Detailed observations of a naturally occurring shear instability

Abstract

Abstract. Simultaneous profiles of microstructure, horizontal velocity, and acoustic backscatter allow one of the most complete descriptions of a naturally occurring shear instability to date. Shear increased rapidly after passing through a lateral constriction which formed a hydraulic control. A kilometer-long set of 20-m-tall billows grew on a middepth density interface where the Richardson number fell below 0.25. The velocity interface thickened steadily after the billows formed, consistent with rapid momentum -mixing across a shear layer with a Reynolds number of 3 x 106. The billows generated large W_ "density overturns and dissipation rates greater than I0-' W kg-1 , even within the first large Woverturn, indicating that these structures were fully turbulent early in their development.1 ,M As the billows grew, a well-mixed layer developed at the interface and survived as an W actively turbulent layer for up to 6 buoyancy periods, 3 times longer than in laboratory 1 5 50 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 interfaces varied with time where ____ the billows first formed. With data from the large overturns within the shear layer, we find e/IN 2 --3 x 10', an average root-mean-square overturn scale (L--' of 2.6 m, and a I%* buoyancy scale (Lb) of 2.7 m. Despite having sampled the billows at varying stages of their evolution, we find no indication that the ratio Lrm/Lb is ever significantly different than 1 for this shear instability.1. Introduction from changes in conditions where the billows first form. The mean density of individual billows varies within While steaming with the current along a deep tidal the set; we propose that this results from changes in channel, we had the good fortune to sample a kilometer-vertical offset of the velocity and density interfaces. The long set of large billows (Plate 1). As the billows faded longitudinal density structure we observe has important on the echo sounder, we turned the ship around to run implications for the final state of the patch of mixed upstream back through them. Because of the increasing fluid produced by the instability. The dissipation rate tidal currents and problems retrieving our microstruc-within the billows varies with billow density, suggesting ture profiler, we fell back and then repositioned several that the interface offset significantly affects the intensity times. But the current swept the billows downstream by of turbulence. us, allowing us to collect a second set of measurements.Section 2 describes our instrumentation and then the This paper is a detailed presentation of our fortuitous setting and spatial organization of the sampling. We encounter. Our simultaneous measurements of micro-give an overview of the data in section 3, including a structure, acoustic backscatter, and horizontal currents simple model of the event that is consistent with the provide a unique realization of energetic, naturally oc-observations. Section 4 a...

Biases in Thorpe-Scale Estimates of Turbulence Dissipation. Part I: Assessments from Large-Scale Overturns in Oceanographic Data

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