| Summary: | We present lidar observations of the middle atmosphere at the South Pole (90$\sp\circ$S), Syowa (69$\sp\circ$S, 39$\sp\circ$E), and Urbana (88$\sp\circ$W, 40$\sp\circ$N). The South Pole stratospheric observations yield a high resolution data set of stratospheric temperature and polar stratospheric cloud (PSC) backscatter ratio profiles during the austral winter and spring of 1990. The observations show that the seasonal development of the clouds is primarily determined by the behavior of the temperature field. The PSCs are composed predominantly of nitric acid trihydrate particles. Correlation with frost-point measurements shows that nitric acid mixing ratios are depressed in the spring. The small-scale structure of the clouds appears to be controlled by gravity waves propagating upward through the clouds. Lidar measurements of the mesospheric Na layer at the South Pole in 1990 and Syowa in 1985 are used to characterize mesopause region gravity wave activity over Antarctica. The structure of the Na layer reflects the general circulation of the high-latitude mesopause. The monochromatic waves observed over Antarctica show the same general characteristics as those reported from other sites. The mean density variance of the gravity wave perturbations at the South Pole is similar to that observed at a variety of lower latitude sites. A distinct feature of the South Pole observations is the presence of strong coherent oscillations in the bottomside density contours of the Na layer close to the inertial frequency. Na Doppler/temperature lidar measurements of Na density and temperature at Urbana yield a high resolution seasonal data set of gravity wave activity. The direct measurement of the Brunt-Vaisala period allows accurate calculation of the horizontal velocity and vertical displacement from the density measurements. The horizontal velocity and vertical displacement m-spectrum magnitudes and indices show considerable seasonal and nightly variability, behaviors which contradict the predictions of Linear Instability Theory and Scale-Dependent Diffusion Theory. We present a detailed comparison of the observations with the predictions of the Scale-Independent Diffusive Filtering Theory. The magnitudes of the m-spectrum, the form of the joint (m,$\omega$) spectrum, the systematic relationships between the monochromatic gravity wave periods, wavelengths, and amplitudes agree remarkably well with those predicted for Scale-Independent Diffusive Filtering Theory. This observational study suggests that the complex nonlinear interactions of the gravity wave field can be modeled successfully as a diffusion process, where the diffusivity is a function of the total wave variance.
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