Internal wave generation in lakes with very slow flow

A High-Resolution Acoustic Doppler Profiler (HR-ADP), deployed at about 8.5 m depth in Lake Soppensee (Switzerland), observed mean velocities, shear, and internal waves over a depth range of about 4 m. The results suggest that, in spite of maximum velocities less than 15 mm/s, there is shear-induced internal wave generation and mixing around the thermocline. In particular, we observe enhanced internal waves underneath the mixed layer at frequencies bounded by the local buoyancy frequency, and a layer, 0.5-m thick, of critical Richardson's number at the base of the mixed layer. The HR-ADP is a pulse-to-pulse coherent Doppler sonar which uses a new, more robust means of signal processing to obtain velocity. Based on the noise spectrum, we conclude the measurement uncertainty in this experiment ranges from 0.3 to 1 mm/s.