Turbulence in the middle atmosphere: A review

Abstract

The stratosphere is characterized for being highly statically stable. lt takes considerable energy to interchange parcels of air from different altitudes; thus, turbulence is normally inhibited, Wind shears of the order of 40 m/sec per kilometer are necessary to overcome the stabilizing effect of negative buoyancy if turbulence is to occur. Shears close to these values do exist in narrow but horizontally exten­sive layers as a consequence of internal gravity waves and the two­dimensional (horizontal) turbulent character of mesoscale stratospheric winds. Shorter wavelength waves, superimposed on these shears, make the local shear exceed the threshold (Richardson number < 0.25) for Kelvin­Helmholtz instabilities to occur and in turn, to break into turbulence. In addition, the shear of these waves may be enhanced by a nonlinear unstable amplification of those gravity waves whose phase velocity matches the local wind velocity. As a consequence, turbulence in the stratosphere occurs intermittently, in extensive layers from ten to a few hundred kilometers in extent, and only a few ten to a few hundred meters in thickness.