Eddies and waves in rotating stratified turbulence

Annick Pouquet
Research Scientist, Laboratory for Atmospheric and Space Physics, Boulder CO Senior Scientist Emeritus, CISL/NCAR

In a stratified fluid, the velocity field couples to density fluctuations, supporting inertia-gravity waves in the presence of rotation, with an anisotropic dispersion relation, and leading to a variety of turbulence regimes resulting from the interactions between nonlinear eddies and waves. What are the delimiting factors of these regimes, and what differentiate them? Several issues will be briefly discussed, such as structures which lead to dissipation related to dual energy cascades.

Using simple classical models of turbulence, it can be demonstrated phenomenologically and numerically that strong localized instabilities lead to an effective dissipation in rotating stratified turbulence which is proportional to the Froude number (ratio of the wave period to the eddy turn-over time). It is correlated to a high kurtosis of the vertical velocity, as found e.g. in the atmosphere. This law also governs the ratio of the amount of energy going, in a constant-flux solution, to the large scales because of rotation to that going to the small scales because of stratification. It thus determines the mixing efficiency in such flows, and it allows to bridge the gap between strong-wave and strong-eddy flow systems in a simple manner.