Scaling the Mixing Efficiency of Sediment-Stratified Turbulence

Abstract

The flux Richardson number Rf, also called the mixing efficiency of stratified turbulence, is important in determining geophysical flow phenomena such as ocean circulation and air-sea transports. Measuring Rf in the field is usually difficult, thus parameterization of Rf based on readily observed properties is essential. Here, estimates of Rf in a strongly turbulent, sediment-stratified estuarine flow are obtained from measurements of covariance-derived turbulent buoyancy fluxes (B) and spectrally fitted values of the dissipation rate of turbulent kinetic energy (ε). We test scalings for Rf in terms of the buoyancy Reynolds number (Reb), the gradient Richardson number (Ri), and turbulent Froude number (Frt). Neither the Reb-based nor the Ri-based scheme is able to describe the observed variations in Rf, but the Frt-based parameterization works well. These findings support further use of the Frt - based parameterization in turbulent oceanic and estuarine environments.