Observational Limits for Lidar, Radar and Airglow Imager Measurements of Gravity Wave Parameters

By examining the observational limits and biases for lidar, radar, and airglow imager measurements of middle atmosphere gravity waves, we provide plausible explanations for the characteristics of the monochromatic wave parameters that have been reported during the past decade. The systematic dependencies of vertical and horizontal wavelength on wave period, reported in many lidar and some radar studies, are associated with diffusive damping. The prominent waves with the largest amplitudes, most often observed by lidars and radars, are those with vertical phase speeds near the diffusive damping limit. The narrow range of horizontal phase velocities of the waves seen by OH imagers is a consequence of the combined effects of the gravity wave spectrum and the OH layer response to wave perturbations. The strongest airglow fluctuations are associated with waves having vertical wavelengths comparable to the width of the OH layer. These waves have fast horizontal phase speeds near 70 m/s. Simple formulas which describe the regions of the wave spectrum observed by each instrument are derived and compared with published data. Lidars, radars, and imagers are often most sensitive to waves in largely different regions of the spectrum so that their measurements are truly complementary. However, these ground-based techniques are often incapable of observing the large-scale waves with periods longer than about 5 hours and both long vertical (\u3e15 km) and horizontal (\u3e1000 km) wavelengths. Spaceborne instruments, such as the high-resolution Doppler imager (HRDI) and wind imaging interferometer (WINDII) on UARS, are the techniques most likely to provide the key observations of the low wavenumber, low-frequency region of the gravity wave spectrum