Observation of local tidal variability and instability, along with dissipation of diurnal tidal harmonics in the mesopause region over Fort Collins, CO (41°N, 105°W)

During the 9-day continuous campaign in September 2003, the Colorado State University sodium lidar observed significant short-term tidal variability in both diurnal and semidiurnal tides above 85 km on days 265-268. Both diurnal and semidiurnal amplitudes dramatically increased on day 267 with a continuous phase advance in diurnal tidal harmonics, causing local atmosphere to become dynamically unstable. Following the dynamical instability associated with tides, we observed an equally dramatic decrease in diurnal amplitude, which was accompanied by rapid and continuous phase retardation at 87 km on day 268; the accompanying diurnal phase profiles changed from propagating mode to evanescent mode. Since the time scale of the observed variability during days 265-268 is less than 1 day, gravity wave/tidal interaction at least is partially responsible for the observed variability. The observed changes in tidal wind amplitudes and phases were observed to correlate with gravity wave (GW) activities and the direction of GW momentum, exhibited and discerned concurrently by an OH all-sky imager at nearby Yucca Ridge station, consistent with well-known models of tidal/GW interactions. The stability analysis in the night of 267, when both diurnal and semidiurnal reached the maximum amplitudes, revealed that the running daily tidal waves alone, superimposed with the associated mean state, could force the atmosphere into local dynamical instability near 90-95 km. The eddy diffusion associated with the instability is believed to have caused a strong dissipation of diurnal tide as observed on day 268.