The Quasi-Biennial Oscillation (QBO) is the dominant pattern of variability in the tropical stratosphere. Despite a well established theory regarding its generation in the atmosphere, the simulation in global climate models remains difficult. A set of metrics assessing the quality of model simulations is presented in this study. The QBO simulations in models submitted to the CMIP5 and CCMVal-2 intercomparison projects are characterised and compared to radiosonde observations and reanalysis datasets. Common model biases and their potential causes are addressed. As the QBO has a long intrinsic period, knowing its influences on other parts of the climate system can be used to improve long range forecasts. These teleconnections of the QBO in observations are investigated using composite analysis, multilinear regression and a novel approach called causal effect networks (CEN). Findings from these analyses confirm previous results of the QBO modulating the stratospheric polar vortex and subsequently the North Atlantic Oscillation (NAO). They also suggest that it is important to take the equatorial zonal mean zonal wind vertical profile into account when studying teleconnections, rather than the more traditional method of using just one single level. While QBO influences on the Northern Hemisphere winter polar vortex and the NAO are more clearly established, interactions within the tropics remain inconclusive. Regression analysis does not show a connection between the QBO and the MJO, whereas the CEN analysis does. Further studies are needed to understand the interaction mechanisms near the equator. Finally, following the unprecedented disruption of the QBO cycle in the winter 2015/16, the event is described and a model analogue from the MPI-ESM-MR historical simulation is presented. Future implications are unclear, although model projections indicate more frequent QBO irregularities in a warming climate.

In der mittleren und oberen Atmosphäre spielen atmosphärische solare Gezeiten eine wichtige Rolle für die Dynamik und den Vertikaltransport von Energie und Impuls aus der Stratosphäre. Angeregt werden sie primär durch Absorption solarer Strahlung in der Troposphäre und Stratosphäre. Dabei entsprechen die Perioden der solaren Gezeiten den harmonischen Anteilen der täglichen Variation solarer Strahlung. Mittlerweile sind die täglichen, halbtägigen und dritteltägigen Gezeiten relativ gut erforscht, was bei der vierteltägigen Gezeit nicht der Fall ist. Die Informationen über diese Gezeit sind bislang rudimentär vor allem bzgl. einer globalen Klimatologie als auch der Details über möglichen Anregungsmechanismen und Wechselwirkungen. Dies ist darauf zurückzuführen, dass die Amplituden sehr klein sind und eine hohe zeitliche Auflösung für die Analyse benötigt wird. Die vierteltägige Gezeit wurde bislang von bodengebundenen Instrumenten und mit Fernerkundungsgsmethoden beobachtet, welche bislang lediglich einen räumlich und zeitlich begrenzten Überblick über die vierteltägige Gezeit boten. Da es nicht möglich ist die Beiträge der einzelnen Anregungen zu messen, muss sich numerischer Modelle als mächtiges Werkzeug bedient werden. Mit numerischen Modellen ist es möglich die verschiedenen Anregungsmechanismen zu separieren und ihre Beiträge für die vierteltägige Gezeit zu analysieren. Modellstudien lieferten bislang kein umfassendes Bild der QDT und berücksichtigten auch keine vierteltägige Schwerwellenanregungen. Diese Arbeit soll das Wissen zu diesem Thema erweitern, indem ein nichtlineares, mechanistisches, globales Zirkulationsmodell genutzt wird. Es wird eine umfassende numerische Studie durchgeführt, um die Wichtigkeit und das Zusammenspiel der drei vierteltägigen Anregungsmechanismen zu untersuchen, das sind die direkte solare Anregung, nichtlineare Wechselwirkung zwischen Gezeiten und Schwerewellen-Gezeiten-Wechselwirkungen. Erstmalig werden Anregungsterme, die über die Erwärmungsraten hinausgehen, selbst analysiert und quantifiziert und die Wechselwirkungen der vierteltägigen Gezeiten aus den unterschiedlichen Quellen untersucht. Darüber hinaus werden verschiedene Gezeitenmoden untersucht, um Interaktionen der vierteltätigen Gezeit aus den unterschiedlichen Anregungsmechanismen zu identifizieren. Darüber hinaus werden mit Hilfe der theoretischen Hough-Moden diejenigen Moden der vierteltägigen Gezeit abgeleitet, die in den Modellsimulationen maßgeblich für die meridionale Struktur verantwortlich sind. Diese aufwändige und umfassende Modellstudie analysiert die Anregungsmechanismen und deren Interaktion der vierteltägigen Gezeit. Die Arbeit hilft somit