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Observations of the movement and distribution of subsurface bubbles reveal the structure of coherent motions near the ocean surface. The measurements were obtained in the open ocean from a freely drifting, self‐contained acoustical instrument equipped with side scan sonar, dual‐frequency echo sounders, and ambient sound recording systems. The bubble clouds are organized into long, narrow plumes aligned with the wind, consistent with windrows caused by Langmuir circulations. They have widths of about 3 to 5 m and lengths up to 100 m, with multiple scales coexisting. Their mean separation depends upon wind speed: below 5 m s−1, mean spacings were about 5 m, increasing to 10 m when winds exceeded 10 m s−1 The maximum depth to which the plumes were observed to penetrate depended to some extent on wind speed, with the greatest penetrations of 12 m occurring at the highest wind speeds (13 m s−1). However, the time‐averaged depth of observed bubble plume penetration bears only a weak dependence on wind speed. Whenever winds exceeded 5 m s−1, a mean penetration depth of 6 m was observed. Associated with these bubble plumes were maximum downward velocities of about 0.06 m s−1. These speeds were detected at 8 m depth in the center of the plumes; magnitudes decreased to zero near the surface and horizontally toward the boundaries of the plumes.

[1] This paper reports on remote acoustic observations of vertical turbulence intensity and vertical suspended sediment flux profiles on a planar beach in 3-4 m water depth. The measurements of suspended sediment concentration and velocity are colocated and simultaneous and extend through the wave bottom boundary layer to the bed with 0.7 cm vertical resolution. Normalized cospectra of the suspended sediment flux and the vertical velocity for different bed states (irregular ripples, cross ripples, linear transition ripples, and flat bed) indicate a small but significant peak at incident wave frequencies but are otherwise rather flat, with weak redness. Estimates of the vertical flux components indicate a general balance between upward fluxes due to waves and turbulence and downward settling. Two exceptions to this balance are found immediately above the bed and for nonmigrating irregular ripples. The contribution from the high-frequency turbulent component is small. Wave phase averages for low-energy bed states exhibit near-bed peaks in the suspended sediment flux following wave phase reversal. Wave phase averages for the high-energy cases do not exhibit a diffusive signature. Observed vertical profiles of turbulence intensity for different bed states reveal that the near-bed turbulence levels are relatively independent of bed state. Friction velocity predictions from presently available models, including a bed stress model and a sediment eddy diffusion model, are compared to measured values of near-bed turbulence intensity. Reasonable agreement is found between measured and predicted bottom friction velocities when wave friction factors from Tolman [1994] are used.

[1] Results are presented from an experimental investigation of rough turbulent oscillatory boundary layers using a prototype wideband bistatic coherent Doppler profiler. The profiler operates in the 1.2 MHz to 2.3 MHz frequency band and uses software-defined radio technologies for digital control of the frequency content and shape of the transmit pulse and for digital complex demodulation of the received signals. Velocity profiles are obtained at sub-millimeter range resolution and 100 Hz profiling rates (each profile being an ensemble average of 10 pulse pairs). The measurements were carried out above beds of fixed sand or gravel particles, with median grain diameters of 0.37 mm and 3.9 mm, respectively, oscillating sinusoidally at a 10 s period through excursions of 0.75 m to 1.5 m. The resulting vertical profiles of horizontal velocity magnitude and phase, with the vertical axis scaled by ' = ku * m =w, are comparable to similarly scaled profiles obtained using laser Doppler anemometry by Sleath (1987) and Jensen (1988). A key objective of the comparisons between the previous experiments and those reported here was to establish how close to the bed reliable velocity measurements can be made with the sonar. This minimum distance above the bed is estimated to be 5 AE 1 mm, a value approaching the 3 to 4 mm limit set by the path of least time.Citation: Hay, A. E., L. Zedel, R. Cheel, and J. Dillon (2012), Observations of the vertical structure of turbulent oscillatory boundary layers above fixed roughness beds using a prototype wideband coherent Doppler profiler: 1. The oscillatory component of the flow,

A pulse-to-pulse coherent acoustic Doppler profiler has been developed for high-resolution particle velocimetry in the ocean, in particular for remote measurements of suspended sediment flux and turbulence in the nearshore and continental shelf bottom boundary layer. Acoustic backscatter estimates of suspended particle concentration and velocity are determined simultaneously from the phase and amplitude of the backscattered signal over an O(1 m)-long profile with subcentimeter resolution, at an ensemble-averaged rate of O(25 Hz). To characterize the performance of the profiler as a remote turbulent flux sensor, laboratory experiments were carried out in a particle-laden high-Reynolds number round jet. The results include comparisons between the acoustic Doppler and standard laboratory sensors, and to previous experimental and theoretical results for turbulent jets. The observed mean radial particle flux profile is found to be consistent with the computed mean flux profile for a turbulent jet. The measured entrainment rate is within 10% of the accepted value for turbulent round jets. Energy spectra of the turbulent motions demonstrate the Ϫ5/3 slope characteristic of the inertial subrange. The kinetic energy spectral densities obtained with the Doppler profiler match observations with a Sontek acoustic Doppler velocimeter and are in qualitative agreement with hotfilm measurements within the jet. Space-time domain images of particle flux exhibit well-defined coherent structures. Cospectral analysis demonstrates that these larger structures dominate the particle flux.