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Jim Thomson

Abstract

Energy dissipation rates during ocean wave breaking are estimated from high-resolution profiles of turbulent velocities collected within 1 m of the surface. The velocity profiles are obtained from a pulse-coherent acoustic Doppler sonar on a wave-following platform, termed a Surface Wave Instrument Float with Tracking (SWIFT), and the dissipation rates are estimated from the structure function of the velocity profiles. The purpose of the SWIFT is to maintain a constant range to the time-varying surface and thereby observe the turbulence in breaking crests (i.e., above the mean still water level). The Lagrangian quality is also useful to prefilter wave orbital motions and mean currents from the velocity measurements, which are limited in magnitude by phase wrapping in the coherent Doppler processing. Field testing and examples from both offshore whitecaps and nearshore surf breaking are presented. Dissipation rates are elevated (up to 10−3 m2 s−3) during strong breaking conditions, which are confirmed using surface videos recorded on board SWIFT. Although some velocity contamination is present from platform tilting and heaving, the structure of the velocity profiles is dominated by a turbulent cascade of eddies (i.e., the inertial subrange). The noise, or uncertainty, in the dissipation estimates is shown to be normally distributed and uncorrelated with platform motion. Aggregated SWIFT measurements are shown to be useful in mapping wave-breaking dissipation in space and time.

Full access
Maricarmen Guerra
and
Jim Thomson

Abstract

Two new five-beam acoustic Doppler current profilers—the Nortek Signature1000 AD2CP and the Teledyne RDI Sentinel V50—are demonstrated to measure turbulence at two energetic tidal channels within Puget Sound, Washington. The quality of the raw data is tested by analyzing the turbulent kinetic energy frequency spectra, the turbulence spatial structure function, the shear in the profiles, and the covariance Reynolds stresses. The five-beam configuration allows for a direct estimation of the Reynolds stresses from along-beam velocity fluctuations. The Nortek’s low Doppler noise and high sampling frequency allow for the observation of the turbulent inertial subrange in both the frequency spectra and the turbulence structure function. The turbulence parameters obtained from the five-beam acoustic Doppler current profilers are validated with turbulence data from simultaneous measurements with acoustic Doppler velocimeters. These combined results are then used to assess a turbulent kinetic energy budget in which depth profiles of the turbulent kinetic energy dissipation and production rates are compared. The associated codes are publicly available on the MATLAB File Exchange website.

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Michael Schwendeman
and
Jim Thomson

Abstract

An algorithm is presented for the stabilization and rectification of digital video from floating platforms. The method relies on a horizon-tracking technique that was tested under a variety of lighting and sea-state conditions for 48 h of video data over 12 days during a research cruise in the North Pacific Ocean. In this dataset, the horizon was correctly labeled in 92% of the frames in which it was present. The idealized camera model assumes pure pitch-and-roll motion, a flat sea surface, and an unobstructed horizon line. Pitch and roll are defined along the camera look direction rather than in traditional ship coordinates, such that the method can be used for any heading relative to the ship. The uncertainty in pitch and roll is estimated from the uncertainties of the horizon-finding method. These errors are found to be of the order 0.6° in roll and 0.3° in pitch. Errors in rectification are shown to be dominated by the uncertainty in camera height, which may change with the heave motion of a floating platform. The propagation of these errors is demonstrated for the breaking-wave distribution Λ(c). A toolbox for implementation of this method in MATLAB is shared via the MATLAB File Exchange.

