Generation of slow large scales in forced rotating stratified turbulence

Numerical simulations are used to study homogeneous, forced turbulence in three-dimensional rotating, stably stratified flow in the Boussinesq approximation, where the rotation axis and gravity are both in the (z) over cap -direction. Energy is injected through a three-dimensional isotropic white-noise forcing localized at small scales. The parameter range studied corresponds to Froude numbers smaller than an O(1) critical value, below which energy is transferred to scales larger than the forcing scales. The values of the ratio N/f range from approximate to 1/2 to infinity, where N is the Brunt-Vaisala frequency and f is twice the rotation rate. For strongly stratified flows (N/f much greater than 1), the slow large scales generated by the fast small-scale forcing consist of vertically sheared horizontal flow. Quasi-geostrophic dynamics dominate, at large scales, only when 1/2 less than or equal to N/f less than or equal to 2, which is the range where resonant triad interactions cannot occur.