Zeitreihen von Monatsmittelwerten des Windes in der Mesosphäre/unteren Thermosphäre über Collm werden auf mögliche Korrelationen mit der Nordatlantischen Oszillation (NAO) und der Südlichen Oszillation (SO) hin untersucht. Während eine positive Korrelation bis in die 1990er Jahre existiert, schwächt sich diese in der Folge ab und kehrt sich teilweise um. Da NAO und SO gekoppelt sind, erfolgen diese Änderungen etwa zur selben Zeit. Die Änderung der Kopplung steht wahrscheinlich in Verbindung mit einer generellen Änderung der Dynamik der mittleren Atmosphäre.

Zeitreihen von Monatsmittelwerten des Windes in der Mesosphäre/unteren Thermosphäre über Collm werden auf mögliche Korrelationen mit der Nordatlantischen Oszillation (NAO) und der Südlichen Oszillation (SO) hin untersucht. Während eine positive Korrelation bis in die 1990er Jahre existiert, schwächt sich diese in der Folge ab und kehrt sich teilweise um. Da NAO und SO gekoppelt sind, erfolgen diese Änderungen etwa zur selben Zeit. Die Änderung der Kopplung steht wahrscheinlich in Verbindung mit einer generellen Änderung der Dynamik der mittleren Atmosphäre.

Mesopause region horizontal winds have been measured over Collm during winter 2000/2001 to investigate the response of the mesosphere/lower thermosphere region to a major stratwarm. The stratospheric warming resulted in a reversal of both the zonal and meridional wind. In the zonal component, this reversal was due to a planetary oscillation with period 10 days, which was even more pronounced in the vertical gradients, so that the stratwarm effect on the mesopause was owing to the superposition of an intensifying planetary wave (PW) and a slow overall decrease of the zonal prevailing winds. An effect of the stratospheric warming on the semidiurnal tide is also visible, namely by a decrease of the tidal amplitudes and rapid phase shifts in the order of 2 – 3 hours to later values. / Um die Auswirkungen von raschen Stratosphärenerwärmungen (Stratwarm) auf das Windfeld der Mesopausenregion zu untersuchen, wurden Messdaten des Horizontalwindes über Collm in den Monaten Januar und Februar 2001 untersucht, in denen eine große Stratosphärenerwärmung auftrat. Der Stratwarm zeigte sich im Windfeld in einer Höhe durch einen Rückgang des zonalen Grundwindes, sowie durch besonders starken Nordwind. Im Zonalwind zeigte sich eine deutliche Welle mit 10-tägiger Periode, die vermutlich den Stratwarm auslöste. Das hatte zur Folge, dass das Windfeld der Mesopausenregion während Stratwarm durch eine planetare Welle, sowie einen überlagerten Rückgang des Vertikalwindgradienten zusammengesetzt war. Ein Effekt des Stratwarm auf die halbtägigen Gezeiten war ebenfalls zu erkennen; die Gezeitenamplituden sind während der Stratosphärenerwärmung reduziert und die Phase liegt in einem kurzen Zeitraum um 2 – 3 Stunden später.

Langzeitmessungen des Windfeldes in der Mesopausenregion (~ 92km) an verschiedenen Stationen in den mittleren Breiten der Nordhemisphäre zeigen systematische zonale Variationen beim (zeitlich) gemittelten Zonal- und Meridionalwind und bei den Amplituden und Phasen der halbtägigen Gezeiten. Als eines der herausragenden Muster, die zonale Variationen in der unteren mittleren Atmosphäre anregen, wird der Einfluß der Geopotentialstörungen zur zonalen Wellenzahl 1 und 2, die mit dem winterlichen stratosphärischen Polarwirbel verbunden sind, auf das Windfeld in der oberen Mesosphäre / unteren Thermosphäre numerisch mit dem COMMA-Modell der mittleren Atmosphäre untersucht. Die Modellergebnisse zeigen eine gute Übereinstimmung der zonalen Variationen des mittleren Zonalwindes, die im Breitenbereich 52ÆN bis 56ÆN beobachtet werden und in der Größenordnung von 10 - 20 m/s liegen. Auch die halbtägigen Gezeitenamplituden und -phasen zeigen qualitative und quantitative Übereinstimmungen zwischen Beobachtungen und Modellergebnissen. / Long-term time series of wind field observations in the upper mesosphere / lower thermosphere region at different locations in the midlatitude region indicate longitudinal variability in the (time-) mean zonal and meridional wind and in the amplitudes and phases of the semidiurnal tide, too. Being one of the prominent patterns forcing zonal inhomogenities in the lower middle atmosphere, the influence of the zonal wavenumber 1 and wavenumber 2 disturbances connected with the winter Northern Hemisphere stratospheric polar vortex on the mesosphere- / lower thermosphere wind field is numerically investigated with the COMMA model. The model results show that the zonal variations through the stationary waves coincide with typical observed mean zonal wind differences between different stations along the midlatitude belt between 52ÆN and 56ÆN with values about 10- 20 m/s. Also, the amplitude and phase variations of the semidiurnal tide show qualitative and quantitative agreements between model results and observations.