Full access
Samuel Harding
,
Levi Kilcher
, and
Jim Thomson

Abstract

High-fidelity measurements of turbulence in the ocean have long been challenging to collect, in particular in the middle of the water column. In response, a measurement technique has been developed to deploy an acoustic Doppler velocimeter (ADV) to midwater locations on a compliant mooring. A variety of instrumentation platforms have been deployed as part of this work with a range of dynamic motion characteristics. The platforms discussed herein include the streamlined StableMoor buoy (SMB), the Tidal Turbulence Mooring (TTM) system based on a conventional 0.9-m spherical buoy, and a 100-lb sounding weight suspended from the stern of a research vessel. The ADV head motion is computed from inertial motion sensors integrated into an ADV, and the spectra of these signals are investigated to quantify the motion of each platform. The SMB with a single ADV head mounted on the nose provided the most stable platform for the measurement of tidal turbulence in the inertial subrange for flow speeds exceeding 1.0 m s−1. The modification of the SMB with a transverse wing configuration for multiple ADVs showed a similar frequency response to the nose configuration in the horizontal plane but with large contamination in the vertical direction as a result of platform roll. While the ADV motion on the TTM was significant in the horizontal directions, the vertical motion of this configuration was the most stable of all configurations tested. The sounding weight measurements showed the greatest motion at the ADV head but are likely to be influenced by both prop-wash and vessel motion.

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Jim Thomson
and
Andrew T. Jessup

Abstract

A Fourier-based method is presented to process video observations of water waves and calculate the speed distribution of breaking crest lengths. The method has increased efficiency and robust statistics compared with conventional algorithms that assemble distributions from tracking individual crests in the time domain. The method is tested using field observations (video images of whitecaps) of fetch-limited breaking waves during case studies with low (6.7 m s−1), moderate (8.5 m s−1), and high (12.6 m s−1) wind speeds. The method gives distributions consistent with conventional algorithms, including breaking rates that are consistent with direct observations. Results are applied to obtain remote estimates of the energy dissipation associated with wave breaking.

Full access
Kristin Zeiden
,
Jim Thomson
, and
James Girton

Abstract

High-resolution profiles of vertical velocity obtained from two different surface-following autonomous platforms, Surface Wave Instrument Floats with Tracking (SWIFTs) and a Liquid Robotics SV3 Wave Glider, are used to compute dissipation rate profiles ϵ(z) between 0.5 and 5 m depth via the structure function method. The main contribution of this work is to update previous SWIFT methods to account for bias due to surface gravity waves, which are ubiquitous in the near-surface region. We present a technique where the data are prefiltered by removing profiles of wave orbital velocities obtained via empirical orthogonal function (EOF) analysis of the data prior to computing the structure function. Our analysis builds on previous work to remove wave bias in which analytic modifications are made to the structure function model. However, we find the analytic approach less able to resolve the strong vertical gradients in ϵ(z) near the surface. The strength of the EOF filtering technique is that it does not require any assumptions about the structure of nonturbulent shear, and does not add any additional degrees of freedom in the least squares fit to the model of the structure function. In comparison to the analytic method, ϵ(z) estimates obtained via empirical filtering have substantially reduced noise and a clearer dependence on near-surface wind speed.

Open access
Michael S. Schwendeman
and
Jim Thomson

Abstract

A new ship-based stereo video system is used to observe breaking ocean waves (i.e., whitecaps) as three-dimensional surfaces evolving in time. First, the stereo video measurements of all waves (breaking and nonbreaking) are shown to compare well with statistical parameters from traditional buoy measurements. Next, the breaking waves are detected based on the presence of whitecap foam, and the geometry of these waves is investigated. The stereo measurements show that the whitecaps are characterized by local extremes of surface slope, though the larger-scale, crest-to-trough steepness of these waves is unremarkable. Examination of 103 breaking wave profiles further demonstrates the pronounced increase in the local wave steepness near the breaking crest, as estimated using a Hilbert transform. These crests are found to closely resemble the sharp corner of the theoretical Stokes limiting wave. Results suggest that nonlinear wave group dynamics are a key mechanism for breaking, as the phase speed of the breaking waves is slower than predicted by the linear dispersion relation. The highly localized and transient steepness, along with the deviation from linear phase speed, explains the inability of conventional wave buoys to observe the detailed geometry of breaking waves.

