Radiating and Nonradiating Modes of Secondary Instability in a Gravity-Wave Critical Layer

Abstract

This paper presents results obtained with high-resolution numerical models of the gravity-wave critical layer. The structure and growth rates of preferred modes of secondary instability—within or near regions of potential temperature overturning in the wave field—are discussed. Model instabilities, which appear to be primarily convective, are of two kinds. The expected mode of convective instability is nonradiating, trapped within the region of overturning. A new “radiating” mode of instability was also obtained that has a preferred zonal scale, grows to observable amplitude prior to the nonradiating mode, and extends into the adjacent stable regions of the wave field. As a result, this mode is important in the transition to turbulence and may affect momentum deposition and turbulent mixing due to gravity-wave breaking.