Publications (31)44.01 Total impact
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ABSTRACT: Previously, we have modeled radar signatures, involving large variations (∼1015 dB) in radar cross section (RCS), that have been observed at strongly convergent ocean fronts and at an estuarine front. In each of these cases, we obtained quantitative agreement with measurement but only by including wavebreaking (WB) effects in an approximate manner. However, in each case, we used the composite scattering (CS) model at a frequency (9.4 GHz) where this model may be deficient. For this reason, questions remain concerning the importance of WB effects in these simulations. In the present study, we monitor the sensitivity of the simulations with respect to this CS approximation by comparing the results of this model with those from an alternative theory, based on simulations of RCS, derived from a common wave spectrum. The spectrum is calculated using a fullspectral treatment of wavecurrent interaction. The resulting simulations are used to model the radar signature of the buoyant plume associated with the efflux of fresh water from the Chesapeake Bay that was observed during the Chesapeake Outflow Plume Experiment 2 (COPEII). In both cases, it is possible to simulate this signature, in quantitative agreement with experiment, but only by including WB effects. We find that the CS model predictions for the behavior of the signature do not agree with the comparable predictions from the remaining model. Additional simulations indicate the difference between the two models, which is rather large because of the large (60°) angle of incidence, occurs because the CS model includes higher order terms that are not included in the Kirchoff Approximation.  [Show abstract] [Hide abstract]
ABSTRACT: An inversion algorithm for inferring the surface velocity field of buoyant plume frontal features from observed radar imagery has been developed. The inversion technique is based upon an assumption, suggested by Alpers and Hennings' (AH) relaxation model (1984), that near strongly convergent fronts, the radar crosssection should be proportional to the component of the local current gradient that is directed along the radarlook direction. However, at Xband, the technique only works when wavebreaking (WB) effects, which are not included in the AH model, are incorporated. This WB model successfully reproduces the magnitude of the signature in images of the plume front at higher frequencies (Xband), where it is known that the AH model is deficient. WB effects play a dominant roˆle in the backscatter associated with frontal regions with strong surface convergence fields. These results suggest that the enhancements of radar backscatter in the vicinity of stronglyconvergent fronts are proportional to the local currentconvergence but that the underlying scattering process involves WB in a manner that cannot be understood from the AH model. Results are presented for the estimated velocity field derived from radar imagery of the Chesapeake Bay plume front. Preliminary considerations of the convergence and uniqueness of the inversion technique are extended by means of a controlled numerical experiment involving the inversion of a prescribed input velocity field 
Conference Paper: On the origin of bright lines and other features in Xband radar imagery of frontal surface velocity fields
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ABSTRACT: Enhanced radar backscatter occurs in radar imagery of strongly convergent ocean currents, but the origin of this phenomenon is not wellunderstood. Although the Alpers and Hennings (AH) relaxation model indicates that variations in radar crosssection (RCS) intensity at convergent fronts are proportional to the magnitude of the local current convergence, at higher radar frequencies, this model significantly underpredicts RCS enhancement and exhibits a nonphysical lookangle dependence. Fullwave spectral modeling incorporating composite Bragg backscatter does not remedy the situation. In developing a procedure for extracting estimates of the near frontal 2dimensional surface velocity fields of convergent ocean frontal features from radar imagery, the authors have identified a plausible explanation, empirically, that accounts for the correlation between the presence of bright lines in Xband radar imagery near convergent fronts and the magnitude of the local current convergence. The authors are able to quantitatively infer a 2dimensional model of the currents, using radar data simulations of wave spectra, and radar crosssection RCS. In the resulting model, enhanced radar intensity occurs where the local convergence increases because of enhanced wavesteepening and wavebreaking. Other effects associated with current structure (for example, shear) are discussed 
Conference Paper: Bathymetry inversion using constituent Boussinesq equations
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ABSTRACT: The phenomenon of ocean waveshoaling, and the associated reduction of ocean wave phase speed with decreased water depth, provides useful information for inferring water depth D (bathymetry) in coastal environments. One strategy for relating D to phase speed C and wavevector k, of long wave length ocean waves, involves using the 1dimensional, linear (gravity wave) dispersion relationship C=(g*tanh(kD)/k)<sup>1/2</sup>. In principle, this approach has limitations, because the approach is based on a WKB approximation. Thus, it cannot be applied when D varies appreciably over the wavelength of a shoalingwave. Also, the approach is restricted to waves that have small waveheight. In the present paper, The authors use a set of marine radar image sequences and apply the linear approximation, via a 3D FFT analysis to the sequences. The authors show that for low to moderate wave heights, the approach does retrieve approximately the correct depth. However, an increase in the RMS waveheight from 1 m to 3.5 m produced a much poorer depth estimate, proving the need for an application of a nonlinear wave model to the problem, with an associated new retrieval approach. They outline a new procedure for extracting bathymetry that uses the recently developed constituent Boussinesq (CB) equations. The inversion procedure is accomplished using a standard (LevenbergMarquardtlike), 1dimensional, cost function minimization procedure  [Show abstract] [Hide abstract]
