No description available.

No description available.

The polar vortex is a large scale cyclone located in the middle atmosphere near to the planet’s geographic poles. These vortices form during the hemispheric winter and break down in the spring of the following year. They may also break down in mid winter, causing a sudden stratospheric warming event (SSW). The vortex is thought to be preconditioned leading up to these warming events, resulting in the breakdown of the vortex. Integral diagnostics are used to investigate the stripping of air from the vortex as part of this preconditioning. Contour diagnostics of mass and circulation are calculated using ERA-40 reanalysis data for the stratosphere. The edge of the vortex is easily identifiable in these diagnostics as a high gradient of Ertel’s potential vorticity (PV), and the warming events are also clearly visible. From these the amount of air removed from the vortex is determined from the balance equation of the mass integral. These terms show that there are significant amounts of air removed from the vortex, with several stripping events identifiable in them through the winter, especially in those during which a major sudden warming event occurred. These stripping events can be seen in corresponding PV maps, where tongues of PV can be seen to be stripped from the vortex and mixed into the surrounding surf zone of turbulent air. From the integral diagnostics a Lagrangian measure of the meridional circulation in the stratosphere is also calculated. In the final part of the thesis a shallow water model is used to investigate a quantitative link between forcing and the amount of stripping of the vortex. It is found that when the forcing is large enough there is significant stripping of mass from the vortex. This does not lead to SSWs in all cases, and the total amount of stripping is not found to be proportional to the maximum amplitude of the forcing.

The wintertime North-Atlantic exhibits enhanced circulation variability relative to other areas of the globe and is a key determinant of weather and climate in the highly populated regions of Europe and Eastern North America. Previous work has linked extreme stratospheric polar vortex and planetary wave heat flux events with variability of the North-Atlantic circulation. To elucidate the role of the stratosphere in driving variability of the North-Atlantic circulation, the goal of this thesis is to clarify the relationship between extreme planetary wave heat flux and vortex events and understand the dynamical mechanisms driving extreme stratospheric planetary wave heat flux events using an idealized model. The relationship between extreme stratospheric planetary wave heat flux and polar vortex events is clarified by comparing and contrasting their composite lifecycles using reanalysis data. Extreme negative heat flux events, defined as those less than the 5th percentile of the wintertime wave-1 distribution, involve stratospheric EP-flux divergence producing an acceleration of the vortex whereas extreme positive heat flux events, defined as those greater than the 95th percentile, involve stratospheric EP-flux convergence producing a deceleration of the vortex. Similar but smaller magnitude heat flux (22th and 78th percentile) events contribute to the development of longer-timescale vortex events. Negative heat flux events precede strong vortex events, showing that strong vortex events are true dynamical events involving wave-mean flow interaction. Conversely, positive heat flux events precede weak vortex events. The tropospheric jet shifts in the North-Atlantic that occur almost simultaneously with extreme stratospheric heat flux events are shown to be comparable if not larger than those that follow extreme vortex events for several weeks. Next, a dry-dynamical core model is configured to capture the lifecycle of extreme positive and negative heat flux events seen in reanalysis. The events are not captured using the standard model setup with idealized wave-1 topography. A modified control simulation captures the key ingredients of the events: 1) the extremes of the stratospheric eddy heat flux distribution, 2) the cross-spectral correlation and phase between the stratosphere and troposphere, 3) the evolution of the eddy heat flux and EP-flux divergence, 4) the stratospheric evolution of the zonal-mean flow, including the NAM, NAM time-tendency, potential temperature time-tendency and stratospheric wave geometry, and 5) the tropospheric evolution, including the high-latitude wave-1 geopotential height pattern and mid-latitude jet shift. Comparison between the model and reanalysis reveals that higher-order planetary wavenumbers play a role prior to the events. Finally, the dry-dynamical core model is used to examine the large-scale dynamical mechanisms driving extreme stratospheric negative heat flux events and their coupling with the tropospheric circulation. An ensemble spectral nudging methodology is used to isolate the role of: 1) the tropospheric wave-1 precursor, 2) the stratospheric zonal-mean flow and 3) the higher-order wavenumbers. The events are partially reproduced when nudging the wave-1 precursor and the zonal-mean flow whereas they are not reproduced when nudging either separately. In contrast, nudging the wave-1 precursor and the higher-order waves reproduces the events, including the evolution of the zonal-mean flow. Mechanism denial experiments show that the higher-order planetary wavenumbers drive the events by modifying the zonal-mean flow and through wave-wave interaction. Nudging all tropospheric wave precursors confirms they are the source of the stratospheric waves. Nudging all stratospheric waves reproduces the coupling with the tropospheric circulation. Taken together, the experiments show that extreme stratospheric negative heat flux events are consistent with downward wave coupling from the stratosphere to the troposphere.

