The gravity wave-critical level interaction

Three models are developed to examine various features of the gravity wave-critical level interaction and some of its possible roles in the dynamics of the atmosphere and the oceans. A linear inviscid steady state model is designed which, though singular at the critical level, illustrates certain important features of the gravity wave-critical level interaction. A linear viscous steady state model is used to examine the effects of dissipation. And a nonlinear viscous time dependent model is developed to evaluate the importance of time dependence and nonlinear effects on the development of the gravity wave-critical level interaction. Results obtained with the three models indicate that a number of effects may be important in correctly modelling the gravity wave-~ritica1 level interaction. Viscosity and heat conduction tend to stabilize the interaction and can, if sufficiently large, prevent the formation of unstable velocity shears. A Viscous Stability Criterion is obtained and its predictions are found to be consistent with observations in a number of laboratory, atmospheric, and oceanic critical level flows. Nonlinear and time dependent effects are found to be important in gravity wave-critical level interactions in which unstable velocity shears are formed. Such an unstable critical level interaction develops long thin nearly horizontal regions of unstable velocity shear which break down through the development and growth of Kelvin-Helmholtz instabilities. This suggests that the gravity wave-critical level interaction may be responsible for some of the thin turbulent layers observed at various levels in the atmosphere and the oceans.U of I OnlyThesi