GENERATION OF INERTIA-GRAVITY WAVES IN A SIMULATED LIFE-CYCLE OF BAROCLINIC INSTABILITY

The excitation and propagation of inertia-gravity waves (IGWs) generat ed by an unstable baroclinic wave was examined with a high-resolution 3D nonlinear numerical model. IGWs arose spontaneously as the troposph eric jetstream was distorted by baroclinic instability and strong parc el accelerations took place, primarily in the jetstream exit region of the upper troposphere. Subsequent propagation of IGWs occurred in reg ions of strong windspeed-in the tropospheric and stratospheric jets, a nd in a cutoff low formed during the baroclinic lifecycle. IGWs on the flanks of these jets were rotated inward by differential advection an d subsequently absorbed by the model's hyperdiffusion. Although absorp tion of IGWs at the sidewalls of the jet is an artifact of the model, IGW propagation was for the most part confined to regions with an intr insic period shorter than the local inertial period. Only a few IGWs w ere able to penetrate the middle stratosphere, due to weak winds or an unfavorable alignment of wavevector with respect to the mean dow. IGW s are important both as a synoptic signal in the jetstream, which may influence subsequent tropospheric developments, and as a source of ise ntropic or cross-isentropic mixing in the lower stratosphere. The auth ors' results demonstrated for the first time numerically a significant isentropic displacement of potential vorticity isopleths due to IGWs above the tropopause. Since conditions for IGW propagation are favorab le within a jet, a region of strong isentropic potential vorticity gra dient, it is likely that inertia-gravity waves affect the permeability of the lower stratospheric vortex and may in some instances lead to s tratosphere-troposphere exchange.