Lidar studies of stratospheric gravity-wave spectra

Lidar observations of Rayleigh backscatter have been used to derive profiles of molecular density in the stratosphere over Aberystwyth (52.4°N, 4.1°W) for a total of 61 nights during the period 25 October 1987-25 September 1990. Vertical profiles of density perturbations believed to be due to gravity waves are examined in terms of the associated fluctuations of potential energy which are spectrally analysed for the height range 30-45 km. The vertical wavenumber spectra in winter are found to be similar in form to those predicted by the theory of Smith et al (1987, J. atmos. Sci. 44, 1404), and suggest characteristic vertical wavelengths near 7.5 km. The observed spectral indices generally have values greater than that predicted for a saturating wave field, -3, except in the winter months, and this is interpreted in terms of the wavenumber regime accessible from the lidar measurements. The seasonal dependence of the spectra is investigated and it is found that maximum power spectral densities at all wavenumbers occur in late January/early February. This seasonal variation is shown to be considerably greater at the lower wavenumbers. The series of consecutive soundings provided by the lidar technique demonstrates that the spectral composition of the wave field can display considerable short-term variability.