Lidar Observations of Gravity Waves and Their Spectra near the Mesopause and Stratopause at Arecibo

Abstract

The UIUC CEDAR Rayleigh/Na lidar was operated for approximately 160 h on 30 nights in January, March, and April 1989 at the Arecibo Observatory (18°N, 67°W) as part of the AIDA Act '89 Campaign. During this period 38 quasi-monochromatic gravity waves were observed in the stratopause region (25-55 km) and 62 waves were observed in the mesopause region (80?105 km). The event rates in both regions are approximately half those observed at the midlatitude site of Urbana. The characteristics of the waves in both regions are similar. Measured vertical wavelengths range from 1.1 to 17 km, vertical phase velocities from ?6 to ?270 cm s?1, observed periods from 5 min to 65 h, and amplitudes (relative atmospheric density variations) 0.4% to 17%. The wave amplitudes in the stratopause region are on average half the values for waves in the mesopause region with similar periods and vertical wavelengths. Vertical wavenumber spectra of density perturbations in both regions exhibit power-law dependencies with slopes near ?3. The magnitudes of the spectra in the stratopause region are typically a factor of 5 to 10 times smaller than the magnitudes of the mesopause region spectra, which is in significant disagreement with the predictions of linear saturation theory. Temporal frequency spectra of density perturbations in the mesopause region also exhibit power-law dependencies with slopes between ?1.5 and ?2.0 (mean slope = 1.85 ± 0.38) for frequencies smaller than the Brunt-Väisälä frequency and slopes near ?3 for frequencies larger than the Brunt-Väisälä frequency. The rms density perturbations averaged 1.2% in the stratopause region and 5.2% in the mesopause region. These results are compared with other radar and lidar observations.