SEASONAL CLIMATOLOGY OF THE NIGHTTIME TIDAL PERTURBATION OF TEMPERATURE IN THE MIDLATITUDE MESOPAUSE REGION

We present a seasonal-average climatology of the nocturnal variation o f temperature and sodium density in the mesopause region (80-110 km) b ased on 300 nights of observation from 1990-1997 by the sodium resonan ce lidar in Fort Collins, Colorado (40.6 degrees N, 105 degrees W). Wa ves with downward phase progression were observed during all seasons, with properties (coherence over many nights, observed half-period, etc .) consistent with the upward-propagating semidiurnal tide. The semidi urnal tide was strong (10 K at 90 km) and stable from November to Marc h with a 30 km vertical wavelength. In April and September, the vertic al wavelengths were longer and the amplitude increased rapidly with he ight above 95 km. From May-August, there is a change in wave structure and low amplitudes at 90 km, possibly due to the diurnal tide. The Th ermosphere Ionosphere-Mesosphere-Electrodynamics General Circulation M odel (TIME-GCM) predicts a dominant semidiurnal tide in good agreement with observations, except for a difference in the phase at equinox.