Observation and Interpretation of the Spectra of Atmospheric Gravity Wave Perturbations with Upward and Downward Phase Progressions

We present over 350 hr of Na lidar measurements of mesospheric (~80-105 km) density perturbations obtained on 50 nights throughout all four seasons at Urbana, Illinois, during the years 1988-1993. For perturbations with frequencies between omega _{o} = 2pi/(120 min) and 2pi/(12 min) or vertical wave numbers between m = 2pi/(10km) and 2pi/(1km), approximately two -thirds of the density variance was associated with perturbations exhibiting downward phase progression (i.e., upwardly propagating waves). The nightly variances were highly correlated, which suggests that the downward and upward phase perturbations originate from the same low-altitude gravity-wave sources. Theoretical models incorporating Doppler effects of the background winds were derived by assuming an intrinsic spectrum of upwardly propagating gravity waves embedded in a uniform horizontal flow. These models were used to predict the m- and omega_{o} -spectra slopes and magnitudes associated with upward and downward phase perturbations. For the omega _{o}-spectra, the measured upward phase spectral slopes (mean = -1.6) were consistently shallower than the downward phase spectral slopes (mean = -1.9). This characteristic is compatible with the predictions of the scale independent diffusive filtering theory of Gardner (1993), but not with the separable gravity wave model of Gardner et al. (1993), which predicts that the slopes of the upward and downward phase spectra will both be approximately equal to the slope of the intrinsic spectrum. Both models predict that the upward phase variances will approach 20 to 30% of the total variance for horizontal wind velocities between 30 and 50 m/s. These are typical values for the mean horizontal winds in the upper mesosphere above Urbana. The data and model predictions strongly suggest that most of the wave energy observed in the mesopause region at Urbana is associated with upwardly propagating gravity waves generated by sources in the lower atmosphere.