Ka-Band Radar Cross-Section of Breaking Wind Waves

Yurovsky, Yu. F.; Kudryavtsev, V. N.; Grodsky, Semyon A.; Chapron, Bertrand

doi:10.3390/rs13101929

Cited by 11 publications

(8 citation statements)

References 54 publications

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“…In particular, they showed that at moderate to high incidence angles, these backscatter spikes represent a non-negligible non-polarized backscatter component associated to quasi specular reflection from breaking facets. Although the contribution of this non-polarized component is shown to be stronger at horizontal polarization than vertical polarization [44], such spikes are also visible in vertical polarization, as shown in [45], and they can also be associated to enhanced roughness of the ocean surface from post wave-breaking events, as described in [46]. Moreover, although the studies of wave breaking effects usually focus on moderate to high wind speed conditions, it was suggested in [38] that the capillary waves observed at Ka-band at low wind speed conditions are generated as parasitic capillary waves from the breaking of short-gravity waves [3], showing a possible breaking contribution at low winds as well.…”

Section: Discussionmentioning

confidence: 90%

“…In this case, the skewness increases from 1.45 to 4.3. Yurovsky et al [44] suggested that the presence of high amplitude spikes, making the distribution strongly skewed in linear space, is likely due to wave breaking events. In particular, they showed that at moderate to high incidence angles, these backscatter spikes represent a non-negligible non-polarized backscatter component associated to quasi specular reflection from breaking facets.…”

Section: Discussionmentioning

confidence: 99%

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A Ka-Band Wind Geophysical Model Function Using Doppler Scatterometer Measurements from the Air-Sea Interaction Tower Experiment

et al. 2022

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Physical understanding and modeling of Ka-band ocean surface backscatter is challenging due to a lack of measurements. In the framework of the NASA Earth Ventures Suborbital-3 Submesoscale Ocean Dynamics Experiment (S-MODE) mission, a Ka-Band Ocean continuous wave Doppler Scatterometer (KaBODS) built by the University of Massachusetts, Amherst (UMass) was installed on the Woods Hole Oceanographic Institution (WHOI) Air-Sea Interaction Tower. Together with ASIT anemometers, a new data set of Ka-band ocean surface backscatter measurements along with surface wind/wave and weather parameters was collected. In this work, we present the KaBODS instrument and an empirical Ka-band wind Geophysical Model Function (GMF), the so-called ASIT GMF, based on the KaBODS data collected over a period of three months, from October 2019 to January 2020, for incidence angles ranging between 40° and 68°. The ASIT GMF results are compared with an existing Ka-band wind GMF developed from data collected during a tower experiment conducted over the Black Sea. The two GMFs show differences in terms of wind speed and wind direction sensitivity. However, they are consistent in the values of the standard deviation of the model residuals. This suggests an intrinsic geophysical variability characterizing the Ka-band surface backscatter. The observed variability does not significantly change when filtering out swell-dominated data, indicating that the long-wave induced backscatter modulation is not the primary source of the KaBODS backscatter variability. We observe evidence of wave breaking events, which increase the skewness of the backscatter distribution in linear space, consistent with previous studies. Interestingly, a better agreement is seen between the GMFs and the actual data at an incidence angle of 60° for both GMFs, and the statistical analysis of the model residuals shows a reduced backscatter variability at this incidence angle. This study shows that the ASIT data set is a valuable reference for studies of Ka-band backscatter. Further investigations are on-going to fully characterize the observed variability and its implication in the wind GMF development.

