Rolf G. Lueck

A New Free-Fall Profiler for Measuring Biophysical Microstructure

This paper evaluates the performance of a newly developed free-falling microstructure profiler. The instrument is equipped with standard turbulence sensors for measuring turbulent velocity shear and temperature gradient, as well as bio-optical sensors for measuring in situ chlorophyll and turbidity variations. Simultaneous measurements with this profiler and an acoustic Doppler velocimeter were carried out in a flow tank, and data from both instruments agreed well. Turbulence spectra computed from both instruments agreed with the Kolmogorov inertial subrange hypothesis over approximately two decades in wavenumber space. Data from field tests conducted with the profiler showed that turbulence spectra measured in situ agreed with the empirical Nasmyth spectrum when corrections were made for the shear probe’s spatial averaging. Dissipation rates as low as 5 3 10210 Wk g 21 were resolved when certain precautions were taken to avoid spectral bias caused by instrument vibrations. By assuming a universal form of the turbulence spectrum, turbulent kinetic energy dissipation rates below 5 3 10210 Wk g 21 can be estimated. The optical sensors resolved centimeter-scale structures of in vivo fluorescence and backscatter in field measurements.

USING A BROADBAND ADCP IN A TIDAL CHANNEL. PART II: TURBULENCE

Abstract A four-transducer, 600-kHz, broadband acoustic Dopple current profiler (ADCP) was rigidly mounted to the bottom of a fully turbulent tidal channel with peak flows of 1 m s−1. Rapid samples of velocity data are used to estimate various parameters of turbulence with the covariance technique. The questions of bias and error sources, statistical uncertainty, and spectra are addressed. Estimates of the Reynolds stress are biased by the misalignment of the instrument axis with respect to vertical. This bias can be eliminated by a fifth transducer directed along the instrument axis. The estimates of turbulent kinetic energy (TKE) density have a systematic bias of 5 × 10−4 m2 s−2 due to Doppler noise, and the relative statistical uncertainty of the 20-min averages is usually less than 20%–95% confidence. The bias in the Reynolds stress due to Doppler noise is less than ±4 × 10−5 m2. The band of zero significance is never less than 1.5 × 10−5 m2 s−2 due to Doppler noise, and this band increases with incre...

Mediterranean outflow mixing and dynamics

The Mediterranean Sea produces a salty, dense outflow that is strongly modified by entrainment as it first begins to descend the continental slope in the eastern Gulf of Cadiz. The current accelerates to 1.3 meters per second, which raises the internal Froude number above 1, and is intensely turbulent through its full thickness. The outflow loses about half of its density anomaly and roughly doubles its volume transport as it entrains less saline North Atlantic Central water. Within 100 kilometers downstream, the current is turned by the Coriolis force until it flows nearly parallel to topography in a damped geostrophic balance. The mixed Mediterranean outflow continues westward, slowly descending the continental slope until it becomes neutrally buoyant in the thermocline where it becomes an important water mass.

Oceanic Velocity Microstructure Measurements in the 20th Century

The science of ocean turbulence was started more than 50 years ago by a small research group using a surplus mine-sweeping paravane to measure the velocity and temperature fluctuations in the ocean. The field has grown considerably and measurements are now conducted by researchers in many countries. A wide variety of sophisticated instrument systems are used to profile horizontally and vertically through the marine environment. Here we review the historical development of velocity micro-structure profiles over the past four decades and summarize the basic requirements for successful measurements. We highlight critical technological developments and glance briefly at some of the scientific discoveries made with these instruments.

The logarithmic layer in a tidal channel

Abstract Twenty-minute averaged velocity profiles taken with a bottom-mounted ADCP in a 30-m deep tidal channel have been fitted to a logarithmic form with 1 % accuracy. The height of the log-layer varies tidally and reaches 20 m above the bottom during peak flows of 1 m s−1. The height is well predicted by 0.04 u*/ω, where u* is the friction velocity and ω is the angular frequency of the dominant tidal constituent. The mean non-dimensional shear,(∂U/∂z)(u*/κz), is within 1 % of unity at the 95% level of confidence inside the log-layer. The friction velocity varies tidally and reachesO(0.05) m s−1 during peak current flow. The bottom drag coefficient referring to the depth-mean flow is 4 × 10−3. The observed log-layer is not connected to the skin friction, but possibly to the form drag. Deviations of the measured velocity from the logarithmic profiles above the log-layer can be explained by the zero-stress boundary condition at surface and by the entrainment of shallow water at mid-depth. The deviations are inconsistent with the effects of acceleration/deceleration and stratification.

