Richardson Number

The Richardson number measures the ratio of stabilizing buoyancy to destabilizing velocity shear in a stratified fluid; the gradient form is N^2 divided by the square of the vertical shear, where N is the Brunt-Vaisala frequency. Below a critical value near 0.25, shear instability overcomes stratification and turbulence and mixing set in. It is the standard predictor for the onset of shear-driven mixing at the base of the mixed layer and in equatorial jets.