A Rayleigh-scatter lidar operated from Utah State University (41.7°N, 111.8°W) for a period spanning 11 years ― 1993 through 2004. Of the 900 nights observed, data on 150 extended to 90 km or above. They were the ones used in these studies related to atmospheric gravity waves (AGWs) between 45 and 90 km. This is the first study of AGWs with an extensive data set that spans the whole mesosphere. Using the temperature and temperature gradient profiles, we produced a climatology of the Brunt-Väisälä (buoyancy) angular frequency squared, N2 (rad/s)2. The minimum and maximum values of N2 vary between 2.2×10-4 (rad/s)2 and 9.0×10-4 (rad/s)2. The corresponding buoyancy periods vary between 7.0 and 3.5 minutes. While for long averages the atmosphere above Logan, Utah, is convectively stable, all-night and hourly profiles showed periods of convective instability (i.e., negative N2). The N2 values were often significantly different from values derived from the NRL-MSISe00 model atmosphere because of the effects of inversion layers and semiannual variability in the lidar data. Relative density fluctuation profiles with 3-km altitude resolution and 1-hour temporal resolution showed the presence of monochromatic gravity waves on almost every night throughout the mesosphere. The prevalent values of vertical wavelength and vertical phase velocity were 12-16 km and 0.5-0.6 m/s, respectively. However, the latter has the significant seasonal variation. Using these two observed parameters, buoyancy periods, and the AGW dispersion relation, we derived the ranges of horizontal wavelength, phase velocity, and source distance. The prevalent values were 550-950 km, 32-35 m/s, and 2500-3500 km, respectively. The potential energy per unit mass Ep showed great night-to-night variability, up to a factor of 20, at all heights. Ep grew at approximately the adiabatic rate below 55-65 km and above 75-80 km. Step function decreases in Ep imply that the AGWs in between gave up considerable energy to the background atmosphere. In addition, Ep varies seasonally. Below 70 km, it has a semiannual variation with a maximum in winter and minima in the equinoxes. At the highest altitudes it has an annual variation with a maximum in winter and a minimum in summer.

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Numerical experiments with the Middle and Upper Atmosphere Model with modified parameterization settings were carried out to study the response of the mesosphere/lower thermosphere (MLT) wind circulation to the non-orographic gravity waves (GWs) originating from the lower atmosphere. The modification of the phase speed spectrum controls the height of the zonal wind reversal due to strengthening of the westerly winds. The simulation results obtained for various latitudinal distributions of the intensity of non-orographic GWs at the source level show that the zonal circulation is most sensitive to GWs variability at high latitudes. The latitudinal distribution of GW intensity, produced by the global distribution of convective processes and seasonal variations in GW sources, and modified phase speed spectrum made it possible to simulate the major zonal circulation structures observed by MLT wind radar. / Numerische Experimente mit dem Middle and Upper Atmosphere Model mit modifizierten Parametriesierungseinstellungen wurden durchgeführt, um die Reaktion der Zirkulation der in der Mesosphäre / unteren Thermosphäre (MLT) auf nicht-orographische Schwerewellen (SW) zu untersuchen, die von der unteren Atmosphäre ausgehen. Die Modifikation des Phasengeschwindikeitsspektrums steuert die Höhe der zonalen Windumkehr aufgrund der Verstärkung der Westwinde. Die Simulationsergebnisse für verschiedene Breitenverteilungen der Intensität nicht-orographischer SW in Quellenhöhe zeigen, dass die zonale Zirkulation am stärksten auf SW-Variabilität in hohen Breiten reagiert. Die Breitenverteilung der SW-Intensität, die durch die globale Verteilung konvektiver Prozesse und saisonaler Variationen der SW-Quellen erzeugt wird, und das modifizierte Phasengeschwindkeitsspektrum ermöglichten es, die wichgsten zonalen Zirkulationsstrukturen zu simulieren, die von einem MLT-Windradar beobachtet wurden.

