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Seth Zippel

Research Assistant




2000-present and while at APL-UW

Surface wave breaking over sheared currents: Observations from the mouth of the Columbia River

Zipple, S., and J. Thomson, "Surface wave breaking over sheared currents: Observations from the mouth of the Columbia River," J. Geophys. Res., EOR, doi:10.1002/2016JC012498, 2017.

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26 Apr 2017

SWIFT website

Measurements of waves and currents from freely drifting buoys are used to evaluate wave breaking parameterizations at the Mouth of the Columbia River, where breaking occurs in intermediate depths and in the presence of vertically sheared currents. Breaking waves are identified using images collected with cameras onboard the buoys, and the breaking activity is well-correlated with wave steepness. Vertical shear in the currents produces a frequency-dependent effective current that modifies the linear dispersion relation. Accounting for these sheared currents in the wavenumber spectrum is essential in calculating the correct wave steepness; without this, wave steepness can be over (under) estimated on opposing (following) currents by up to 20%. The observed bulk steepness values suggest a limiting value of 0.4. The observed fraction of breaking waves is in good agreement with several existing models, each based on wave steepness. Further, a semispectral model designed for all depth regimes also compares favorably with measured breaking fractions. In this model, the majority of wave breaking is predicted to occur in the higher frequency bands (i.e., short waves). There is a residual dependence on directional spreading, in which wave breaking decreases with increasing directional spread.

On the modeling of wave-enhanced turbulence nearshore

Moghimi, S., J. Thomson, T. Özkan-Haller, L. Umlauf, and S. Zippel, "On the modeling of wave-enhanced turbulence nearshore," Ocean Model., 103, 118-132, doi:10.1016/j.ocemod.2015.11.004, 2016.

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1 Jul 2016

A high resolution k – ω two-equation turbulence closure model, including surface wave forcing was employed to fully resolve turbulence dissipation rate profiles close to the ocean surface. Model results were compared with observations from Surface Wave Instrument Floats with Tracking (SWIFTs) in the nearshore region at New River Inlet, North Carolina USA, in June 2012. A sensitivity analysis for different physical parameters and wave and turbulence formulations was performed. The flux of turbulent kinetic energy (TKE) prescribed by wave dissipation from a numerical wave model was compared with the conventional prescription using the wind friction velocity. A surface roughness length of 0.6 times the significant wave height was proposed, and the flux of TKE was applied at a distance below the mean sea surface that is half of this roughness length. The wave enhanced layer had a total depth that is almost three times the significant wave height. In this layer the non-dimensionalized Terray scaling with power of –1.8 (instead of –2) was applicable.

Wave breaking turbulence in the ocean surface layer

Thomson, J., M.S. Schwendeman, S.F. Zippel, S. Moghimi, J. Gemmrich, and W.E. Rogers, "Wave breaking turbulence in the ocean surface layer," J. Phys. Oceanogr., 46, 1857-1870, doi:10.1175/JPO-D-15-0130.1, 2016.

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1 Jun 2016

Observations of winds, waves, and turbulence at the ocean surface are compared with several analytic formulations and a numerical model for the input of turbulent kinetic energy by wave breaking and the subsequent dissipation. The observations are generally consistent with all of the formulations, although some differences are notable at winds greater than 15 m/s. The depth dependence of the turbulent dissipation rate beneath the waves is fit to a decay scale, which is sensitive to the choice of vertical reference frame. In the surface following reference frame, the strongest turbulence is isolated within a shallow region of depths much less than one significant wave height. In a fixed reference frame, the strong turbulence penetrates to depths that are at least half of the significant wave height. This occurs because the turbulence of individual breakers persists longer that the dominant period of the waves, and thus the strong surface turbulence is carried from crest to trough with the wave orbital motion.

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Air–sea interactions in the marginal ice zone

Zippel, S., and J. Thomson, "Air–sea interactions in the marginal ice zone," Elem. Sci. Anth., 4, 000095, doi:10.12952/journal.elementa.000095, 2016.

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31 Mar 2016

The importance of waves in the Arctic Ocean has increased with the significant retreat of the seasonal sea-ice extent. Here, we use wind, wave, turbulence, and ice measurements to evaluate the response of the ocean surface to a given wind stress within the marginal ice zone, with a focus on the local wind input to waves and subsequent ocean surface turbulence. Observations are from the Beaufort Sea in the summer and early fall of 2014, with fractional ice cover of up to 50%. Observations showed strong damping and scattering of short waves, which, in turn, decreased the wind energy input to waves. Near-surface turbulent dissipation rates were also greatly reduced in partial ice cover. The reductions in waves and turbulence were balanced, suggesting that a wind-wave equilibrium is maintained in the marginal ice zone, though at levels much less than in open water. These results suggest that air-sea interactions are suppressed in the marginal ice zone relative to open ocean conditions at a given wind forcing, and this suppression may act as a feedback mechanism in expanding a persistent marginal ice zone throughout the Arctic.

