Geophysical Doppler shifts measured by synthetic aperture radar (SAR) are influenced by the combination of ocean surface winds, waves, and currents. Accurate retrieval of radial surface current velocities therefore requires precise estimation and removal of the wave-induced Doppler contributions. To address this challenge, we developedWaveDop, a hybrid physics-guided and data-driven model based on the eXtreme Gradient Boosting (XGBoost) machine learning algorithm, designed to estimate Doppler shift contributions arising from wind and waves effects. The model was trained on a comprehensive dataset comprising Sentinel-1 SAR Doppler measurements acquired over global coastal regions from January 2015 to October 2022, in conjunction with WAVEWATCH III wave model outputs and drifter-observed surface currents. Our analysis reveals that wave-induced Doppler shifts are strongly modulated by the radar incidence angle, surface wind fields, and wave characteristics, with both wind waves and swell making significant contributions. By accounting for these wave effects, and correcting for non-geophysical artifacts such as antenna electronic mis-pointing, satellite attitude variations, and azimuthal scalloping, we derive radial current velocities from SAR observations. Validation against independent drifter measurements indicates strong agreement, with a correlation coefficient of 0.82, negligible bias, and a root-mean-square error (RMSE) of 0.16 m/s. These results demonstrate thatWaveDopeffectively estimates wave-induced Doppler shifts and enables reliable retrieval of ocean surface current velocities from SAR data.

This study examines the impact of wave breaking (WB) and Bragg scattering on the normalized radar cross section (NRCS) to reveal radar scattering features during high wind conditions. This is conducted by decomposing C-band quasi-synchronous wide-swath quad-polarization synthetic aperture radar (SAR) observations acquired from the RADARSAT Constellation Mission (RCM) and RADARSAT-2 (RS-2). The analysis results clearly demonstrate that the polarization difference (PD) associated with Bragg scattering saturates at high wind speeds, while still maintaining azimuthal modulation. Notably, the radar returns from breaking waves at cross-polarization (HV or VH) exhibit higher sensitivity to wind speeds but lower sensitivity to wind direction, compared to co-polarization (HH or VV). Moreover, our analyses show that WB contributes 40%, 80%, and 90% of VV-, HH-, and HV-polarized NRCS, respectively. This study highlights the unique capabilities provided by collocated RCM and RS-2 observations for investigating radar scattering features under high wind conditions. Results of this study can be further used to develop empirical models for estimating co- and cross-polarization radar backscatters induced by WB under high wind speeds.

We present the characteristics of the sea surface height (SSH) response to tropical cyclones (TCs), based on a statistical analysis of long time series of satellite altimeter data. SSH anomalies on both along-and cross-track of TC are found to be dependent on storm intensity and translation velocity. Larger sea surface troughs correspond to the stronger and slower moving TCs. The most striking trough features are located in the vicinity of TC centers. The amplitudes of wedge-shaped troughs decay in the direction normal to the storm tracks. In extreme weather conditions, these troughs are primarily caused by the vertical displacement of isopycnals in the thermocline. For very strong and slow-moving TCs, the magnitude of the sea surface depressions can reach -21 cm. The spatial distribution of SSH anomalies tends to be asymmetric for faster moving TCs, comparing both sides of the TC tracks, becoming more symmetric for slower storms. The conventional altimeter has large gaps between tracks, especially in the tropics, which leads to the absence of SSH anomaly observations. To address this inherent limitation, we apply a semi-empirical model to estimate SSH anomalies, using TC and ocean stratification parameters. When applying the maximum wind speed and radius of the maximum wind derived from high-resolution synthetic aperture radar (SAR) gridded surface wind fields, the semi-empirical model estimates are in good overall agreement with collocated altimeter observations. These results suggest that indirect calculations of SSH anomalies may serve as an important complement for TC inner-core areas, when altimeter observations are not available.

