Middle Atmosphere Structure And Dynamics Lidar Studies And Gravity Wave Spectral Modeling

We present lidar observations of the middle atmosphere at the South Pole (90$spcirc$S), Syowa (69$spcirc$S, 39$spcirc$E), and Urbana (88$spcirc$W, 40$spcirc$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.

A new mobile lidar was used to obtain data on the variations that occur in the high latitude atmosphere between 20 and 85 km. Data were obtained on 26 nights during the program. Standard meteorological balloon and rocket payloads also measured the density, for comparison with the lidar data. More than a thousand profiles of atmospheric density were obtained with the lidar and 20 meteorological rockets were launched. Comparisons between the lidar data and the meteorological rocket data showed generally good agreement. However, the lidar data from the February period is offset 12-14 percent from the rocket data. The March and April data, including 14 sets of overlapping data, generally agree to within 3 percent. Keywords: Lidar; Atmospheric density; Middle atmosphere; Atmospheric variations; High latitude atmosphere.

PAGEOPH, stratosphere, these differences provide us with new evidence, interpretation of which can materially help to advance our understanding of stratospheric dynamics in general. It is now weil established that smaller-scale motions-in particular gravity waves and turbulence-are of fundamental importance in the general circulation of the mesosphere; they seem to be similarly, if less spectacularly, significant in the troposphere, and probably also in the stratosphere. Our understanding of these motions, their effects on the mean circulation and their mutual interactions is progressing rapidly, as is weil illustrated by the papers in this issue; there are reports of observational studies, especially with new instruments such as the Japanese MV radar, reviews of the state of theory, a laboratory study and an analysis of gravity waves and their effects in the high resolution "SKYHI" general circulation model. There are good reasons to suspect that gravity waves may be of crucial significance in making the stratospheric circulation the way it is (modeling experience being one suggestive piece of evidence for this). Direct observational proof has thus far been prevented by the difficulty of making observations of such scales of motion in this region; in one study reported here, falling sphere observations are used to obtain information on the structure and intensity of waves in the upper stratosphere.

Published by the American Geophysical Union as part of the Geophysical Monograph Series, Volume 87. This volume provides a review of progress made in recent years in experimental and theoretical investigation of the upper mesosphere and lower thermosphere and coupling between these regions and the ionosphere. Detailed study of the mesosphere/lower thermosphere/ionosphere (MLTI) region has historically been difficult because of its relative inaccessibility to direct measurement techniques and the complex and highly coupled processes which occur there. Although we have still not successfully unraveled all these complex interactions, we have made significant recent progress toward a fuller understanding of the basic state of the MLTI and of the dominant wave and coupling processes. This monograph includes a set of tutorial papers, which review our current understanding of aspects of the MLTI. These tutorials are interspersed with a selection of papers describing research progress on various topics of current interest in this region. The book should therefore be useful both to the newcomer, as an introduction to this field of research, and to the more experienced researcher, providing an overview of research in progress as well as a convenient reference collection of papers describing our current understanding.

This dissertation presents new studies of waves and turbulence in the Arctic middle atmosphere. The study has a primary focus on wintertime conditions when the largescale circulation of the middle atmosphere is disrupted by the breaking of planetary waves associated with sudden stratospheric warming (SSW) events. We used ongoing Rayleigh lidar measurements of density and temperature to conduct a multi-year study of gravity waves in the upper stratosphere-lower mesosphere (USLM) over Poker Flat Research Range (PFRR) at Chatanika, Alaska. We analyzed the night-to-night gravity wave activity in terms of the wind structure and the ageostrophy. We find that the weak winds during disturbed conditions block the vertical propagation of gravity waves into the mesosphere. The gravity wave activity is correlated with the altitudes where the winds are weakest. During periods of weak winds we find little correlation with ageostrophy. However, during periods of stronger winds we find the USLM gravity wave activity is correlated with the ageostrophy in the upper troposphere indicating that ageostrophy in this region is a source of the gravity waves. Inter-annually we find the wintertime gravity wave activity is correlated with the level of disturbance of the middle atmosphere, being reduced in those winters with a higher level of disturbance and weaker winds. We used rocket-borne ion gauges to measure turbulence in the wintertime middle atmosphere while documenting the larger meteorological context from Rayleigh lidar and satellites. This investigation of turbulence was called the Mesosphere-Lower Thermosphere Turbulence Experiment (MTeX). During MTeX we found a highly disturbed atmosphere associated with an SSW where winds were weak and gravity wave activity was low. We found low levels of turbulence in the upper mesosphere. The turbulence was primarily found in regions of convective instability in the topside of mesospheric inversion layers (MILs). The strongest and most persist turbulence was found in a MIL that is associated with the breaking of a monochromatic gravity wave. These MTeX observations indicate that turbulence is generated by gravity wave breaking as opposed to gravity wave saturation. These MTeX findings of low levels of turbulence are consistent with recent model studies of vertical transport during SSWs and support the view that eddy transport is not a dominant transport mechanism during SSWs.

This book examines the origins and dynamical characteristics of atmospheric inertia-gravity waves in the Antarctic mesosphere. Gravity waves are relatively small-scale atmospheric waves with a restoring force of buoyancy that can transport momentum upward from the troposphere to the middle atmosphere. In previous studies, the dynamical characteristics of mesospheric gravity waves have not been fully examined using numerical simulations, since performing a numerical simulation with a high resolution and a high model-top requires considerable computational power. However, recent advances in computational capabilities have allowed us to perform numerical simulations using atmospheric general circulation models, which cover the troposphere to the mesosphere with a sufficiently fine horizontal resolution to resolve small-scale gravity waves. The book first describes the simulation of mesospheric gravity waves using a high-resolution non-hydrostatic atmospheric model with a high model top. The accuracy of the numerical results was confirmed by the first Mesosphere-Stratosphere-Troposphere/Incoherent Scattering (MST/IS) radar observation in the Antarctic. It also depicts the origins and propagation processes of mesospheric gravity waves on the basis of the results of the high-resolution numerical model. The behaviors of mesospheric gravity waves can be clearly explained using both fundamental and cutting-edge theories of fluid dynamics

The NATO Advanced Research Workshop on Coupling Processes in the Lower and Middle atmosphere held in Loen, Norway in May 1992 was, in the estimation of apparently all participants, an enormous success. The 18 invited speakers included many of the leaders in the field and resulted in the attendance of a large number of contributing speakers and observers. The subject of the workshop was itself very timely, given the increasing awareness within the international community of the sensitivity of the atmosphere to coupling between adjacent layers, different latitudes, and various scales of motion. It was also very beneficial to bring together researchers with different approaches to the same or similar problems. For example, experimentalists benefitted from the inputs of modelers and theoreticians concerning the needs of current models and the most pressing problems and unknowns. Likewise, theoreticians were challenged to apply themselves to realistic problems and saw their theories tested against geophysical data. These discussions led to meaningful exchanges of ideas and challenges to or displacement of conventional wisdom in some areas. Indeed, possibly the greatest benefit of the workshop was the exposure of many participants to other areas of research or approaches to problems relevant to their own work. Workshop topics were confined to dynamical coupling processes in order to examine progress in a relatively focussed area. Nevertheless, the results presented spanned spatial scales from molecular to global and temporal scales from seconds to decades.