ABSTRACT: Using simulations of radar cross section (RCS) based on wavecurrent interaction calculations, we investigate the origin of a prominent enhancement in Lband from signals that were transmitted and received, respectively, with horizontal (H) and vertical (V) polarization radar return. This was observed in imagery of the northern boundary of the Gulf Stream (GS) during the first Shuttle Radar Laboratory (SRL1) mission. The calculations of surface roughness are based on a onedimensional (1D) surface current model that closely resembles a current shear that was observed in in situ current measurements, taken at both sides of the GS at the time SRL1 imaged the GS boundary. In agreement with trends observed in the imagery, significant enhancements in Lband HV polarization cross section occur in the neighborhood of the GS thermal boundary, relative to comparable vertical polarization (VV) cross section signatures at X, C, and Lband.We also find reasonably good agreement between the simulated and observed magnitudes of the GS signatures (based on calculations of wave action) using two different radar imaging models, and we provide an overview of a number of additional submesoscale features associated with the GS that were present in the image of the GS boundary.  [Show abstract] [Hide abstract]
ABSTRACT: Bright linear features have been observed in radar imagery taken near the Gulf Stream (GS) boundary on two separate occasions. In each case, these have been observed directly over strong current convergences. Progress has been made in understanding the origin of these signatures through simulations that incorporate environmental forcing from the winds and currents. These simulations significantly underestimate the backscatter unless wavebreaking (WB) effects are included at least approximately. Using a new, quasistatistical procedure that generalizes and quantifies earlier procedures for including WB effects, the authors have been able to successfully simulate the magnitude and behavior of these signatures. The approach combines the statistically based, composite model of radar backscatter with a deterministic feature model that relates backscatter from breaking waves to a particular geometrical model of a spilling breaker. This is accomplished using localized criteria, defined by local wave crest acceleration, to determine the probability of breaking, and by extending the feature model so that its unknown parameters may be evaluated directly from wavecurrent interaction calculations. The new approach provides an estimate of the critical crest acceleration of a potentially breaking wave, as a function of wind speed, that agrees with independent measurements  [Show abstract] [Hide abstract]
ABSTRACT: Previously, the authors have modeled radar signatures, involving large (~1015 dB) variations in radar crosssection (RCS), that have been observed at strongly convergent ocean fronts and at an estuarine front. In each of these cases, they obtained quantitative agreement with measurement but only by including wavebreaking (WB) effects in an approximate manner. However, in each case, they used the composite scattering (CS) model at a frequency, where this model may be deficient. For this reason, questions remain concerning the importance of WB effects in these simulations. In the present study, they monitor the sensitivity of the simulations with respect to this CS model approximation by comparing the results from three different radar model simulations of RCS, derived from a common wave spectrum. The spectrum is calculated using a fullspectral treatment of wavecurrent interaction. The resulting simulations are used to model the radar signature of the buoyant plume associated with the efflux of fresh water from the Chesapeake Bay that was observed during the COPE2 experiment. In each case, it is possible to simulate this signature, in quantitative agreement with experiment, but only by including WB effects. However, CS model predictions for the behavior of the signature do not agree with the comparable predictions from the two remaining models. Additional simulations indicate these differences result from higher order terms that are not included in the Kirchoff approximation  [Show abstract] [Hide abstract]
ABSTRACT: A version of the integral equation method is developed, which applies to scattering from a surface of a substance with a large dielectric constant ε. An ``impedance'' boundary condition for the tangential components of electric and magnetic fields on the scattering surface is used, and the integral equation for the tangential components of the total magnetic field on a surface is formulated. This equation is applied to the problem of electromagnetic scattering from a slightly rough surface. It is demonstrated that the commonly used perfect conductor approximation (PCA) (ε=∞) can adequately describe scattering in the case of extremely large ε only. The greatest sensitivity in scattering occurs when the incident and/or scattered waves are vertically polarized. For such cases the PCA does not work until ε>104. In the particular case of scattering from the ocean (ε~65), the PCA fails to provide an adequate description of the phenomena for either horizontally or vertically polarized waves, for practically all incidence and scattering angles. 
