During strong atmospheric perturbation events, such as sudden stratospheric warmings, planetary wave breaking and mean flow change have large impacts on gravity wave activities. On one hand, as a consequence of large-scale wave breaking, catastrophic generation of small-scale perturbations would cause enhancement in gravity wave activities. On the other hand, background flow changes during the sudden warming events could introduce different filtering to existing gravity waves propagating upward from the lower atmosphere. The interactions among planetary waves, mean flow, and gravity waves are of great interest to understand how the atmospheric system responds to a strong perturbation as a whole. The following example shows two stratosperic warming (mesospheric cooling) events during UARS day 80-130 (Nov.30, 1991-Jan.19,1992). Apparently, planetary wave 1 enhancment is the precursor to these warming events. Enhanced gravity wave activities follow immediately the planetary wave event in the first case while occur at about same time in the second event. The time lags among gravity wave activities at different altitudes are evident, showing a later enhancement at a lower altitude.