LIDAR OBSERVATIONS OF LONG-PERIOD GRAVITY-WAVES IN THE STRATOSPHERE

Lidar observations of Rayleigh backscatter have been used to derive molecular densities at stratospheric heights over Aberystwyth (52.4-degrees-N, 4.1-degrees-W) for a total of 50 nights during the period 25/10/87-25/9/90. The variations of density perturbation, represented in time-height contours, are used to identify the presence of atmospheric gravity waves in the height range 30-60 km. The results suggest the frequent occurrence of quasi-monochromatic waves with vertical wavelengths ranging from 4 to 15 km and apparent periods exceeding the inertial value in about 50% of cases. The variation of vertical wavelength with height on a given night is generally found to be greater than that expected from the corresponding variation in the Brunt-Vaisala frequency. The wave perturbations are considered in terms of the potential energy per unit mass and the height variation of this quantity suggests there is considerable energy dissipation in winter, but little in summer. The potential energy shows a marked seasonal variation with maximum values being observed in winter at all heights, but a semi-annual component is also apparent at the greater heights. Vertical wave-number spectra show that the major part of the wave energy is associated with low wavenumbers in all seasons.