On the Critical Richardson Number in Stably Stratified Turbulence

<p> The critical Richardson number, <i> Ri </i> _{<i> c </i>} , is used in studies of stably stratified turbulence as a measure of flow laminarization. The accepted range of <i> Ri </i> _{<i> c </i>} is between 0.2 and 1. A growing body of experimental and observational data indicates, however, that turbulence survives for <i> Ri </i> &gg; 1. This result is supported by a new spectral theory of turbulence that accounts for strong anisotropy and waves. The anisotropization results in the enhanced horizontal mixing of both momentum and scalar. Internal wave contribution preserves vertical momentum mixing above its molecular level. In the absence of laminarization, <i> Ri </i> _{<i> c </i>} becomes devoid of its conventional meaning. Copyright &copy; 2007 Royal Meteorological Society</p>