Seasonal variation of diurnal perturbations in mesopause regiontemperature, zonal, and meridional winds above Fort Collins, Colorado (40.6°N, 105°)

On the basis of lidar observations from May 2002 through April 2003, covering both day and night, we performed a harmonic analysis to extract the diurnal perturbations in mesopause region temperature, zonal and meridional winds over Fort Collins, Colorado (40.6°N, 105°W), binned every 2 months. The results were compared to predictions of the 2000 and 2002 versions of Global-Scale Wave Model (GSWM00 and GSWM02). The diurnal tidal period oscillations showed a mixture of propagating and evanescent (trapped) modes, but the propagating modes dominated for most of the year. The agreement in temperature diurnal phases between observation and GSWM prediction is marginal. On the other hand, other than July-August meridional winds, the observed diurnal phases in both wind components are in good agreement with GSWM predictions for most of the altitude range reported. The diurnal amplitude predictions of GSWM00 were reasonably close to lidar observations, while other than January-February, the GSWM02 amplitude prediction overestimated the observations, typically by a factor of two. We also conducted comparisons on tidal perturbations in zonal wind between radar campaigns and our lidar observations. The lidar data agreed reasonably well with the MF radar data from 2000 to 2001 at nearby Platteville, Colorado (40.2°N, 104.7°W), but showed considerable differences with the data from other midlatitude stations from 1992 to 1993. The dominance of the evanescent mode in the temperature diurnal tidal oscillation during the early winter (November and December), which reached a peak value at midnight, was interesting and anomalous. By invoking the more recent data (November and December in 2003), as well as the diurnal temperature observations from December 1998, we report that the evanescent (trapped) diurnal tidal perturbations were robust and persisted from one year to the next