Abstract

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.

Abstract

The Lagrangian properties of a high-resolution, three-dimensional, direct numerical simulation of Kelvin– Helmholtz (K–H) instability are examined with the goal of assessing the ability of Lagrangian measurements to determine rates and properties of ocean mixing events. The size and rotation rates of the two-dimensional K–H vortices are easily determined even by individual trajectories. Changes in density along individual trajectories unambiguously show diapycnal mixing. These changes are highly structured during the early phases of the instability but become more random once the flow becomes turbulent. Only 36 particles were tracked, which is not enough to usefully estimate volume-averaged fluxes from the average rates of temperature change. Similarly, time-and volume-averaged vertical advective flux can be estimated to only 20% accuracy. Despite the relatively low Reynolds number of the flow, R _λ ≈ 100, the dissipation rates of energy ɛ and density variance χ are correlated with the spectral levels of transverse velocity and density in an inertial subrange, as expected for high-Reynolds-number turbulence. The Kolmogorov constants are consistent with previous studies. This suggests that these inertial dissipation methods are the most promising techniques for making useful measurements of diapycnal mixing rates from practical Lagrangian floats because they converge rapidly and have a clear theoretical basis.

Abstract

A practical method for determining the CTD thermal-lag correction amplitude α and time constant τ is presented. The method is based upon minimizing the salinity separation of temperature-salinity curves from upcasts and downcasts of a yo-yo sequence of CTD profiles. For the Sea-Bird 9 CTD operated at the 1989 Coordinated Eastern Arctic Experiment O Camp with a 1.75 m s⁻¹ water velocity through the conductivity cell, the optimum coefficients are α = 0.0245 and τ = 9.5 s. These results combined with those of Lueck and Picklo and results obtained from other Sea-Bird CTDs operating at lower flow rates confirm the flow dependence of α and τ predicted by Lueck but indicate that the theoretical constants are too high. Based on the empirical results, the formulas for α and τ as a function of the average velocity V through the cell are found to be α = 0.0264 V ⁻¹ + 0.0135 and τ = 2.7858 V ^−1/2 + 7.1499. where V is in units of meters per second.

Abstract

The development and testing of a new, fast response, profiling gas tension device (GTD) that measures total dissolved air pressure is presented. The new GTD equilibrates a sample volume of air using a newly developed (patent pending) tubular silicone polydimethylsiloxane (PDMS) membrane interface. The membrane interface is long, flexible, tubular, and is contained within a seawater-flushed hose. The membrane interface communicates pressure to a precise pressure gauge using low dead-volume stainless steel tubing. The pressure sensor and associated electronics are located remotely from the membrane interface. The new GTD has an operating depth in seawater of 0–300 m. The sensor was integrated onto an upper-ocean mixed layer, neutrally buoyant float, and used in air–sea gas exchange studies. Results of laboratory and pressure tank tests are presented to show response characteristics of the device. A significant hydrostatic response of the instrument was observed over the depth range of 0–9 m, and explained in terms of expulsion (or absorption) of dissolved air from the membrane after it is compressed (or decompressed). This undesirable feature of the device is unavoidable since a large exposed surface area of membrane is required to provide a rapid response. The minimum isothermal response time varies from (2 ± 1) min near the sea surface to (8 ± 2) min at 60-m depth. Results of field tests, performed in Puget Sound, Washington, during the summer of 2004, are reported, and include preliminary comparisons with mass-spectrometric analysis of in situ water samples analyzed for dissolved N₂ and Ar. These tests served as preparations for deployment of two floats by aircraft into the advancing path of Hurricane Frances during September 2004 in the northwest Atlantic. The sensors performed remarkably well in the field. A model of the dynamical response of the GTD to changing hydrostatic pressure that accounts for membrane compressibility effects is presented. The model is used to correct the transient response of the GTD to enable a more precise measurement of gas tension when the float was profiling in the upper-ocean mixed layer beneath the hurricane.

Abstract

Seven subsurface Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats measured the voltage induced by the motional induction of seawater under Typhoon Fanapi in 2010. Measurements were processed to estimate high-frequency oceanic velocity variance associated with surface waves. Surface wave peak frequency f _p and significant wave height H _s are estimated by a nonlinear least squares fitting to , assuming a broadband JONSWAP surface wave spectrum. The H _s is further corrected for the effects of float rotation, Earth’s geomagnetic field inclination, and surface wave propagation direction. The f _p is 0.08–0.10 Hz, with the maximum f _p of 0.10 Hz in the rear-left quadrant of Fanapi, which is ~0.02 Hz higher than in the rear-right quadrant. The H _s is 6–12 m, with the maximum in the rear sector of Fanapi. Comparing the estimated f _p and H _s with those assuming a single dominant surface wave yields differences of more than 0.02 Hz and 4 m, respectively. The surface waves under Fanapi simulated in the WAVEWATCH III (ww3) model are used to assess and compare to float estimates. Differences in the surface wave spectra of JONSWAP and ww3 yield uncertainties of <5% outside Fanapi’s eyewall and >10% within the eyewall. The estimated f _p is 10% less than the simulated before the passage of Fanapi’s eye and 20% less after eye passage. Most differences between H _s and simulated are <2 m except those in the rear-left quadrant of Fanapi, which are ~5 m. Surface wave estimates are important for guiding future model studies of tropical cyclone wave–ocean interactions.

Abstract

The design and Operation of neutrally buoyant floats that attempt to track the three-dimensional motion of water parcels in highly turbulent regions of the ocean, such as the upper mixed layer, are described. These floats differ from previous floats by combining high drag, a compressibility that nearly matches that of seawater, rapid (1 Hz) sampling, and short-range, high-precision acoustic tracking. Examples of float data are shown with the twin goals of demonstrating the utility of the floats and estimating the accuracy to which they are “Lagrangian.”

The analysis indicates that these floats follow the motion of the surrounding water to better than 0.01 m s⁻¹ under most circumstances. Both the net buoyancy of the float and its finite size contribute to the error. The float's buoyancy is controlled by making its compressibility very close to that of seawater, by making its drag large, by reducing air pockets and bubbles on the float, and by carefully controlling variations in the float's mass and volume between deployments. The float accurately follows that part of the velocity field with Scales much larger than its own size (1 m) but does not follow components with scales smaller than itself. A model of this dependence is presented for turbulent flows.

Several unique measurements are possible with these floats. They measure vertical displacement using pressure and therefore accurately filter out the vertical velocity of surface waves, since linear surface waves have no pressure fluctuations along Lagrangian trajectories. Accurate measurements of vertical velocity in the oceanic mixed layer are therefore possible. This, combined with temperature, can be used to measure vertical heat flux. A compass measures the spin rate of the float and thus the vertical vorticity. In fully turbulent flows with outer scales much larger than the float size, the spectra of both vertical velocity and vorticity scale with ε (the turbulent kinetic energy dissipation) over a wide range of ε values, thus allowing ε to be measured. The floats con, in principle, therefore measure many important properties of turbulent flows even in the presence of surface waves.