Thomas E. Evans

United States Naval Research Laboratory, Washington, Washington, D.C., United States

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Publications (26)57.13 Total impact

  • The Journal of the Acoustical Society of America 01/2011; 129. · 1.65 Impact Factor
  • Colin Y. Shen, Thomas E. Evans, Steven Finette
    Journal of Atmospheric and Oceanic Technology 01/2010; 27(6):1059-1071. · 1.69 Impact Factor
  • Journal of Physical Oceanography 01/2009; 39. · 3.18 Impact Factor
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    ABSTRACT: Internal solitary waves referred to as solitons are common occurrences in the South China Sea. The Asian Seas International Acoustics Experiment (ASIAEX) experiment was carried out in April-May 2001 to perform measurements on these solitons, which are often highly energetic, having isopycnal displacements well over 100 m and phase speeds greater than 2.5 m/s. Of particular interest is the interaction of a soliton with the sloping shelf bottom that occurs as the soliton shoals to water depths less than its wave height. Observations during the experiment show that at such shallow depths, a soliton undergoes strong refraction and transformation. In this article, we present hindcast simulation of a particular soliton observed during the experiment, using a fully nonlinear, nonhydrostatic, three-dimensional model and the actual bathymetry from the ASIAEX area. The computation begins with the soliton at a distance about 100 km from the shelf and obtains the propagation and evolution of the soliton over the shelf-slope. The three-dimensional hindcast is able to reproduce the refraction of the soliton propagation observed during the experiment as well as the propagation speed and direction in the range observed. The final nonlinear transformation of the soliton from symmetrical to skewed elongated waveform is also obtained in the model consistent with the observations. The hindcast simulation reveals that the relative position of soliton's vorticity core to the local water depth is a crucial indicator of the onset of transformation and formation of elevation waves.
    Journal of Geophysical Research 01/2009; 114. · 3.17 Impact Factor
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    ABSTRACT: We focus on inverting the surface temperature (or heat) equation to obtain the surface velocity field in the coastal ocean and compare the results with those from the maximum cross correlation (MCC) technique and with the in situ velocity fields measured by the Rutgers University Coastal Ocean Dynamics Radar (CODAR). When compared with CODAR fields, velocities from the heat equation and MCC have comparable accuracies, but the heat equation technique better resolves the finer scale flow features. We use the results to directly calculate the surface divergence and vorticity. This is possible because we convert the traditionally underdetermined heat inversion problem to an overdetermined one without constraining the velocity field with divergence, vorticity, or energy statements. Because no a priori assumptions are made about the vorticity, it can be calculated directly from the velocity results. The derived vorticity field has typical open-ocean magnitudes ( ~ 5 times 10<sup>-5</sup>/s) and exhibits several structures (a warm core ring, Gulf Stream filament, and a diverging flow) consistent with the types of flows required to kinematically deform the sea surface temperature patterns into the observed configurations.
    IEEE Transactions on Geoscience and Remote Sensing 12/2008; · 3.47 Impact Factor
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    ABSTRACT: To the extent that sea surface temperature and colors can be considered passive tracers, their motions can be tracked to estimate the current velocities, or a conservation equation can be invoked to relate their temporal variations to the velocities. We investigate the latter, the so-called tracer inversion problem, with a particular focus on (1) the conditions under which the problem can be rendered over-determined for least squares solutions, (2) the possibility of using the tracer conservation equation within the “velocity projection” framework to estimate subsurface current profiles in shallow coastal waters, and (3) the accuracy of the tracer inversion calculation in terms of the data resolution and noise. The velocity projection framework refers to relating surface motion, either measured directly or made visible by tracers, to the subsurface current motion through the equations of motion. The accuracy of the tracer inversion calculation is quantified in terms of the spatial and temporal resolution of the tracer distribution. In the presence of irreducible tracer noise, the accuracy of the inversion rapidly degrades, and it is shown that the inversion with velocity projection can help improve accuracy. The tracer inversion method developed in this study is applied to the satellite sea surface temperature data, and the velocity result is compared to the velocity measurements made with the shore-based HF Coastal Current Radar. The potential of improving the velocity estimation with the present approach is indicated.
