Gaspar R. Valenzuela

Theories for the interaction of electromagnetic and oceanic waves — A review

This paper reviews analytical methods in electromagnetic scattering theory (i.e., geometrical and physical optics, perturbation, iteration, and integral-equation) which are applicable to the problems of remote sensing of the ocean. In dealing with Earths surface (in this case, the weakly non-linear ocean), it is not possible to have a complete and exact description of its spatial and temporal statistics. Only the first few moments are generally available; and in the linear approximation the statistics are assumed homogeneous, stationary and Gaussian. For this case, the high-frequency methods (geometrical and physical optics) and perturbation (Rayleigh-Rice), or a combination of them, provide tractable analytical results (i.e., the specular-point, the slightly-rough Bragg scattering and the composite-surface models). The applicability and limitations of these models are discussed. At grazing incidence and for higher frequencies, other scattering mechanisms become significant; and shadowing, diffraction and trapping must be considered.The more exact methods (integral-equation and Greens function) have not been as successful in yielding tractable analytical solutions, although they have the potential to provide improved theoretical scattering results in the future.

Gulf Stream surface convergence imaged by synthetic aperture radar

On July 20, 1990, the north edge of the Gulf Stream (36.7°N, 72.0°W) was sampled by the R/V Cape Henlopen and simultaneously imaged by the Jet Propulsion Laboratorys airborne synthetic aperture radar (SAR). Hydrographic measurements show an abrupt surface front separating warm, salty Gulf Stream water in the south from a filament of cool, fresh (<33 practical salinity unit (psu)) water to the north. The filament lies within the stream and is likely water entrained from the continental shelf. The southern boundary of the filament is marked by increased surface wave breaking in a 100- to 200-m-wide zone, accumulations of Sargassum, and an orthogonal velocity change of 20 cm/s. The front is manifested in a sequence of SAR images as a narrow line having returns 1–2 dB higher than background. (A second, transient SAR line occurs near the northern filament boundary.) The observations are compared with model calculations of the surface wave hydrodynamics and radar scattering. The ocean waves are driven by southwesterly 8-m/s winds and interact with the front to produce primarily an enhancement of 2- to 3-m waves over a ≲200-m-wide region centered downwind of the front. Using a composite scattering radar model along with measured breaking-wave statistics, we show that the observed modulations in the radar backscatter can be accounted for through breaking-wave and tilted Bragg wave scattering effects. These results further show that SAR images of the ocean surface can be exploited for detailed study of particular ocean processes.

INSAR imagery of surface currents, wave fields, and fronts

The authors demonstrate the ability of interferometric radar imagery to determine both relative and absolute surface velocities in the open ocean. Absolute phase calibration is accomplished by noting the azimuthal displacement of range-travelling targets-demonstrating for the first time that under favourable circumstances phase calibration can be achieved in open-ocean in the absence of ground truth. The high resolution of radar imagery permits observation of sharp velocity discontinuities, e.g. the Gulf Stream boundary and the wave field. The recent SIR-C/X-SAR shuttle missions dramatically emphasize the experimental and observational aspects of space-based radar. The combination of absolute velocities, high spatial resolution, and wide-area coverage suggest that interferometric radar imagery can provide a unique and powerful aid both for studies of global circulation patterns and detailed analysis of slope/shelf water interactions with ocean currents. In particular, the authors employ this measurement of the surface currents and wave field near a velocity front to help refine and bound results of their modeling of calculated radar images of the front. The results of this paper are compared with available ground truth. >

Radar surface signatures for the two‐dimensional tidal circulation over Phelps Bank, Nantucket shoals: A comparison between theory and experiment

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 currents over the Phelps Bank (Greenberg et al., 1989), with ⅛ × ⅛ min of arc resolution, are used as input to the surface signature models: the Alpers and Hennings (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.

