Length scales of turbulence in stably stratified mixing layers

Turbulence resulting from Kelvin-Helmholtz instability in layers of localiz ed stratification and shear is studied by means of direct numerical simulat ion. Our objective is to present a comprehensive description of the turbule nce evolution in terms of simple, conceptual pictures of shear-buoyancy int eraction that have been developed previously based on assumptions of spatia lly uniform stratification and shear. To this end, we examine the evolution of various length scales that are commonly used to characterize the physic al state of a turbulent flow. Evolving layer thicknesses and overturning sc ales are described, as are the Ozmidov, Corrsin, and Kolmogorov scales. The se considerations enable us to provide an enhanced understanding of the rel ationships between uniform-gradient and localized-gradient models for shear ed, stratified turbulence. We show that the ratio of the Ozmidov scale to t he Thorpe scale provides a useful indicator of the age of a turbulent event resulting from Kelvin-Helmholtz instability. (C) 2000 American Institute o f Physics. [S1070-6631(00)02206-6].