Stratospheric inertia-gravity waves generated in a numerical model of frontogenesis .2. Wave sources, generation mechanisms and momentum fluxes

Using a dry two-dimensional numerical model, we investigate the source regions and generation mechanisms for stratospheric inertia-gravity waves accompanying frontogenesis. Attention is focused on run 3 from Part I of this paper (Griffiths and Reeder 1996), here referred to as our 'standard calculation'. Ray-tracing techniques are used to identify the source regions of the waves. Although these results are strictly valid only in the far field away from the frontal region, they suggest that the waves originate in the frontal region. The ray equations also show that the horizontal wavelength decreases exponentially with time because of the action of the basic-state deformation, whereas the vertical wavelength increases linearly with time because of the shear of the basic-state cross-front wind. It is argued that the cross-front circulation may be viewed, for the most part, as the unsteady source of stratospheric gravity-waves in the standard calculation. This position is supported by a linear forced-wave calculation, in which the forcing is provided by the cross-front circulation from the standard calculation. Comparison of the vertical-motion fields in the stratosphere for the standard and forced-wave calculations show good agreement. An estimate of the nonlinear interaction between the frontal circulation and the wave field is made. It is shown that the back-reaction of the waves on the forcing may be neglected, at least, qualitatively. The time-space mean of the vertical flux of horizontal momentum in the stratosphere accompanying the modelled waves is approximately 1.4 x 10(-4) N m(-2) where the average is taken over -2000 km less than or equal to x less than or equal to 3000 km, 15 km less than or equal to z less than or equal to 29 km and 24 h less than or equal to t less than or equal to t 48 h.