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

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.

Multi-year observations of gravity wave momentum fluxes at low and middle latitudes inferred by all-sky meteor radar

“…Placke et al (2011b) in their analysis of GW activity over Andenes (69.3 • N, 16 • E) and Juliusruh (54.6 • N, 13.4 • E) report SAO with maxima around the solstice. As stated earlier, annual and semiannual oscillations in GW activity are anticipated and have been documented in earlier work (e.g., Antonita et al,2008;Clemesha et al, 2009;Mitchell and Beldon, 2009;Beldon and Mitchell, 2009;Senft and Gardner, 1991;Placke et al, 2011b;De Wit et al, 2014b; and references therein); however, this appears to be the first report of a 4-month oscillation in GW momentum fluxes.…”

“…Mitchell and Beldon (2009) observed a SAO with maxima at the solstices at Rothera (68 • S, 68 • W) and Beldon and Mitchell (2009) observed the same behavior for Esrange (68 • N) for the fluctuating radial velocities. Senft and Gardner (1991) observed annual and SAO in GW activity at Urbana (40 • N, 88 • W) using Na lidar data to infer atmospheric density perturbations and their spectra. These results are similar to those found by us for middle latitudes, corresponding to SM and CP, the latter mainly in the meridional component of the variances.…”

Climatology and modeling of quasi‐monochromatic atmospheric gravity waves observed over Urbana Illinois

“…Measured amplitudes of a few tenths of percent are consistent with wave amplitudes incident on the duct of a few percent, which seems reasonable. Indeed, typical AGW relative density amplitudes are 5% as measured by Senft and Gardner [1991] with a Na lidar over Urbana. Perhaps, more relevant data are those of Gardner and Voelz [1987] who measured the seasonal characteristics of monochromatic waves seen over Urbana using a Na Table 3 …”

Simultaneous, common‐volume lidar observations and theoretical studies of correlations among Fe/Na layers and temperatures in the mesosphere and lower thermosphere at Boulder Table Mountain (40°N, 105°W), Colorado

“…[35] Gardner and Shelton [1985] and Senft and Gardner [1991] showed that the solutions to (A3) and (A4) are accurately represented by the equations…”