The Excitation of Atmospheric Gravity Waves by Unstable Shears.

Abstract

Abstract : Research performed under this AFOSR contract addressed two aspects of gravity wave excitation by unstable shear layers in the atmosphere; the non-linear excitation of radiating gravity waves and the response of the mean velocity profile to gravity wave growth and decay. Gravity excitation via the nonlinear interaction of two smaller-scale Kelvin-Helmholtz (KH) instabilities was found to be significantly faster than that predicted by linear stability analysis, accounting for the geophysical significance of such waves. Gravity waves excited in this manner were observed to achieve large amplitudes and to be very localized during excitation. Mean velocity profiles were found to respond in several ways to evolving, unstable gravity waves. Radiating waves excited by the nonlinear interaction of two KH modes were observed to produce relatively small mean flow accelerations away from the shear layer because of source shear stabilization by the KH modes. Large mean flow accelerations were induced by unstable modes evolving in isolation. These results suggest that gravity waves generated by unstable shears may be important in the transport of energy and momentum throughout the atmosphere.