    Continental Shelf Research - CONT SHELF RES. 01/2008; 28(7):849-864.
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    ABSTRACT: Amplitude and phase variability in acoustic fields are simulated within a canonical shelf-break ocean environment using sound speed distributions computed from hydrodynamics. The submesoscale description of the space and time varying environment is physically consistent with tidal forcing of stratified flows over variable bathymetry and includes the generation, evolution and propagation of internal tides and solibores. For selected time periods, two-dimensional acoustic transmission examples are presented for which signal gain degradation is computed between 200 and 500 Hz on vertical arrays positioned both on the shelf and beyond the shelf break. Decorrelation of the field is dominated by the phase contribution and occurs over 2-3 min, with significant recorrelation often noted for selected frequency subbands. Detection range is also determined in this frequency band. Azimuth-time variations in the acoustic field are illustrated for 100 Hz sources by extending the acoustic simulations to three spatial dimensions. The azimuthal and temporal structure of both the depth-averaged transmission loss and temporal correlation of the acoustic fields under different environmental conditions are considered. Depth-averaged transmission loss varies up to 4 dB, depending on a combination of source depth, location relative to the slope and tidally induced volumetric changes in the sound speed distribution.
    The Journal of the Acoustical Society of America 06/2007; 121(5 Pt1):2575-90. · 1.65 Impact Factor
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    ABSTRACT: Ocean submesoscale features appear to be widespread in the surface mixed layer and thus may be an important link in the energy pathway from large to small scales. An example is the "spiral eddy," for which several theories have been proposed. High-resolution radar imagery should be useful in testing these theories, but there have as yet been no simulations of radar imagery from first principles. As a step in this direction, we developed a capability to simulate imagery using a full-spectral calculation that includes the effects of both wave-current interaction and wave damping due to a surface film. A particular model of a spiral eddy is used to specify the surface velocity field and film distribution. Imagery is then simulated for a range of radar frequencies, wind speeds, initial film pressures, and relative radar view directions. For winds of 3-8 m/s and an initial film pressure of 0.5 mN/m, imagery for shorter radar wavelengths (X- and C-band) is dominated by the effects of film damping. For longer wavelengths (L- and P-band) wave-current interactions and film damping are of comparable magnitude; but for higher initial film pressures, the L- and P-band images also become dominated by film damping. L-band imagery, in particular, is highly sensitive to the initial value of film pressure, and such a result may have implications for determining properties of seawater films. Overall, the radar simulations produce surface patterns having characteristics that resemble radar imagery of real ocean spiral eddies.
    IEEE Transactions on Geoscience and Remote Sensing 11/2005; · 3.47 Impact Factor
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    ABSTRACT: A modeled continental shelf-break region is used to assess the combined and relative impact of bathymetry and internal wave/bore influences on horizontal array performance in a littoral environment. Tidal forcing over a smooth break varying from a maximum depth of 100-m flat bathymetry to 50 m is used in conjunction with a submesoscale hydrodynamic model to produce internal wave activity in a fully stratified water column. A continuous wave source (100 Hz, for example) is sited at different aspects relative to the shelf break, and the acoustic field is propagated to broadside arrays that are placed at various bearings. A three-dimensional parabolic equation code propagated the field to the arrays. Time-variable three-dimensional acoustic effects such as beam wander due to horizontal refraction, resulting from either slope or internal waves, are both illustrated. The time-dependent bathymetric effects can be either enhanced or reduced by the internal tidal activity, depending upon the relative geometry of propagation, bathymetry, and internal tide. The influence of bathymetry versus the full evolving environment due to hydrodynamic action can be compared using array correlation calculations, signal gain degradation, and beam wander. [This research is sponsored by the ONR.]
