Rayleigh Lidar Measurements of the Temporal Frequency and Vertical Wavenumber Spectra in the Mesosphere over the Rocky Mountain Region

Temporal and spatial spectral analysis techniques were applied to lidar data collected over a period of 18 months above the Rocky Mountain region at an altitude range from 45 to 70 km by a Rayleigh lidar system located in Logan, Utah (41.7øN, 111.8øW). Examination of the averaged temporal frequency F(co) and vertical wavenumber F(m) spectra showed spectral slope values of1.49 + 0.03 and -2.3 + 0.1, respectively. The observed slope for the overall averaged F(m) spectrum is considerably more positive than the value of-3 predicted by the linear instability theory but close to the value of--2 that is predicted by the scale independent diffusive filtering theory using the measured F(co) slope parameter. However, examination of the monthly averaged F(m) spectra for the transition from winter to summer showed the spectra became flatter suggesting that Doppler shift effects caused by the seasonal change in the magnitude and direction of the back•ound wind field are significant. The characteristic vertical wavelength X,, was found to be --12 km for the altitude region of 45-70 km. Comparison of this value with the characteristic wavelengths from other lidar observations at lower and higher altitude ranges showed an overall increase of • with height. The observed enhancement of the F(m) spectral magnitude in winter is believed to be caused in part by the low-frequency wave activity observed in the temperature profiles.