HIGH-FREQUENCY INTERNAL WAVES IN LAKE-GENEVA

Trains of high-frequency internal waves ha ire been observed in the th ermocline of Lake Geneva within a few hours after the passing of distu rbances which cause a rapid increase or 'jump' in the depth of the the rmocline. Three possible mechanisms for the source of the internal wav es are proposed and compared with available data, (i) that the waves a re a soliton packet following the jump, (ii) that the waves are genera ted as the jump passes over or around a local region of rough topograp hy, and (iii) that the jump produces a moving disturbance near the sho reline with internal waves forming a wake behind it. The investigation of the last possibility involves the solution of the problem of the f orm of an internal wave wake in the presence of mean currents which va ry in depth both in their strength and direction. The wave pattern of the wake in the observed current distribution and stratification of th e lake is described and compared with that in the absence of shear. An expression for the energy of internal waves propagating in a shear fl ow 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 resultin g from the co-existence of the mean shear and the internal waves is ma rginally greater than 1/4 suggesting that the waves may be limited by shear-flow instability. However, none of the mechanisms for their gene ration and propagation is fully compatible with the available data.