F. Askari

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.

The July 1990 Gulf Stream Experiment

The specific scientific tasks addressed in the July 1990 Gulf Stream (GS) experiment were the following: (1) Kelvin wake behavior across fronts at various ship speeds, (2) the physics of temperature front/radar cross section (RCS) mismatch, (3) wave-current interactions in curvature fronts, and (4) the hydrodynamic structure and origin of synthetic-aperture-radar (SAR) slick-like features. Overall, the GS Experiment was most successful, and about 60 percent of the planned data was collected. On-going efforts concentrate on the analysis and interpretation of the data. An overview of the experiment and preliminary results of the data analysis are given.

An occluded coastal oceanic front

Field observations, including hydrographic, microwave imaging radar, and HF radar measurements, reveal the evolution of a complicated frontal interaction between three water masses on the continental shelf near Cape Hatteras, North Carolina, during a period of incursion of water from the Gulf Stream. The water masses were found to be separated by intersecting frontal lines configured in a manner analogous to an occluded atmospheric front. The densest water lay between inshore and offshore fronts that gradually merged or occluded in the generally downstream direction, leaving a single surface front. The overall frontal structure appeared as a distinct Y-shaped feature in the radar imagery, similar to historical imagery of the study area. The interpretation of the observations is aided by the use of a two-dimensional numerical model. The model is initialized with two fronts idealized from the ocean measurements. The model fronts quickly sharpen and begin to move together, eventually occluding into a single surface front. As a result of the occlusion, the water mass having intermediate density subducts and intrudes under the most buoyant water, carrying with it strong horizontal and vertical shears, and a frontal band of diverging currents is created in the densest water mass. The model thus suggests that in the ocean there will be an increase in hydrographic and velocity fine structure downstream of the frontal occlusion point.

Radar imaging of sand waves on the continental shelf east of Cape Hatteras, NC, U.S.A.

Abstract Imaging radars, under certain environmental conditions, can provide an extensive description of shallow submarine topography. In this investigation, sand waves were observed in shallow water and under light winds, weak flow, and highly stratified conditions with an L band synthetic aperture radar and X band real aperture radar. An analysis of the radar data reveals that regularly spaced modulations seen in the imagery are a result of bathymetric forcing. These modulations appear as a group of bright linear east-west trending features approximately 5 km in length and spaced 230 m apart with observed peak modulations exceeding predicted modulations by 7 dB. Bathymetric measurements extracted from shipboard ADCP data confirm the existence of sand waves in this region. Results from the ADCP data reveal an east-west orientation of the sand wave crest with lee slopes facing north. Mean wavelengths are 230 m and the heights are roughly 2.5 m. The radar modulations lead the sand wave crest, by approximately 135 m suggesting a possible upstream hydrodynamic effect, which is consistent with an observed Froude number less than one. This study shows that bathymetric effects are observed in radar imagery at low current speed, light winds, and strong stratification, demonstrating the critical nature that topographic and stratified hydrodynamic effects have on radar image interpretation in the littoral environment.

Radar backscatter from breaking waves in Gulf Stream current convergence fronts

Bright linear features have been observed in radar imagery taken near the Gulf Stream (GS) boundary on two separate occasions. In each case, these have been observed directly over strong current convergences. Progress has been made in understanding the origin of these signatures through simulations that incorporate environmental forcing from the winds and currents. These simulations significantly underestimate the backscatter unless wave-breaking (WB) effects are included at least approximately. Using a new, quasistatistical procedure that generalizes and quantifies earlier procedures for including WB effects, the authors have been able to successfully simulate the magnitude and behavior of these signatures. The approach combines the statistically based, composite model of radar backscatter with a deterministic feature model that relates backscatter from breaking waves to a particular geometrical model of a spilling breaker. This is accomplished using localized criteria, defined by local wave crest acceleration, to determine the probability of breaking, and by extending the feature model so that its unknown parameters may be evaluated directly from wave-current interaction calculations. The new approach provides an estimate of the critical crest acceleration of a potentially breaking wave, as a function of wind speed, that agrees with independent measurements.

Detection of oceanic fronts at low grazing angles using an X band real aperture radar

We examine the radar signatures and changes in the surface roughness associated with oceanic features in the low grazing angle (LGA) scattering regime. The X band (HH) radar signatures consist of high-amplitude sea spikes, step changes in the normalized radar cross-section (NRCS) modulations, and bright narrowbanded frontal structures. Using in situ observations coupled with airborne precision radiation thermometer (PRT-5) data, we show that the step changes in radar cross-section modulations are associated with either thermal stability-induced stress variations or current velocity variations. Superimposed on the step changes are additional modulations that result from wave breaking and hydrodynamic straining. The amplitudes of the NRCS LGA measurements are compared with the predictions of four backscattering models: the Bragg, the tilted-Bragg, the wedge, and the plume model. It is shown that while the simple Bragg model can describe the measurements to a limited degree, it generally tends to underpredict the results. Agreement is improved by including the tilt contribution from the longwave surface waves in the context of the composite scattering model. We use the wedge and plume models as the basis for explaining the cross sections associated with the high-amplitude sea spikes. The wedge model is used to describe scattering from sharply crested waves, and the plume model is used to describe the extreme cross sections that are associated with breaking waves near the fronts. In describing the probability density function characteristics we show that the backscattering statistics exhibit “K distribution” behavior for the Gulf Stream current region and near-frontal regions, while the cooler shelf waters have characteristics of an exponential distribution.