    The Journal of the Acoustical Society of America 01/2005; 118:2003-2003. · 1.65 Impact Factor
  • C.Y. Shen, T.E. Evans
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    ABSTRACT: Remote sensing technology has made it possible to rapidly observe wide areas of the ocean surface. To determine subsurface conditions from these observations requires the application of hydrodynamic principles. For coastal currents, the viscous dynamics can be applied to efficiently project surface data downward to obtain the current's vertical structure in real time. This method has been developed for use with remotely sensed surface current data obtained from HF radars, and now is being extended to use remote sensing images. In this case, the surface currents are determined together with subsurface currents from the dynamic and conservation equations, as opposed to tracking or correlating image patterns in time. Alternatively, the surface data can be assimilated into a numerical coastal ocean model that solves for depth-dependent currents. But, presently, efficient assimilative computation appears to be feasible only with depth-integrated ocean models. In this case, a method can be developed to obtain current structure from the depth-integrated solution via the use of surface and bottom boundary stresses and associated shear equations. This method of determination can be practical as it requires only the depth-integrated solution and surface and bottom boundary conditions at the location of interest as input, in contrast to conventional modeling approach of determining currents over the whole model domain. Demonstration of the use of the various methods noted here will be given by means of numerical examples and comparisons with some field experimental data
    OCEANS, 2005. Proceedings of MTS/IEEE; 01/2005
  • The Journal of the Acoustical Society of America 01/2004; 116(4):2506-. · 1.65 Impact Factor
  • Colin Y. Shen, Thomas E. Evans
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    ABSTRACT: A non-hydrostatic density-stratified hydrodynamic model with a free surface has been developed from the vorticity equations rather than the usual momentum equations. This approach has enabled the model to be obtained in two different forms, weakly non-hydrostatic and fully non-hydrostatic, with the computationally efficient weakly non-hydrostatic form applicable to motions having horizontal scales greater than the local water depth. The hydrodynamic model in both its weakly and fully non-hydrostatic forms is validated numerically using exact nonlinear non-hydrostatic solutions given by the Dubriel–Jacotin–Long equation for periodic internal gravity waves, internal solitary waves, and flow over a ridge. The numerical code is developed based on a semi-Lagrangian scheme and higher order finite-difference spatial differentiation and interpolation. To demonstrate the applicability of the model to coastal ocean situations, the problem of tidal generation of internal solitary waves at a shelf-break is considered. Simulations carried out with the model obtain the evolution of solitary wave generation and propagation consistent with past results. Moreover, the weakly non-hydrostatic simulation is shown to compare favorably with the fully non-hydrostatic simulation. The capability of the present model to simulate efficiently relatively large scale non-hydrostatic motions suggests that the weakly non-hydrostatic form of the model may be suitable for application in a large-area domain while the computationally intensive fully non-hydrostatic form of the model may be used in an embedded sub-domain where higher resolution is needed.
    Journal of Computational Physics 01/2004; · 2.14 Impact Factor
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    ABSTRACT: The technique of 'velocity projection' (J. Geophys. Res. 106 (2001) 6973) is used to estimate the sea-surface height field and its change over time from measurements of surface velocity made using a shore-based HF Doppler radar over a 30 Â 30-km region of the continental shelf located near the mouth of the Chesapeake Bay (USA). Projected current profiles are compared with measured currents from an array of acoustic Doppler current profilers, and the consistency and sensitivity of the projections to model assumptions are also examined. Using projected values of the local surface slope, a model sea-surface Zðx; yÞ is least-squares fit over the study region at each measurement time. The error associated with these fits provides an internal check on the validity of the projection results. The slope of the model sea-surface shows a set-up toward the mouth of the Chesapeake Bay during downwelling-favorable winds and a counterclockwise rotation over the tidal cycle that is consistent with linear, shallow-water dynamics. A time series of sea-level difference extracted from the Z maps shows a dominant M 2 tidal signal that compares well with measurements of bottom pressure made at two moorings. With proper attention to limits of applicability, such projection-based sea-surface slope fields (as well as other projection results) may be useful in diagnostic calculations or as nowcasts for use with prognostic models.
