The Cloud Imaging and Particle Size (CIPS) instrument on the Aeronomy of Ice in the Mesosphere satellite images in the nadir at the UV wavelength of 265 nm. The camera array has an approximately 120° along track (2000 km) by 80° cross track (1000 km) field of view at a horizontal resolution of 1 by 2 km in the nadir. The satellite is in a sun synchronous orbit with an approximately noon local time equator crossing. The observed albedo is due to Rayleigh scattered sun light from an altitude of approximately 50 km and sunlight scattered from Polar Mesospheric Clouds (PMC) which occur in the summer mesosphere at 83 km. The goal of the CIPS instrument is to retrieve high horizontal resolution maps of PMC albedo and the mode radius of the particle size distribution. The first manuscript analyzes the uncertainties involved in the retrieval. The ability to infer mode radius from the PMC signal is made significantly harder by the presence of the Rayleigh signal. Much of the difference between PMC signals of different mode radii is also consistent with possible changes in the Rayleigh signal. The signal is decomposed into components which isolate the portion of the PMC signal's dependence on radius which is not consistent with changes in the Rayleigh signal. This isolated component is compared with the measurement noise to estimate and understand the uncertainties in the CIPS retrieval. The presence of the Rayleigh signal is a difficulty in the PMC retrieval, but it is also a valuable data product. The second manuscript highlights the initial findings of a new gravity wave data set developed by the author. The data set provides relative ozone variations at the stratopause with a horizontal resolution of 20 by 20 km. An abundance of gravity wave signatures can be seen in the data which appear to emanate from weather events like thunderstorms and hurricanes as well as orographic sources such as the Andes and the Antarctic Peninsula. The data set fills a gap that presently exists in our observational coverage of gravity waves, so the data set should help significantly in constraining Global Climate Models. / Ph. D.

The middle atmosphere is a very important part of the Earth system. Until recently, we did not realize the importance of the structure of this vaporous shell and of the fundamental role it plays in both creating and sustaining life on the planet. Thanks to the development and improvement of new sounding methods and techniques, our knowledge of the composition of the atmosphere has become more detailed than ever before. We have also learned how to reveal complex interactions between different species and how they react to the incoming solar radiation. The middle part of the Earth’s atmosphere serves as a host for the Polar Mesospheric Clouds. These clouds consist of a thin layer of water-ice particles, only exsisting during the summer months and only close to the poles. There are indications that the occurrence of Polar Mesospheric Clouds may be linked to climate change. It has been pointed out that the first sightings coincide with the industrial revolution. Satellite observations have shown that Polar Mesospheric Clouds have become brighter and possibly more widely distributed during the 20th century. The clouds might therefore be suited as indicators of the variability of the climate - a good reason for studying this night-shimmering phenomena. The clouds can also be used as a proxy for middle atmospheric dynamics. In order to fully utilize Polar Mesospheric Clouds as tracers for atmospheric variability and dynamics, we need to better understand their local properties. The Optical Spectrograph and Infra-Red Imager System (OSIRIS) is one of two instruments installed on the Odin satellite. The optical spectrograph of this instrument observes sunlight scattered by the atmosphere and thus the Polar Mesospheric Clouds. This thesis deals with a tomographic technique that can reconstruct both horizontal and vertical structures of the clouds by using observations from various angles of the atmospheric region. From this information, microphysical properties such as particle sizes and number densities are obtained. The tomographic technique presented in this thesis also provides a basis for a new satellite concept - MATS. The idea behind the MATS satellite mission is to analyze wave activity in the atmosphere over a wide range of spatial and temporal scales, based on the scientific heritage from Odin/OSIRIS and the tomographic algorithms presented in this thesis. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper3: Submitted. Paper 4: Manuscript.</p>

The deep convection from monsoons is known to be a major source of gravity waves in the Earth's summer troposphere. While propagating through the middle atmosphere, these waves can carry their momentum up to the mesosphere, following either a vertical or an oblique path. This upward and oblique propagation of gravity waves refers to the latitudinal propagation, away from their low-latitude tropospheric source and towards the polar summer mesosphere. Their dissipation in this atmospheric region plays an important role in the global dynamical structure of the middle atmosphere and yet, the oblique propagation of gravity waves is not included in the present global models. Understanding the influence of the obliquely propagating monsoon gravity waves on the polar summer mesosphere, as well as the hemispheric and seasonal variations of this phenomenon, can improve the gravity-wave parameterization schemes used in the global models. My dissertation relies upon the atmosphere theory and the gravity-wave observations, first, to perform an observational analysis of the oblique propagation of gravity waves in the summer hemisphere. In response to temperature anomalies in the winter northern stratosphere, the distribution of the gravity-wave pseudomomentum flux in the opposite summer mesosphere appeared to be altered. This in turn changes the gravity-wave oblique propagation and its influence on the temperature variations seen in the polar mesospheric clouds. After the development of a 4-D non-hydrostatic ray-tracing model for the simulation of the gravity-wave propagation, my dissertation explores the hemispheric and seasonal differences in the propagation and dissipation of more than 40,000 gravity waves from the low-latitude troposphere. These ray-tracing simulations show the southern hemisphere to be more conducive to both the vertical and the oblique propagation of tropospheric to mesospheric gravity waves. This analysis also highlighted a strong wave filtering at the northern tropopause where a significant number of gravity waves were vertically reflected before reaching the stratosphere. / Doctor of Philosophy / The propagation of waves throughout the Earth's atmosphere is a key phenomenon to understanding the global atmosphere dynamics. These atmospheric waves are known to change the temperature, the pressure, the density and the composition of the middle atmosphere. As a wave propagates upward, the density of the atmospheric background exponentially decreases, resulting in an exponential increase in the wave amplitude and thus, an exponential increase in the energy carried by the wave. When the wave breaks, this energy is released and transferred to the background flow. Gravity waves are part of the atmospheric wave spectrum that is of interest to the scientific community. While small-scale gravity waves can form from tropospheric instabilities such as an unbalanced flow over the mountains or a deep convection from monsoon or thunderstorms, they can propagate up to the upper mesosphere where they can break and transfer a significant amount of energy to the background flow. Although the significant role of these gravity waves in the coupling mechanisms between atmospheric regions is without dispute, their horizontal scale is too small to be resolved by most of the global-scale atmospheric models. The deep convection from monsoon regions is known to be a major source of mesospheric GWs and previous studies on summer northern hemisphere have shown that monsoon GWs tend to propagate obliquely from the low-latitude stratopause up to the high-latitude mesopause. We focus the observational study on the summer southern hemisphere and the Inter-Hemispheric Coupling (IHC) between the summer mesopause, where Polar Mesospheric Clouds (PMCs) form, and the winter stratosphere where sudden warmings occur. PMCs are excellent indicators of atmospheric changes. Their correlations with wind, temperature and GW pseudomomentum flux highlight the consequences of anomalies in the winter stratosphere, such as warmings, on the oblique propagation of GWs that influence the PMC formation in the summer southern hemisphere. After the computation of a ray-tracing model for the simulation of the gravity-wave propagation, a hemispheric and seasonal comparison of the tropospheric to mesospheric gravity-wave propagation based on four simulations highlights the spectral nature of this phenomenon.