Article: Polarimetric analysis and modeling of multifrequency SAR signatures from Gulf Stream fronts
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ABSTRACT: Using airborne synthetic aperture radar data from the 1990 Gulf Stream Experiment, this paper investigates the polarization and wavelength dependence of radar signatures for narrow fronts with converging flows occurring within the Gulf Stream. The signaltobackground ratios of the crosspolarization backscatter return from a convergent front were found much higher than those of copolarization returns, when the flight path is crossing the front. However, a second convergent front, imaged at 45°, showed that the signaltobackground ratios are nearly equal for co and crosspolarizations. A polarimetric procedure, which has been successfully used to measure terrain slopes and to generate elevation maps, is applied to the convergent front to explain the polarization and imaging geometry dependence of these radar responses. A theoretical modeling of radar modulation using an ocean wave model and a compositeBragg scattering model, which incorporates the effect of breaking waves, was developed. Calculations with the model agree reasonably well with the radar measurements at various polarizations for three radar frequencies: Pband (68 cm in wavelength), Lband (24 cm), and Cband (5.7 cm)  [Show abstract] [Hide abstract]
ABSTRACT: In this paper we investigate the underlying dynamics associated with a strong, lineshaped submesoscale feature that was observed in radar imagery at the boundary between Gulf Stream (GS) and shelf water near Cape Hatteras during the first Naval Research Laboratory HighResolution Remote Sensing Experiment (HIRES 1). The lineshaped feature, which appears as a pronounced (~10 dB) increase in radar cross section, extends several kilometers in the eastwest direction. In situ current measurements have shown that this feature coincides with the boundary of a sharp current convergence front. These measurements also indicate that the frontal dynamics is associated with the subduction of denser GS water under lighter shelf water. Using the observation that the convergence can be attributed to a hydrodynamic instability at the water interface, we have modeled the resulting subsurface hydrodynamics on the basis of a rigidlid, twodimensional solution of the Navier Stokes equation. The calculations of subsurface current flow were used as input to a spectral (wave action) model of wavecurrent interaction to obtain the surface wave field, which in turn was used to provide input for modeling of radar backscatter. The resulting description also includes the effects of surfactantinduced wave damping on electromagnetic backscatter. Our predictions are compared with real aperture radar imagery and in situ measurements from the HIRES 1 experiment. 
