Shear Excitation of Atmospheric Gravity Waves. Part II: Nonlinear Radiation from a Free Shear Layer

Abstract

This paper addresses the efficiency and characteristics of two mechanisms that have been proposed to account for the excitation of radiating gravity waves by Kelvin-Helmholtz (KH) instabilities at a free shear layer in a stratified atmosphere. These mechanisms are the vortex pairing or subharmonic interaction observed to occur at the interface between two homogeneous fluid layers and the KH interaction or “envelope radiation” mechanism found to occur in the presence of propagating unstable modes. Vortex pairing in a stratified environment is found to be highly dependent on the minimum mean Richardson number, being very efficient when the subharmonic is itself a KH mode and relatively unimportant when the subharmonic has propagating character. The envelope radiation mechanism, in contrast, is observed to provide efficient radiating wave excitation in the absence of propagating unstable modes, as anticipated by Fritts. It is suggested that this latter mechanism may lead naturally to the excitation of large-scale gravity waves due to the horizontal inhomogeneity of unstable shear layers and may therefore constitute an important source of atmospheric gravity waves.