Hydrodynamic and radar modeling of surface features observed in Gulf Stream boundary regions

The perturbed ocean wave field and the radar return associated with Gulf Stream boundary current convergence fronts (Rips) are modeled. The surface wave field is simulated using the full-wave action density equation including the effects of wind input, wave/current interaction, and dissipation. Radar return from the ocean surface is computed using a tilted-Bragg scattering model and wave breaking statistics obtained using modelled wave dissipation results. The effect of wave breaking is examined for a range of commonly observed Rip conditions and compared with observations made during the NRL Gulf Stream 90 (Valenzuela, 1991), and Hi-Res Ocean experiments (Mied, 1992).<<ETX>>

Microwave Sensing of the Ocean Surface

A review of electromagnetic (e.m.) and hydrodynamic processes pertinent to the backscatter of microwaves from water/ocean waves is presented. The understanding of e.m. scattering from the ocean has increased rapidly in the past two decades. The enhanced knowledge has allowed the probing of ocean waves with radio/microwaves, paving the way for the success of the first oceanographic satellite in 1978, SEASAT. In the future, as the calibration of microwave sensors improve, satellite measurements of the world oceans should become routine. In particular, the forthcoming 1989–1994 World Ocean Circulation Experiment (WOCE) will rely on microwave measurements from space for wind stress and geostrophic currents information.

INSAR imagery of surface currents, wave fields and fronts

Interferometric synthetic aperture radar (INSAR) has lead to a resurgence of interest in space-based and airborne remote radar sensing. The recent SIR-C/X-SAR shuttle missions dramatically emphasize the experimental and observational aspects of space-based radar. The information obtain from INSAR studies is of high quality, detailed and oft times unique. The authors employ an INSAR measurement of the surface currents and wave field near a velocity front to refine the modeling of calculated radar images of the front.<<ETX>>

Radar surface signatures based on the two-dimensional tidal circulation of Phelps Bank

Numerical results for the M/sub 2/ tidal currents over Phelps Bank, Nantucket Shoals, Massachusetts, with 1/8*1/8 min arc resolution are used to predict the radar surface signatures (cross sections sigma ) of Phelps Bank using the first-order relaxation model of W. Alpers and I. Hennings (1984). The numerical predictions of sigma incorporate all two-dimensional contributions from the tidal currents and include both the residual and first-harmonic term. Strong correlation between spatially resolved contour plots of sigma over a wide range of directions and time intervals within the tidal cycle, with variations in the bottom topography of the Phelps Bank, is obtained even when Bragg wave scattering alone is included. The predictions for sigma are contrasted with previous measurements in the Phelps Bank by the US Naval Research Laboratory, and the importance of including the off-diagonal elements from the current stress tensor in the evaluation of sigma is discussed. >

DETERMINING THE CURRENTS OVER PHELPS BANK

The M2 tidal currents over Phelps Bank are examined using a fully nonlinear model. Simulations with resolutions of 2’ × 2’ and 0.5’ × 0.5’ were made of the full area covering Nantucket Shoals south of Cape Cod. A fine resolution model (0.125’ × 0.125’) was also run over a limited area surrounding Phelps Bank. Agreement was good in amplitude and phase when compared to surface elevation data. The agreement with current meter data was fair in amplitude and poor in phase with the exception that agreement was good at the one Phelps Bank mooring when compared with models of the full area. The limited area model showed signs of instability that were damped out with a horizontal eddy viscosity. Agreement of this model with data from the one available current meter mooring was worse. Different options are available to go from model predictions to the currents necessary for calibration of microwave data obtained over Phelps Bank.

Modeling of Gulf Stream boundary features in SAR imagery

Current measurements of a Gulf Stream boundary region off Cape Charles, VA, identified two current fronts with shear and convergent flows. Using these measured currents the authors have made predictions of the HH and VV radar return for P-, L-, and C- bands and compared them with simultaneously collected polarimetric SAR data. The ocean wave spectrum is computed using the wave-action equation with a Hughes-type source term. Radar cross sections are derived from the predicted wave field using a composite scattering Bragg model. Across the fronts wind waves are both refracted and shortened leading to modulations in their spectral density which is evident in the radar imagery. These current fronts produce significant densities of breaking waves which cannot be ignored in the radar predictions. The authors estimates of the radar cross section including breaking wave contributions are found to be consistent with the SAR data.<<ETX>>