das Verständnis über die Wellenausbreitung der mittleren Atmosphäre auf ein neues Niveau zu heben.:1. Tides in the Middle Atmosphere - An Introduction 2. Quarterdiurnal Solar Tides 2.1. Forcing of Quarterdiurnal Tides 2.1.1. Overview of the different Forcing Mechanisms 2.1.2. Theoretical Consideration of the Nonlinear Forcing Mechanism 2.2. Observations and Model Study of the QDT 2.3. Summary and Outlook 3. The Middle and Upper Atmosphere Model (MUAM) 3.1. Introduction 3.2. Numerical Properties 3.3. Model Physics 3.4. Parameterizations 3.5. Background Climatology 4. Mathematical and Numerical Methods 4.1. Fast Fourier Transform 4.2. Harmonic Analysis 5. MUAM: Sensitivity Studies 5.1. Influence of Horizontal Resolution on the Background Climatology and QDT amplitudes 5.2. Influence of the Initial Conditions on the Background Climatology and QDT amplitudes 5.3. Influence of temporal resolution on the Background Climatology and QDT amplitudes 6. MUAM: Climatology of the Quarterdiurnal Tide 6.1. Amplitudes 6.2. Phases and Vertical Wavelengths 6.3. QDT reconstruction with Hough modes 7. MUAM: The Quarterdiurnal Tide Forcing Mechanisms 7.1. The Quarterdiurnal Forcing Terms 7.2. Model Experiments and Single Forcing Mechanisms 7.2.1. The Solar Forcing 7.2.2. The Gravity Wave Forcing 7.2.3. The Nonlinear Forcing 7.2.4. No Gravity Wave Forcing 7.2.5. No Nonlinear Forcing 7.3. Hough modes in Model experiments 7.3.1. SOL Hough modes 7.3.2. GW Hough modes 7.3.3. NLIN Hough modes 7.3.4. Hough modes: Seasonal cycle 7.4. Nonlinear Tidal Interactions 7.4.1. Model run without SDT/SDT interaction 7.4.2. Model run without DT/TDT interaction 7.4.3. Model run without tide-tide interaction 7.5. Solar Tidal Interactions 7.6. Interactions of Different Forcing Mechanisms 7.6.1. Interaction between Nonlinear and Solar Forcing 7.6.2. Interaction between Gravity wave and Solar Forcing 7.7. Influence of Enhanced Forcing Mechanisms 7.7.1. Influence of Enhanced Solar Forcing Mechanisms 7.7.2. Influence of Enhanced Gravity Wave Forcing Mechanisms 7.7.3. Influence of Enhanced Nonlinear Forcing Mechanisms 8. Summary and Conclusion 9. Outlook / In the middle and upper atmosphere atmospheric solar tides play an important role in the dynamics and vertical transport of energy and momentum from the stratosphere. They are primarily excited by absorption of solar radiation in the troposphere and stratosphere. The periods of the solar tides correspond to the harmonic components of the daily variation of solar radiation. Meanwhile, the diurnal, semidiurnal and terdiurnal tides have been relatively well studied, which is not the case with the quarterdiurnal tide. The knowledge about this tide is so far rudimentary, especially with regard to global climatology and details of possible excitation mechanisms and interactions. This is due to the fact that the amplitudes are very small and a high temporal resolution is required for the analysis. The quarterdiurnal tide has been observed by ground-based instruments and remote sensing methods, which until now have only provided a spatially and temporally limited overview of the quarterdiurnal tide. Since it is not possible to measure the contributions of the individual excitations, numerical models must be used as a powerful tool. With the numerical models it is possible to separate the different excitation mechanisms and to analyse their contributions for the quarterdiurnal tide. Model studies so far did not provide a comprehensive picture of QDT and did not consider QDT gravity wave excitation. This work is intended to extend the knowledge on this topic by using a nonlinear, mechanistic, global circulation model. A comprehensive numerical study will be carried out to investigate the importance and the interaction of the three quarterdiurnal excitation mechanisms, i.e. direct solar excitation, nonlinear tidal interactions and gravity wave tidal interactions. For the first time, excitation terms beyond the heating rates will be analyzed and quantified and the interactions of the quarterdiurnal tides from different sources will be investigated. Furthermore, different tidal modes will be investigated to identify quarterdiurnal tide interactions from the different excitation mechanisms. Furthermore, the theoretical Hough modes are used to derive those quarterdiurnal modes that are significantly responsible for the meridional structure in the model simulations. This elaborate and comprehensive model study analyses the excitation mechanisms and their interaction of the quarter-day tide. The work thus helps to raise the understanding of wave propagation