Gravity Wave Dynamics Near The Mesospause Over Poker Flat, Alaska (Spectra)

Abstract

The characteristics of mesospheric gravity waves are studied using racket and radar soundings. The soundings are examined in two different approaches: identification of individual, nearly monochromatic waves, and through the statistical view offered by spectral analysis of wind velocity fluctuations. Simultaneous rocket and radar data has provided a complete overview of the saturation of a large-scale inertio-gravity wave near the summer mesopause. The 3m scale-size turbulence, to which the Poker Flat MST radar responds, was most intense in that phase of the wave that was most nearly unstable. However, the wave amplitude was not sufficient to produce instabilities by itself. Other waves, identified in radar data from the winter mesophere, also lacked sufficient amplitude to become unstable yet were clearly saturated as shown by their lack of growth with height. A comparison of vertical wavenumber spectra, of high spatial resolution (300 m) radar measurements, with a gravity wave model proposed by Van Zandt demonstrates that gravity waves dominate the mesospheric wind spectrum. These spectra also show that the wave amplitudes are approximately 1/2 of the saturation threshold. These observations point towards wave superposition as the cause of the reduced saturation threshold. The mesospheric spectra agree in shape and amplitude with similar spectra obtained in the stratosphere and troposphere. It is concluded that small-scale gravity waves are saturated throughout the lower and middle atmospere. Observations of the growth of wave amplitudes with height as seen in frequency spectral studies are compatible with the conclusion that the small-scale waves are saturated since the vertical wavelength of the largest saturating wave increases with height. Waves with scales greater than the largest saturated wave are at less than saturation amplitudes and do grow with height.