The Evolution and Stability of Finite-Amplitude Mountain Waves. Part II: Surface Wave Drag and Severe Downslope Windstorms

Abstract

The transient evolution of internal waves which are forced by the flow of stably stratified fluid over two-dimensional topography exhibits several pronounced nonlinear effects for geophysically relevant values of the governing parameters. For homogeneous flows in which the internal Froude number is constant, the importance of nonlinearity is determined by the aspect ratio of the topography and the flow in the steady-state regime is as predicted by Long's model. When the background flow is inhomogeneous, Long's model no longer applies and new nonlinear effects may occur. One example of such an effect is the marked increase in the efficiency with which resonant lee waves are excited beyond the linear efficiency. A second example concerns the possibility of the trapping and subsequent amplification of the internal wave beneath its own level of supercritical steepening. The latter process appears to be important in understanding the strong downslope windstorm which occurred at Boulder, Colorado, on 11 January 1972.