Diapycnal diffusivities in homogeneous stratified turbulence

Quantifying diapycnal mixing in stably stratified turbulence is fundamental to the understanding and modeling of geophysical flows. Data of diapycnal mixing from direct numerical simulations of homogeneous stratified turbulence and from grid turbulence experiments, are analyzed to investigate the scaling of the diapycnal diffusivity. In these homogeneous flows the instantaneous diapycnal diffusivity is given exactly by K-d = epsilon(rho)/(partial derivative(rho) over bar/partial derivative z)(2) where epsilon(rho) is the dissipation rate of density fluctuations, and partial derivative(rho) over bar/partial derivative z is the mean density gradient. The diffusivity K-d may be expressed in terms of the large scale properties of the turbulence as K-d = gamma L-E(2)/T-L, where L-E is the Ellison overturning length-scale, T-L is the turbulence decay time-scale, and gamma is half the mechanical to scalar time-scale ratio. Our results show that L-E and T-L can explain most of the variations in K-d over a wide range of shear and stratification strengths while gamma remains approximately constant. Citation: Stretch, D. D., and S. K. Venayagamoorthy (2010), Diapycnal diffusivities in homogeneous stratified turbulence, Geophys. Res. Lett., 37, L02602, doi:10.1029/2009GL041514.