Can limited observations recover irreversible turbulent fluxes? A test case using DNS*

Turbulent mixing plays a key role in setting the distribution of heat and other tracers in the ocean. However, quantifying irreversible turbulent mixing in a stratified flow is complicated by reversible motions associated, for example, with internal waves. Given limited space and time coverage, can observations meaningfully recover the irreversible mixing associated with a turbulent event?

Winters & D'Asaro (J. Fluid Mech.(317), 1996) established a framework in which the irreversible heat flux is computed by first globally sorting the buoyancy field and then interpreting turbulent dissipation quantities in that sorted space. This approach has been successfully applied to numerical simulations in which the full flow field is known. Here we apply such an approach to moored turbulence observations by subsampling the output of direct numerical simulations of shear instability in a manner that mimics real instruments. We compare the heat flux inferred from the subsampled data to the true heat flux computed using the full three-dimensional sorted buoyancy field. This comparison lets us evaluate our ability to infer, and refine our estimates of, irreversible heat fluxes in a turbulent flow using limited observational measurements.