Conference Paper: Integral equation method for electromagnetic scattering from theocean
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ABSTRACT: The integral equation method (IEM) is an important tool for a theoretical study of electromagnetic scattering from the ocean. However, in its current form, the method can be used only for a perfect conductor approximation (PCA), that considers the relative dielectric constant of seawater to be ∞. This paper extends the IEM so that it can be used in the cases of finite but large dielectric constant, and accounts automatically for a neargrazing anomaly. Radar crosssections are determined 
Conference Paper: Wavebreaking effects in radar signatures from 2dimensionalmodelling of the HIRES1 rip feature
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ABSTRACT: Fullspectral modelling of a twodimensional current structure is used to understand the role of wavecurrent interaction and wavebreaking (WB) effects in the radar signatures of the riplike submesoscale feature that were observed during the First High Resolution Remote Sensing (HIRES1) experiment. It is found that the large variations in radar crosssection (RCS) in the neighborhood of the cusplike features within the rip can be reproduced with or without incorporation of wavebreaking (WB) effects. However, when WB effects are not included, consistent with previous models that have used 1dimensional current structures, in regions away from these cusplike structures, the composite scattering (CB) model significantly underpredicts the magnitude of the signature. As a consequence, somewhat surprisingly, within the rip, the CB model overpredicts the magnitude of the cusp signature relative to the signature from the rip in noncusplike regions. By including WB effects, this deficiency is overcome, and good agreement is obtained. The resulting agreement occurs when the WB effect is based on an estimate of the local critical crest acceleration Ω<sub>c</sub>=0.4 g (g=9.8 m/s<sup>2</sup>) that accompanies the onset of wavebreaking. This value for Ω<sub>c </sub> is in good agreement with independently measured values obtained in wavetank and field experiments. Incorporation of WB effects also eliminates a nonphysical dependence on lookangle that occurs when the CB model alone is used 
Conference Paper: Microwave scattering from a slightly rough surface of a medium possessing a finite large dielectric constant, and applications to an airwater scattering
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ABSTRACT: A theory for electromagnetic scattering from a slightly rough surface is developed that is based on application of an effective boundary condition. The theory generalizes previous approaches that apply only to the infinite dielectric constant (ε→∞) limit to cases of finite but large ε. Also, the theory yields the known results by J. Wright. In a particularly important case of scattering from the ocean, the backscatter crosssection is dramatically different from that calculated for ε→∞ over a wide range of incident angles, for both vertical and horizontal polarization 
Conference Paper: Study of Gulf Stream features with a multifrequency polarimetric SAR from the space shuttle
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ABSTRACT: The authors use simulations of radar crosssection, based on wavecurrent interaction calculations, to investigate the origin of a prominent enhancement in Lband, HV polarization radar return that was observed in imagery of the northern boundary of the Gulf Stream (GS) during the first Shuttle Radar Laboratory (SRL1) mission. The calculations of surface roughness are based on a 1dimensional surface current model that closely resembles a current convergence that was observed in insitu current measurements, taken at both sides of the Stream at the time SRL1 imaged the GS boundary. In agreement with trends observed in the imagery, significant enhancements in Lband HV polarization crosssection occur in the neighborhood of the GS boundary, relative to comparable VV polarization crosssection signatures at X, C and Lband. This occurs despite the fact that the magnitude of the Lband HV crosssection is significantly reduced relative to the comparable X, C, and Lband VV crosssections. These results indicate that the associated Lband HV enhancement occurs from tiltinduced modulation in the radar backscatter, which preferentially alters the relative modulation in Lband HV backscatter in regions where considerable variation in surface slope takes place. The authors also provide an overview of a number of additional submesoscale features associated with the Gulf Stream that were present in the image of the GS boundary  [Show abstract] [Hide abstract]
ABSTRACT: A new nesting technique has been developed for computing solutions of the steadystate form of the wave action equation. The technique is especially useful for investigating the effects of resolution on the accuracy and stability of the computation. This has importance in the problem of determining ocean wave spectra under the influence of ambient wind fields and current distributions. The technique enables extremely high resolution computations to be performed with minimal computer storage requirements. It is especially useful for applications in modelling radar imagery of the ocean surface. Investigations of the convergence, stability, and accuracy of the procedure are made possible by introducing a fixed grid point location which is common to all the nested grids. In order to display the method, we apply it to a particular model of an oceanographic current rip feature that was recently observed during the first High Resolution Remote Sensing Experiment. Limitations of the method are also discussed. 
