Turbulence energetics in stably stratified geophysical flows: strong and weak mixing regimes

Details Turbulence energetics in stably stratified geophysical flows: strong and weak mixing regimes Turbulence energetics in stably stratified geophysical flows: strong and weak mixing regimes

2008-07-11

arxiv.org/abs/0807.1873v1

Quarterly Journal of Royal Meteorological Society 134, 793-799 (2008)

Physics

Atmospheric and Oceanic Physics

23 pages, 4 figures, Quarterly Journal of Royal Meteorological Society, in press

Scientific paper

10.1002/qj.264

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>1, clarify principal difference between turbulent boundary layers and free flows, and provide basis for improving operational turbulence closure models.

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