This dissertation comprises multiple studies surrounding the middle atmosphere's chemistry, composition, and dynamics. The middle atmosphere refers to the region from ~ 10 km to ~ 100 km and consists of the Stratosphere, Mesosphere, and Lower Thermosphere. The Stratosphere, Mesosphere, and Thermosphere are bounded by pauses where the strongest changes in chemical composition, movement, density, and thermal behavior take place. While several studies in the past have investigated the chemical composition of the middle atmosphere and quantified the distribution of various species from the stratosphere to the lower thermosphere, seasonal variations and redistribution of species resulting from transport events make it important to continuously monitor the middle atmosphere. Dynamic events such as Sudden Stratospheric Warmings (SSW) impact the temperature gradient and the zonal mean wind pattern in the stratopause. Descent events triggered by SSWs result in enhanced transport of species from the lower thermosphere to the stratosphere. Temperature increments during SSWs have an important impact on Polar Stratospheric Clouds (PSCs), resulting in Antarctic ozone enhancement and a smaller ozone hole. The middle atmosphere is, thus, home to a diverse range of dynamics and chemistry, making it a critical subject that warrants attention from the science community. The continuous monitoring of the middle atmosphere is important to this end. Several satellite missions in the past have been dedicated to monitoring the middle atmosphere and collecting data for decades. However, continual revisions and revaluations of measurement approaches and the introduction of novel space instruments are necessary to compensate for the limitations associated with existing missions, expand the extant specimen database, and improve phenomenon-centric observations. The Solar Occultation for Ice Experiment (SOFIE) is one of the two instruments on the Aeronomy of Ice in the Mesosphere (AIM) spacecraft. The studies presented in this dissertation primarily focus on the use of SOFIE observations combined with results from other science missions, an atmospheric model, and other datasets. Chapter I is an overview of the research goals and the motivations that propelled this research. In Chapter II, a validation study of the Version 1.3 SOFIE ozone data against the Atmospheric Chemistry Experiment (ACE) and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) ozone data is presented. The SOFIE-ACE and SOFIE-MIPAS data pairs demonstrate similar variability in the ozone vertical profile. SOFIE vertical ozone profiles agree best with ACE from 30 - 70 km and MIPAS from 30-64 km. The mean difference values averaged over all seasons and both hemispheres are typically < 24% with ACE and < 20 % with MIPAS. Atomic oxygen is an important species in the mesopause region (~ 80 – 100 km) that impacts the region's ozone photochemistry and radiative balance. In Chapter III, SOFIE ozone measurements used to derive daytime atomic oxygen are compared to coincident retrievals from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument and the Naval Research Laboratory Mass Spectrometer Incoherent Scatter radar (NRLMSIS 2.0) model. The datasets agree qualitatively. Results indicate a strong seasonal variation of atomic oxygen with summer and wintertime maxima at ~ 84 km and 94 km, respectively. The middle atmospheric composition is redistributed by the transport of species during SSWs. In Chapter IV, the 2019 SSW in the northern hemisphere that triggered a large transport event from the lower thermosphere to the stratosphere is evaluated using SOFIE, ACE, and the Modern-Era Retrospective analysis for Research and Applications (MERRA-2) observations. The event was similar to the major SSW-triggered descent events in the northern hemisphere since 2004 and led to the enhancement of nitric oxide produced by Energetic Particle Precipitation, attributed to unusual meteorology. The transport peak descended by ~ 5-6 km every 10 days. An SSW event occurred in the southern hemisphere in 2019 and led to enhanced ozone in the stratosphere. In Chapter V, satellite instruments, ground station data, and measurements from NASA Ozone Watch are used to conclude that large temperature increments evaporated PSCs, resulting in the lower conversion of halogen reservoir species into ozone-destroying forms. Thus, a large ozone enhancement was recorded in 2019. Chapter VI concludes all findings and Chapter VII summarizes future work. / Doctor of Philosophy / The middle atmosphere is the region between ~ 10 and 100 km in the atmosphere and is comprised of the Stratosphere, Mesosphere, and Lower Thermosphere. The middle atmosphere is a dynamic region, and the chemistry of this region is subject to variations occurring naturally or those triggered by anomalous events such as Sudden Stratospheric Warmings (SSW). Several species in the middle atmosphere need to be measured continuously or reevaluated for improved understanding. Dynamical events in the middle atmosphere are responsible for transporting and redistributing species in the middle atmosphere. Thus, the continuous monitoring of the middle atmosphere is necessary. Novel approaches with improved techniques and approaches are thus important to explore the middle atmosphere and quantify the chemistry of the region. The Solar Occultation for Ice Experiment (SOFIE) instrument is an instrument onboard the Aeronomy of Ice in the Mesosphere (AIM) spacecraft. SOFIE typically measures at high latitudes and looks at a wide range of wavelengths. This dissertation uses SOFIE and other datasets to evaluate the varying chemistry and dynamics of the middle atmosphere. The dissertation addresses four research problems and assimilates them to evaluate the middle atmosphere. Ozone is an important species in the middle atmosphere, which is present in the highest quantity in the stratosphere, followed by the lower thermosphere (~ 85 – 100 km). Ozone is important as it absorbs ultraviolet radiations and impacts the stratospheric radiative balance. Missions in the past have monitored ozone in the middle atmosphere. Novel approaches and improved observation techniques to compensate for the limitations of past missions and the continuous measurement of ozone are necessary. Thus, ozone retrievals from SOFIE are validated against independent and established datasets to demonstrate the robustness and usability of the SOFIE ozone data product within the atmospheric science community. Atomic oxygen is an important species in the mesopause region (~ 80 – 100 km) because of its role in ozone photochemistry and impact on the radiative balance of the region. It is technologically challenging to make direct measurements of atomic oxygen; thus, most conventionally, derived measurements and model results are used. To this date, atomic oxygen has been understood in a limited capacity with several inaccuracies. To improve the understanding of atomic oxygen and fill the current knowledge gaps, atomic oxygen is derived from SOFIE ozone measurements during the daytime using the Chapman equations for ozone photochemistry. Further, the derived atomic oxygen is compared to other established datasets from satellite instrument-derived measurements and model predictions. The seasonal variability of atomic oxygen is evaluated with a focus on the difference in its behavior during summer and winter. Lastly, inter-hemispheric differences in atomic oxygen distribution are evaluated. Apart from the natural atmospheric variation in species, SSW-triggered transport events redistribute species in the atmosphere. The 2019 SSW event in the northern hemisphere was similar to those in 2004, 2006, 2009, and 2013. Large quantities of nitric oxide were transported from the lower thermosphere to the stratosphere. Air poor in water vapor and methane was also transported. Atomic oxygen was transported from the lower thermosphere to several kilometers below in amounts higher than usual. The increased nitric oxide concentration in the stratosphere due to the transport catalytically destroyed the ozone in the region. The vertical transport rates were calculated to understand the speed of the descent. The low geomagnetic index in 2019, like in all years besides 2004, indicates that these events are attributed to unusual meteorology. An SSW event took place in the southern hemisphere in 2019 during the Antarctic winter. This led to a large increase in temperature, which evaporated the Polar Stratospheric Clouds (PSCs). PSCs provide their surface for converting halogen reservoir species into ozone-destroying reactive forms. The absence of PSCs during and immediately after the SSW event led to a lower conversion of halogen reservoir species into reactive forms. Satellite instrument measurements agree with theoretical expectations. The 2002 SSW in the SH led to similar outcomes and are compared to the 2019 event. Large enhancements in ozone in 2019 led to the smallest ozone hole since ~ 1982.

