Abstract
A criterion for the viscous stabilization of gravity wave critical level flow is derived to be Z¼≤ 2.5 Zr where Z¼is the vertical thickness of the unstable region in the vicinity of a gravity wave critical level calculated with an inviscid linear theory and Zy is the viscous length scale for gravity wave critical level interaction. The stability implications of this criterion are examined for gravity wave critical level flows in the laboratory, the atmospheric planetary boundary layer, the upper atmosphere and the ocean thermocline region. Laboratory flows, where no turbulence in the vicinity of a gravity wave critical level was observed, are found to meet our criterion for stability. Applying this criterion to atmospheric boundary layer flows implies that no such regions of instability should be observed in the fully developed turbulent convective region, but such turbulent regions should exist in stably stratified cases. Using observed gravity wave parameters for the upper atmosphere the stability criterion gives the result that molecule viscosity should stabilize gravity wave critical level flows above about 130 km altitude. Using gravity wave parameters appropriate to the ocean in the theory suggests that gravity wave critical level instabilities might give rise to some of the thin mixed layers that are observed in the thermocline region.