Internal Gravity Wave Emission into the Middle Atmosphere from a Model Tropospheric Jet

Abstract

A mechanism is investigated whereby large amplitude internal gravity waves (IGWs) may be excited by the tropospheric jet stream when this is driven to parallel shear instability following a rapid external forcing of the mean flow a circumstance that might be realized, for example, through ageostrophic effects in the process of baroclinic wave development. A series of mean states are examined, first on the basis of linear theory, to determine the characteristics of the most unstable normal mode, which is expected to dominate the initial stages of flow evolution. Two-dimensional nonlinear numerical simulations of stratified, incompressible, Boussinesq flow in a periodic channel are also performed. On the basis of linear theory, the authors show that it is possible to assess whether IGWs will be strongly excited by examining whether the initial instability satisfies an easily calculable criterion that has been previously termed the “penetration condition.” In cases in which the penetration condition is satisfied, large amplitude IGWs with the same horizontal phase speed and wavelength as the most unstable mode of linear theory are shown in the nonlinear simulations to radiate freely into the stratosphere, and the process of stabilization of the mean state is thereafter significantly influenced by the extraction and upward vertical transport of horizontal momentum by IGWs away from the mixing region. In cases in which the penetration condition is not satisfied only very weak internal wave emission is observed. These small amplitude waves evidently develop through nonlinear mechanisms. On the basis of model calculations performed using initial conditions intended to simulate realizable midlatitude zonal flows, the authors demonstrate that the vertical flux of horizontal momentum delivered into the stratosphere by such sheer instability could reach levels comparable to the fluxes associated with topograhically forced internal waves that develop during severe down-slope windstorm events. This raises the question as to whether there may be circumstances in which the action of gravity wave drag on the general circulation could be affected by the process of emission rather than by the process of absorption. The basis of all current IGW drag parameterization schemes employed in large-scale models is that momentum is being deposited into the mean flow by IGW breaking. The authors question the universal validity of this assumption.