In the companion paper <Langford et al., this issue> clear signatures of ozone modulation by gravity waves in the lower and middle troposphere have been described from lidar observations over Fritz Peak Observatory (39.9 ¿N, 105.3 ¿W) during September 3--4, 1993, and analyzed using a linear model. It is emphasized in that paper that the linear analysis breaks down at the tropopause level. The purpose of this paper is to interpret the ozone observation in the upper troposphere using a high-resolution mesoscale model and analyze the physical processes responsible for the structures in the upper troposphere and lower stratosphere. In particular, it is shown that topographically excited gravity waves that break at the tropopause level are responsible for an irreversible exchange of ozone from the stratosphere to the troposphere. ¿ American Geophysical Union 1996