摘要:

The synthetic aperture radar (SAR) flown on the Seasat satellite in 1978 yielded a rich collection of radar imagery exhibiting surface expressions from a variety of oceanic phenomena. These images have stimulated substantial research efforts to develop a quantitative understanding of the process by which surface waves, bathymetric features, internal waves, fronts, eddies, and ship wakes modify microwave energy back‐scattered from the sea surface and manifest their presence as intensity variations in SAR images.Conceptually, the process can be subdivided into three generic problem areas: (1) hydrodynamics, which concentrates on describing how the phenomenon of interest alters the surface wave roughness characteristics; (2) microwave backscattering, which addresses the manner in which surface roughness variations are encoded in electromagnetic energy reflected from the sea surface; and (3) the SAR image formation process, which accounts for the effects of imaging geometry and scatterer motion on the coherence properties of the backscattered radiatio

ISSN:0148-0227    

DOI:10.1029/JC093iC10p12217

年代:1988

数据来源: WILEY

摘要:

An overview of the Georgia Strait Experiment ( Joint Canada‐U.S. Ocean Wave Investigation Project, or JOWIP), 1983, is presented containing a list of participants, objectives, procedures, and main results. JOWIP was designed to provide quantitative comparisons between surface measurements and remote SAR imagery of internal waves and narrow V surface ship wakes. First‐order Bragg modeling gives factor of 3 accuracy forLband internal wave modulation; the composite model and possibly nonlinear surface wave‐internal wave interaction theory are required atXband. Narrow V wakes were imaged in Dabob Bay, Washington, under low‐wind conditions atLband. Surface slopes in the Kelvin wakes are not large enough to produce specular reflections even at a radar incidence angle of 23°. Modulations along the V arms are strongly evident and have the same wavelength as the transverse Kelvin wake component. These results and others are described in detail in other papers in this sa

ISSN:0148-0227    

DOI:10.1029/JC093iC10p12219

年代:1988

数据来源: WILEY

摘要:

A shipboard continuous wave coherent dual‐polarized focused radar operating at 9.23 GHz, an optical device which detects the occurrence of specular reflection at the radar incidence angle, and a laser wave slope gauge have been used to investigate mechanisms governing radar backscatter from surface water waves as part of the Joint Canada‐U.S. Ocean Wave Investigation Project ( also known as the Georgia Strait Experiment ). The relative contributions of Bragg and specular backscatter mechanisms have been identified using the output of the optical specular detector and the radar backscatter polarization ratios. Analysis of the radar data together with data from the laser wave slope gauge ( Hughes and Dawson, this issue ) has been used to further identify the mechanisms governing radar backscatter and modulations of radar backscatter from surface water waves. It is found that specular reflection is very significant at 20°incidence angle and not negligible at 40°. It is found that when specular contributions are absent, the relationship between the surface wave slope and the radar backscatter is well described by Bragg theory. Specular contributions, when they occur, appear as large spikes on top of the Bragg return. Analysis of the wave slope data shows that the wave slope modulations at “Xband ” and “Lband ” Bragg wave frequencies are comparable, in contradiction to simple Bragg wave modulation theories which predict that “Xband ” waves should exhibit much weaker modulations. Modulations of the radar backscatter in low seas are due primarily to Bragg scattering. As the wind speed increases, the hydrodynamic modulations tend to decrease, but the radar modulations can be enhanced by specular reflection. Possible additional contributions due to wedge diffraction type scattering at higher wind speeds may also be important but have not been addressed in this experiment. Exceptionally low radar returns were observed in some cases and may be due to the presence of organic fil

ISSN:0148-0227    

DOI:10.1029/JC093iC10p12235

年代:1988

数据来源: WILEY

摘要:

Calculations of surface wave and radar cross section modulation induced by a spatially varying surface current have been compared with field measurements made with synthetic aperture radar (XandLbands), real aperture radar (Xband), laser slope gauge, and charge‐coupled device video camera during the Joint Canada‐U.S. Ocean Wave Investigation Project (JOWIP), which took place in August 1983. The comparison reveals that (1) many existing wind relaxation models underpredict the hydrodynamic effect of the current, (2) the simple Bragg scattering model underpredicts radar backscatter modulation for higher‐frequency radars (e.g.,Xband) but appears acceptable at lower radar frequencies (e.g.,Lband), and (3) the discrepancy between measurements and calculations at high radar frequency is reduced, but not eliminated, when the effects of long surface waves have been accounte

