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

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 N⁴ m³. The observed spectra appear to be compatible with the recently published non-linear wave-induced diffusion and non-linear wave-induced Doppler spreading theories.