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Section: Discussionmentioning

confidence: 90%

Section: Discussionmentioning

confidence: 99%

A Ka-Band Wind Geophysical Model Function Using Doppler Scatterometer Measurements from the Air-Sea Interaction Tower Experiment

et al. 2022

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“…The spectrum can be speculatively partitioned into a low Doppler frequency, f d , "core", associated with orbital motions of surface waves (|V d | < 2 m s −1 ), and high | f d | transient tails, largely associated with radar signal spikes (mostly positive for upwind look geometry). Most of these spikes can also be associated with peaks in the "core" and/or wave breaking events (see more examples in the dedicated study [42]). However, some events occur independent of wave characteristics within the radar footprint.…”

Section: Sea Spray Signaturesmentioning

confidence: 99%

Ka-Band Doppler Scatterometry: A Strong Wind Case Study

et al. 2022

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Global joint measurements of sea surface winds and currents are planned using satellite-based Doppler scatterometers operating in the Ka-band to achieve improved spatial resolution and retrieval accuracy. Still, the knowledge of sea surface Ka-band backscatter properties is poor, particularly, at high winds (>20 m s−1). Sea surface radar cross-section in the Ka-band, in contrast to that in the lower frequency Ku-/X-/C-/L-bands, is likely more sensitive to sea spray, small-scale particles typically present at high winds. In this paper, tower-based field data collected by a continuous dual-co-polarized Ka-band radar during a strong offshore wind event (with wind speed reaching 33 m s−1) are analyzed. This katabatic wind event (≈12 h long) was also recorded by supplementary wave, wind, and current sensors. At the wave fetch of ≈1 km, the maximum wavelength of observed offshore waves was ≈10 m. For such extremely young wind–sea conditions, an apparent sea spray generation was observed during wind gusts. Radar measurements were performed at 20∘ and 45∘ incidence angles, mostly for cross- and up-wind azimuth look geometry. Based on these high wind measurements, the previously developed Ka-band empirical model is tested and compared with other published geophysical model functions. Dual-co-polarized measurements are used to infer resonant Bragg and non-Bragg scattering components and assess the short wind wave spectrum, which shows a clear tendency for saturation at high winds. The presence of sea spray signatures is apparent in the high-frequency tails of radar Doppler spectra, but their overall contribution to the Doppler centroid frequency is weak. Hence, the standard modulation transfer function approach developed for moderate winds is still applicable at high winds for interpreting the wave-induced Doppler velocity and inferring sea surface currents. These results can also be useful for understanding Doppler scatterometry measurements in tropical cyclones.

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“…This investigation started with a manual inspection of breaker Doppler properties [52]. Following that, a model for the breaker normalized radar cross-section (NRCS) was developed [55] using automated methods for breaker detection. In this paper, the same dataset was used to evaluate breaker Doppler shifts at various observation look geometries using computer vision breaker detection with far richer statistics than that for manual detection.…”

Section: Introductionmentioning

confidence: 99%

On Doppler Shifts of Breaking Waves

et al. 2023

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Field-tower-based observations were used to estimate the Doppler velocity of deep water plunging breaking waves. About 1000 breaking wave events observed by a synchronized video camera and dual-polarization Doppler continuous-wave Ka-band radar at incidence angles varying from 25 to 55 degrees and various azimuths were analyzed using computer vision methods. Doppler velocities (DVs) associated with breaking waves were, for the first time, directly compared to whitecap optical velocities measured as the line-of-sight projection of the whitecap velocity vector (LOV). The DV and LOV were found correlated; however, the DV was systematically less than the LOV with the ratio dependent on the incidence angle and azimuth. The largest DVs observed at up-wave and down-wave directions were accompanied by an increase of the cross-section polarization ratio, HH/VV, up to 1, indicating a non-polarized backscattering mechanism. The observed DV was qualitatively reproduced in terms of a combination of fast specular (coherent) and slow non-specular (incoherent) returns from two planar sides of an asymmetric wedge-shaped breaker. The difference in roughness and tilt between breaker sides (the front face was rougher than the rear face) explained the observed DV asymmetry and was consistent with previously reported mean sea surface Doppler centroid data and normalized radar cross-section measurements.

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Ka-Band Radar Cross-Section of Breaking Wind Waves

Cited by 11 publications

References 54 publications

A Ka-Band Wind Geophysical Model Function Using Doppler Scatterometer Measurements from the Air-Sea Interaction Tower Experiment

A Ka-Band Wind Geophysical Model Function Using Doppler Scatterometer Measurements from the Air-Sea Interaction Tower Experiment

Ka-Band Doppler Scatterometry: A Strong Wind Case Study

On Doppler Shifts of Breaking Waves

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