Potential and Limitations of a Commercial Broadband Echo Sounder for Remote Observations of Turbulent Mixing
Julia MuchowskiLars UmlaufLars ArneborgPeter HoltermannElizabeth WeidnerChristoph HumborgChristian Stranne
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Abstract Stratified oceanic turbulence is strongly intermittent in time and space, and therefore generally underresolved by currently available in situ observational approaches. A promising tool to at least partly overcome this constraint are broadband acoustic observations of turbulent microstructure that have the potential to provide mixing parameters at orders of magnitude higher resolution compared to conventional approaches. Here, we discuss the applicability, limitations, and measurement uncertainties of this approach for some prototypical turbulent flows (stratified shear layers, turbulent flow across a sill), based on a comparison of broadband acoustic observations and data from a free-falling turbulence microstructure profiler. We find that broadband acoustics are able to provide a quantitative description of turbulence energy dissipation in stratified shear layers (correlation coefficient r = 0.84) if the stratification parameters required by the method are carefully preprocessed. Essential components of our suggested preprocessing algorithm are 1) a vertical low-pass filtering of temperature and salinity profiles at a scale slightly larger than the Ozmidov length scale of turbulence and 2) an automated elimination of weakly stratified layers according to a gradient threshold criterion. We also show that in weakly stratified conditions, the acoustic approach may yield acceptable results if representative averaged vertical temperature and salinity gradients rather than local gradients are used. Our findings provide a step toward routine turbulence measurements in the upper ocean from moving vessels by combining broadband acoustics with in situ CTD profiles.Keywords:
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If a fluid's potential density decreases continuously with height then it can support internal waves that, like interfacial waves, move up and down due to buoyancy forces but which are not confined to an interface: they can move vertically through the fluid. This chapter focuses upon the dynamics of small-amplitude internal waves in uniformly stratified, stationary fluid. It also examines some effects of shear in non-uniform stratification, with a more general treatment given in Chapter 6.
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Geophysical fluid dynamics
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Pycnocline
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The effects of three distinct stratifications on density current dynamics are investigated using a nonhydrostatic numerical model: (i) a stably stratified layer underneath a deep neutrally stratified flow, representing a nocturnal boundary layer over land; (ii) a neutrally stratified layer underlying a deep stably stratified flow, representing a daytime boundary layer; and (iii) a continuously stratified atmosphere. In the first case, a weak or intermediate stratification decreases the height of density currents and increases the propagation speed. The same result holds in strongly stratified situations as long as the generated disturbances in the neighborhood of the head do not propagate away. Classical density currents occur in weak stratification, multiheaded density currents in intermediate stratification, and multiheaded density currents with solitary wave–like or borelike disturbances propagating ahead of the current in strong stratification. In the second case, the upper-layer stratification consistently reduces the density-current height and its propagation speed. The simulated system resembles laboratory density currents and is not much affected by the overlying stratification. Finally, in continuously stratified flow, the effect of stratification is similar to the second case. The density current becomes shallower and moves more slowly as the stratification is increased. The modeled system has the basic features of density currents if the stratification is weak or moderate, but it becomes progressively less elevated as stratification increases. In strong stratification the density current assumes a wedgelike structure. The simulation results are compared with the authors' previously obtained analytical results, and the physical mechanisms for the effect of stratification are discussed.
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An internal soliton in two-layered stratified fluids has been investigated by means of flow visualization. The relationship between the phase speed of soliton and its amplitude has been obtained experimentally. It has been also found that steady waves form behind the internal soliton traveling alongthe interface of a stratified fluid. The waves are of similar characteristics tothe lee waves which are observed behind a mountain in stratified atmosphericflow.
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The internal wave brings in non-trivial influence in the ocean. A large number of investigations have focused on the internal wave in linear stratification, which has a constant buoyancy frequency representing a typical and widespread stratification configuration in the ocean. For the non-constant buoyancy frequency configuration, a typical situation in pycnocline in the ocean, the behavior of the internal wave has been far less understood and is experimentally investigated in this study. We use the synthetic schlieren technique to study the internal wave in three cases of characteristic stratification. The cross-beam profiles and the intensity decay of the internal wave are particularly examined and compared with those of the linear stratification.
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Based on the measurements of the velocity and the environmental factors of seawater at the P2 anchor station on the East China Sea inner continental shelf in spring,it was found that strong stratification(with a value of buoyancy frequency of 1×10?3 s?2) is caused by the interface between coastal water and shelf mixing water.The variations of temperature and salinity in the water column are related to the flow field.Under the influence of stratification,distribution of environmental factors presents a two-layered structure,above and below the pycnocline.Internal waves consisted of semidiurnal internal tides and near inertial waves are simultaneously generated by the stratification.The time when stratification layer jumps up and Ri?1 4 exists in the pycnocline corresponds to that when cross-stratification of chlorophyll and dissolved oxygen takes place.This indicates internal waves provide environmental factors that energize the cross-stratification mixing.Stratification and internal waves work together toward vertical mixing of environmental factors;the former inhibits,and the latter enhances mixing.
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Temperature salinity diagrams
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When ocean's internal tidal waves “beach” at underwater topography, they transform from more or less linear into highly nonlinear waves that can break with generation of vigorous turbulent mixing. Although most mixing occurs in the half hour around a steep (bottom-)front leading the upslope moving internal tide phase, relatively large mixing also occurs some distance of several tens of meters off the bottom, just prior to the downslope moving internal tide phase and initiated by high-frequency “small-scale” internal waves. Details of this off-bottom small-scale mixing in a stratified natural environment and some of its variability per tidal period are presented here in two case studies using high-resolution temperature observations (61 sensors at 1 m intervals; <10−3 °C precision) at a 969 m deep site south of New Zealand. The observations shed some light on stratified turbulence that is generated in a relatively thick (∼30 m) weakly stratified layer and in the strongly stratified interfaces above and below. The interfacial internal waves generate turbulence with largest dissipation rate and temperature variance at the edge of the upper interface and the weakly stratified layer. When these waves steep nonlinearly, immediate moderate turbulence generation is observed below, throughout the weakly stratified layer. Largest turbulence is generated by 25 m high asymmetric Holmboe overturns.
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Mixed layer
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The flow field of a three-dimensional, time-dependent thermal plume in stably stratified ambient above a heated square plate was elucidated by solving numerically governing equations. Phenomena characterizing this plume are as follows : (a) Velocity vectors on a horizontal plane above the heated plate have a X-shape distribution at any stratification. (b) The local Nusselt number distribution on the plate was affected by the X-shape distribution. (c) In Velocity vectors on the vertical plane, a V-shape occurred near the plate, and a horizontal imterface occurred between inward and outward flows. Time-dependency in the plume is resulted from instability near the front with negative buoyancy, and became weak with increasing stratification. At the extremely strong stratification, flow became steady state. The flow at strong stratification differed clearly from that at medium stratification as follows : At the medium stratification, the upward flow interfered with the stratified air, and a Π-shape of the velocity vector distribution occurred near the front. However, at strong stratification, the stratification effect is more dominant than the upward flow effect, and a Λ-shape occurred. With increasing stratification, the flow velocity near the plate became small, and the mean Nusselt number decreased.
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