in the middle atmosphere to a new level.:1. Tides in the Middle Atmosphere - An Introduction 2. Quarterdiurnal Solar Tides 2.1. Forcing of Quarterdiurnal Tides 2.1.1. Overview of the different Forcing Mechanisms 2.1.2. Theoretical Consideration of the Nonlinear Forcing Mechanism 2.2. Observations and Model Study of the QDT 2.3. Summary and Outlook 3. The Middle and Upper Atmosphere Model (MUAM) 3.1. Introduction 3.2. Numerical Properties 3.3. Model Physics 3.4. Parameterizations 3.5. Background Climatology 4. Mathematical and Numerical Methods 4.1. Fast Fourier Transform 4.2. Harmonic Analysis 5. MUAM: Sensitivity Studies 5.1. Influence of Horizontal Resolution on the Background Climatology and QDT amplitudes 5.2. Influence of the Initial Conditions on the Background Climatology and QDT amplitudes 5.3. Influence of temporal resolution on the Background Climatology and QDT amplitudes 6. MUAM: Climatology of the Quarterdiurnal Tide 6.1. Amplitudes 6.2. Phases and Vertical Wavelengths 6.3. QDT reconstruction with Hough modes 7. MUAM: The Quarterdiurnal Tide Forcing Mechanisms 7.1. The Quarterdiurnal Forcing Terms 7.2. Model Experiments and Single Forcing Mechanisms 7.2.1. The Solar Forcing 7.2.2. The Gravity Wave Forcing 7.2.3. The Nonlinear Forcing 7.2.4. No Gravity Wave Forcing 7.2.5. No Nonlinear Forcing 7.3. Hough modes in Model experiments 7.3.1. SOL Hough modes 7.3.2. GW Hough modes 7.3.3. NLIN Hough modes 7.3.4. Hough modes: Seasonal cycle 7.4. Nonlinear Tidal Interactions 7.4.1. Model run without SDT/SDT interaction 7.4.2. Model run without DT/TDT interaction 7.4.3. Model run without tide-tide interaction 7.5. Solar Tidal Interactions 7.6. Interactions of Different Forcing Mechanisms 7.6.1. Interaction between Nonlinear and Solar Forcing 7.6.2. Interaction between Gravity wave and Solar Forcing 7.7. Influence of Enhanced Forcing Mechanisms 7.7.1. Influence of Enhanced Solar Forcing Mechanisms 7.7.2. Influence of Enhanced Gravity Wave Forcing Mechanisms 7.7.3. Influence of Enhanced Nonlinear Forcing Mechanisms 8. Summary and Conclusion 9. Outlook

SIW, Stratospheric Inferred Winds, is one of the upcoming Swedish research satellites, aiming to study the winds in the middle atmosphere by measuring the Doppler shifts in molecules such as O3 with a limb viewing sub-mm spectrometer. By conducting simulations on the expected observations, the pointing sensitivity requirement of the satellite was investigated to be able to optimise the matching of the wind vectors. The development of SIW is in the early phases and therefore few studies of this topic has been conducted. The Python program AMATERASU (Advanced Model for Atmospheric TeraHertz Radiation Analysis and Simulation) developed by P. Baron has been used to calculate the radiative transfer through the atmosphere with the aim of simulating the satellite observations and the induced pointing errors, both horizontal and vertical. The results indicate that the effects of the horizontal pointing errors are neglectable due to their small contributions to the measurements. If horizontal pointing errors were induced the scan bias could be corrected for in the retrieval to a good degree for all species and the wind. If not corrected for, H2O and O3 were the most affected species and something to take into consideration during further development of the satellite design. The optimisation of the wind vectors showed that the bias of the matching could get decreased by changing the angle between the satellite antennas. / SIW, Stratospheric Inferred Winds, är en av de nästkommande svenska forskningssatelliterna som ämnar att studera vindarna i mellanatmosfären genom att mäta dopplerskift från molekyler så som O3 med en sub-mm spektrometer som skannar atmosfären från sidan. Simuleringar av de förväntade observationerna genomfördes för att bestämma gränsen för känsligheten i pekningen i kravspecifikationen och optimera matchningen av vindvektorerna. Utvecklingen av SIW är fortfarande i ett tidigt stadie och studier kring detta är få. Pythonprogrammet AMATERASU (Advanced Model for Atmospheric TeraHertz Radiation Analysis and Simulation) som är utvecklat av P. Baron har använts för att räkna ut strålningstransfern genom atmosfären och därmed kunna simulera satellitens förväntade observationer och de inducerade pekfelen, både horisontella och vertikala. Resultaten indikerar att effekterna av de horisontella pekfelen är försumbara på grund av deras låga påverkan på mätningarna. Vid vertikala pekfel kunde pekfelet korregeras för i återvinningen i hög grad för alla ämnen och vinden. Om pekfelet inte korregerades för var H2O and O3 de mest påverkade molekylerna och något som måste tas i beaktning under vidare utveckling av satellitdesignen. Optimeringen av vindvektorerna visade att skillnaden för matchningen kan minskas genom att ändra vinkeln mellan satellitantennerna.

