Mixing and turbulent fluxes in the metallimnion of a stratified lake

Turbulent mixing within the metalimnion of a stratified lake was investigated using a portable flux profiler (PFP) capable of resolving all three components of the velocities, the conductivity, and the temperature microstructure. Presented is a detailed description of the techniques used in the data processing, particularly in the separation of the turbulence from the internal wave signal. The sampling, carried out in Lake Kinneret (Israel) during the summer for 3 consecutive years, showed that most of the time the vertical flux through the metalimnion was negligible, but, at times, the eddy diffusivity did reach values as high as 10(-2) m(2) s(-1). A comparison between direct measurement of the vertical fluxes obtained from the PFP data with that from indirect estimates of the fluxes shows good agreement for the 6 less than or equal to Fr(gamma) < 100 range. Scaling of the turbulence based on Fr() and Ri reveal two classes of turbulent regimes: (1) due to traumata characteristic of internal wave-wave interaction and, another, (2) more energetic and due to shear-driven turbulence. The PFP penetrated the water relatively slowly (0.1 m s(-1)) allowing the measurement of temperature fluctuations down to 1 mm and, at the same time, also providing information of the velocity fluctuations. This is different from previous oceanographic measurements, which are always gathered with instrument traversing the water column at velocities closer to 1 m s(-1), preventing regime (1) from being detected.