Daniel C. Senft

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

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.

Characteristics of gravity wave activity and spectra in the upper stratosphere and upper mesosphere at Arecibo during early April 1989

Abstract Twenty-nine hours of Rayleigh/Na lidar observations were obtained on six different nights from 6 to 11 April 1989 during the AIDA-89 Campaign. The lidar data were used to calculate the relative atmospheric density perturbations and their spectra. The average r.m.s. density perturbations for early April at Arecibo are 6.2% in the upper mesosphere (~ 83–103 km) and 1.4% in the upper stratosphere (20–45 km). In the upper mesosphere, the average r.m.s. vertical wind velocity, vertical shear variance of horizontal winds and Richardson number are, respectively, 2.8 m/s [28.5 (m/s)/km]2 and 0.63. In the stratosphere the average shear variance and Ri number are, respectively [7.9 (m/s)/km]2 and 9.7. The temporal frequency spectra of the mesopause region density perturbations exhibit power-law shapes with an average slope of −1.76 and magnitude at ω = 2π (1h) of 1.2 (cycles/s)−1. The vertical wave number spectra also exhibit power-law shapes with average slopes of −2.66 in the upper stratosphere and −3.15 in the upper mesosphere. The magnitudes of the stratospheric spectra were consistently smaller than the mesopause spectra at all wave numbers less than m = 2π/(1 km). At m = 2π (4 km) the density spectra magnitudes were 5–12 times smaller in the upper stratosphere. The observed altitude variations in m-spectra magnitudes are inconsistent with linear instability theory which predicts that density spectra in the saturation regime vary only as N 4 m 3 . The observed spectra appear to be compatible with the recently published non-linear wave-induced diffusion and non-linear wave-induced Doppler spreading theories.

Observations of a 12 H wave in the mesopause region at the South Pole

In December 1989 a Na lidar was installed at the Amundsen-Scott South Pole Station and was used to measure aerosol, stratospheric temperature and mesospheric Na profiles through October 1990. The mesospheric Na data are used to characterize the gravity wave field in the mesopause region. These first lidar observations of Na layer dynamics at the South Pole show strong wave activity during the Antarctic winter. Data for 25 June and 19 August 1990 UT are presented here. The total wave induced variances in atmospheric density are respectively 29 and 35 (%)2. The Na layer centroid height is very low during both observation periods. On 25 June a strong 12 h oscillation is observed in the bottomside of the Na layer which extends to altitudes as low as 74 km. The vertical displacement and temperature amplitudes associated with the 12 h oscillation are respectively 1.9 km and 19 K. The characteristics of the 12 h wave are similar to the pseudotide observed at Svalbard by Walterscheid et al. [1986].

Seasonal variations of the thermal structure of the mesopause region at Urbana, IL (40° N, 88° W) and Ft. Collins, CO (41° N, 105° W)

Beginning in 1991, Na lidars were used to make routine measurements of the temperature structure between 80 and 105 km altitude during 65 nights at Urbana, IL and 116 nights at Ft. Collins, CO. More than 13,000 temperature profiles representing more than 1100 h of observations were used to characterize the seasonal temperature variations in the upper mesosphere and lower thermosphere. The seasonal behavior is quite similar at the two sites. The mesopause altitude is near 100 km in winter during Nov–Feb and near 86 km in summer during May–Jul. The mesopause temperature varies from 190 K in mid-winter to 175 K in mid-summer. The transitions between winter and summer mesopause structures occur rapidly in Mar–Apr and Aug–Sep. The temperature differences between the two sites are less than ±15 K and appear to be the result of atmospheric planetary, tidal, and gravity wave perturbations.