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.

<p>The results presented in this PhD thesis are mainly based on measurements collected by the advanced sub-mm radiometer (SMR) aboard the Odin satellite in 2002-2007. The primary data are series of temperature and ozone profiles in the middle atmosphere up to 68 km. These data are used to estimate global properties of planetary wave propagation in both horizontal and vertical directions. As good-quality retrievals from Odin are not available above 68 km, additional data sources have been considered in order to extend coverage of planetary wave properties to higher levels. These sources are temperature observations at 85-90 km obtained by the ground-based meteor radars located in the polar region in the Northern Hemisphere in Scandinavia at Esrange and at Andenes, and in Canada at Resolute Bay and at Yellowknife. Also, the series of ozone profiles from the ground-based Kiruna mm-wave radiometer, KIMRA, are used in order to compare the wave properties in ozone fields measured globally by Odin and locally by KIMRA.</p><p>The main task of this PhD thesis is to study the 5-day planetary wave characteristics in the Earth’s atmosphere. The influence of waves on the atmospheric circulation causes, for example, substantial local departures from radiative equilibrium, observed in the winter stratosphere and close to the summer mesopause. Seasonal variations of the 5-day planetary wave properties and physical phenomena related to these variations are also studied in this thesis.</p><p>During winter, planetary waves propagate freely in the vertical direction, and maximal wave amplitudes are found in the extratropical stratosphere. The Northern Hemisphere (NH) winter periods of 2002-2003 and 2005 have been examined and a comparison has been carried out between the planetary wave properties in temperature and ozone variations. In general, the results show an expected in-phase behavior between the temperature and ozone fields in the lower stratosphere (due to dynamic effects) and an out-of-phase pattern in the upper stratosphere (which is expected as a result of photochemical effects).</p><p>Earlier theoretical and experimental studies have shown that, despite unfavourable summertime wind conditions, 5-day planetary waves can be registered not only in the stratosphere but also at higher altitudes in the mesosphere. The NH summers of 2003-2005 and 2007 have been considered and results have confirmed the existence of 5-day planetary waves up to the mesopause level (85-90 km). The results demonstrate that, for different periods, the possible source of the observed waves could be located at lower altitudes in both hemispheres with successive propagation into the summer mesosphere, or the waves could be generated in-situ as a result of the baroclinic instability of summer easterly jet.</p>

The results presented in this PhD thesis are mainly based on measurements collected by the advanced sub-mm radiometer (SMR) aboard the Odin satellite in 2002-2007. The primary data are series of temperature and ozone profiles in the middle atmosphere up to 68 km. These data are used to estimate global properties of planetary wave propagation in both horizontal and vertical directions. As good-quality retrievals from Odin are not available above 68 km, additional data sources have been considered in order to extend coverage of planetary wave properties to higher levels. These sources are temperature observations at 85-90 km obtained by the ground-based meteor radars located in the polar region in the Northern Hemisphere in Scandinavia at Esrange and at Andenes, and in Canada at Resolute Bay and at Yellowknife. Also, the series of ozone profiles from the ground-based Kiruna mm-wave radiometer, KIMRA, are used in order to compare the wave properties in ozone fields measured globally by Odin and locally by KIMRA. The main task of this PhD thesis is to study the 5-day planetary wave characteristics in the Earth’s atmosphere. The influence of waves on the atmospheric circulation causes, for example, substantial local departures from radiative equilibrium, observed in the winter stratosphere and close to the summer mesopause. Seasonal variations of the 5-day planetary wave properties and physical phenomena related to these variations are also studied in this