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 ˆ-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 ≈1/2 to ∞, where N is the Brunt–Väisälä frequency and f is twice the rotation rate. For strongly stratified flows ( N / f Gt 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 les N / f les 2, which is the range where resonant triad interactions cannot occur.