Mixing and turbulent fluxes in the metallimnion of a stratified lake

Turbulent mixing within the metalimnion of a stratified lake was investigat ed using a portable flux profiler (PFP) capable of resolving all three comp onents of the velocities, the conductivity, and the temperature microstruct ure. Presented is a detailed description of the techniques used in the data processing, particularly in the separation of the turbulence from the inte rnal wave signal. The sampling, carried out in Lake Kinneret (Israel) durin g the summer for 3 consecutive years, showed that most of the time the vert ical flux through the metalimnion was negligible, but, at times, the eddy d iffusivity did reach values as high as 10(-2) m(2) s(-1). A comparison betw een direct measurement of the vertical fluxes obtained from the PFP data wi th 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 bas ed on Fr-<gamma> 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 temperat ure fluctuations down to 1 mm and, at the same time, also providing informa tion of the velocity fluctuations. This is different from previous oceanogr aphic measurements, which are always gathered with instrument traversing th e water column at velocities closer to 1 m s(-1), preventing regime (1) fro m being detected.