The Rossby number, Ro=U/L2Ω, is the non-dimensional number characterizing rotating flows. Here U is the characteristic velocity, L the characteristic length scale, and 2Ω is the Coriolis parameter. When Ro → 0 the nonlinearity of the equations of motion becomes weak, and the theories of weak wave interactions apply. The normal modes of the flow can be decomposed into zero-frequency 2D large scale structures and inertial waves (3D).

Rotating turbulent flow experiments and simulations are known to generate large-scale two-dimensional (2D) columnar structures from initially isotropic turbulence. Decaying turbulence simulations show this generation to be dependent on Rossby number, with three distinct regimes appearing. These are the weakly rotating Ro regime, for which the turbulent flow is essentially unaffected by rotation, the intermediate Ro range, characterized by a strong transfer of energy from the wave to the 2D modes (with a peak at around Ro 0.2), and the small Ro range for which the 2D modes receive less and less energy from the wave modes as Ro → 0.