Full access
Morteza Derakhti
,
Jim Thomson
, and
James T. Kirby

Abstract

We examine how Eulerian statistics of wave breaking and associated turbulence dissipation rates in a field of intermittent events compare with those obtained from sparse Lagrangian sampling by surface following drifters. We use a polydisperse two-fluid model with large-eddy simulation (LES) resolution and volume-of-fluid surface reconstruction (VOF) to simulate the generation and evolution of turbulence and bubbles beneath short-crested wave breaking events in deep water. Bubble contributions to dissipation and momentum transfer between the water and air phases are considered. Eulerian statistics are obtained from the numerical results, which are available on a fixed grid. Next, we sample the LES/VOF model results with a large number of virtual surface-following drifters that are initially distributed in the numerical domain, regularly or irregularly, before each breaking event. Time-averaged Lagrangian statistics are obtained using the time series sampled by the virtual drifters. We show that convergence of statistics occurs for signals that have minimum length of approximately 1000–3000 wave periods with randomly spaced observations in time and space relative to three-dimensional breaking events. We further show important effects of (i) extent of measurements over depth and (ii) obscuration of velocity measurements due to entrained bubbles, which are the two typical challenges in most of the available in situ observations of upper ocean wave breaking turbulence. An empirical correction factor is developed and applied to the previous observations of Thomson et al. Applying the new correction factor to the observations noticeably improves the inferred energy balance of wind input rates and turbulence dissipation rates. Finally, both our simulation results and the corrected observations suggested that the total wave breaking dissipation rates have a nearly linear relation with active whitecap coverage.

Free access
Michael Schwendeman
,
Jim Thomson
, and
Johannes R. Gemmrich

Abstract

Coupled in situ and remote sensing measurements of young, strongly forced wind waves are applied to assess the role of breaking in an evolving wave field. In situ measurements of turbulent energy dissipation from wave-following Surface Wave Instrument Float with Tracking (SWIFT) drifters and a tethered acoustic Doppler sonar system are consistent with wave evolution and wind input (as estimated using the radiative transfer equation). The Phillips breaking crest distribution Λ(c) is calculated using stabilized shipboard video recordings and the Fourier-based method of Thomson and Jessup, with minor modifications. The resulting Λ(c) are unimodal distributions centered around half of the phase speed of the dominant waves, consistent with several recent studies. Breaking rates from Λ(c) increase with slope, similar to in situ dissipation. However, comparison of the breaking rate estimates from the shipboard video recordings with the SWIFT video recordings show that the breaking rate is likely underestimated in the shipboard video when wave conditions are calmer and breaking crests are small. The breaking strength parameter b is calculated by comparison of the fifth moment of Λ(c) with the measured dissipation rates. Neglecting recordings with inconsistent breaking rates, the resulting b data do not display any clear trends and are in the range of other reported values. The Λ(c) distributions are compared with the Phillips equilibrium range prediction and previous laboratory and field studies, leading to the identification of several inconsistencies.

Full access
Seth F. Zippel
,
Jim Thomson
, and
Gordon Farquharson

Abstract

Observations of surface waves, currents, and turbulence at the Columbia River mouth are used to investigate the source and vertical structure of turbulence in the surface boundary layer. Turbulent velocity data collected on board freely drifting Surface Wave Instrument Float with Tracking (SWIFT) buoys are corrected for platform motions to estimate turbulent kinetic energy (TKE) and TKE dissipation rates. Both of these quantities are correlated with wave steepness, which has previously been shown to determine wave breaking within the same dataset. Estimates of the turbulent length scale increase linearly with distance from the free surface, and roughness lengths estimated from velocity statistics scale with significant wave height. The vertical decay of turbulence is consistent with a balance between vertical diffusion and dissipation. Below a critical depth, a power-law scaling commonly applied in the literature works well to fit the data. Above this depth, an exponential scaling fits the data well. These results, which are in a surface-following reference frame, are reconciled with results from the literature in a fixed reference frame. A mapping between free-surface and mean-surface reference coordinates suggests 30% of the TKE is dissipated above the mean sea surface.

Open access