Chih Y. Peng

Observation of wave refraction at an ice edge by synthetic aperture radar

In this note the refraction of waves at the ice edge is studied by using aircraft synthetic aperture radar (SAR). Penetration of a dominant swell from open ocean into the ice cover was observed by SAR during the Labrador Ice Margin Experiment (LIMEX), conducted on the marginal ice zone (MIZ) off the east coast of Newfoundland, Canada, in March 1987. At an ice edge with a large curvature, the dominant swell component disappeared locally in the SAR imagery. Six subscenes of waves in the MIZ from the SAR image have been processed, revealing total reflection, refraction, and energy reduction of the ocean waves by the ice cover. The observed variations of wave spectra from SAR near the ice edge are consistent with the model prediction of wave refraction at the ice edge due to the change of wave dispersion relation in ice developed by Liu and Mollo-Christensen (1988).

Wave effects on ocean-ice interaction in the marginal ice zone

The effects of wave train on ice-ocean interaction in the marginal ice zone are studied through numerical modeling. A coupled two-dimensional ice-ocean model has been developed to include wave effects and wind stress for the predictions of ice edge dynamics. The sea ice model is coupled to the reduced-gravity ocean model through interfacial stresses. The main dynamic balance in the ice momentum is between water-ice stress, wind stress, and wave radiation stresses. By considering the exchange of momentum between waves and ice pack through radiation stress for decaying waves, a parametric study of the effects of wave stress and wind stress on ice edge dynamics has been performed. The numerical results show significant effects from wave action. The ice edge is sharper, and ice edge meanders form in the marginal ice zone owing to forcing by wave action and refraction of swell system after a couple of days. Upwelling at the ice edge and eddy formation can be enhanced by the nonlinear effects of wave action; wave action sharpens the ice edge and can produce ice meandering, which enhances local Ekman pumping and pycnocline anomalies. The resulting ice concentration, pycnocline changes, and flow velocity field are shown to be consistent with previous observations.

Wave‐current interaction near the Gulf Stream during the Surface Wave Dynamics Experiment

This paper presents a case study on the wave-current interaction near the local curvature of a Gulf Stream meander. The wave data were obtained from in situ measurements by a pitch-roll discus buoy during the Surface Wave Dynamics Experiment (SWADE) conducted off Wallops Island, Virginia, from October 1990 to March 1991. Owing to the advection of the Gulf Stream by the semidiurnal tide, the discus buoy was alternately located outside and inside the Gulf Stream. The directional wave measurements from the buoy show the changes in wave direction, wave energy, and directional spreading when waves encountered the current in the Gulf Stream meanders. A wave refraction model, using the ray-tracing method with an estimated Gulf Stream velocity field and meandering condition, was used to simulate wave refraction patterns and to estimate wave parameters at relative locations corresponding to buoy measurements. The numerical simulation shows that a focusing zone of wave rays was formed near the boundary and behind the crest of a simulated Gulf Stream meander. The focusing of wave rays causes changes in wave direction, increases in wave energy, and decreases in wave directional spreading, which are in good agreement with the results from the buoy measurements.

Mystery ship detected in SAR image

Ships and their wakes can often be detected in the high-resolution synthetic aperture radar (SAR) imagery provided by satellites. Image processing techniques can be used to enhance extremely weak ship wake patterns, but occasionally, the ship in the SAR image remains invisible. In the SAR image below (Figure 1), two trailing, dark, turbulent wakes are seen. In addition to the dark, turbulent wakes, one side of a ships Kelvin wake can sometimes be seen as a bright line. One of the ships is seen as a bright spot caused by backscattering at the end of the trail above A in Figure 1. The other ship remains hidden, however.

Wavelet Analysis of SAR Images in the Marginal Ice Zone

Ocean-ice interaction processes in the marginal ice zone (MIZ) by waves and mesoscale features, such as upwelling and eddies, have been studied by using ERS-i SAR imagery and wave-ice interaction models (Liu and Peng 1992,1993a, 1994; Liu et al. 1993,1997). Satellite observations of mesoscale features and ice motion can play a crucial role in ocean-ice interaction study. Ocean surface waves from the open sea may penetrate into the MIZ and contribute to the breakup of floes and to other processes that modify the ice cover. Using SAR imagery, the spatial variability of the wave and current field can be estimated through spectral analysis and wave models. The wave-ice interaction models take into account the physical characteristics of the ice cover and the wave parameters to predict wave dispersion, attenuation, refraction, and possibly ice thickness (Liu et al. 1991,1992; Liu 1995).

Detection and analysis of ship waves in ERS-1 SAR imagery

From an ERS-1 SAR image over the East China Sea in May 31, 1995, transverse ship waves are detected and analyzed. The original Hough transform is applied to determine the location of the ship its direction of motion and the linear boundary of the V-shaped wake region. Then the one-parameter Hough transform is applied to detect the transverse ship wave pattern in the SAR image. Once detected, the hazy transverse ship wave crests in the SAR image can be best fitted to analytic solutions. From the fitted wave crests, one can estimate the wavelength of the transverse ship wave and hence the speed of the ship.

Wave evolution in the marginal ice zone using ERS-1 SAR

Using SAR imagery, the spatial variability of the wave and current field has been estimated through spectral analysis and wave modelling. SAR spectra of waves-in-ice in the Bering Sea were computed from C-band ERS-1 SAR images for the study of wave evolution in the marginal ice zone (MIZ). Based on the change of wavelength and energy, two kinds of ice near the ice edge can be identified. Wave attenuation in the MIZ has also been estimated and compared reasonably well with the wave model. The change of SAR backscatters of new ice and compact ice is consistent with the change of wave energy.<<ETX>>

SAR application for ocean eddy monitoring

High resolution ERS-1 synthetic aperture radar (SAR) images are used for mesoscale eddy detection which aids in monitoring the recruitment of fish in the Gulf of Alaska. During high sea states and high winds, the direct surface signature of the eddy was not clearly visible, but the wave refraction in the eddy area was observed. The rays of the wave field are traced-out directly from the SAR image.<<ETX>>

Wave Attenuation in the Marginal Ice Zone During Limex

The effect of ice cover on ocean-wave attenuation is investigated for waves under flexure in the marginal ice zone (MIZ) with SAR image spectra and the results of models. Directional wavenumber spectra are taken from the SAR image data, and the wave-attenuation rate is evaluated with SAR image spectra and by means of the model by Liu and Mollo-Christensen (1988). Eddy viscosity is described by means of dimensional analysis as a function of ice roughness and wave-induced velocity, and comparisons are made with the remotely sensed data. The model corrects the open-water model by introducing the effects of a continuous ice sheet, and turbulent eddy viscosity is shown to depend on ice thickness, floe sizes, significant wave height, and wave period. SAR and wave-buoy data support the trends described in the model results, and a characteristic rollover is noted in the model and experimental wave-attenuation rates at high wavenumbers.