environment, allows to realize that they provide qualitatively correct but quantitatively incorrect predictions. One of the possibilities here explored to correct for it, is by accounting for the sources of energy at ±90° to the wind direction, which could be associated with the breaking of long waves. This source term on its own does not explain the shapes of the observed directional distributions. Other mechanisms could come into play that future investigations will help clarify., Les vagues courtes sont omniprésentes à la surface des océans, avec des longueurs de quelques dizaines de mètres au mètre typiquement. Connaitre leurs directions de propagation en mer est important à plusieurs titres, notamment pour la compréhension de la dynamique de l'état de mer, des échanges air-mer ou de la dérive de particules en surface. Ces distributions directionnelles sont étudiées ici au regard des progrès récents réalisés en techniques d'instrumentation. L'analyse du bruit sismo-acoustique enregistré en grandes profondeurs permet d'extraire un comportement quasi-universel qui dépend indirectement de cette distribution à travers ladite intégrale de recouvrement. Il est cohérent avec des observations directes du champ de vagues obtenues à partir de reconstructions tridimensionnelles de la surface de l'océan. Alors que la direction de propagation des vagues longues s'aligne avec celle du vent, les vagues courtes s'en détachent d'autant plus à mesure que leurs échelles diminuent (bimodalité).La comparaison de ces observations avec les prédictions d'un modèle numérique de vagues, basé sur l'environnement WAVEWATCH®III, permet de constater que ces modèles sont qualitativement valables mais encore quantitativement incorrects. Une des possibilités explorées pour corriger cet effet est la prise en compte de sources de vagues courtes à ±90° de la direction du vent, qui pourraient être associées au déferlement des vagues longues. Une telle source à elle seule n'explique pas les formes des distributions directionnelles observées. D'autres mécanismes pourraient intervenir que de futures investigations pourront tenter de clarifier.