Turbulence and mixing estimates using a towed horizontal microstructure profiler.

Paul Macoun

The Ocean Turbulence Laboratory

University of Victoria

The Purpose

Mixing is the key element in the redistribution of salt, heat and energy in the ocean. Understanding the mechanisms and magnitude of turbulence and other mixing processes is crucial to the improvement of models that predict oceanic and atmospheric change. Our elusive goal is the parameterization of the vertical fluxes of salt and heat based upon the physics of oceanic mixing processes.

The horizontal profiler TOMI (Towed Ocean Microstructure Instrument) is used to make simultaneous measurements of temperature, conductivity, vertical velocity, and their fluctuations. The measurement of vertical velocity provides a means of estimating the rate of dissipation of turbulent kinetic energy. The respective products of instantaneous velocity and temperature, and that of velocity and conductivity, provide a direct measure of the vertical fluxes of heat and salt.

Simultaneous vertical profiles of current and its shear from a ship mounted ADCP (Acoustic Doppler Current Profiler) provide a means to relate the measured fluxes to the Froude Number and the Buoyancy Reynolds Number over a wide range of these parameters. The efficiency of mixing, the ratio of buoyancy-to-kinetic energy production, reaches 0.5 in near coastal regions and is generally much larger than the value of 0.2 assumed to apply to the open ocean.

This analysis is to be the first task to undertake with respect to the data set collected at Race Rocks on August 17th, 2000. At present, the processing is at a preliminary stage.

The Instrument

TOMI is a towed oceanographic vehicle designed to measure turbulence in both shallow and deep water. TOMI can support a multitude of instruments, from sonar to underwater cameras. Its primary oceanographic and electronic sensors are as follows:

1) Four airfoil shear probes and two fast-response thermistors on the nose.

2) A seabird temperature and conductivity sensor adjacent to the nose probes.

3) A seabird temperature and conductivity sensor, and a flowmeter, on the upper mast.

4) A seabird temperature and conductivity sensor, and a flowmeter, on the lower mast.

5) A pressure sensor mounted mid-body, internally.

6) An inertial motion sensor package, internal.

The sample rates for these instruments vary from 64 samples per second up to 512 samples per second. The airfoil shear probe is a unique instrument that measures the cross-stream component of velocity. The force generated by cross-stream flow bends a tiny ceramic bi-morph beam, which generates a voltage that is amplified and recorded. The motion sensor contains accelerometers and gyros. Its presence is required because of low frequency body motion contamination that finds its way into the shear probe signals. By having a record of the vehicle’s behaviour, the probe signals can be decontaminated through processing.

(Photos to be added)

Return to P. Macoun- Oceanographic Research