Effects of current on electromagnetic scattering signal from one dimensional sea surface
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The emergence of sea surface current has a significant impact on electromagnetic (EM) backscattering signals. This may be one of important synthetic aperture radar (SAR) imaging mechanisms of ocean currents. Fractal sea surface ocean wave-current model is derived based on the mechanism of wave current interaction in this paper. A effective EM backscattering model of one dimensional drifting fractal sea surface is presented. numerical results show that both magnitude and direction of ocean current have effects on EM backscattering signals from one dimensional ocean wave-current coupled fractal sea surface. The existence of ocean currents which paralleling to the direction of wave can weaken EM backscattering signal intensity. EM backscattering signal intensity can be strengthened by ocean currents propagating in the opposing direction of wave.Keywords:
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A series of experiments were conducted to adjust and normalize the acoustic backscatter acquired by Reson 8111 and 8160 systems. The dependency of the backscatter on the receiver gain, transmit power, pulse width and acquisition mode was analyzed. Empirical beam patterns are calculated as the difference between the backscatter measured by the sonars and the expected backscatter. Expected acoustic backscatter is estimated based on a mathematical model
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The paper presents an attempt to estimate short wave generation by wave breaking. A breaking wave crest disturbs the surface and generates sub‐surface turbulence. The locally disturbed area further disperses and feeds short wave energy to the surroundings. The total short wave energy results from summing up the effect of wave breaking events randomly distributed over the sea surface. The rate of short wave generation is determined by the frequency of wavebreaking events per unit area. The source of short wave energy is isotropic, thus it generates waves at cross and opposite wind directions. The model qualitatively reproduces observed directional property of short wind wave spectra and mean square slope.
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The paper presents the theory on first order interaction between HF radio waves and oceanic waves, generation and characterization of ocean waves on the surface, first order mechanism for scattering of radio waves from ocean waves, influence of water depth, and methods for calculating Bragg frequency and first order spectra radar cross section, and discusses the application of HF ground wave radar in remote sensing of seastate, for example, extracting surface wind direction and surface current vectors.
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Image sequences of the ocean have been collected at long range and low grazing angle with an airborne infrared system. The images are geographically registered, and 3D frequency-wavenumber spectra are calculated and shown to have a strong 2D dispersion surface that is characteristic of wind waves and swell. Wave directions compare well with in situ measurements, and their speeds are consistent with the water depth. The authors conclude that temporal sequences of IR images acquired from long distance can provide information for extracting surface wave parameters.
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The damping of ocean surface waves by a monomolecular oleyl alcohol film of about 1.5–3 km 2 in area is measured in the North Sea by wave staffs, a coherent X band microwave scatterometer mounted on a sea‐based platform, and an incoherent K u band microwave scatterometer carried by an aircraft under moderate wind conditions (wind speed u 10 = 3.5–7.7 m s −1 ). The observed wave attenuation by the monomolecular surface film measured by a wave staff in the frequency band between 3.2 and 16 Hz is in the range of about 40–60%, with only a slight increase with frequency. From this result it can be predicted that slicks affect microwave backscattering similarly in the L band (λ 0 ≈ 20 cm) as in the K u and X bands (λ 0 ≈ 2 cm). It is shown by additional wave tank experiments that a direct influence of oleyl alcohol surface films on wave damping is confined to frequencies f ≥ 2 Hz, but a further indirect effect of oleyl alcohol films on the damping of ocean waves in the frequency range between 0.12 and 0.7 Hz by modifying the wind input and wave‐wave interaction mechanisms is indicated from our results. A possible directional dependence of the wave‐damping effect caused by surface films is discussed.
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For several years, suggestions have been made that sea surface roughness cannot be adequately explained in terms of weakly interacting, wind-generated waves. Rather, ripples that are generated by longer waves, are tilted by them, and are moving at nearly the speed of the longer wave have been postulated to coexist with waves that are directly generated by the wind, that is, with free waves. Effects of these bound waves have been detected in microwave backscatter on the ocean and in wind-wave tanks, and in surface-slope probability density functions (PDFs) in wind-wave tanks. Here we show that Cox and Munk's sea-surface slope PDFs are fully consistent with this bound wave/free wave model. From the fits of these PDFs to the model, we conclude that probabilities of finding bound waves are nearly the same in wind-wave tanks and on the ocean, and both are much higher than the probability of observing whitecaps. Variances of both bound and free waves are larger on the ocean than in tanks.
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Marine energy
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Energy transformation
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The influences of surface waves on ocean currents in the coastal waters of the South Atlantic Bight are investigated by using a coupled wave‐current modeling system. The ocean circulation model employed is the three‐dimensional Princeton Ocean Model (POM), and the wave model invoked is an improved third‐generation wave model (WAM). The coupling procedure between the POM and the WAM and the simulated coastal ocean circulation driven by uniform surface winds are presented. The simulated results show that wind waves can significantly affect coastal ocean currents not only through an enhancement of wind stress but also through a modification of bottom stress. Wave‐induced wind stress increases the magnitude of currents both at the surface and near the seabed. On the other hand, wave‐induced bottom stress weakens the currents both at the sea surface and near the seabed. Therefore the net effect of surface wind waves on currents depends on the relative importance of current modulations by wave‐induced wind stress and bottom stress. The results further indicate that at a fixed location, the relative importance of wave‐induced surface and bottom shear stresses in coastal ocean circulation depends on the surface wind field. For the constant wind cases considered in this study, the effect of wave‐induced bottom stress is more significant in along‐shore wind conditions than in cross‐shore wind conditions.
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Stokes drift
Circulation (fluid dynamics)
Seabed
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In the framework of the SAR imaging process simulator we are developing, the modification of the short wave spectrum by ocean surface is of great importance. Short waves at the sea surface produce the backscattering of the radar wave. So the modulation of the short wave distribution by ocean surface induces the modulation of the SAR image. In this paper we present a two-step algorithm designed to simulate the modulation of the short waves by both current and long waves. First of all the modulation of the long waves by current is taken into account. Then the modification of the short wave spectrum by long-wave orbital velocity and current is considered. To achieve the calculation we use the action balance equation (ABE) based on a weak interaction theory. ABE theory and the two-step algorithm are described here
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Electromagnetic spectrum
Wave radar
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