The Doppler shift obtained from synthetic aperture radar (SAR) measurements comprises the combined contribution to the radial motion of the ocean surface induced by the sea state (wind waves and swell) and underlying surface currents. Hence, to obtain reliable estimates of the ocean surface current, the sea-state-induced Doppler shifts must be accurately estimated and eliminated. In this study, we use a semiempirical dual co-polarization Doppler velocity (DPDop) model, presented in the companion paper, to calculate sea-state-induced Doppler shifts using buoy-measured wind speed, wind direction, and ocean wave spectra. The DPDop model-simulated Doppler shifts are compared with the collocated Sentinel-1B SAR Wave (WV) mode observations at the 24° and 37° incidence angles, showing a bias of -0.24 Hz and a root-mean-square error (RMSE) of 5.55 Hz. This evaluation is also implemented on a simplified DPDop model at the same incidence angles. The model inputs include wind fields from the European Center for Medium-Range Weather Forecasts (ECMWF) and wave characteristic parameters (e.g., significant wave height, mean wave direction, and mean wavenumber) from WAVEWATCH III (WW3). The estimated Doppler shifts are validated using the ascending and descending observations of Sentinel-1B WV over the global ocean. Furthermore, the comparisons show that the bias and RMSE are -0.71 Hz and 9.25 Hz, respectively. Based on accurate wave bias correction, we obtain the radial current speeds of the ocean surface from the Doppler shift measurements. The estimated current speeds are compared with the collocated high-frequency radar measurements, with a bias of -0.04 m/s and an RMSE of 0.15 m/s. These results suggest that the original and simplified DPDop models can be used to estimate sea-state-induced Doppler shifts and, thus, derive accurate surface current retrievals.

This article proposes a Doppler velocity (DV) model based on dual co-polarized (co-pol) decomposition of a normalized radar cross section of an ocean surface on polarized Bragg scattering and nonpolarized (NP) radar returns from breaking wave components. The dual co-pol decomposition provides a quantitative description of resonant and NP scattering, as well as their dependence on the incident angle, azimuth, and wind speed. Subsequently, the contributions of the facet (resonant Bragg waves and breakers) velocities, tilt, and hydrodynamic modulations due to long waves to the resulting DV can be quantified. The tilt modulation contributions to DV are estimated using the measured/empirical tilt modulation transfer function (MTF). The hydrodynamic modulations are mostly dominated by wave breaking and are estimated using a semiempirical model based on in situ measurements. In addition to the VV and HH radar data, which are required for dual co-pol decomposition and tilt MTF estimates, the surface wave spectrum is required in the DV determination for a given radar observation geometry. In this article, qualitative and quantitative consistencies are presented between the model simulations and the empirical CDOP model. In a companion paper, a DV analysis is presented to analyze the Sentinel-1 synthetic aperture radar measurements and collocated in situ measurements of surface wind and wave spectra.

International audience

Results of investigations of surface waves generated by Polar Lows (PLs) in the Barents and Norwegian seas are presented. A parametric wave model originally developed in [1] is applied to reproduce surface waves in Arctic seas generated under non-stationary and non-uniform wind field of PLs, observed by altimeters. The ERA-5 wind field reanalysis is used as the input parameter. The solution of model equations provides hourly fields of wave parameters (significant wave height, SWH, mean wavelength, and mean direction) on the regular grid. Satellite altimeter observations and analysis of model simulations reveal appearance of abnormal high surface waves, resulting from the group velocity resonance between developing waves and moving wind field. This resonance effect (or effect of wave trapping) occurs when the zone of strong wind inside the PL is located in the right sector, where the direction of the wind velocity coincides with PL heading.

In this study we demonstrate high potential of synergistic satellite data usage to investigate the Arctic saline sea ice. A simple geometrical optics model is explored to show the possibility to derive the First Year sea ice parameters best fitting the experimental data. Satellite Sentinel-1 Synthetic Aperture Radar (SAR) data are used to analyze the mesoscale sea ice structure and detect the age of the sea ice samples. Sentinel-3 altimeter data along with the Metop Advanced scatterometer (ASCAT) measurements of the normalized radar cross section (NRCS) are jointly used to build the NRCS dependencies on the incidence angle for the selected sea ice samples. Sea ice drift fields are used to investigate only those regions where the drift is negligible on a single day scale. It is shown that the suggested approach presents a powerful tool to study the Arctic sea ice properties.

The CFOSAT mission provides a unique data set of collocated nadir, near-nadir and moderate angle dual-polarization Ku-band radar measurements. These combined measurements open new opportunities to revisit existing interpretation and approaches to analyze ocean radar backscatter signals. In particular, actual scatterometer wind vector retrieval algorithms and definitions of Geophysical Modulation Functions (GMFs) can be revisited, including sensitivity to additional parameters: wave state, currents, etc. The present work describes an alternative GMF approach based on the analysis of signal polarization differences, and interpretation in terms of ocean surface wind-roughness spectra. Based on CFOSAT measurements, the polarization-difference GMF reproduces main properties of standard empirical radar GMFs. However, it is more directly related to a theoretical short wave spectral model. This approach naturally enables the inclusion of additional sea state variables. Results of this work is intended to be implemented in wind retrieval processors to complement existing methods.

