  Traditionally, turbulence energetics is characterized by turbulent kinetic
energy (TKE) and modelled using solely the TKE budget equation. In stable
stratification, TKE is generated by the velocity shear and expended through
viscous dissipation and work against buoyancy forces. The effect of
stratification is characterized by the ratio of the buoyancy gradient to
squared shear, called Richardson number, Ri. It is widely believed that at Ri
exceeding a critical value, Ric, local shear cannot maintain turbulence, and
the flow becomes laminar. We revise this concept by extending the energy
analysis to turbulent potential and total energies (TPE and TTE = TKE + TPE),
consider their budget equations, and conclude that TTE is a conservative
parameter maintained by shear in any stratification. Hence there is no
"energetics Ric", in contrast to the hydrodynamic-instability threshold,
Ric-instability, whose typical values vary from 0.25 to 1. We demonstrate that
this interval, 0.25<Ri<1, separates two different turbulent regimes: strong
mixing and weak mixing rather than the turbulent and the laminar regimes, as
the classical concept states. This explains persistent occurrence of turbulence
in the free atmosphere and deep ocean at Ri>>1, clarify principal difference
between turbulent boundary layers and free flows, and provide basis for
improving operational turbulence closure models.
