This paper describes the instrumentation and techniques for long-term targeted observation of the centimeter-scale velocity structure within the oceanic surface boundary layer, made possible by the recent developments in capabilities of autonomous platforms and self-contained pulse-coherent acoustic Doppler current profilers (ADCPs). Particular attention is paid to the algorithms of ambiguity resolution (“unwrapping”) of pulse-coherent Doppler velocity measurements. The techniques are demonstrated using the new Nortek Signature1000 ADCP mounted on a Lagrangian float, a combination shown to be capable of observing ocean turbulence in a number of recent studies. Statistical uncertainty of the measured velocities in relation to the ADCP setup is also evaluated. Described techniques and analyses should be broadly applicable to other autonomous and towed applications of pulse-coherent ADCPs.

Upper-ocean turbulence is central to the exchanges of heat, momentum, and gasses across the air/sea interface, and therefore plays a large role in weather and climate. Current understanding of upper-ocean mixing is lacking, often leading models to misrepresent mixed-layer depths and sea surface temperature. In part, progress has been limited due to the difficulty of measuring turbulence from fixed moorings which can simultaneously measure surface fluxes and upper-ocean stratification over long time periods. Here we introduce a direct wavenumber method for measuring Turbulent Kinetic Energy (TKE) dissipation rates, ϵ, from long-enduring moorings using pulse-coherent ADCPs. We discuss optimal programming of the ADCPs, a robust mechanical design for use on a mooring to maximize data return, and data processing techniques including phase-ambiguity unwrapping, spectral analysis, and a correction for instrument response. The method was used in the Salinity Processes Upper-ocean Regional Study (SPURS) to collect two year-long data sets. We find the mooring-derived TKE dissipation rates compare favorably to estimates made nearby from a microstructure shear probe mounted to a glider during its two separate two-week missions for (10−8) ≤ ϵ ≤ (10−5) m2 s−3. Periods of disagreement between turbulence estimates from the two platforms coincide with differences in vertical temperature profiles, which may indicate that barrier layers can substantially modulate upper-ocean turbulence over horizontal scales of 1-10 km. We also find that dissipation estimates from two different moorings at 12.5 m, and at 7 m are in agreement with the surface buoyancy flux during periods of strong nighttime convection, consistent with classic boundary layer theory.

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Acoustic Doppler velocimeters (ADVs) are a valuable tool for making high-precision measurements of turbulence, and moorings are a convenient and ubiquitous platform for making many kinds of measurements in the ocean. However, because of concerns that mooring motion can contaminate turbulence measurements and that acoustic Doppler profilers make middepth velocity measurements relatively easy, ADVs are not frequently deployed from moorings. This work demonstrates that inertial motion measurements can be used to reduce motion contamination from moored ADV velocity measurements. Three distinct mooring platforms were deployed in a tidal channel with inertial-motion-sensor-equipped ADVs. In each case, motion correction based on the inertial measurements reduces mooring motion contamination of velocity measurements. The spectra from these measurements are consistent with other measurements in tidal channels and have an slope at high frequencies—consistent with Kolmogorov’s theory of isotropic turbulence. Motion correction also improves estimates of cross spectra and Reynolds stresses. A comparison of turbulence dissipation with flow speed and turbulence production indicates a bottom boundary layer production–dissipation balance during ebb and flood that is consistent with the strong tidal forcing at the site. These results indicate that inertial-motion-sensor-equipped ADVs are a valuable new tool for making high-precision turbulence measurements from moorings.

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