Enhancement of turbulence through lateral spreading in a stratified-shear flow: Development and assessment of a conceptual model

Abstract

A conceptual model for the impact of lateral spreading processes on local stratified-shear turbulence is presented and evaluated using field data from the rapidly spreading near-field region of the Merrimack River plume. The conceptual model addresses increases in turbulent kinetic energy associated with the stretching of a Kelvin-Helmholz billow along its rotational axis. A non-dimensional relationship is derived with two fit parameters, including a parameter representing the magnitude of mixing under the no spreading case, and a non-dimensional time scale. For the model to be viable, this time scale, representing the period of time that a Kelvin-Helmholz billow is subject to stretching should be of the same order as the evolution time scale for the billow. Observational data from field experiments are used to evaluate the two fit parameters, indicating a stretching time scale of the same order as evolution time scales reported in the literature. In addition, application of the model to the field data reduces observed variability. We conclude that spreading is an important mechanism capable of enhancing local stratified-shear turbulence. Copyright 2012 by the American Geophysical Union.