Research during the last two decades has established that variability of the winter polar stratospheric vortex can significantly influence the troposphere, affecting the likelihood of extreme weather events and the skill of long-range weather forecasts. This influence is particularly strong following the rapid breakdown of the vortex in events known as sudden stratospheric warmings (SSWs). This thesis addresses some outstanding issues in our understanding of the dynamics of this stratospheric variability and its influence on the troposphere. First, a geometrical method is developed to characterise two-dimensional polar vortex variability. This method is also able to identify types of SSW in which the vortex is displaced from the pole and those in which it is split in two; known as displaced and split vortex events. It shown to capture vortex variability at least as well as previous methods, but has the advantage of being easily applicable to climate model simulations. This method is subsequently applied to 13 stratosphere-resolving climate models. Almost all models show split vortex events as barotropic and displaced vortex events as baroclinic; a difference also seen in observational reanalysis data. This supports the idea that split vortex events are caused by a resonant excitation of the barotropic mode. Models show consistent differences in the surface response to split and displaced vortex events which do not project stongly onto the annular mode. However, these differences are approximately co-located with lower stratospheric anomalies, suggesting that a local adjustment to stratospheric potential vorticity anomalies is the mechanism behind the different surface responses. Finally, the predictability of the polar stratosphere and its influence on the troposphere is assessed in a stratosphere-resolving seasonal forecast system. Little skill is found in the prediction of the strength of the Northern Hemisphere vortex at lead times beyond one month. However, much greater skill is found for the Southern Hemisphere vortex during austral spring. This allows for forecasts of interannual changes in ozone depletion to be inferred at lead times much beyond previous forecasts. It is further demonstrated that this stratospheric skill descends with time and leads to an enhanced surface skill at lead times of more than three months.