ISSN:0148-0227    

DOI:10.1029/JC093iC10p12249

年代:1988

数据来源: WILEY

摘要:

During the Joint Canada‐U.S. Ocean Wave Investigation Project (JOWIP), video and laser slope meter observations of surface waves perturbed by solitonlike internal waves were obtained from the Canadian research shipEndeavour. Two instances were observed of resonant interaction between wind waves that had been turned through a large angle, and the advancing internal wave current. When the wind makes a relatively small angle with the internal wave crest, the effect of the internal wave current is to split the (unperturbed) wind wave spectrum into a resonant system propagating with the interval wave, and a weaker wave system propagating against it. This phenomenon was clearly observed in the measurements and qualitatively confirmed by theoretical calculation

ISSN:0148-0227    

DOI:10.1029/JC093iC10p12265

年代:1988

数据来源: WILEY

摘要:

Comparisons are made ofLband andXband SAR images with in situ measurements of surface currents and surface wave spectral perturbations for two of the Joint Canada‐U.S. Ocean Wave Investigation Project internal wave data sets. Predicted and observed perturbations show agreement to within approximately a factor of 2 forLband, while the agreement atXband is much less satisfactory. Comparison of predicted microwave backscatter atXband with near‐surface radar measurements indicates large discrepancies even when surface tilt and specular reflection effects are included in the model. Synthetic aperture radar (SAR)Xband modulations compare favorably with near‐surface radar measurements for range‐traveling internal waves, but for other directions, the SAR modulations are smaller than the near‐surface radar measurements. SimultaneousXandLband SAR images show comparable internal wave mo

ISSN:0148-0227    

DOI:10.1029/JC093iC10p12283

年代:1988

数据来源: WILEY

摘要:

Synthetic aperture radar (SAR) images of moving ships often exhibit characteristic patterns associated with various wake phenomena. These features can be classified into three general categories which include (1) surface waves generated by the ship, (2) turbulent or vortex wakes, and (3) internal waves. In this paper an overview of the present status of SAR ship wake imaging is presented.

ISSN:0148-0227    

DOI:10.1029/JC093iC10p12293

年代:1988

数据来源: WILEY

摘要:

The SAR Internal Wave Signature Experiment (SARSEX) was conducted in the New York Bight in late summer 1984 to investigate synthetic aperture radar (SAR) imaging of oceanic internal waves. The experiment was designed to acquire adequate in situ data to test hydrodynamic theories for the interaction of surface waves and currents, as well as theories for radar imaging of internal wave surface manisfestations. This paper provides an overview of the experiment and highlights from results obtained to date. Excellent agreement has been found between measured and calculated surface wave modulations at wavelengths from 20 to 100 cm. Internal wave signatures in SAR images atXandLband were found to have comparable magnitudes. Calculated SAR intensity modulations were in reasonable agreement with observed modulations at both radar frequencies.

ISSN:0148-0227    

DOI:10.1029/JC093iC10p12304

年代:1988

数据来源: WILEY

摘要:

This paper presents the results on internal wave analysis of the current meter mooring data in the New York Bight from the SAR Internal Wave Signature Experiment (SARSEX). Selective sets of current and temperature measurements are analyzed for a parametric study of internal wave characteristics and shear effects on wave parameters are studied based on the limited data from moorings. The solitary wave theory has been extended for the wave evolution on a continental shelf by including dissipation and shoaling effects; a series of numerical simulations are performed to demonstrate the relative importance of dissipation and shoaling effects. From an observed initial wave packet at the upstream mooring, the numerical evolution simulation compares reasonably well with the measurements at the distant mooring for the leading two large solitons.

ISSN:0148-0227    

DOI:10.1029/JC093iC10p12317

年代:1988

数据来源: WILEY

摘要:

Video images of surface waves were recorded from a research vessel to characterize the internal wave‐induced surface roughness modulations during the SAR Internal Wave Dynamics Experiment (SARSEX). These images have yielded two‐dimensional slope spectra during the vessel's traversal of two internal wave packets. By sampling the spectra along the internal wave propagation direction, spectral modulation time series were generated for surface waves of 20‐ to 100‐cm wavelength which clearly show roughness modulations correlated with the internal wave activity. Surface waves of 60‐cm wavelength or longer were found to experience the largest modulations. The maximum roughness increases and decreases occur near regions where the internal Wave surface current gradients are largest, with increased roughness occurring forward of the peak internal wave dis

ISSN:0148-0227    

DOI:10.1029/JC093iC10p12330

年代:1988

数据来源: WILEY