Based on energy and momentum conservation laws, a new simple 2D parametric model using ray tracing analysis is used to assess high sea states from tropical cyclones (TCs). From self-similar solutions of model equations, different combinations of TC parameters (maximum wind speed, TC radius and translation velocity) are performed and simplified analytical expressions are suggested for robust first-guess estimates of maximal wave length and height. A 2D geophysical model function (TC-wave GMF) is then constructed to map 2D fields for the dominant wave energy, wavelength and direction within the TC area. Results from the full model and simplified expressions are compared to airborne scanning radar altimeter measurements during hurricane Bonnie (1998) and WW3 estimates. Excellent agreement is found for both field measurements and wave spectral numerical model. While the direct model calculations are more accurate and detailed, TC-wave GMF provides useful immediate estimates of wave fields for practical nowcasting and scientific applications.

Field Ka-band radar measurements of breaking wind waves, identified with synchronized video recordings, are presented. Separating video frames with and without breakers, the breaker-free (background) and breaker-disturbed surface normalized radar cross-section (NRCS) are estimated, and analyzed along long wave profiles. Breaker NRCS signals depict clear non-polarized features near long wave crests. Simultaneous breaker radar footprint fraction measurements enables the precise estimation of pure breaker NRCS, suitable to model both wave breaking and Doppler signatures.

RADARSAT-2 C-band quad-polarization ocean synthetic aperture radar (SAR) scenes are decomposed into resonant Bragg scattering from regular (no-breaking) surface and scattering from breaking waves. Analysis of the surface current signatures in dual co- and cross-pol SAR images revealed that governing imaging mechanism is modulations of wave breakings which are very sensitive to the presence of current non-uniformities. As found, due to small relaxation scale, short Bragg waves do not "feel" the current. Thus routinely observed current signatures in quad-pol SAR images originate essentially from wave breaking modulations, and modulation of Bragg waves does not matter this issue.

This study provides quantitative estimates of wind waves breaking contribution to dual co- and cross-polarized radar scattering. Extending approaches suggested by [1], [2], 1696 RADARSAT-2 quad-polarized Synthetic Aperture Radar measurements collocated with 65 in situ National Data Buoy Center buoys observations are analyzed. For VV polarization, wave breaking contribution decreases from 60% to 20% with increasing incidence angle, while for HH polarization and cross-polarization wave breaking contribution is about 60%-70% at all incidence angles. Empirical dependencies of wave breaking contributions to co- and cross-pol signals on wind speed, incidence angles and azimuth are then suggested.

This paper provides improved quantitative estimates of the wind-ruffled roughness contributions to dual co- and cross-polarized radar signals. Expanding previous approaches, 1696 RADARSAT-2 quad-polarized synthetic aperture radar (SAR) measurements, co-located with 65 in situ National Data Buoy Center (NDBC) buoy observations, are analyzed. Considering all wind conditions, the impact of breaking and near-breaking waves on dual co- and cross-polarized radar signals is robustly documented. For VV polarized measurements, the contribution of breaking waves decreased from 60% to 20% with increasing incidence angle, whereas for HH polarization and cross-polarization measurements, it can amount to about 60%-70% for all incidence angles. Building on the large analyzed data set, robust empirical dependencies between breaking waves and their impact on co- and cross-pol signals are then derived, as functions of wind speeds, incidence angles, and azimuth directions.

Polar lows are the most intense subclass of mesocyclones developing at high latitudes above ice-free sea surface. These dangerous phenomena are capable of generating high wind waves even when cyclone is stationary. Moreover, as in case of tropical cyclones, a trapped-fetch effect may occur, i.e., when a polar low translates, waves in its right sector remain under high wind forcing for a prolonged time and grow to anomalous heights. Here we provide a model evaluation of the probability of appearance of such anomalous waves using a recently suggested wave enhancement criterion for moving extreme events and a new polar low climatology. As found, a significant part of the observed polar lows is capable of generating anomalously high waves. Frequency of appearance of abnormally high waves with heights exceeding given level (e.g. 6, 8, 10m) in the area of interest is presented.

The reconstruction of ocean surface wave characteristics, i.e. directional energy distribution and dispersion, from ocean sunglitter images is discussed. Wave spectral analysis is performed, through the definition of linear transfer functions, to relate image brightness variations and sea surface slopes. Surface-current Doppler shift modifies the wave dispersion relation and can be obtained from cross-correlating two images and tracking glint changes over a known time. The technique is tested on photographs, taken from an aircraft in the Gulf of Mexico, and from a drone in the Black sea. The method is further extended to satellite sunglitter imagery, taking advantage of the technical peculiarities of Sentinel- 2 sensors helping to infer both the brightness gradient in the along-track direction. Wave dispersion information through the inter-channel time delay allows to reconstruct the current vector. The results demonstrate encouraging prospects of using the sunglitter imagery for the sea state monitoring, including formation of abnormally high waves and the study of wave-current interactions at different scales.