thesis. During winter, planetary waves propagate freely in the vertical direction, and maximal wave amplitudes are found in the extratropical stratosphere. The Northern Hemisphere (NH) winter periods of 2002-2003 and 2005 have been examined and a comparison has been carried out between the planetary wave properties in temperature and ozone variations. In general, the results show an expected in-phase behavior between the temperature and ozone fields in the lower stratosphere (due to dynamic effects) and an out-of-phase pattern in the upper stratosphere (which is expected as a result of photochemical effects). Earlier theoretical and experimental studies have shown that, despite unfavourable summertime wind conditions, 5-day planetary waves can be registered not only in the stratosphere but also at higher altitudes in the mesosphere. The NH summers of 2003-2005 and 2007 have been considered and results have confirmed the existence of 5-day planetary waves up to the mesopause level (85-90 km). The results demonstrate that, for different periods, the possible source of the observed waves could be located at lower altitudes in both hemispheres with successive propagation into the summer mesosphere, or the waves could be generated in-situ as a result of the baroclinic instability of summer easterly jet.

Une grande partie de la variabilité naturelle de l’atmosphère et du climat est liée à la variabilité solaire. L’un des modes d’action du forçage solaire repose sur des perturbations de la moyenne atmosphère (stratosphère, mésosphère), notamment par l’intermédiaire de variations d’ozone (processus photochimiques) qui ensuite se propagent dans la troposphère jusqu’à la surface. La thèse se focalise sur la première étape de ce mode d’action, i.e. les perturbations de l’ozone associées à la variabilité solaire et plus particulièrement aux échelles de temps du cycle à 27 jours. Cette relation entre ozone et variabilité solaire est étudiée non seulement à partir de plusieurs séries temporelles de données satellitaires (MLS et GOMOS) mais également de résultats d’un modèle de chimie-climat (LMDz-Reprobus) sur des fenêtres d’analyse variant de 1 à 15 ans. La sensibilité moyenne d’ozone au cycle solaire à 27 jours (% de variation d’ozone pour 1% de variation du forçage solaire) se caractérise par des valeurs positives de 10 à 1 hPa avec un maximum de 0.4 vers 3 hPa. Cette sensibilité varie beaucoup selon la taille de la fenêtre d’analyse au point d’être masquée par la variabilité dynamique, même pendant les périodes de forte activité solaire. La dispersion des résultats apparaît aussi anti-corrélée à l’amplitude des fluctuations solaires rotationnelles qui est liée à la phase du cycle solaire à 11 ans. Dans la mésosphère, l’ozone est anti-corrélé à la variabilité solaire avec un maximum autour de 80 km. Il correspond exactement à l’altitude où la réponse de OH (le radical dominant dans la destruction de l’ozone mésosphérique) à la variabilité solaire est maximum. / A large part of the natural variability of the atmosphere and climate is related to solar variability. One of the forcing mechanisms of solar variability is based on perturbations of the middle atmosphere (stratosphere, mesosphere), particularly through ozone variations (photochemical processes), that then propagate through the troposphere to the surface. The thesis focuses on the first stage of this forcing mechanism, i.e. perturbations of ozone associated with solar variability and more specifically at the 27-day solar rotational time scales. The relationship between ozone and solar variability is studied not only using several time series of satellite data (MLS and GOMOS) but also results of a chemistry-climate model (LMDz-Reprobus) over analysis windows varying from 1 to 15 years. The mean ozone sensitivity to the 27-day solar cycle (% of ozone variation for 1% change in solar forcing) is characterized by positive values from 10 to 1 hPa with a maximum of 0.4 at 3 hPa. This sensitivity varies strongly depending on the size of the analysis window indicating that the solar signal can be masked by the dynamical variability, even during periods of strong solar activity. The dispersion of the results is found to be anti-correlated with the amplitude of the solar rotational fluctuations that are related to the phase of the 11-year solar cycle. In the mesosphere, ozone is found to be anti-correlated with solar variability with a maximum around 80 km. This corresponds exactly to the altitude of the maximum in the solar-induced enhancement of OH, the dominant radical in the destruction of mesospheric ozone.