    Continental Shelf Research 01/2004; 24:353-374. · 2.12 Impact Factor
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    ABSTRACT: We describe a methodology for estimating subsurface velocity structure in a buoyant outflow plume from a set of available observations. The observational data include HF Doppler radar, SST and sea surface color. In addition, plume-specific temperature and salinity information from in situ observations are used minimally, if available. Detailed application of the methodology is shown via a case study for the Chesapeake Bay during November 1997. The proposed methodology depends on developing a zero-order dynamical feature model for a typical plume. Theoretical models and past synoptic observational data sets are used to design the 'plume feature model'. The feature model's primary parameters include the location and extent of the frontal boundary, a simplified gravity current structure in the vertical with prescribed (or inferred) density stratification, and spatial gradient of salinity across the plume. These parameters are inferred from remote sensing or minimal strategic in situ observations. For the Chesapeake Bay case study, a previously developed velocity projection method by Shen and Evans (2002), which obtains subsurface current structure within the Ekman layer depth from surface currents (HF Doppler radar) and wind observations, is employed in a modified configuration. The 'feature model' density stratification in shallow water is incorporated now in the dynamical projection equations. The resulting subsurface projected currents are compared with available ADCP profiles. The difference between the density-stratified estimate and ADCP is further used to calibrate and improve the zero-order dynamical feature model parameters. This synergistic approach can now be applied to other shallow water features such as salt lenses and other anomalous entities.
    OCEANS 2003. Proceedings; 10/2003
  • Steven Finette, Colin Y. Shen, Thomas E. Evans
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    ABSTRACT: A nonhydrostatic, hydrodynamic model of the sound speed field in a continental shelf-break environment has been developed and implemented. The model is based on a vorticity formulation of the equations of motion for an incompressible fluid with a free ocean surface, and it is capable of simulating the generation and propagation of internal tides and solibores under tidal forcing. The model has been benchmarked with an exact numerical solution for a soliton. A set of space and time evolving sound speed distributions is integrated with a parabolic equation code to compute time and frequency dependent pressure fields. Two-dimensional examples of broad-band signal gain degradation on vertical arrays in this environment are presented, as well as range-frequency maps that illustrate the structure of the waveguide invariant in a shelf-break environment that is changing in time. Implications for source localization are considered. [Work supported by ONR.]
    The Journal of the Acoustical Society of America 01/2003; 114:2429-2430. · 1.65 Impact Factor
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    Steven Finette, Thomas Evans, Colin Shen
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    ABSTRACT: A coupled oceanographic/acoustic simulation model is under development for studying the relationship between acoustic field variability and dynamic oceanographic processes in a continental shelf/slope environment. The oceanographic component of the model involves numerical integration of the non-linear hydrodynamic equations of motion describing density, temperature and salinity distributions as a function of space and time. This component includes sub-mesoscale dynamics, allowing for the generation and propagation of non-hydrostatically generated phenomena such as tidally driven internal tides and solitary waves. Results are mapped into the corresponding sound speed distribution, and the resulting set of time evolved sound speed fields is used as input to a wide-angle parabolic equation that computes the acoustic field propagating through the environment. The general approach is discussed, and an illustrative result is presented that links acoustic field variability to specific oceanographic features.
    01/2002;
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    Colin Y. Shen, Thomas E. Evans
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    ABSTRACT: An approach is described that projects surface current observations downward to obtain subsurface current structure consistent with the interior current dynamics. The projection approach is unrestricted by water depth. However, this study emphasizes well-mixed constant density flow in water depths appropriate for mid and outer shelf, where viscous and inertial processes are comparably important. By means of twin experiments, in which the projected current profiles are compared to the known simulated current profiles, it is shown that both the projection time step and the time domain affect the accuracy of the projection. At minimum, the time domain needs to span the dominant period of current oscillation while the time step resolves this oscillation. When both are achieved, the projected current profiles converge to the simulated profiles, and this convergence can be faster than that by assimilating surface observations to correct model spin-up from an inaccurately known initial condition. Furthermore, the projection is shown to be robust in the presence of data noise, and with appropriate weighting of the data constraints, the effect of noise on the projection accuracy can be minimized. In the present projection problem, the sea surface slope is assumed unknown and obtained together with the current profile. The use of variable eddy viscosity in velocity projection is illustrated as well with an iterative procedure.
