Seasonal variability of gravity wave activity and spectra in the mesopause region at Urbana

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Abstract
The nightly and seasonal variations in gravity wave activity in the mesopause region are examined by analyzing 60 nights of Na lidar observations obtained during a 5‐year period at Urbana, Illinois. The lidar data were used to calculate the atmospheric density perturbations and their spectra. The atmospheric density variances, density vertical shear variances, vertical wind variances, ω spectra magnitudes, and m spectra magnitudes all exhibit considerable nightly variability as well as strong annual and semiannual variations with the largest values in summer. The annual mean values of the rms density perturbations, density shear variance, and rms vertical wind velocity are respectively 5.6%, 37 (%/km)2, and 1.3 m/s. The midsummer values for these parameters are typically 2 to 3 times larger than the annual means. the equivalent Richardson number for the wave field varies between 1/2 and 2 for most of the year. However, during summer, Ri decreases appreciably and sometimes falls well below 1/4. The ω spectra exhibit power law shapes with slopes varying between −1.28 and −2.45. The m spectra also exhibit power law shapes with slopes varying between −2.20 and −3.55. The annual mean slopes are −1.82 for the ω spectra and −2.90 for the m spectra. The magnitudes of both the ω spectra and m spectra vary by more than a factor of 10 throughout the year at all periods between 5 min and 4 hours and vertical scales between 1 and 10 km, with the largest values in summer. The observed variability of the m spectra slopes and magnitudes is not consistent with the predictions of linear instability theory and the concept of a universal vertical wave number spectrum. The characteristic periods (T* = 2π/ω*) vary between 3 and 50 hours with an annual mean of 9.7 hours. The characteristic vertical wavelengths (λz* = 2π/m*) vary between 8.9 and 27 km with an annual mean of 14.1 km. The characteristic wavelengths λz* exhibit a weak seasonal variation with smaller values in summer ( ) compared to winter ( ). The rms bandwidths of the wave field have mean values of and . The two‐dimensional density spectra are not separable.