Ka-band Radar Doppler velocity is compared to the optical velocity of deep water breaking wind waves observed in field conditions from a stationary oceanographic platform. One typical record is analyzed: wind speed is 11 m/s, dominant wave frequency is 0.24 Hz, radar incidence angle is 53°, and radar-to-wind azimuth is upwind. This 30 minute record includes 42 breaking wave events. The Doppler velocity is estimated as the first moment of instantaneous Doppler spectrum. The optical velocity of breakers is measured using spatially collocated and temporally synchronized video records of the radar footprint. As found the Doppler velocity of breakers is remarkably slower (by a factor of 4) than the “optical” velocity of whitecaps. This suggests that whitecaps (translated with phase velocities of breaking waves) represent a moving source of the enhanced surface roughness providing radar returns. Since this roughness is “embedded” into the water, its Doppler velocity is much smaller than the phase velocity.

This paper presents dual copolarized (VV and HH) Ka-band radar measurements of joint modulation of normalized radar cross section (NRCS) and Doppler velocity (DV) performed from a sea research platform. NRCS and DV modulations are well correlated. NRCS modulations exhibit a spiky structure. HH modulations are stronger than VV ones leading to modulations of the polarization ratio. This suggests that an important portion of NRCS modulations is produced by nonpolarized radar returns from modulated wave breaking facets. DV modulations reveal that at incidence angles <;50°, NRCS spikes are attributable to rather slow moving facets, which may be interpreted as short wave breaking disturbances embedded in the water at crests of modulating waves. Using the DV as a proxy for wave gauge, a modulation transfer function (MTF) is estimated for both polarizations. The hydrodynamics component of the total MTF, hydro-MTF, combines NRCS modulations supported by Bragg waves and wave breaking. The contribution of each type of facets to the hydro-MTF is weighted by its partial contribution to the NRCS, and thus, hydro-MTF becomes dependent on radar polarization. Using hydro-MTF for HH and VV, Bragg wave and wave breaking modulations are separated. Wave breaking modulations are significant, with MTF amplitude varying from 5 to about 30 depending on wind speed. Ka-band Bragg waves are strongly modulated at low winds, but their modulation almost vanishes at moderate winds. Finally, we propose an empirical MTF parameterization based on polynomial fitting as a function of observation geometry and wind.

The first results of internal solitary wave (ISW) observations over the ice-free Laptev Sea derived from 354 ENVISAT Advanced Synthetic Aperture Radar (ASAR) images acquired in May-October 2011 are reported. Analysis of the data reveals the key regions of ISW distribution that are primarily found over the outer shelf/slope regions poleward the M2 critical latitude. Most of the ISWs are observed in regions where enhanced tide-induced vertical mixing and heat fluxes have been previously reported. This suggests that spaceborne SAR observations may serve as a tool to infer local mixing hot spots over the ice-free Arctic Ocean.

This paper presents dual copolarized (PP) (VV and HH) Ka-band sea surface backscattering measurements taken from the Black Sea research platform at incidence angles ranging from 25° to 65° and in the wind speed range from 3 to 18 m/s. These measurements are corrected for radar antenna pattern and geometry of observations. The resulting normalized radar cross section (NRCS) is parameterized in a form of truncated azimuthal Fourier series with coefficients dependent on the incidence angle and wind speed. This dual PP empirical model (KaDPMod) is consistent with the Ku-band NSCAT-4 model. However, some remarkable differences are revealed. They are apparent when analyzed using a decomposition of VV and HH measurements into polarized Bragg backscattering (polarization difference, PD = VV-HH) and nonpolarized (NP) backscattering from breaking waves. The polarization difference (PD) has strong azimuth and wind dependencies, with the wind exponent ranging from 2.5 to 3. The saturation wave spectra derived from multifrequency PD (based on KaDPMod and Ku- and C-band empirical models) have a notable peak in the capillary-gravity range. The relative contribution of NP radar return to the Ka-band NRCS is significant. In the upwind direction, it reaches up to 60%-80% and 25%-50% for HH and VV, respectively. It is found that the NP wind exponent is lower than that for Bragg backscattering. Therefore, the relative contribution of the NP to Ka-band NRCS decreases with increasing wind speed at both polarizations. Such a behavior is the opposite of that observed in the Ku-band.