Atmospheric solar tides play an important role in the vertical transport of energy and momentum from the troposphere to the middle and upper atmosphere. They are primarily excited by the absorption of solar heating in the troposphere and stratosphere. The periods of solar tides are according to the harmonics of the diurnal solar radiation variations. While the diurnal and semidiurnal tides are relatively well investigated, the terdiurnal tide has gained less attention to date, especially with regard to its possible excitation mechanisms. These become more complex for higher harmonics because the direct solar forcing is smaller and further possible excitation mechanisms such as nonlinear tidal interactions, gravity wave-tide interactions or tidal-mean flow interactions come into play. The terdiurnal tide has been observed from various ground-based instruments and by remote sensing techniques, but these measurements only provide an overview of the total terdiurnal tide as a product of all forcing mechanisms. At present, it is not possible to measure the contribution from individual forcings. Therefore, numerical models provide a powerful tool to separate the different forcing mechanisms and to analyze their contribution and interplay. A few model studies exist about this topic but they do not provide a comprehensive picture, and they are partly contradicting. Possible reasons are the respective model features and setups or narrowly focused analyses. The terdiurnal gravity wave forcing, for example, has never been considered in these studies even though gravity waves are known to have a large impact on the middle atmosphere dynamics. To extend the knowledge of that topic, a nonlinear mechanistic global circulation model is used in this thesis. This is a comprehensive numerical study to investigate the relative importance of three different terdiurnal forcing mechanisms and their interplay, including the direct solar forcing, nonlinear tidal interactions and gravity wave-tide interactions. For the first time, the forcing terms itself are analyzed and quantified. Different tidal modes are correlated to identify tidal interactions. Model simulations are presented that show the contribution of individual forcings on the observed wave amplitude in the mesosphere and lower thermosphere. Finally, new coupling features between the different forcings are discovered that have not been reported before. All together, this modeling study is the most extensive and comprehensive analysis about the forcing mechanisms of the terdiurnal tide, and it helps to fill a significant gap in the understanding of wave propagation in the middle atmosphere.:Bibliographische Beschreibung Bibliographic Description Acronyms 1. Introduction 2. Terdiurnal Solar Tides 2.1. Review of the Climatology of Terdiurnal Tides 2.2. Forcing of Terdiurnal Tides 2.2.1. Theory of Forcing Mechanisms 2.2.2. Model Studies 2.3. Summary and Open Questions 3. The Middle and Upper Atmosphere Model (MUAM) 3.1. Overview 3.2. Numerical Properties 3.3. Dynamics 4. Climatology of the Terdiurnal Tide in MUAM 4.1. Amplitudes 4.2. Phases and Vertical Wavelengths 5. The Terdiurnal Forcing Mechanisms in MUAM 5.1. The Terdiurnal Forcing Terms – In-Situ Excitation 5.1.1. The Solar Forcing 5.1.2. The Gravity Wave Forcing 5.1.3. The Nonlinear Forcing 5.2. Nonlinear Tidal Interactions - Common Analysis Methods 5.2.1. The Wavelength Criterion 5.2.2. The Correlation Analysis 5.3. Propagating Terdiurnal Tides 5.3.1. The Removal of Excitation Mechanisms in a MUAM Simulation 5.3.2. Simulation Overview 5.3.3. Seasonal and Global Structure of the Terdiurnal Tide from Individual Forcing Mechanisms 5.3.4. Interactions Between Different Forcing Mechanisms 5.4. The Impact of Terdiurnal Forcing Terms on the Background Circulation 5.5. Summary of the Terdiurnal Forcing Mechanisms 6. Summary and Conclusion 7. Outlook References A. Appendix: MUAM Reference Simulation B. Appendix: MUAM Simulations with