Dependence of polarimetric Doppler spectra on breaking-wave energy
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Cited by 5 publications
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Experiments on Bragg and non‐Bragg scattering using single‐frequency and chirped radars
Abstract. A collection of laboratory experiments on Bragg and non-Bragg scattering, mainly from water surfaces, are conducted using a radar system which can be operated in a frequency-chirped, range-resolved mode, or a single-frequency mode without range resolution. A Bragg wave generator is used to generate monochromatic, plane gravitycapillary water waves with which Bragg resonance and Rice's theory are examined in some detail at small grazing angles. The Bragg resonance, which is sharp for single-frequency operation, is broadened for a chirped system where the broadening is proportional to the chirp bandwidth. For single-frequency operation, Bragg resonances are found to be Lorentzian; the resonance width has been used to infer the spatial decay rate (the imaginary wave number) of the Bragg waves, and the results are in agreement with that obtained from radar RCS and wave-height probe measurements. For scattering from water surfaces, Bragg and non-Bragg scattering are distinguished by the fact that the former process yields polarization by diffraction where HH is always less than VV, while the latter process usually yields polarization by reflection where HH is usually greater than VV. Fresnel reflection, a prime example of non-Bragg scattering, is also studied using metal dihedral and labyrinth targets. We point out that although a fine range resolution is desirable, in some cases, it may lead to "phantom binning." Other physics issues related to non-Bragg scattering using chirped systems, such as the effects of multiple scattering and the dependence of frequency chirp on the dielectric constant, are also examined and discussed. IntroductionIt is well known that Bragg scattering is one of the major mechanisms which contribute to radar returns in scattering from water waves or sea surfaces fine range resolution, a large chirp bandwidth is required; however, several questions pertaining to the effects of chirp on scattering from water waves have not been addressed. For example, what is the Braggresonant water wavelength for a chirped system? In order to answer these questions, we perform experiments in a large wave tank, using a Bragg wave generator to generate monochromatic plane waves, and compare the scattering results of a single-frequency system and a chirped system. Physics issues peculiar to non-Bragg scattering, e.g., Fresnel reflection and multipath scattering using a chirped system, are also investigated and discussed. Fresnel reflection is one of the fundamental processes which give rise to the many manifestations of what is collectively called "non-Bragg" scattering from rough water surfaces [Lee et al., 1995a]. The difference between non-Bragg scattering and Bragg scattering is that the latter process is due to coherent, constructive interference from periodic structures, which is basically diffraction, while the former process is not. A list of possible mechanisms to explain non-Bragg backscatter was 1725
Experiments on Bragg and non‐Bragg scattering using single‐frequency and chirped radars
Abstract. A collection of laboratory experiments on Bragg and non-Bragg scattering, mainly from water surfaces, are conducted using a radar system which can be operated in a frequency-chirped, range-resolved mode, or a single-frequency mode without range resolution. A Bragg wave generator is used to generate monochromatic, plane gravitycapillary water waves with which Bragg resonance and Rice's theory are examined in some detail at small grazing angles. The Bragg resonance, which is sharp for single-frequency operation, is broadened for a chirped system where the broadening is proportional to the chirp bandwidth. For single-frequency operation, Bragg resonances are found to be Lorentzian; the resonance width has been used to infer the spatial decay rate (the imaginary wave number) of the Bragg waves, and the results are in agreement with that obtained from radar RCS and wave-height probe measurements. For scattering from water surfaces, Bragg and non-Bragg scattering are distinguished by the fact that the former process yields polarization by diffraction where HH is always less than VV, while the latter process usually yields polarization by reflection where HH is usually greater than VV. Fresnel reflection, a prime example of non-Bragg scattering, is also studied using metal dihedral and labyrinth targets. We point out that although a fine range resolution is desirable, in some cases, it may lead to "phantom binning." Other physics issues related to non-Bragg scattering using chirped systems, such as the effects of multiple scattering and the dependence of frequency chirp on the dielectric constant, are also examined and discussed. IntroductionIt is well known that Bragg scattering is one of the major mechanisms which contribute to radar returns in scattering from water waves or sea surfaces fine range resolution, a large chirp bandwidth is required; however, several questions pertaining to the effects of chirp on scattering from water waves have not been addressed. For example, what is the Braggresonant water wavelength for a chirped system? In order to answer these questions, we perform experiments in a large wave tank, using a Bragg wave generator to generate monochromatic plane waves, and compare the scattering results of a single-frequency system and a chirped system. Physics issues peculiar to non-Bragg scattering, e.g., Fresnel reflection and multipath scattering using a chirped system, are also investigated and discussed. Fresnel reflection is one of the fundamental processes which give rise to the many manifestations of what is collectively called "non-Bragg" scattering from rough water surfaces [Lee et al., 1995a]. The difference between non-Bragg scattering and Bragg scattering is that the latter process is due to coherent, constructive interference from periodic structures, which is basically diffraction, while the former process is not. A list of possible mechanisms to explain non-Bragg backscatter was 1725
Space‐time properties of radar sea spikes and their relation to wind and wave conditions
Abstract. Low-grazing angle (LGA) radar sea spikes were observed with a highresolution, dual-polarization, X band imaging radar deployed on the floating instrument platform (FLIP) during the Marine Boundary Layer Experiment (MBLEX) held off the California coast during April-May 1995. Spatiotemporal statistics of observed sea spike events are presented, including duration, velocity, and directional distributions. The dependence of these parameters on wind and wave conditions is compared to theoretical predictions and to similar measurements of breaking wave signatures obtained with passive acoustic techniques by Ding and Farmer [1994]. The density of sea spikes (events per unit time per unit area) and the fractional surface coverage are estimated as a function of friction velocity and compared to theoretical predictions. Though we find consistency between the dynamical aspects of sea spike events and of acoustic wavebreaking signatures, we do not observe a predicted cubic u, dependence of sea spike density. Differences may be a consequence of the specific nature of low-grazing angle scattering at X band (and higher) frequencies. We observe an approximately quadratic dependence on u, of sea spike fractional surface coverage which is insensitive to the choice of backscattered power threshold over a 9 dB range. IntroductionBreaking waves on the ocean surface are believed to be an important part of air-sea interactions. They limit the height of ocean waves, mix surface waters, transfer energy from the wave field to currents, and enhance the fluxes of heat, mass, and momentum through the generation of turbulence and entrainment of air [Melville, 1996]. Because wave breaking is a nonlinear and intermittent process, direct measurement of wave breaking in the field is extremely difficult. For this reason the development of optical, acoustical, and microwave remote sensing methods is desirable.Breaking events have long been attributed as a source for microwave backscatter, especially for horizontally polarized radiation at near-grazing angles [Wetzel, 1990]. "Sea spikes," the colloquial term for high-intensity bursts of radar backscatter typically observed at low grazing angles, are attributable both to actively breaking waves and to scattering features (i.e., wedges, bores, plumes) bound near the crests of steep waves. A number of radar studies of wave breaking in the laboratory have attempted to identify the specific mechanisms for scattering and to measure the dependence of the microwave signature on breaking strength. Kwoh and Lake [1984] attributed discrete bursts of backscatter from short gravity waves at moderate to high incidence angles to "gentle" breaking. Banner and Fooks [1990, 1991a, b] used a CW scatterometer and boresighted video to study both the detection of breaking waves and the dependence on environmental parameters. They found an approximately cubic relation between u, and the number of sea spikes observed at moderate incidence attributable to breaking, in agreement with theoretical predictions by ...
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