In this work taking the advantage of high resolution spaceborne synthetic aperture radar (SAR) measurements we present first preliminary results of short-period internal waves (IW) observations in the Barents, Kara and White seas based on ENVISAT ASAR data for summer-autumn months in 2007–2011. Altogether more than 2000 IW packets were identified in about 1400 SAR images. Detailed maps of internal wave's occurrences in the three Arctic seas have helped to identify main sites of regular IW generation.

In this study we exploit the polarization sensitivity of high-resolution SAR measurements in C- and L-bands using a methodology suggested in [1, 2] to study the physics of radar imaging of upper ocean dynamics. Using samples of RADARSAT-2, ALOS PALSAR and ALOS-2 PALSAR-2 quad-polarization ocean SAR scenes, we decompose dual co-polarized (VV and HH) radar data into polarization difference (PD) and non-polarized (NP) components. Following [1, 2] these quantities provide means to distinguish Bragg scattering and non-polarized radar returns related to the quasi-specular reflection from breaking waves and from slopes of “regular” large-scale waves, and assess their relative role in the formation of surface current signatures in C- and L-band SAR ocean images. While C-band SAR features of ocean currents are fully dominated by non-resonant and non-polarized radar scattering from breaking waves, their manifestation in L-band SAR images are formed by Bragg waves, wave breaking and large-scale “regular” waves modulations by the currents.

In this paper, we compare satellite-borne Cand X-band synthetic aperture radar (SAR) data for marine oil spill observation. During large-scale oil-on-water exercises in the North Sea, quad-polarization Radarsat-2 (C-band) and dual-polarization TerraSAR-X (X-band) data were acquired with temporal distances of less than 24 min. The objective is to characterize and quantify differences in the Radarsat-2 and TerraSAR-X measurements. Three scene pairs are compared in terms of data quality and signal characteristics, including statistical properties and selected multipolarization (HH, VV) parameters. The signal characteristics are also compared among low-backscatter features of various origin within the individual pairs. No viable argument for selecting one sensor above the other is identified in the data quality study. In the statistical analysis, investigation of logcumulants indicates a larger deviation from Gaussian statistics in the TerraSAR-X data compared with Radarsat-2 measurements. Log-cumulant diagrams are also shown to be a useful tool for discrimination between oil spills and a simulated biogenic slick in both sensors. Multipolarization features show enhanced slick-sea contrasts and a better discrimination between mineral oil spills and other low-backscatter features in Radarsat-2 compared with TerraSAR-X. The presence of a non-Bragg scattering component in the data is revealed for both sensors. The relative contribution of non-Bragg scattering to the total backscatter is found to be higher in the TerraSAR-X data than in the Radarsat-2 data. In general, the non-Bragg component is found to account for a larger part of the backscatter in slick-covered areas compared with clean sea.

An effective methodology using satellite high-resolution polarized information to interpret and quantitatively assess various surface ocean phenomena is suggested. Using a sample RADARSAT-2 quad-polarization ocean synthetic aperture radar (SAR) scene, the dual co-polarization (VV and HH) radar data are combined into polarization difference, polarization ratio, and nonpolarized components. As demonstrated, these field quantities provide means to distinguish Bragg scattering mechanism and radar returns from breaking waves. As shown, quantitative characteristics of the surface manifestation of ocean currents, slicks, and wind field features in these dual co-polarization properties are very different and may be effectively used in the development of new SAR detection and discrimination algorithms.

Centerline wakes of ships in synthetic aperture radar (SAR) images were modeled in 2-D with the computational fluid dynamics (CFD) software Fluent and a radar-imaging algorithm. We initialized the model with a pair of vortices generated by a ship hull and applied wind stress perpendicular to the ship wake. Results of the CFD simulation using a nonhydrostatic model have demonstrated ship-wake asymmetry with respect to the wind-stress direction relative to the ship course. Due to the wind stress, flow convergence increased on the upwind side of the centerline wake and reduced on the downwind side of the wake. The radar-imaging algorithm processed with the surface velocity field produced by the CFD model revealed ship-wake asymmetry relative to the wind direction. These results are qualitatively consistent with SAR images from the TerraSAR-X satellite and representative statistics of photographic images of the ship wake collected from a volunteer observing ship.

We propose here to use a combination of radar observations in HH and VV polarizations at C-Band to study the different contributions to the sea surface backscattering - polarized and non-polarized parts- and to characterize the relation between azimuth anisotropy of the signal and the surface properties.