Hidekatsu Yamazaki

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.

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 vertical trajectories of motile phytoplankton in a wind-mixed water column

Abstract Motile phytoplankton representing a diatom, two dinoflagellates and a ciliate were studied using a random walk model in a dynamically active mixing layer where turbulence was driven by a range of wind speeds acting on a weakly stratified water column. The simulations extend the results reported in Yamazaki and Osborn (1989) by incorporating a broad range of more complex phytoplankton behavior and by examining the differential Photosynthetically Active Radiation (PAR) exposure of different individuals within a species population under a range of mixing conditions. The respective motility capabilities of the modeled species clearly influence their vertical distributions under the same mixing conditions even at the highest wind condition considered. The wind energy required to distort the diel vertical migration significantly is related directly to their swimming capability. The portion of the initial phytoplankton population that is mixed to the bottom of the Ekman layer can experience PAR stress. The simulations show that turbulence in the mixing layer does not necessarily create a physiologically uniform state after 24 h; this differs from the conventional wisdom concerning the effect of turbulence in mixed layers. The difference reflects depth-dependent turbulence intensity inferred from the Ekman layer model, as well as the definition of the Lagrangian diffusion coefficient discussed in our study.

Effects of turbulent shear flow on zooplankton distribution

Abstract Dynamics of the physical environment should help regulate interactions within zooplankton communities that determine their structure and function. Using a submarine, we have made the first simultaneous measurements of turbulent dissipation rate, vertical velocity shear, temperature, salinity, and zooplankton distribution and abundance. Our results suggest that energetic turbulent regimes due to strong winds, combined with an anomalous region of high vertical shear, broaden the vertical distributions of some zooplankton species, resulting in the selective mixing together of species that are vertically separated under less energetic conditions.

Why Oceanic Dissipation Rates Are Not Lognormal

Abstract In their derivation of the lognormal probability density function for volume-averaged dissipation rates, Gurvich and Yaglom assumed explicitly that these dissipation rates are statistically homogeneous and that the averaging scale is small compared to the domain scale of the turbulent flow and large compared to the Kolmogorov scale. Estimates of dissipation rates in the oceanic thermocline reported by various researchers do not, in general, distribute lognormally because these datasets are often not homogeneous, nor is the averaging scale small compared to the scale of the turbulent patches. The conventional method of computing dissipation rates, a spectral technique, is incompatible with the assumptions for a lognormal distribution. Dissipation rates do distribute lognormally when they are computed with an alternative method that is consistent with the assumptions made by Gurvich and Yaglom. The shortest averaging scale that produced a lognormal distribution is three Kolmogorov length scales.

Direct numerical simulation of buoyancy-driven turbulence in stably stratified fluid

We investigate the role of buoyancy force on the generation and decay of random motion in a homogeneously stratified fluid by means of direct numerical simulations (DNS) of the dynamic and thermodynamic equations. The simulations start from a fluid which is at rest but has appreciable temperature fluctuations. Therefore the flow initially evolves by extracting energy from the potential energy field. Three free parameters, the Reynolds number Re , the Prandtl number Pr and the stratification number St , characterize the flow. Among these numbers the stratification number, St = ( l T 0 / T ′ 0 ) (d T R /dz), is the most crucial one for the investigated problem. Here T ′ 0 and l T 0 are the initial r.m.s. temperature and the initial integral temperature lengthscale, respectively, and d T R /dz is the background stratification. St is a measure of the strength of background-temperature gradient compared to the initial mean fluctuating temperature gradient in the fluid. A critical stratification number of order one is found to separate an oscillating, non-turbulent flow from flow states which exhibit features of turbulence. When St > 1, the statistics reveal a nearly linear and strongly anisotropic flow as typical for gravity waves but the flow-field variables behave randomly. When St The stratification number is an easily measurable parameter in field experiments in the ocean as well as in the atmosphere. Therefore St may be a useful indicator of whether a flow regime contains sufficient potential energy to create turbulence.

Stratified Turbulence near a Critical Dissipation Rate

Abstract Turbulence in the thermocline was investigated using data observed by the submarine Dolphin. Various length scales, the Reynolds and the Froude numbers of the Dolphin data were compared to those values of a laboratory experiment presented in Itsweire et al. In addition to these datasets, the data obtained by the sub-mersible Pisces IV provide the range of ϵ/νN2 above the thermocline values. Our thermocline data demonstrate that the dynamical condition of the laboratory experiment is confirmed as identical to the turbulence in the thermocline when ϵ/νN2<100. Therefore, the critical dissipation rate, ϵC = 16νN2, is applicable to the thermocline. However, the applicability of laboratory experiments for high values of ϵ/νN2, such as the generating stage of turbulence, requires a cautious interpretation for turbulence in the thermocline. The power spectrum of velocity closely follows the Nasmyths empirical universal spectrum in the high wavenumbers even when the dissipation rate is as low as ϵC. Addi...

Direct Estimation of Heat Flux in a Seasonal Thermocline

Abstract This paper reports on a direct measurement of the turbulent heat flux. The sampling was from a submarine that used a conventional airfoil probe to measure the vertical component of turbulent velocity and a thermistor probe to measure the temperature fluctuations in a well-defined, shear-driven, turbulent weakly stratified layer. Error analyses indicate that these heat flux estimates are the first statistically significant results. A comparison between the direct measurements and an eddy diffusivity model shows that the flux Richardson number in the shear layer is about 0.05. The dominant heat flux contribution is in the neighborhood of 0.7-cpm wavenumber.

Preferential concentration of marine particles in isotropic turbulence

Abstract The effect of small-scale turbulence on marine and aquatic particle transport has traditionally been to act as a means for creating homogeneous distributions. However, previous numerical simulations of heavy particle transport in turbulent flows have shown that particles are preferentially concentrated by turbulence and that effects of preferential concentration are most pronounced for particle parameters comparable to the Kolmogorov scales. Therefore, the focus of the present work is examination of the preferential concentration of marine particles. Application of Kolmogorov scaling indicates that effects of preferential concentration may be important for marine particles with diameters of order 1 mm in the upper mixed layer. Numerical simulations of unstratified isotropic turbulence are then used to support the notion that preferential concentration of particles possessing material characteristics representative of those encountered in marine environments can occur. In the simulations, particles of order 1 mm diameter are idealized as rigid spheres with a density ratio of 1.005. Simulation results demonstrate preferential concentration with peak particle number densities ranging from 10 to 60 times the global mean value. Implications of preferential concentration are also discussed, together with the limitations of the approach employed in the present study.

Turbulence in the California Undercurrent

Abstract Vertical profiles of microstructure velocity over the San Diego Trough showed enhanced levels of kinetic energy dissipation in the intrusive region between the California Undercurrent and the surface California Current. If the observed rate of dissipation is typical, then the kinetic energy of the undercurrent is extracted with a minimum time scale of 11 days. The time scale for the dissipation of total mechanical energy (kinetic plus potential) and the transit time from southern California to Vancouver Island are comparable. The vertical eddy diffusivity is less than 1.9 × 10−5 m−2 s−1 and is not a factor in the mixing of the undercurrent. The most frequently observed thickness of a turbulent layer is 1–2 m. Layers thinner than 6 m contribute the most to the total dissipation, while thicker and less frequent layers are noticeable contributors.