Observations of mixing near the sides of a deep lake in winter

The dissipation rate of turbulent kinetic energy per unit mass, «, vertical eddy diffusivity, Kz, and the rate of dissipation of temperature variance, x, are estimated over the sloping sides of Lake Geneva both far from and near the bed, using temperature data from horizontal tracks of a submarine during periods of winter cooling. The estimated values are about one order of magnitude greater near the slope than those distant from it. The mean dissipation rate per unit mass averaged over the epilimnion varies between O(10210) m2 s23 in calm weather to O(1028) m2 s23 in winds of 8 m s21, for surface buoyancy fluxes of 6 3 1029 m2 s23 and 1.1 3 1028 m2 s23, whereas near the slope « has an average value of 3 3 1028 m2 s23. The relation between Kz and «N22 (where N is the buoyancy frequency) is examined. Values of the mixing efficiency, G 5 «N22K, of 0.15 6 0.1 and 0.16 6 0.121z are obtained in the upper 10-m layer for calm and windy conditions, respectively. Near the slope, G is found to be 0.22 6 0.2, slightly larger than in surface layers under windy conditions. Different mixing mechanisms in different regions, near the slope and in the surface waters of the epilimnion and relatively calm deeper layers, can be identified in the diagram of overturn Froude number versus overturn Reynolds number. Mixing near the slope in the epilimnion appears to be related to the gravitational winter cascading of cold water down the sloping sides of the lake. It is known from measurements that mixing is often more intense in the vicinity of sloping boundaries in the stratifie