DETAILED OBSERVATIONS OF A NATURALLY-OCCURRING SHEAR INSTABILITY

Simultaneous profiles of microstructure, horizontal velocity, and acou stic backscatter allow one of the most complete descriptions of a natu rally occurring shear instability to date. Shear increased rapidly aft er passing through a lateral constriction which formed a hydraulic con trol. A kilometer-long set of 20-m-tall billows grew on a middepth den sity interface where the Richardson number fell below 0.25. The veloci ty interface thickened steadily after the billows formed, consistent w ith rapid momentum mixing across a shear layer with a Reynolds number of 3 x 10(6). The billows generated large density overturns and dissip ation rates greater than 10(-5) W kg-1, even within the first large ov erturn, indicating that these structures were fully turbulent early in their development. As the billows grew, a well-mixed layer developed at the interface and survived as an actively turbulent layer for up to 6 buoyancy periods, 3 times longer than in laboratory studies at low Reynolds number. Variations in the mean density of the billows lead us to infer that the vertical offset of the velocity and density interfa ces varied with time where the billows first formed. With data from th e large overturns within the shear layer, we find epsilon/nuN2 almost- equal-to 3 x 10(4), an average root-mean-square overturn scale (L(rms) BAR) of 2.6 m, and a buoyancy scale (L(b)) of 2.7 m. Despite having sa mpled the billows at varying stages of their evolution, we find no ind ication that the ratio L(rms)/L(b) is ever significantly different tha n 1 for this shear instability.