GRAVITY-WAVE EXCITATION BY GEOSTROPHIC ADJUSTMENT OF THE JET-STREAM .1. 2-DIMENSIONAL FORCING

Fourier integral and Green's function techniques are used to examine the gravity wave field and mean state arising from an initially unbalanced Gaussian jet. Approximate solutions specifying no mean motion reveal a wave field that is nearly inertial in character. Horizontal motions are symmetric about the jet axes, while vertical motions and the thermal field are antisymmetric. Full solutions of the initial-value problem yield a wave field that is qualitatively like that obtained in the absence of mean state adjustment and a mean jet structure with a balanced thermal field that conserves potential vorticity and is somewhat more extended vertically than the original unbalanced jet. The mean adjustment occurs within an inertial period and the scales and frequencies of the forced wave motions are controlled by the horizontal and vertical extent of the initial perturbation. We suggest that the excitation of inertio-gravity waves in this manner may explain some observations of such motions in association with jet streams and may contribute to the preponderance of gravity wave energy at near-inertial frequencies in the atmosphere.