High-frequency internal waves in Lake Geneva

Trains of high-frequency internal waves have been observed in the thermocline of Lake Geneva within a few hours after the passing of disturbances which cause a rapid increase or `jump' in the depth of the thermocline. Three possible mechanisms for the source of the internal waves are proposed and compared with available data, (i) that the waves are a soliton packet following the jump, (ii) that the waves are generated as the jump passes over or around a local region of rough topography, and (iii) that the jump produces a moving disturbance near the shoreline with internal waves forming a wake behind it. The investigation of the last possibility involves the solution of the problem of the form of an internal wave wake in the presence of mean currents which vary in depth both in their strength and direction. The wave pattern of the wake in the observed current distribution and stratification of the lake is described and compared with that in the absence of shear. An expression for the energy of internal waves propagating in a shear flow is also derived, and it is found that the rate of energy loss from the jump to the waves is small. The minimum Richardson number resulting from the co-existence of the mean shear and the internal waves is marginally greater than $\frac{1}{4}$, suggesting that the waves may be limited by shear-flow instability. However, none of the mechanisms for their generation and propagation is fully compatible with the available data.