Airborne scatterometer detection of winds and sea surface roughness changes across the gulf stream front

Abstract This article investigates the scatterometer radar cross-section distributions and wind stress spatial variability across a sea surface temperature (SST) front under different wind directions and synoptic scale atmospheric forcings. Shipboard meteorological measurements in concert with airborne K u -band (14.0 Ghz) rotating scatterometer data show evidence of mesoscale circulation near the front during low ambient winds (4 m/s) blowing from the cold side of the SST front to the warm side. The mesoscale signature is characterized by a 25–35° counterclockwise shift in the wind direction and a maximum of 4.9 dB increase in the backscattering cross section across the front. The mesoscale circulation is reduced when the wind direction reverses, and the ambient wind flow increases to 13.6 m/s. The maximum cross-section change across the front is reduced to 3.6 dB. The azimuthal characteristics of the scatterometer data are compared with the predictions of two scatterometer model functions. According to the models, the anisotropy of the short waves is inversely related to the wind speed, such that the short wave directional spreads tend to broaden as the wind speed increases. The scatterometer data, however, show greater short-wave anisotropy at the higher wind speeds. This measured difference could be caused by current velocity changes or by longwave directional changes across the front.

Study of Gulf Stream features with a multifrequency polarimetric SAR from the Space Shuttle

Using simulations of radar cross section (RCS) based on wave-current interaction calculations, the authors investigate the origin of a prominent enhancement in L-band from signals that were transmitted and received, respectively, with horizontal (H) and vertical (V) polarization radar return. This was observed in imagery of the northern boundary of the Gulf Stream (GS) during the first Shuttle Radar Laboratory (SRL-1) mission. The calculations of surface roughness are based on a one-dimensional (1D) surface current model that closely resembles a current shear that was observed in in situ current measurements, taken at both sides of the GS at the time SRL-1 imaged the GS boundary. In agreement with trends observed in the imagery, significant enhancements in L-band HV polarization cross section occur in the neighborhood of the GS thermal boundary, relative to comparable vertical polarization (VV) cross section signatures at X-, C-, and L-band. The authors also find reasonably good agreement between the simulated and observed magnitudes of the GS signatures (based on calculations of wave action) using two different radar imaging models, and they provide an overview of a number of additional submesoscale features associated with the GS that were present in the image of the GS boundary.

Observations of the creation and evolution of small-scale oceanic frontal cusps and slicks

Airborne microwave radar imagery and coincident in situ data collected off Cape Hatteras, NC (USA) are used to examine the small-scale horizontal structure of a frontal region, which formed through intrusion of relatively dense Gulf Stream water onto the continental shelf. The frontal outcrop is shown to have a kilometer-wavelength scalloped structure consisting of sharp angular features (cusps) alternating with broad, gently curved regions (troughs). There is also an associated pattern of slicks lying on the buoyant side of the front and asymmetrically offset from the cusps. These slicks appear to originate from biophysical processes associated with the front itself and to trace out cyclonic trajectories of surface fluid particles. It is conjectured that the distinctive horizontal pattern of frontal cusps and slicks arises from shear-flow instability modified by the requirement for convergence of buoyant water along the front.

Study of Gulf Stream features with a multi-frequency polarimetric SAR from the space shuttle

The authors use simulations of radar cross-section, based on wave-current interaction calculations, to investigate the origin of a prominent enhancement in L-band, HV polarization radar return that was observed in imagery of the northern boundary of the Gulf Stream (GS) during the first Shuttle Radar Laboratory (SRL-1) mission. The calculations of surface roughness are based on a 1-dimensional surface current model that closely resembles a current convergence that was observed in in-situ current measurements, taken at both sides of the Stream at the time SRL-1 imaged the GS boundary. In agreement with trends observed in the imagery, significant enhancements in L-band HV polarization cross-section occur in the neighborhood of the GS boundary, relative to comparable VV polarization cross-section signatures at X-, C- and L-band. This occurs despite the fact that the magnitude of the L-band HV cross-section is significantly reduced relative to the comparable X-, C-, and L-band VV cross-sections. These results indicate that the associated L-band HV enhancement occurs from tilt-induced modulation in the radar backscatter, which preferentially alters the relative modulation in L-band HV backscatter in regions where considerable variation in surface slope takes place. The authors also provide an overview of a number of additional sub-mesoscale features associated with the Gulf Stream that were present in the image of the GS boundary.