Vertical structure of currents in western Lake Constance

Simultaneous measurements of current velocity and density profiles at the S ill of Mainau (western Lake Constance) enabled the observation of the inter nal response of the lake and the calculation of gradient Richardson number over the entire water column at the sampling site. Periodic changes of curr ent direction corresponded with predicted periods of the two-layer oscillat ion and could be confirmed with thermocline oscillation data. In general, t he current profiles confirmed a two-layer structure. However, an activating wind of speeds higher than 4 m/s produced three-layer profiles for a perio d of several hours at the sampling site, when the velocities due to the two -layer oscillation traversed zero. At times the implied current shear produ ced supercritical values of gradient Richardson number on a vertical scale of a few meters. This occurred during winds in the epilimnion and after str ong (> 4 m/s) winds in the lower thermocline and the hypolimnion. In the 20 m above the lake bed, topographical conditions helped the gradient Richard son number to go supercritical sporadically. Across the thermocline, no sup ercritical gradient Richardson numbers could be confirmed at the measuring site. A lower boundary for vertical mixing in the measuring site due to cur rent shear could be evaluated.