Diese Arbeit beschäftigt sich mit der chemischen Ozonzerstörung im arktischen und antarktischen stratosphärischen Polarwirbel. Diese wird durch Abbauprodukte von anthropogen emittierten Fluorchlorkohlenwasserstoffen und Halonen, Chlor- und Bromradikale, verursacht. Studien in denen der gemessene und modellierte Ozonabbau verglichen wird zeigen, dass die Prozeße bekannt sind, der quantitative Verlauf allerdings nicht vollständig verstanden ist. <br><br> Die Prozesse, die zur Ozonzerstörung führen sind in beiden Polarwirbeln ähnlich. Allerdings fällt als Konsequenz unterschiedlicher meteorologischer Bedingungen der chemische Ozonabbau im arktischen Polarwirbel weniger drastisch aus als über der Antarktis. Der arktische Polarwirbel ist im Mittel stärker dynamisch gestört als der antarktische und weist eine stärkere Jahr-zu-Jahr Variabilität auf. Das erschwert die Messung des chemischen Ozonabbaus. Zur Trennung des chemischen Ozonabbaus von der dynamischen Umverteilung des Ozons im arktischen Polarwirbel wurde die Matchmethode entwickelt. Bei dieser Methode werden Luftpakete innerhalb des Polarwirbels mehrfach beprobt, um den chemischen Anteil der Ozonänderung zu quantifizieren. Zur Identifizierung von doppelt beprobten Luftpaketen werden Trajektorien aus Windfeldern berechnet. Können zwei Messungen im Rahmen bestimmter Qualitätskriterien durch eine Trajektorie verbunden werden, kann die Ozondifferenz zwischen beiden Sondierungen berechnet und als chemischer Ozonabbau interpretiert werden. Eine solche Koinzidenz wird Match genannt. Der Matchmethode liegt ein statistischer Ansatz zugrunde, so dass eine Vielzahl solcher doppelt beprobter Luftmassen vorliegen muss, um gesicherte Aussagen über die Ozonzerstörung gewinnen zu können. So erhält man die Ozonzerstörung in einem bestimmten Zeitintervall, also Ozonabbauraten. Um die Anzahl an doppelt beprobten Luftpackten zu erhöhen wurde eine aktive Koordinierung der Ozonsondenaufstiege entwickelt. <br><br> Im Rahmen dieser Arbeit wurden Matchkampagnen während des arktischen Winters 2002/2003 und zum ersten Mal während eines antarktischen Winter (2003) durchgeführt. Aus den gewonnenen Daten wurden Ozonabbauraten in beiden Polarwirbeln bestimmt. Diese Abbauraten dienen zum einen der Evaluierung von Modellen, ermöglichen aber auch den direkten Vergleich von Ozonabbauraten in beiden Polarwirbeln. <br><br> Der Winter 2002/2003 war zu Beginn durch sehr tiefe Temperaturen in der mittleren und unteren Stratosphäre charakterisiert, so dass die Matchkampagne Ende November gestartet wurde. Ab Januar war der Polarwirbel zeitweise stark dynamisch gestört. Die Kampagne ging bis Mitte März. Für den Höhenbereich von 400 bis 550 K potentieller Temperatur (15-23 km) konnten Ozonabbauraten und der Verlust in der Gesamtsäule berechnet werden. Die Ozonabbauraten wurden in verschiedenen Tests auf ihre Stabilität überprüft. <br> Der antarktische Polarwirbel war vom Beginn des Winters bis Mitte Oktober 2003 sehr kalt und stellte Ende September kurzzeitig den Rekord für die größte bisher aufgetretene Ozonloch-Fläche ein. Es konnten für den Kampagnenzeitraum, Anfang Juni bis Anfang Oktober, Ozonabbauraten im Höhenbereich von 400 bis 550 K potentieller Temperatur ermittelt werden. Der zeitliche Verlauf des Ozonabbaus war dabei auf fast allen Höhenniveaus identisch. Die Zunahme des Sonnenlichtes im Polarwirbel mit der Zeit führt zu einem starken Anwachsen der Ozonabbauraten. Ab Mitte September gingen die Ozonabbauraten auf Null zurück, da bis zu diesem Zeitpunkt das gesamte Ozon zwischen ca. 14 und 21 km zerstört wurde. <br><br> Im letzten Teil der Arbeit wird ein neuer Algorithmus auf Basis der multivariaten Regression vorgestellt, mit dem Ozonabbauraten aus Ozonprofilen verschiedener Sensoren gleichzeitig berechnet werden können. Dabei können neben der Ozonabbaurate die systematischen Fehler zwischen den einzelnen Sensoren bestimmt werden. Dies wurde exemplarisch am antarktischen Winter 2003 für das 475 K potentielle Temperatur Niveau gezeigt. Neben den Ozonprofilen der Sonden wurden Daten von zwei Satellitenexperimenten verwendet. Die mit der multivariaten Matchtechnik berechneten Ozonabbauraten stimmen gut mit den Ozonabbauraten der Einzelsensor-Matchansätze überein. / <br>The subject of this thesis is the destruction of ozone in the stratospheric polar vortex of the Arctic and Antarctic. It is caused by decomposition products of anthropogenic emitted Chlorofluorocarbons and Halons, radicals of chlorine and bromine. Studies which are dealing with the comparison of measured and modelled ozone loss show that the processes are known but that the quantitative development is not fully understood yet. <br><br> The processes that lead to ozone destruction are similar in both polar vortices. But as a consequence of different meteorological conditions the chemical ozone loss in the arctic polar vortex is less dramatic than over the Antarctic. On average the Arctic polar vortex is stronger perturbed and exhibit a stronger annual variability. In order to distinguish between chemical ozone loss and the dynamical redistribution of ozone in the Arctic vortex the Match method was developed. Air parcels in the polar vortex are probed several times in order to quantify the chemical change in ozone. To identify those air parcels trajectories are calculated using wind fields. When it is possible to connect two measurements by a trajectory within certain quality criteria the difference in ozone can be calculated and is interpreted as chemical ozone loss. Such a coincidence is called a Match. The Match method is a statistical approach which needs many of those doubly probed air parcels in order to draw significant conclusions about the destruction of ozone. So the ozone destruction can be calculated for a certain period in time which gives ozone loss rates. In order to enhance the number of doubly probed air masses an active coordination was established. <br><br> Within the scope of the thesis Match campaigns were performed during the Arctic winter 2002/2003 and for the first time during the Antarctic winter 2003. The achieved data was used in order to determine ozone loss rates in both polar vortices. The loss rates serve for the evaluation of numerical models but allow as well the direct comparison of ozone loss rates of both polar vortices. <br><br> The beginning of the winter 2002/2003 is characterized by very low temperatures in the middle and lower stratosphere. Hence, the Match campaign started at the end of November. From January on the polar vortex was strongly dynamically disturbed at certain times. For the height region of 400 to 500 K potential temperature (15-23 km) ozone loss rates and the column loss were determined. The robustness of the ozone loss rates was checked with a variety of different tests. <br><br> From beginning of the winter until October 2003 the Antarctic polar vortex was very cold and the expansion of the ozone hole area reached record values in late September. From the beginning of June until the beginning of October ozone loss has been calculated in a height region from 400 to 550 K potential temperature. The development of the ozone loss was almost identical on the different height levels. The increase in sunlight led to an increase in ozone loss rates. From mid September the ozone loss rates decreased rapidly and the ozone was completely destroyed between approx. 14 and 21 km. <br><br> In the last part of the thesis a new algorithm is presented which is based on a multivariate regression in order to calculate ozone loss rates from ozone profiles made by different sensors. At the same time the systematic error between different sensors has to be considered. As an example the approach is shown for the Antarctic Winter 2003 on the 475 K potential temperature level. Beside the ozone profiles from the sondes, data from two satellites experiments have been used. The agreement between the ozone loss rates calculated by the multivariate regression method and those calculated by the single match approach is very good.

