Winter cascading of cold water in Lake Geneva

Abstract

During the winters of 1998 and 1999, observations were made of the cascading of cold water from the nearshore, shallow “shelf” zones and down the sloping sides of Lake Geneva. Cascading starts on the average 10 hours after the onset of surface cooling. The draining cold water descends like a gravity current, and the downslope speed of the head of these slugs of cold water, U , has a mean value of 5.2 cm s −1 , with slugs persisting, on the average, for 8 hours. When the Monin‐Obukov length scale at the water surface, L , is negative, implying convection occurs, and /| L |> 1,where is the mean shelf depth, the nondimensionalized speed of the front of “slugs,” U / b 1/3 is found to be 1.3 ± 0.4, where b is the surface buoyancy flux integrated over the time period from one slug to the next. Each slug is unsteady, the head being followed by several fronts in which the temperature of the current decreases and its thickness increases. These fronts travel faster than the mean flow by a factor of r = 1.38 ± 0.3. Dynamical similarities are found with roll waves observed in turbulent open channel flows. The circulation induced by the cascade is found to give a positive skewness to the time derivatives of near‐surface temperature in shallow waters, in contrast with negative values close to the slope. The volume of cold water carried by a slug increases with downslope distance as a consequence of turbulent entrainment and the contribution of convectively unstable plumes from the surface. The average volume carried by the slug across the 21 m depth contour is about 1.9 times the volume of water in shallower water (i.e., that on the shelf between shore and a depth of 21 m), implying that cascading is an efficient means of flushing shelf water. Integrated around the lake the mean total volume flux amounts to 11.5 the average winter river inflow.