Some new insights for inferring diapycnal (irreversible) diffusivity in stably stratified turbulence

New insight for inferring diapycnal diffusivity in stably stratified turbulent flows is obtained based on physical scaling arguments and tested using high-resolution direct numerical simulation data. It is shown that the irreversible diapycnal diffusivity can be decomposed into a diapycnal length scale that represents an inner scale of turbulence and a diapycnal velocity scale. Furthermore, it is shown that the diapycnal length scale and velocity scale can be related to the measurable Ellison length scale (L-E) that represents outer scale of turbulence and vertical turbulent velocity scale (w ') through a turbulent Froude number scaling analysis. The turbulent Froude number is defined as Fr=epsilon/Nk, where epsilon is the rate of dissipation of turbulent kinetic energy, N is the buoyancy frequency, and k is the turbulent kinetic energy. The scaling analysis suggests that the diapycnal diffusivity K-rho similar to w ' L(E )in the weakly stratified regime (Fr > 1) and K-rho similar to(w ' L-E)xFr for the strongly stratified regime (Fr < 1). Published under an exclusive license by AIP Publishing.