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 ϫ 10 Ϫ10 W kg Ϫ1 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 ϫ 10 Ϫ10 W kg Ϫ1 can be estimated. The optical sensors resolved centimeter-scale structures of in vivo fluorescence and backscatter in field measurements.
Microstructure measurements were made in the Mixed Water Region of the Oyashio/Kuroshio/Tsugaru currents system where both turbulence and double diffusion are involved in mixing. While intense turbulence is observed near the front between the Oyashio and the Tsugaru Current, double diffusion occupies a noticeable fraction in both the Tsugaru Water and the Mixed Water between the Oyashio and the Kuroshio. After determining a criterion to distinguish double diffusion from turbulence, vertical diffusivities and buoyancy fluxes are estimated using microstructure data. When turbulence is weak, double diffusion is observed around temperature and salinity anomalies, partly due to interleaving, and dominates the buoyancy flux. Vertical diffusivities due to double diffusion are parameterized as a function of the 10-m-scale density ratio. The 10-m-scale diffusivity estimates are consistent with the microstructure data when an appropriate criterion to reproduce a probability density function for the Turner angle is applied. A weighted-average diffusivity model is proposed to account properly for turbulence and double diffusion simultaneously.
The ability to estimate the rate of dissipation (ε) of turbulent kinetic energy at middepth in a high-speed tidal channel using broadband acoustic Doppler current profilers (ADCPs) is assessed by making comparisons to direct measurements of ε obtained using shear probes mounted on a streamlined underwater buoy. The investigation was carried out in Grand Passage, Nova Scotia, Canada, where the depth-averaged flow speed reached 2 m s−1 and the Reynolds number was 8 × 107. The speed bin–averaged dissipation rates estimated from the ADCP data agree with the shear probe data to within a factor of 2. Both the ADCP and the shear probe measurements indicate a linear dependence of ε on the cube of the flow speed during flood and much lower dissipation rates during ebb. The ebb–flood asymmetry and the small-scale intermittency in ε are also apparent in the lognormal distributions of the shear probe data. Possible sources of bias and error in the ε estimates are investigated, and the most likely causes of the discrepancy between the ADCP and shear probe estimates are the cross-channel separation of the instruments and the high degree of spatial variability that exists in the channel.
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