Removed Forcings Acknowledgements Curriculum Vitae Affirmation / Atmosphärische solare Gezeiten spielen eine bedeutende Rolle für den Vertikaltransport von Energie und Impuls aus der Troposphäre in die mittlere und obere Atmosphäre. Sie werden primär durch Absorption solarer Strahlung in der Troposphäre und Stratosphäre angeregt. Die Perioden der solaren Gezeiten entsprechen den harmonischen Anteilen der täglichen Variation solarer Strahlung. Während die täglichen und halbtägigen Gezeiten relativ gut erforscht sind, haben die dritteltägigen Gezeiten bisher weniger Aufmerksamkeit, insbesondere in Bezug auf ihre Anregungsmechanismen, auf sich gezogen. Diese werden für höhere harmoni-sche Anteile komplexer, da die solare Anregung geringer ist und weitere Anregungsmechanismen wie nichtlineare Wechselwirkungen zwischen Gezeiten untereinander, mit Schwerewellen oder mit der mittleren Strömung ins Spiel kommen. Die dritteltägigen Gezeiten wurden bereits vielfach von bodengebundenen Instrumenten und mit Fernerkundungsmethoden beobachtet, jedoch bieten diese Messungen lediglich einen Überblick über die gesamten dritteltägigen Gezeiten als Produkt aller Anregungsmechanismen. Bis heute ist es nicht möglich, die Beiträge einzelner Anregungen zu messen. Deshalb sind numerische Modelle ein mächtiges Werkzeug, diese verschiedenen Anregungen zu separieren und ihre Beiträge und ihr Zusammenspiel zu analysieren. Es gibt einige wenige Modellstudien zu diesem Thema, aber diese bieten kein umfassendes Bild und sind teilweise widersprüchlich. Mögliche Gründe sind die entsprechenden Modelleigenschaften und -einstellungen und die schmal fokussierte Analyse. So wurde z.B. die dritteltägige Schwerewellenanregung bisher nie in die Betrachtungen einbezogen, obwohl bekannt ist, dass Schwerewellen einen großen Einfluss auf die Zirkulation der mittleren Atmosphäre haben. Um das Wissen zu diesem Thema zu erweitern, wird in dieser Arbeit ein nichtlineares, mechanistisches, globales Zirkulationsmodell genutzt. Es handelt sich um eine umfassende numerische Studie, um die relative Wichtigkeit und das Zusammenspiel von drei dritteltägigen Anregungsmechanismen zu untersuchen: die direkte solare Anregung, nichtlineare Wechselwirkungen zwischen Gezeiten und Schwerewellen-Gezeiten-Wechselwirkungen. Zum ersten Mal werden die Anregungsterme selbst analysiert und quantifiziert. Verschiedene Gezeitenmoden werden korreliert, um Interaktionen zwischen Gezeiten zu identifizieren. Es werden Modellsimulationen vorgestellt, welche die Beiträge der einzelnen Anregungen zu den beobachteten Wellenamplituden in der Mesosphäre und unteren Thermosphäre zeigen. Schließlich werden neue Kopplungsmechanismen zwischen den verschiedenen Anregungen entdeckt, wovon zuvor noch nicht berichtet wurde. Zusammenfassend ist diese Modellstudie die aufwändigste und umfassendste Analyse zu den Anregungsmechanismen der dritteltägigen Gezeiten und sie hilft eine entscheidende Lücke zum Verständnis der Wellenausbreitung in der mittleren Atmosphäre zu füllen.:Bibliographische Beschreibung Bibliographic Description Acronyms 1. Introduction 2. Terdiurnal Solar Tides 2.1. Review of the Climatology of Terdiurnal Tides 2.2. Forcing of Terdiurnal Tides 2.2.1. Theory of Forcing Mechanisms 2.2.2. Model Studies 2.3. Summary and Open Questions 3. The Middle and Upper Atmosphere Model (MUAM) 3.1. Overview 3.2. Numerical Properties 3.3. Dynamics 4. Climatology of the Terdiurnal Tide in MUAM 4.1. Amplitudes 4.2. Phases and Vertical Wavelengths 5. The Terdiurnal Forcing Mechanisms in MUAM 5.1. The Terdiurnal Forcing Terms – In-Situ Excitation 5.1.1. The Solar Forcing 5.1.2. The Gravity Wave Forcing 5.1.3. The Nonlinear Forcing 5.2. Nonlinear Tidal Interactions - Common Analysis Methods 5.2.1. The Wavelength Criterion 5.2.2. The Correlation Analysis 5.3. Propagating Terdiurnal Tides 5.3.1. The Removal of Excitation Mechanisms in a MUAM Simulation 5.3.2. Simulation Overview 5.3.3. Seasonal and Global Structure of the Terdiurnal Tide from Individual Forcing Mechanisms 5.3.4. Interactions Between Different Forcing Mechanisms 5.4. The Impact of Terdiurnal Forcing Terms on the Background Circulation 5.5. Summary of the Terdiurnal Forcing Mechanisms 6. Summary and Conclusion 7. Outlook References A. Appendix: MUAM Reference Simulation B. Appendix: MUAM Simulations with Removed Forcings Acknowledgements Curriculum Vitae Affirmation

