Kazuo Ouchi

Analysis of the Contribution of Wind Drift Factor to Oil Slick Movement under Strong Tidal Condition: Hebei Spirit Oil Spill Case

The purpose of this study is to investigate the effects of the wind drift factor under strong tidal conditions in the western coastal area of Korea on the movement of oil slicks caused by the Hebei Spirit oil spill accident in 2007. The movement of oil slicks was computed using a simple simulation model based on the empirical formula as a function of surface current, wind speed, and the wind drift factor. For the simulation, the Environmental Fluid Dynamics Code (EFDC) model and Automatic Weather System (AWS) were used to generate tidal and wind fields respectively. Simulation results were then compared with 5 sets of spaceborne optical and synthetic aperture radar (SAR) data. From the present study, it was found that highest matching rate between the simulation results and satellite imagery was obtained with different values of the wind drift factor, and to first order, this factor was linearly proportional to the wind speed. Based on the results, a new modified empirical formula was proposed for forecasting the movement of oil slicks on the coastal area.

Direct evidence and implications of Brewster's angle damping observed by synthetic aperture radar images

This paper describes the direct evidence of Brewsters angle damping and its effects on polarimetric analyses observed in the synthetic aperture radar (SAR) images of concrete constructions such as seawalls. The reflectivity of concrete material is nearly zero at the Brewster angle for X-band VV-polarization microwave. The TerraSAR-X image acquired over the Tokyo Bay, Japan at small incidence angles showed strong double-bounce reflection by the sea and construction surfaces with HH-polarization, while with VV-polarization, very little radar backscatter was observed. The same effect was seen in the images of concrete walls on ground, and a swamp area covered by reeds. This effect is illustrated with HH/VV intensity and phase difference images, and ground survey data; its implication is also suggested for better understanding of polarimetric SAR images.

Application of Velocity Bunching Model to Estimate Wave Height of Ocean Waves using Multiple Synthetic Aperture Radar Data

ABSTRACT Yang, C.-S. and Ouchi, K., 2017. Application of velocity bunching model to estimate wave height of ocean waves using multiple synthetic aperture radar data. In: Lee, J.L.; Griffiths, T.; Lotan, A.; Suh, K.-S., and Lee, J. (eds.), The 2nd International Water Safety Symposium. Journal of Coastal Research, Special Issue No. 79, pp. 94–98. Coconut Creek (Florida), ISSN 0749-0208. The purpose of this study is to investigate the images of ocean waves produced by synthetic aperture radar (SAR) through the image modulations based on the normalized radar cross section (NRCS) and wave orbital motions, leading to the estimation of ocean wave height. The former is known as the tilt or NRCS modulation and the latter as velocity bunching. In general, the dominant contribution to the image modulation of range travelling waves is the NRCS modulation, and that of azimuth travelling waves is the velocity bunching mechanism. In this study, these two wave imaging mechanisms are investigated using the airborne Pi-SAR (Polarimetric-Interferometric SAR) X-band VV-polarization images of ocean waves around the Miyake Island, Japan. Two images in a same region were produced at approximately 20 minutes interval from two orthogonal directions. One image shows the images of dominant range travelling waves, and the other shows a different wave pattern of azimuth traveling waves. This difference can be caused by the different image modulations of NRCS and velocity bunching. In this study, 18 sub-images are extracted from the two sets of Pi-SAR data, and the directional wave spectra of these sub-images in the orthogonal look directions are compared. We have estimated the dominant wavelength from the images of range waves, and the wave phase velocity from the dispersion relation; the image intensity is also computed by using the velocity bunching model and compared with the data. The comparison of the results strongly suggests that the latter images of azimuth waves are produced by velocity bunching. Furthermore, it is shown that the wave height of 1.4 m estimated from the velocity bunching model is similar to the height of 1.5 m simulated using the MM5 numerical weather model and data assimilation by JWA (Japan Weather Association).

Accuracy improvement of the radar backscatter simulation from sea surface covered by oil slick using fetch-dependent waveheight spectrum: Comparison with the 2007 Heibei Spirit Case in the Yellow Sea

In this paper, results are presented on the comparison of X-band radar backscattering coefficient (RBC) from an oilcovered sea surface that features the Elfouhaily and Durden-Vesecky waveheight spectra. The Durden-Vesecky spectrum applies to a fully-developed sea, while the Elfouhaily spectrum accounts for the fetch of arbitrary length. Using these two waveheight spectra, a one-dimensional random rough surface is simulated by the Monte Carlo method, and the method of moments (MoM) is applied to yield the RBC. Comparison of the results with TerraSAR-X synthetic aperture radar (SAR) data acquired over the coastal waters polluted by the Hebei Spirit oil tanker shows that the Elfouhaily spectrum yields better agreement than the Durden-Vesecky spectrum for the fully-developed sea, and that the fetch-dependent Elfouhaily spectrum improves the agreement with SAR data in comparison with the fetch-independent spectrum for the fully developed sea. A possible application to estimate the amount of spilled oil is also suggested.

Analysis of internal waves around the Korean Peninsula using RADARSAT-1 data

Surface signatures of oceanic internal waves (IWs) are often observed in synthetic aperture radar (SAR) images since IWs change surface roughness through the interaction of small-scale ocean waves with varying currents induced by IWs. If we know the relationship between the vertical and horizontal parameters governing IWs, the mixed layer depth (MLD) can be estimated from SAR data since IWs propagate along a pycnocline where the water density changes rapidly. This study presents a method for estimating MLD from SAR data using IW images observed by RADARSAT-1 over the east waters of the Korean Peninsula. To interpret IWs using SAR data, the backscatter radar cross section is computed by applying the small perturbation model to the sea surface perturbed by varying surface currents. Wave height spectrum is computed by varying the upper layer depth of mixed layer from 5 m to 100 m with 1 m depth interval. The surface current field is assumed to move with the IWs, and is described by the KdV equation. The computed RCS is then compared with the RADARSAT-1 data, resulting in satisfactory agreement of the estimated depth of IWs with other data.