Article: Nonlinear Coherent Multiwave Modes: Universal Integrals of Motion and Effective Hamiltonian
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ABSTRACT: It is shown that a resonant nonlinear interaction of waves may result in a variety of collective multiwave modes of coherent resonantly coupled waves. The equations of motion for the modes have a universal set of integrals ({open_quote}{open_quote}collision constants{close_quote}{close_quote}) reflecting a {open_quote}{open_quote}particle balance{close_quote}{close_quote} (in quantummechanical terms) in wave collisions. The existence of these integrals ensures that the collective modes are completely integrable. Any multiwave mode can be described by an effective single degree of freedom conservative Hamiltonian, similar to the one known for threewave coherent modes (triads). {copyright} {ital 1996 The American Physical Society.} 
Conference Paper: Issues surrounding surfactant effects upon radar imagery of convergent ocean surface flows with application to the HiRes I rip feature
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ABSTRACT: Xband radar signatures of a current rip convergence, resulting from denser Gulf Stream fluid interacting with fresh coastal shelf water near Cape Hatteras, were observed during the First High Resolution (HiRes I) experiment. These signatures, which appeared as intense (~10 dB) enhancements in radar cross section (RCS) in the form of meandering linear segments, were accompanied by secondary parallel meandering segments of reduced (~510 dB) RCS on the shelf water side. The effects of wavecurrent interaction scale with the surface velocity (u) while the effects of surfactants scale as the ratio of surface current to phase velocity (u/c). Unlike internal waves which `graze' upon the ambient surface film material, current rips `herd' the ambient surface film material to a convergence point where u/c=1. The latter features can induce singular behaviour for a monomolecular surface film. A number of fundamental issues need to be resolved including: continuity of the surface film in regions of wave breaking; buckling of the monomolecular film; and subduction of the surfactant at the frontal boundary. These issues are highlighted through application to the HiRes I rip feature using a simplified onedimensional model of the feature as well as the surface manifestation of currents derived from the associated depthdependent structure  [Show abstract] [Hide abstract]
ABSTRACT: Examines the origin of the strong radar signatures from the riplike feature observed during the First High Resolution Remote Sensing (HIRES1) experiment, based on an “endtoend” strategy in which radar signatures are modelled from a fullspectrum wave approach, using surface and subsurface currents, derived from the underlying hydrodynamics. The authors find that it is necessary to include wavebreaking (WB) effects to obtain satisfactory agreement with experiment for the magnitude of the observed radar signature. They accomplish this using an earlier WB model and by introducing an improved WB model. The new model combines the statisticallybased, composite model of radar backscatter with a deterministic, feature model that relates the predominant backscatter from breaking waves to a particular geometrical model of a spilling breaker. This is accomplished by using localized WB criteria, based on critical crest acceleration Λ<sub>c</sub> information, to determine the probability of breaking, and by extending the feature model so that its assumed geometry may be determined statistically. This allows the authors to eliminate all of the unknown parameters of the feature model using calculations of waveheight spectra  [Show abstract] [Hide abstract]
ABSTRACT: The authors demonstrate the ability of interferometric radar imagery to determine both relative and absolute surface velocities in the open ocean. Absolute phase calibration is accomplished by noting the azimuthal displacement of rangetravelling targetsdemonstrating for the first time that under favourable circumstances phase calibration can be achieved in openocean in the absence of ground truth. The high resolution of radar imagery permits observation of sharp velocity discontinuities, e.g. the Gulf Stream boundary and the wave field. The recent SIRC/XSAR shuttle missions dramatically emphasize the experimental and observational aspects of spacebased radar. The combination of absolute velocities, high spatial resolution, and widearea coverage suggest that interferometric radar imagery can provide a unique and powerful aid both for studies of global circulation patterns and detailed analysis of slope/shelf water interactions with ocean currents. In particular, the authors employ this measurement of the surface currents and wave field near a velocity front to help refine and bound results of their modeling of calculated radar images of the front. The results of this paper are compared with available ground truth 
Conference Paper: Remote sensing of currentwave interactions with SIRC/XSAR during SRL1 and SRL2 at the Gulf Stream Supersite
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ABSTRACT: The Shuttle Imaging Radar, (SIR)C/XSAR, first and second Shuttle Radar Laboratory (SRL) missions, SRL1 and SRL2, took place April 920 and Sept. 30Oct. 10, 1994. The authors report on a major, multi organizational series of experiments designed to investigate oceanographic phenomena at the Gulf Stream (GS) Supersite off the east coast of the US during these two missions. The investigations emphasized currentwave and airsea interactions with extensive ground/sea/air truthing. The authors summarize a number of detailed findings associated with SRL1 and provide a preliminary description of SRL2 results
Publication Stats
171  Citations  
44.01  Total Impact Points  
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Institutions

1998

Northwestern University
Evanston, Illinois, United States


19941997

SpecTIR™ Remote Sensing Division
Reno, Nevada, United States