Thermal Inertia of Conductivity Cells: Observations with a Sea-Bird Cell

Abstract We have examined the magnitude and relaxation time of the thermal anomaly of the fluid flowing through the conductivity cell manufactured by Sea-Bird Electronics (SBE) that is induced by the heat stored in the wall of this cell using oceanic data collected in a thermohaline staircase. The relaxation is 9 to 10 s, about twice the value predicted by Lueck, while the initial magnitude of the conductivity error is 2.8%, about 35% smaller than predicted. The error in the measured conductivity is significant and long-lived and results in density errors detectable for 45 s after the sensor enters an isopycnal layer. An efficient numerical algorithm removes the anomaly from the measured conductivity signal and clears the resulting error from the computed salinity and density.

On Measuring the Terms of the Turbulent Kinetic Energy Budget from an AUV

Abstract The terms of the steady-state, homogeneous turbulent kinetic energy budgets are obtained from measurements of turbulence and fine structure from the small autonomous underwater vehicle (AUV) Remote Environmental Measuring Units (REMUS). The transverse component of Reynolds stress and the vertical flux of heat are obtained from the correlation of vertical and transverse horizontal velocity, and the correlation of vertical velocity and temperature fluctuations, respectively. The data were obtained using a turbulence package, with two shear probes, a fast-response thermistor, and three accelerometers. To obtain the vector horizontal Reynolds stress, a generalized eddy viscosity formulation is invoked. This allows the downstream component of the Reynolds stress to be related to the transverse component by the direction of the finescale vector vertical shear. The Reynolds stress and the vector vertical shear then allow an estimate of the rate of production of turbulent kinetic energy (TKE). Heat flux ...

Using a Broadband ADCP in a Tidal Channel. Part I: Mean Flow and Shear

This paper discusses the principles of measuring the mean velocity and its vertical shear in a turbulent flow using an acoustic Doppler current profiler (ADCP), and presents an analysis of data gathered in a tidal channel. The assumption of horizontal homogeneity of the first moments is fundamental to the derivation of the mean velocity vector because the velocity is never homogeneous over the span of the beams in a turbulent flow. Two tests of this assumption are developed—a comparison of the mean error velocity against its standard deviation and against the mean speed. The fraction of the samples that pass these tests increases with increasing spatial averaging and exceeds 95% for distances longer than 55 beam separations. The statistical uncertainty of the velocity and shear vector, averaged over 10 min and longer, stems from turbulent fluctuations rather than Doppler noise. Estimation of the vertical velocity requires a correction for the bias in the measured tilt. The mean velocity and shear estimates from this natural tidal channel show more complex depth‐time variations than found in idealized one-dimensional channel flow, which seldom occurs in nature. The ADCP measurements reveal the secondary circulation, bursts of up- and downwelling, shear reversals, and transverse velocity shear.

Stress on the Mediterranean Outflow Plume: Part II. Turbulent Dissipation and Shear Measurements

Abstract Bottom and interfacial stresses on the Mediterranean outflow plume are estimated using vertical profiles of turbulent dissipation and velocity collected in the Gulf of Cadiz. Turbulent dissipation is high throughout the plume, with a local minimum often present near the plume nose (depth of maximum downstream velocity). Bottom stresses are estimated by applying a log-layer. model to the dissipation measurements. The dissipation measurements are also divided by plume-scale vertical shear from the horizontal velocity profiles to construct profiles of stress within the plume. The mean stress estimates in the bottom layer agree well with those calculated in the log layer from the dissipation measurements alone. The bottom-layer means are slightly larger than those of the interfacial layer. The maximum stresses in each layer are uncertain, since they depend on the ill-defined shape of the stress profiles within the plume. Dissipation-derived stress estimates in the log layer and those from dissipation...

Turbulent Dissipation Over the Continental Slope Off Vancouver Island

Abstract Thirteen profiles of the rate of viscous dissipation of turbulent kinetic energy &epsi¯ were made over the continental slope off Vancouver Island between 12 and 14 May 1980 in conjunction with CTD and moored current-meter observations. Systematic variability was observed in the vertical but not in the horizontal direction. Above 200 m depth numerous salt-stabilized temperature inversions were seen and dissipation rates were significantly larger than below 200 m. Dissipation rates below 200 m are the lowest ever reported and coincide with a low level of energetics revealed by the current meter moorings. Comparison with the Garrett-Munk internal wave spectrum indicates an e-folding decay time of internal wave energy of ∼50 days at depths below 200 m.