Turbulence measurements from moving platforms

Thomson, J., J. Talbert, A. de Klerk, S. Zippel, M. Guerra, and L. Kilcher, "Turbulence measurements from moving platforms," Proc. 11th IEEE/OES Current, Waves and Turbulence Measurement (CWTM) Workshop, 2-6 March, St. Petersburg, FL, doi:10.1109/CWTM.2015.7098107 (IEEE, 2015).

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2 Mar 2015

Two recent methods for making high-fidelity turbulence measurements from moving platforms are described and demonstrated. The first is a method for measuring profiles of near-surface turbulence from a wave-following 'SWIFT' buoy. The second is a method for measuring time series of turbulence from a submerged compliant mooring. Both approaches use coherent Doppler instruments and inertial motion units (IMUs). In the buoy method, wave motions (e.g., pitch, roll, and heave) are quantified via GPS and IMU measurements. These wave motions are not present in the turbulence observations, because buoy follows the wave orbital motion, and thus the turbulent velocities are processed in the wave-following reference frame. In the mooring method, IMU measurements track the mooring motions (e.g., strum and kiting) and these motions are removed in post-processing to obtain turbulent velocities in the fixed earth reference frame. These approaches successfully quantify turbulence in regions previously unavailable or limited by the noise and spatial aliasing of sampling from bottom-mounted platforms.

Wave breaking and turbulence at a tidal inlet

Zippel, S., and J. Thomson, "Wave breaking and turbulence at a tidal inlet," J. Geophys. Res., 120, 1016-1031, doi:10.1002/2014JC010025, 2015.

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1 Feb 2015

Field measurements collected with surface drifters at New River Inlet (NC, USA) are used to characterize wave breaking and turbulence in the presence of currents. Shoreward wave evolution is affected by currents, and breaking is observed in deeper water with opposing currents (ebb tides) relative to the following currents (flood tides). Wave dissipation models are evaluated with observed cross-shore gradients in wave energy flux. Wave dissipation models that include the effects of currents are better correlated with the observations than the depth-only models. Turbulent dissipation rates measured in the breaking regions are used to evaluate two existing scaling models for the vertical structure and magnitude of turbulent dissipation relative to wave dissipation. Although both describe the rapid decay of turbulence beneath the surface, exponential vertical scaling by water depth is superior to power law vertical scaling by wave height.

Wave breaking turbulence at the offshore front of the Columbia River Plume

Thomson, J., A.R. Horner-Devine, S. Zippel, C. Rusch, and W. Geyer, "Wave breaking turbulence at the offshore front of the Columbia River Plume," Geophys. Res. Lett., 41, 8987-8993, doi:10.1002/2014GL062274, 2014.

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28 Dec 2014

Observations at the Columbia River plume show that wave breaking is an important source of turbulence at the offshore front, which may contribute to plume mixing. The lateral gradient of current associated with the plume front is sufficient to block (and break) shorter waves. The intense whitecapping that then occurs at the front is a significant source of turbulence, which diffuses downward from the surface according to a scaling determined by the wave height and the gradient of wave energy flux. This process is distinct from the shear-driven mixing that occurs at the interface of river water and ocean water. Observations with and without short waves are examined, especially in two cases in which the background conditions (i.e., tidal flows and river discharge) are otherwise identical.

Video recognition of breaking waves

Rusch, C., J. Thomson, S. Zippel, and M. Schwendeman, "Video recognition of breaking waves," Proc., OCEANS'14, 14-19 September, St. John's, Newfoundland (MTS/IEEE, 2014).

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15 Jul 2014

An algorithm is presented to automate the identification of breaking waves in images collected with a camera on a drifting buoy. Each image is given a score from four separate analysis techniques: brightness detection, pixel histogram, entropy (texture) analysis, and glare identification. By combining these in a composite score, potential breaking wave images are detected and the number of images requiring manual review is a small fraction of the original set. Most of the images with false breaking wave signals due to sun glare are identified and removed. The final output is the wave-breaking rate over the length of the video capture.

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center