Abstract. In this paper we investigate the underlying dynamics associated with a strong, line-shaped submesoscale feature that was observed in radar imagery at the boundary between Gulf Stream (GS) and shelf water near Cape Hatteras during the first Naval Research Laboratory High-Resolution Remote Sensing Experiment (HIRES 1). The lineshaped feature, which appears as a pronounced (---10 dB) increase in radar cross section, extends several kilometers in the east-west direction. In situ current measurements have shown that this feature coincides with the boundary of a sharp current convergence front. These measurements also indicate that the frontal dynamics is associated with the subduction of denser GS water under lighter shelf water. Using the observation that the convergence can be attributed to a hydrodynamic instability at the water interface, we have modeled the resulting subsurface hydrodynamics on the basis of a rigid-lid, twodimensional solution of the Navier Stokes equation. The calculations of subsurface current flow were used as input to a spectral (wave action) model of wave-current interaction to obtain the surface wave field, which in turn was used to provide input for modeling of radar backscatter. The resulting description also includes the effects of surfactant-induced wave damping on electromagnetic backscatter. Our predictions are compared with real aperture radar imagery and in situ measurements from the HIRES 1 experiment. IntroductionThe ability to infer the underlying current and depth structure from microwave frequency radar imagery of the ocean surface is a long-standing problem of considerable interest. Efforts to infer subsurface structure are inherently limited since radar signals at best penetrate the subsurface at the level of tens of centimeters or less. As a consequence, in the absence of ground truth, it is possible to understand subsurface structure in this manner only on the basis of some understanding and modeling of the underlying dynamics. On the other hand, interpretation of features apparent in ocean imagery is possible only if the link between radar modulation and variations in the small-scale surface roughness due to currents, depth, wind, and the associated momentum fluxes is correctly described. Further complicating the problem, especially at low wind speed, is the effect of wave damping from surfactants. A complete analysis of this problem is manifold, involving subsurface and surface hydrodynamics and electromagnetic backscatter. shown that this signature occurs along the boundary of a sharp current convergence front. These measurements also indicate that the frontal dynamics is associated with the subduction of denser Gulf Stream (GS) water under lighter shelf water. The goal of this paper is to marry subsurface current flow, surface wave-current interaction, and radar imaging, on the basis of a full-spectral representation, to obtain an understanding of this high radar backscatter feature, which will be referred to as a current "rip." As shown in Plate 1, the resul...

The Rayleigh-Taylor instability is investigated for a nonsteady basic state. A model of a magnetically imploded cylindrical metallic liner compressing an axial magnetic field is constructed and used as the basis of a linear stability analysis. The liner, idealized to be without energy loss mechanisms, can be given an initial rotation about its axis. Analytic and numerical techniques are used to study the stability of flutelike (∼eimφ) irrotational perturbations about this state. Stability is quantified in terms of the tendency of the liner to disrupt or to encroach toward the axis, and is determined as a function of mode number m, the form of initial disturbance, liner thickness and the amount of rotation. It is shown that thickening the liner tends to stabilize against both encroachment and disruption, while increasing rotational velocity tends to stabilize against encroachment. Implications for experimental designs are discussed, in particular for experiments with deep compressions (large ratio of initial to final liner radius) of possible interest in the Linus concept of a pulsed power device.

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A comparison is made between real aperture radar (RAR) measurements and simulations (based on modeled tidal currents) of radar cross section over a complicated tidal basin (in the vicinity of the Phelps Bank region of the Nantucket shoals) in order to more fully understand the origin of radar signatures that are observed at the ocean surface as a consequence of variations in the topography of the ocean bottom. The Phelps Bank region was mapped under two extreme wind speed conditions' in high winds, in excess of 15 m/s, and in low winds, of the order of 2-3 m/s. For the light-wind case the measured radar cross section over the west side of the Phelps Bank was enhanced by as much as 20 dB relative to the clutter background. For the high-wind case, no discernible bathymetric signature was found in the highclutter background. Numerical results for the two-dimensional M2 (semidiurnal) tidal ß ! icurrents over the Phelps Bank (Greenberg et al, 1989), w•th-x • m•n of arc resolution, are used as input to the surface signature models: the Alpers and Henrungs (1984) first-order Bragg relaxation model; a generalized form of this relaxation model (in which wind directional effects are incorporated in an approximate manner); and the full-spectrum model of Lyzenga and Bennett (1988). Comparisons between the models (which do not include wave breaking) and an extreme case of 2-3 m/s winds (where strong wave breaking could become important) reveal that although the models predict correlation between variations in bottom topography and surface signature, they significantly underpredict the magnitude of the observed effect. The model calculations also are very sensitive at low (<2 m/s) wind speeds to the functional form that is assumed for the wind-wave forcing in the wave action equation. Prior visual observations and measurements of wave spectra (and wave shoaling) in the vicinity of Phelps Bank strongly suggest that the deficiencies of the modeled results that occur explicitly at light winds are due to wave breaking. A number of additional experiments and measurements are suggested for more normal environmental conditions for further theory assessments.

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