    Journal of Geophysical Research 01/2002; 107. · 3.17 Impact Factor
  • Colin Y. Shen, Thomas E. Evans
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    ABSTRACT: The ubiquitous occurrence of submesoscale cyclonic spirals in the sea as inferred from space imagery is interpreted in terms of the inertial instability of a horizontally sheared current in the oceanic mixed layer. The instability is shown to weaken anticyclonic current shear while enhancing cyclonic shear, which, in turn, becomes unstable and creates a cyclonic vortex; concurrently, surface tracer particles concentrated along the evolving cyclonic shear are wound up into a spiral, mimicking the spiral slick patterns seen in the imagery. The entire process, investigated with a fully nonlinear nonhydrostatic 3D numerical model, is contrasted with the baroclinic frontal process considered previously. The differences point to a clear need for field observations of this significant phenomenon, which are presently almost totally lacking.
    Geophysical Research Letters 01/2002; 29(23). · 3.98 Impact Factor
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    Colin Y. Shen, Thomas E. Evans
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    ABSTRACT: Sea surface currents in coastal oceans are accessible to continuous direct observations by shore-based high-frequency Doppler radar systems. Inferring current structure in shallow water from such surface current observations is attempted. The approach assumes frictionally dominated flow and vertically varying current velocity on the scale of the Ekman boundary layer. The approximation of the velocity variation with depth is consequently derivable in terms of orthogonal basis functions from the sea surface kinematic and dynamic boundary conditions; specifically, the viscous momentum and shear equations evaluated at the sea surface. The inference procedure developed is demonstrated with sea surface data obtained in the coastal High-Resolution Remote Sensing Experiment on the continental shelf off Cape Hatteras. Despite uncertainties in the surface measurements, qualitative agreement is obtained between the inferred subsurface current and the current measured in situ. The sensitivity of the inference to the measurement uncertainties as well as to the model assumptions is investigated, and the inferred result is found to be generally robust.
    Journal of Geophysical Research 01/2001; 106:6973-6984. · 3.17 Impact Factor
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    ABSTRACT: In situ observation and remote sensing imagery reveal the presence of longitudinal velocity convergences over bathymetric channels in tidal estuaries. We present the results of numerical experiments designed to investigate the cause of these convergences for channels possessing shallow shoal regions and a deeper central region. The equations of motion for a homogeneous fluid on a rotating Earth are solved using a fully spectral code in the across-estuary (i.e., the vertical or x-z) plane, while no alongestuary flow variations (in the y direction) are permitted. A Gaussian-shaped bottom bathymetry is chosen. In the along-channel (y) direction we impose a pressure gradient which is the sum of constant and fluctuating parts to simulate the steady and tidally oscillating parts of the estuarine flow. The details of the transient response can be complicated, but we observe that for most (~80%) of the tidal cycle there exists a cross-estuary recirculation cell collocated with a localized along-channel jet. Both of these are situated over the bottom bathymetric groove; the circulation is always clockwise when facing down current. This feature results from the generation of stream-wise vorticity through the tilting of planetary vorticity by the vertical shear of the along-estuary flow. A surface convergence-divergence pair is associated with the flow. The maximum value of each is seen to occur on the edge of the bathymetric feature but may migrate toward or away from the center as long as the current continues in the same direction. When the tide reverses, the feature reappears on the opposite shoal, and the migration of the convergence and divergence extrema begins again. We also find that the responses are qualitatively similar for all bathymetric grooves, even asymmetrically situated ones, provided that the estuary width-to-depth ratio is of order 100 or larger, the Rossby numbers are of order unity, and the Ekman layer thickness-to-channel-depth ratio is greater than ~0.65.
    Journal of Geophysical Research 01/2001; 106:27145-27162. · 3.17 Impact Factor