Cynthia Bluteau

Estimating Turbulent Dissipation from Microstructure Shear Measurements Using Maximum Likelihood Spectral Fitting over the Inertial and Viscous Subranges

AbstractA technique is presented to derive the dissipation of turbulent kinetic energy ϵ by using the maximum likelihood estimator (MLE) to fit a theoretical or known empirical model to turbulence shear spectral observations. The commonly used integration method relies on integrating the shear spectra in the viscous range, thus requiring the resolution of the highest wavenumbers of the turbulence shear spectrum. With current technology, the viscous range is not resolved at sufficiently large wavenumbers to estimate high ϵ; however, long inertial subranges can be resolved, making spectral fitting over both this subrange and the resolved portion of the viscous range an attractive method for deriving ϵ. The MLE takes into account the chi-distributed properties of the spectral observations, and so it does not rely on the log-transformed spectral observations. This fitting technique can thus take advantage of both the inertial and viscous subranges, a portion of both, or simply one of the subranges. This flexi...

Determining Mixing Rates from Concurrent Temperature and Velocity Measurements

AbstractOcean mixing has historically been estimated using Osborn’s model by measuring the rate of dissipation of turbulent kinetic energy ϵ and the background density stratification N while assuming a value of the flux Richardson number . A constant is typically assumed, despite mounting field, laboratory, and modeling evidence that varies. This challenge can be overcome by estimating the turbulent diffusivity of heat using the Osborn–Cox model. This model, however, requires measuring the rate of dissipation of thermal variance χ, which has historically been challenging, particularly in energetic flows because the high wavenumbers of the temperature gradient spectra are unresolved with current technology. To overcome this difficulty, a method is described that determines χ by spectral fitting to the inertial-convective (IC) subrange of the temperature gradient spectra. While this concept has been exploited for moored time series, particularly near the bottom boundary, it has yet to be adapted to vertical...

Acquiring Long-Term Turbulence Measurements from Moored Platforms Impacted by Motion

AbstractFor measurements from either profiling or moored instruments, several processing techniques exist to estimate the dissipation rate of turbulent kinetic energy ϵ, a core quantity used to determine oceanic mixing rates. Moored velocimeters can provide long-term measurements of ϵ, but they can be plagued by motion-induced contamination. To remove this contamination, two methodologies are presented that use independent measurements of the instrument’s acceleration and rotation in space. The first is derived from the relationship between the spectra (cospectra) and the variance (covariance) of a time series. The cospectral technique recovers the environmental (or true) velocity spectrum by summing the measured spectrum, the motion-induced spectrum, and the cospectrum between the motion-induced and measured velocities. The second technique recovers the environmental spectrum by correcting the measured spectrum with the squared coherency, essentially assuming that the measured signal shares variance with...

Determining Near‐Bottom Fluxes of Passive Tracers in Aquatic Environments

In aquatic systems, the eddy correlation method (ECM) provides vertical flux measurements near the sediment-water interface. The ECM independently measures the turbulent vertical velocities w′ and the turbulent tracer concentration c′ at a high sampling rate (> 1 Hz) to obtain the vertical flux w′c′ from their time-averaged covariance. This method requires identifying and resolving all the flow-dependent time (and length) scales contributing to w′c′. With increasingly energetic flows, we demonstrate that the ECM’s current technology precludes resolving the smallest flux-contributing scales. To avoid these difficulties, we show that for passive tracers such as dissolved oxygen, w′c′ can be measured from estimates of two scalar quantities: the rate of turbulent kinetic energy dissipation ε and the rate of tracer variance dissipation χc. Applying this approach to both laboratory and field observations demonstrates that w′c′ is well resolved by the new method and can provide flux estimates in more energetic flows where the ECM cannot be used.