Gravity wave (GW) breaking and associated GW drag is not uniformly distributed among latitudes and longitudes. In particular, regions of enhanced GW breaking, so-called GW hotspots, have been identified, major Northern Hemisphere examples being located above the Rocky Mountains, the Himalayas and the East Asian region. These hotspots influence the middle atmosphere circulation both individually and in combination. Their interference is here examined by performing simulations including (i) the respective single GW hotspots, (ii) two GW hotspots, and (iii) all three GW hotspots with a simplified global circulation model. The combined GW hotspots lead to a modification of the polar vortex in connection with a zonal mean flow decrease and an increase of the temperature at higher latitudes. The different combinations of GW hotspots mainly prevent the stationary planetary wave (SPW) 1 from propagating upward at midlatitudes leading to a decrease in energy and momentum transfer in the middle atmosphere caused by breaking SPW 1, and in turn to an acceleration of the zonal mean flow at lower latitudes. In contrast, the GW hotspot above the Rocky Mountains alone causes an increase in SPW 1 amplitude and Eliassen–Palm flux (EP flux), inducing enhanced negative EP divergence, decelerating the zonal mean flow at higher latitudes. Consequently, none of the combinations of different GW hotspots is comparable to the impact of the Rocky Mountains GW hotspot alone. The reason is that the GW hotspots mostly interfere nonlinearly. Depending on the longitudinal distance between two GW hotspots, the interference between the combined Rocky Mountains and East Asian GW hotspots is more additive than the interference between the combined Rocky Mountains and Himalaya GW hotspots. While the Rocky Mountains and the East Asian GW hotspots are longitudinally displaced by 105°, the Rocky Mountains are shifted by 170° to the Himalayas. Moreover, while the East Asian and the Himalayas are located side by side, the interference between these GW hotspots is the most nonlinear because they are latitudinally displaced by 20°. In general, the SPW activity, e.g., represented in SPW amplitudes, EP flux or Plumb flux, is strongly reduced, when the GW hotspots are interacting with each other. Thus, the interfering GW hotspots mostly have a destructive effect on SPW propagation and generation.

