Nonlinear Internal Waves in Synthetic Aperture Radar Imagery

Abstract

USAGEPermission is granted to copy this article for use in teaching and research. Republication, systematic reproduction, or collective redistribution of any portion of this article by photocopy machine, reposting, or other means is permitted only with the approval of The Oceanography Society. Send all correspondence to: info@tos.org or The Oceanography Society, PO Box 1931, Rockville, MD 20849-1931 S p e c i a l i S S u e O N O c e a N R e m O t e S e N S i N g w i t h S y N t h e t i c a p e R t u R e R a d a R B y c h R i S t O p pROpeRtieS Of iNteRNal waVe SigNatuReS theory of SaR imagingIn order for nonlinear internal waves to appear on SAR imagery, the internal wave must interact with the ocean surface and modify it at roughness scales that interact with the observing radar signal. As manifested on SAR images, a nonlinear internal wave packet typically appears as an alternating pattern of quasi-periodic bright and dark bands against a gray background. These radar bands result from enhanced and reduced radar backscatter, with the bright bands representing a convergence (rough) zone and the dark bands representing a divergent (smooth) zone. The convergence and divergence zones are the result of variations in the subsurface currents associated with the internal waves interacting with ocean surface. The most common pattern is a bright band followed by a dark band representing a nonlinear internal wave of depression (Figure 1, packets 1-6). However, a number of factors can affect the characteristics of this signature pattern, including the environment at the ocean surface (wind speed, wind direction, presence of surface films) and the properties of the internal wave itself (mode, half-width, amplitude, and currents).Theoretical models describing the modulation of short-scale sea surface roughness by variable surface currents have been developed in the framework of weak hydrodynamic interaction theory (Alpers, 1985). When using this theory together with Bragg scattering theory, figure 1. a Seasat l-band synthetic aperture radar (SaR) image from the gulf of california acquired September 29, 1978, at 18:11 utc (Rev 1355. The image contains a variety of internal wave packet signatures, with the most prominent labeled 1-8. it shows many distinctive internal wave features: alternating bright/dark band signatures grouped into packets, packets from multiple tidal cycles present on a single image, and the nonlinear interaction between packets. The series of bright "dots" arrayed in a line across the image are the result of system calibration pulses (fu and holt, 1982 which relates spectral values of the ocean surface waves to the normalized radar cross section (NRCS), the relationship between NRCS and surface current gradient dUx dx can be written as:where σ denotes the total NRCS, σ 0 is the NRCS of the background,x is the coordinate in the look direction of the SAR antenna projected onto the horizontal plane, and A is a constant that depends on radar wavelength, incidence angle, and relaxation rate. The relaxa...

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