Using a mechanistic global circulation model, we analysed the trends of solar tides in the middle atmosphere. Forced by monthly mean assimilation of reanalysis data in the lower atmosphere and monthly adjusted CO2 and ozone distributions, the simulations represent a time period from January 1980 to May 2019. The time series of monthly mean wind and temperature amplitudes of all tidal components have been extracted from these data. Trend analyses by linear regression show prevailing negative trends in July and October for all tides and for all latitudes in the mesosphere and lower thermosphere. In April and January, however, trends are positive or negative, depending on the tidal component. Furthermore, the data set has been examined on possible trend changes via a statistical trend algorithm. A large part of those break points for the zonal wind amplitudes were found from 1985 to 1988 and from 2012 to 2015 for the investigated months January and April. Therefore, a clear relation between changes in the atmospheric ozone concentration and trends of the amplitudes of solar tides is not evident for the presented variables. / Unter Verwendung eines mechanistischen globalen Zirkulationsmodells wurden Trends von solaren Gezeiten in der mittleren Atmosphäre analysiert. Die Simulationen, die in den unteren Atmosphärenschichten mit monatlich gemittelten Reanalysedaten angetrieben wurden sowie mit angepassten CO2 und Ozonverteilungen, decken einen Zeitraum von Januar 1980 bis Mai 2018 ab. Aus diesen Daten wurden Zeitreihen für Monatsmittel in den Amplituden des Windes und der Temperatur für alle vier Gezeiten herausgefiltert. Die über lineare Regression gewonnen Trends ergeben -global betrachtet in der Mesosphäre und unteren Thermosphäre - vorwiegend negativsignifikante Trends im Juli und Oktober. Im April und Januar können jedoch je nach Gezeit und Parameter positive wie auch negative Trends vorkommen. Weiterhin wurden die Datenreihen auf mögliche Trendänderungen mit Hilfe eines statistischen Algorithmus untersucht. Ein Großteil dieser Trend-Wendepunkte in den Zonalwindamplituden liegen für die untersuchten Monate Januar und April in den Jahren von 1985 bis 1988 und von 2012 bis 2015. Eine direkte Verbindung zwischen Änderungen in der atmosphärischen Ozonkonzentration und Trends in den Amplituden solarer Gezeiten lassen sich in den hier behandelten Größen daher nicht ableiten.

The thesis deals with the manifestation of coupling processes of various timescales in the middle atmosphere. Longer and shorter timescales are represented here by the 11-year solar cycle (SC) and orographic gravity waves (oGWs) considered on the intraseasonal timescale of the north hemisphere winter, respectively. The first two chapters deal with the application of rigorous attribution the variability of temperature, ozone and circulation characteristics in the stratosphere and lower mesosphere with regard to the SC using multiple nonlinear techniques (support vector regression and neural networks) besides the multiple linear regression approach. The aliasing of the SC with volcanic eruptions or the El Niňo Southern Oscillation is qualitatively assessed and its impact on conclusions about the top-down coupling mechanisms discussed. The last chapter examines the role of parametrized oGWs in the lower stratosphere. The Himalayan hotspot reveals common features with sudden stratospheric warmings such as the residual circulation amplification leading to a warming and ozone enrichment in the polar latitudes of the lower stratosphere.