Dynamic Systems Theory (DST) and spectral analysis are employed to study the tropospheric jet stream and the stratospheric polar vortex. The objective is to investigate the relationship between Rossby wave activity and inverse persistence and dimensionality of geopotential height at 250 hPa and 10 hPa, as these two dynamical indicators are expected to show a characteristic behavior of Rossby wave harmonics. The results show that persistent states exhibit suppressed Rossby wave activity for eastward-propagating Rossby waves, whereas it is increased for the westward counterpart. Positive anomalies of spectral power at positive phase speeds are present for less persistent states. Events with low dimensionality relate to the suppression of most Rossby waves, while an increase in spectral power is present during high dimensional states. The results were more pronounced in the stratosphere compared to the troposphere with different spatial patterns of geopotential height anomalies due to additional factors influencing the location of Rossby waves. Furthermore, Sudden Stratospheric Warmings (SSWs) are connected to a decrease in persistence up to 2 weeks prior, followed by a significant increase in persistence and dimensionality, and reduced integrated spectral power. Strong Polar Vortex events (SPVs) exhibit the opposite behavior with an increase in persistence before and a decrease in persistence and dimensionality, and higher ISP afterward. Additionally, SSWs (SPVs) exhibit a suppression (enhancement) of Rossby wave activity in the stratosphere and to a lesser extent in the troposphere for eastwards traveling waves.

Master of Science / Department of Electrical and Computer Engineering / Anil Pahwa / The unusual weather events during the polar vortex of 2014 illuminated the needs for fuel diversity for power generation in order to allow reliable operation of the electricity grid. A system wide reliability assessment for winter months should be undertaken in addition to the summer months to ensure reliable operation of the electricity grid throughout the year. Severe weather conditions that lead to equipment malfunction during the polar vortex should be thoroughly investigated and remediations to ensure satisfactory future performance of the grid must be undertaken. Environmentally unfriendly emissions from power plants must be minimized but diversity of generation fuel must be maintained. Future energy policies must be formulated with consideration that approximately 14 GW of coal generation in Pennsylvania Jersey Maryland Regional Transmission Organization’s control area available during the polar vortex will be retired by 2015 and replaced with plants that utilize fuel types other than coal.

The mean strengths of the wintertime stratospheric polar vortices are known to be related to the phase of the quasi-biennial oscillation (QBO) in the tropical stratosphere from circulation statistics - the "Holton-Tan relationship". The principal topic of this thesis is improving understanding of the mechanism behind the QBO's influence. Following the example of previous studies, the QBO influence on the Northern Hemisphere (NH) extratropics on monthly time scales in an observational reanalysis is examined, and is shown to closely resemble the stratospheric Northern annular mode (NAM). It is argued that this may not be informative about the mechanism, as the response could be NAM-like for many different mechanisms. It is suggested that examining the transient response of the NH extratropics to forcing by the QBO would be much more informative, particularly on time scales of a few days. In a primitive equation model of the middle atmosphere, the long-term stratospheric NH response to imposed zonal torques is often found to be NAM-like under perpetual January conditions, with wave feedbacks making a very important contribution. However, the response in runs with a seasonal cycle is not NAM-like. Investigation of the transient responses indicates the wave feedbacks are qualitatively similar in each case but only strong enough under perpetual January conditions to make the long-term response NAM-like. This supports the hypothesis that feedbacks from large-scale dynamics tend to make the stratospheric response to arbitrary forcings NAM-like, and therefore indicates that the long-term response is not generally useful for understanding forcing mechanisms. Examining the short-term transient response to known torques is found to be more successful at inferring information about the torques than several other previously proposed methods. Finally, the short-term transient response of the NH extratropics to forcing by the easterly QBO phase in a general circulation model is found to be consistent with the proposed mechanism of Holton and Tan (1980), indicating that this mechanism plays a role in the Holton-Tan relationship.