Gravity waves have a significant dynamic effect in the mesosphere. In particular, they drive the mesospheric circulation and are the reason that the summer polar mesosphere is cooler than the winter polar mesosphere. This thesis examines whether the effects of gravity waves are largely determined by filtering effects which allow only gravity waves with certain properties to propagate into the atmosphere. The filtering of gravity waves above Scott Base, Antarctica is examined using a radiosonde derived gravity wave source function, an MF-radar derived mesospheric gravity wave climatology, and a model derived filtering function. Least squares fitting of the source function and filtering function to the observed mesospheric gravity wave climatology allows us to determine which gravity wave phase velocities and propagation direction are likely to be present in the mesosphere and the relative importance of filtering and sources in this region. It is concluded the blocking of eastward gravity waves is important in winter and westward waves in summer.

Gravity waves have a significant dynamic effect in the mesosphere. In particular, they drive the mesospheric circulation and are the reason that the summer polar mesosphere is cooler than the winter polar mesosphere. This thesis examines whether the effects of gravity waves are largely determined by filtering effects which allow only gravity waves with certain properties to propagate into the atmosphere. The filtering of gravity waves above Scott Base, Antarctica is examined using a radiosonde derived gravity wave source function, an MF-radar derived mesospheric gravity wave climatology, and a model derived filtering function. Least squares fitting of the source function and filtering function to the observed mesospheric gravity wave climatology allows us to determine which gravity wave phase velocities and propagation direction are likely to be present in the mesosphere and the relative importance of filtering and sources in this region. It is concluded the blocking of eastward gravity waves is important in winter and westward waves in summer.