Svein Vagle

Structure and Generation of Turbulence at Interfaces Strained by Internal Solitary Waves Propagating Shoreward over the Continental Shelf

Abstract Detailed observations of the structure within internal solitary waves propagating shoreward over Oregons continental shelf reveal the evolving nature of interfaces as they become unstable and break, creating turbulent flow. A persistent feature is high acoustic backscatter beginning in the vicinity of the wave trough and continuing through its trailing edge and wake. This is demonstrated to be due to enhanced density microstructure. Increased small-scale strain ahead of the wave trough compresses select density interfaces, thereby locally increasing stratification. This is followed by a sequence of overturning, high-density microstructure, and turbulence at the interface, which is coincident with the high acoustic backscatter. The Richardson number estimated from observations is larger than 1/4, indicating that the interface is stable. However, density profiles reveal these preturbulent interfaces to be O(10 cm) thick, much thinner than can be resolved with shipboard velocity measurements. By as...

Waveguide propagation of ambient sound in the ocean‐surface bubble layer

Measurements of the ambient sound generated by breaking waves over the range 40–20 000 Hz reveal well‐defined spectral peaks, the frequency of which may remain generally consistent from one breaking event to the next, but which can change significantly over the course of a storm, or from one storm to another. A theory is proposed, based on the concept of trapping of a portion of the sound in the waveguide formed by the ocean‐surface bubble layer. Simultaneous measurements of the bubble population and size distribution as a function of depth and time were obtained with a multifrequency inverted echo sounder, allowing calculation of the resulting (dispersive) sound‐speed anomaly profile. Theoretical predictions of the spectral peaks, which are associated with modal cutoff frequencies, are in good agreement with the observations. It is suggested that this result might have application to the remote determination of ocean‐surface bubble fields relevant to the study of wave breaking, turbulence, and the air–se...

Structure and variability of Langmuir circulation during the Surface Waves Processes Program

A cooperative, multiplatform field experiment was conducted in the eastern North Pacific during February and March of 1990 as part of the Surface Waves Processes Program (SWAPP). One of the experimental objectives was to investigate Langmuir circulation so that its role in the evolution of the oceanic surface boundary layer could be better understood. The concurrent use of different observational techniques, ranging from simple surface drifters to complex Doppler sonar systems, resulted in new information about Langmuir circulation structure and variability. Estimates of Langmuir cell spacing indicated that a broad range of scales, from about 2 to 200 m, was excited during periods of strong surface forcing and that the energy containing scales evolved with time. Estimates of cell spacing based on Doppler velocities from a surface-scanning sonar directed crosswind showed this scale evolution, but estimates based on backscattered intensity did not. This was attributed to the fact that the intensity-based estimates were only indirectly related to circulation strength. The near-surface convergent velocities from the sonar were used to form an objective, quantitative measure of the temporal variations in Langmuir circulation strength. As expected, the circulation strength increased dramatically during strong wind events. However, circulation strength and wind stress did not decrease simultaneously, and Langmuir circulation was detectable for up to a day after abrupt reductions in wind stress. Energy from the surface wave field, which decayed more slowly than the wind, was apparently responsible for maintaining the circulation. The variation of circulation strength was found to be better related to (u*Us)½ than to u*, where u* = (τ/ρ)½ is the friction velocity, τ is the wind stress, and Us is the surface wave Stokes drift. This scaling is consistent with wave-current interaction theories of Langmuir cell generation.

The Measurement of Bubble-Size Distributions by Acoustical Backscatter

Abstract A multifrequency acoustical-backscatter technique is described for determining the size distribution of bubbles with radii between 8 and 130 µm. The method makes use of the resonance in the microbubbles and operates at six frequencies ranging from 28 to 400 kHz. It has the advantage that vertical profiles of the bubble-size distribution can be obtained for extended periods without need for in situ instrumentation. Algorithms have been developed for real-time calculation, including correction for wave orbital displacement, bubble attenuation of the transmitted pulse, and other effects.

Stratified flow over topography: upstream influence and generation of nonlinear internal waves

Results are presented from a recent experiment in Knight Inlet, British Columbia, focusing on the generation and propagation of nonlinear internal waves near the sill during ebb tide. High–quality echo–sounder and velocity measurements were obtained in a novel fashion, using instrumentation carried aboard a small inflatable boat. Complementing these data, a set of photographic images of the surface signature of the internal waves was acquired from both a fixed mountainside location and a helicopter. These photographs situate the observations within the larger–scale structure of the internal response found in the inlet. The observations reveal the development of strongly nonlinear internal waves on a bore occurring upstream of the sill crest. As the tidal flow relaxes, these waves propagate upstream, at first slowly, and later rapidly against the gradually waning flow. A simulation of wave generation with a fully nonlinear numerical model is discussed. The simulation shows that upstream influence, associated with the rapidly increasing tidal forcing, leads to formation of an undular bore upstream of the sill crest. As the tidal flow relaxes, the bore subsequently evolves into a group of upstream–propagating solitary–like internal waves. Taken together, the observations and numerical simulation describe a process for the generation of nonlinear internal waves involving upstream influence, which likely has application in other environments for stratified flow over topography.

A Free-Flooding Acoustical Resonator for Measurement of Bubble Size Distributions

Abstract An instrument for the measurement of bubble size distributions is described. The sensing element exploits the free-flooding resonator design of Medwin with modifications to overcome the limitations in the original implementation, especially those due to a sensitivity to ambient pressure fluctuations in the surrounding medium. A mathematical model of the resonator provides insight into the factors affecting its performance and motivates application of appropriate signal processing algorithms. Comparison of different bubble size calculation methods shows the direct approach of Commander and MacDonald to be most successful. The stability of this new implementation of the resonator facilitates accurate measurement of the complex dispersion relation. Comparison of the real and imaginary components then leads to the definition of a measurement quality factor that may be calculated for each sample. Practical considerations are discussed for implementation of autonomous battery-powered resonator arrays f...

A comparison of four methods for bubble size and void fraction measurements

We compare the performance of four different bubble-sensing techniques in a range of environment from the surf zone to the open ocean: a remote sensing method using high-frequency backscatter, two in situ methods using an acoustical resonator and a pulse propagation sensor, and a bulk method using electrical conductivity. Comparisons between the techniques show general consistency within the appropriate operational bubble density ranges, although spatial variability in bubble clouds introduces substantial variance. Each technique has its strengths and limitations. Our acoustical resonator is suitable for bubble concentrations with air fractions greater than approximately 10/sup -9/ and the pulse propagation sonar for air fractions from 10/sup -6/; the upper limit for both is constrained by attenuation and the validity of the Foldy scattering approximation. Both sensors can be implemented to encompass a wide frequency range with high resolution, corresponding to resonant bubble radii of 10/spl sim/1200 /spl mu/m. For air fractions higher than /spl sim/5/spl times/10/sup -4/, bulk measurement using electrical conductivity provides a measure of air fraction. Sufficient overlap in operational air-fraction range exists between in situ acoustical techniques and conductivity measurement to permit comparison and demonstrate consistency in the measurement. Single- and multi-frequency backscatter sonars may be used for low air fractions (<1/spl times/10/sup -5/) and provide a continuous vertical profile from a deployment beneath the active surface zone, but are subject to masking by dense bubble clouds and are unable to resolve high air fractions close to the surface. This study suggests that the best approach is to use a combination of sensors to probe the bubble field.

An Evaluation of the WOTAN Technique of Inferring Oceanic Winds from Underwater Ambient Sound

Abstract The potential of the WOTAN technique to estimate oceanic winds from underwater ambient sound is thoroughly evaluated. Anemometer winds and sound spectrum levels at 11 frequencies in the range 3–25 kHz from the FASINEX Experiment are used to establish both the frequency and wind speed dependencies of ambient sound. These relationships are then tested using independent data from four other deployments, and found to hold in the deep ocean in the OCEAN STORMS but not in shallow coastal waters. The OCEAN STORMS ambient-sound wind speed estimates are within ±0.5 m s−1 of anemometer values for wind speeds between 4 and 15 m s−1. Causes of differences, including disequilibrium of the surface wave field, are discussed and it is argued that they are no larger than expected. The procedure for processing ambient-sound data is developed. It includes temperature dependent calibration detection of shipping and precipitation contamination, and standardization of measurements to 1 m depth. The latter procedure al...

Measurements of bubble plumes and turbulence from a submarine

Abstract An experiment using turbulence probes and an array of side‐scan and vertically pointing pencil beam sonars mounted on the U.S. submarine Dolphin was carried out to measure turbulence in near‐surface regions of acoustic scattering, in particular, those caused by subsurface bubbles produced by breaking wind waves. The dataset collected during winds of 5–9 m s−1 reveals the banded patterns of bubbles associated with Langmuir circulation, even though no surface manifestations were visible. A forward‐pointing side‐scan sonar determined the “age” of bubble clouds after their generation by breaking waves. There is enhanced turbulent dissipation in the bubble clouds, and the dissipation rate close to the surface exceeds that predicted using conventional calculations based on the law of the wall and buoyancy flux. The correspondence between bubbles and turbulence implies a horizontally patchy turbulent structure near the surface. Below the base of the bubble clouds the distance between turbulent patches i...

Acoustical measurements of microbubbles within ship wakes

High‐frequency sonar measurements of target strength due to microbubbles were obtained within the wakes of three oceanographic vessels. Two self‐contained, high‐frequency acoustics instruments suspended at 25‐m depth were used to measure the wake acoustic properties during three separate sea trials. The backscatter cross section per unit volume, Mv, as a function of depth and time was calculated from the echo intensity of six upward‐looking, conical beam sonars (28–400 kHz). Four 100‐kHz steerable sidescans allowed measurement of wake locations, widths, and persistence. In the near‐surface core of the wake Mv reached peak values of approximately 0.3 m−1 for the 120‐ and 200‐kHz sonars. The volumetric scattering cross sections were observed to be roughly constant at all frequencies within the top 5–6 m of the wake, suggesting a roughly homogeneous vertical bubble distribution. However, differences in the volumetric backscatter at different acoustical frequencies suggest a higher relative concentration of l...