<p>The Southeastern United States (SE US) is one of the fastest developing regions of the nation, where summer precipitation becomes increasingly important to sustain population and economic growth. In recent decades, the variability of SE US summer precipitation has significantly intensified, leading to more frequent and severe climate extremes. However, the processes that have caused such enhanced climate variability have been poorly understood. By analyzing atmospheric hydrological cycle, diagnosing atmospheric circulation dynamics, and performing regional climate simulations, this dissertation investigates the mechanisms responsible for SE US summer precipitation variability. </p><p>Analysis of regional moisture budget indicates that the variability of SE US summer precipitation is primarily controlled by moisture transport processes associated with the variation of the North Atlantic Subtropical High (NASH) western ridge, while local water recycling is secondary. As the ridge moves northwestward (NW) into the US continent, moisture transport pathway is away from the SE US and the upward motion is depressed. Thus, rainfall decreases over the SE US, leading to dry summers. In contrast, when the ridge moves southwestward (SW), moisture convergence tends to be enhanced over the SE US, facilitating heavier rainfall and causing wetter summers. However, as the ridge is located relatively eastward, its influence on the summer precipitation is weakened. The intensified precipitation variability in recent decades is attributed to the more frequent occurrence of NW- and SW-type ridges, according to the "NASH western ridge - SE US summer precipitation" relationship. </p><p>In addition, the "NASH western ridge - SE US summer precipitation" relationship acts as a primary mechanism to determine general circulation model (GCM) and regional climate model (RCM) skill in simulating SE US summer precipitation. Generally, the state-of-the-art GCMs that are capable of representing the abovementioned relationship perform better in simulating the variability of SE US summer precipitation. Similarly, the RCM simulated summer precipitation bias over the SE US is largely caused by the errors in the NASH western ridge circulation, with the physical parameterization playing a secondary role. </p><p>Furthermore, the relationship between the NASH western ridge and SE US summer precipitation well explains the projected future precipitation changes. According to the projection by the ensemble of phase-5 of Coupled Model Intercomparison Project (CMIP5) models, summer precipitation over the SE US will become more variable in a warming climate. The enhancement of precipitation variability is due mainly to the atmospheric circulation dynamics, resulting from the pattern shift of the NASH western ridge circulation. In a warming climate, the NASH circulation tends to intensify, which forces its western ridge to extend further westward, exerting stronger impact on the SE US summertime climate. As the ridge extends westward, the NW- and SW-type ridges occur more frequently, resulting in an increased occurrence of extreme summers over the SE US. </p><p>In summary, the studies presented in this dissertation identify the NASH western ridge as a primary regulator of SE US summer precipitation at seasonal scale. The "NASH western ridge - SE US summer precipitation" relationship established in this study serves as a first order mechanism for understanding and simulating processes that influence the statistics of extreme events over the SE in the current and future climate.</p> / Dissertation

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Two approaches have been used in this investigation. The first involved the employment of the technique of molecular modulation kinetic (MMK) spectroscopy in the study of reaction kinetics of transient species. Previous research at Aberdeen had established a systematic error in both the kinetic (<I>k</I>_obs) and spectroscopic (σ_λ) parameters determined. Therefore it was intended to investigate the source of this error through the study of the kinetics of the association of the trichloromethyl radical with oxygen, producing the trichloromethyl peroxy (CCl₃O₂) radical. However, over a period of thirty-three months, no sensible results were obtained, as a result of severe instrumental component problems and the lack of in-house technical expertise. The project ultimately had to be abandoned. Since the kinetic investigation and the attempted rectification of the many problems encountered with the MMK spectrometer constituted a major portion of the allocated research time, they are reported in the final chapter of this thesis. In the second approach, two computational procedures, applying the Rice-Ramsperger-Kassel-Marcus (RRKM) theory of unimolecular reactions, have been used to evaluate kinetic parameters for the decomposition of two distinct chemical systems, namely tert-butoxy radical and peracetic acid. The programmes used are the Rabinovitch-RRKM programme and the programme UNIMOL (Gilbert <I>et al</I>), the latter of which is the more sophisticated. Fall-off data for the decomposition of tert-butoxy radical have been modelled over the temperature range 402-443 K relative to the experimental Arrhenius parameters, evaluated from both thermal and photolysis rate data obtained in previous investigations. The reaction was found to be pressure dependent over the range 1-1500 Torr. The refined high-pressure limit rate parameters obtained in this work are found to be in reasonable agreement with related rate constants reported from different laboratories. A value of the decomposition rate constant is extrapolated for atmospheric conditions. The efficiencies of energy transfer for three inert bath-gases, namely sulphur hexafluoride, carbon tetrafluoride and nitrogen, at 402 K, are examined using the weak collision model. RRKM modelling predicts very little difference in the efficiencies of these bath-gases - an unexpected result.

<p> An analysis was conducted to assess the relationship between the Arctic Oscillation (AO) and wintertime climatology of the Midwest and Tennessee Valley regions of the USA. In particular, this study focuses on variation in wintertime temperatures and snowfall totals during the top ten most positive and negative AO winters for the aforementioned regions. In addition, NCEP/NCAR reanalysis composite maps were created and examined to evaluate the relationship between certain atmospheric parameters and the opposing phases of the AO. In the Midwest and Tennessee Valley regions, variation in wintertime mean temperature and snowfall totals are associated with strong phases of the AO. The top ten most negative AO winters resulted in below average temperatures and above average snowfall totals. In contrast, the top ten most positive AO winters resulted in above average temperatures and below average snowfall totals. In addition, variation in mean wintertime temperature and snowfall totals is less significant during positive AO winters in comparison to negative AO winters. The top 10 most positive and negative AO winters appear to provide a significant link between anomalous middle and upper atmospheric circulation and atypical surface weather patterns across the Midwest and Tennessee Valley regions.</p>

<p>Ground level ozone is a harmful air pollutant to humans and is not directly emitted. It is formed from the combination of nitrogen oxides (NO_x) and volatile organic compounds (VOCs) in the presence of warm temperatures and sunlight. The St. Louis metropolitan area is home to different types of industry and the citizens of the area rely on the interstate network to commute to and from work. A spatial analysis of the St. Louis metropolitan area's 2012 ozone season (April 1 - October 31) was conducted to investigate the relationships between ground level ozone and meteorological and climatological variables at the micro- and synoptic scales. Previous studies addressed these relationships but may not have accounted for the issue of autocorrelation. The some of the study variables experienced autocorrelation; however, by calculating the effective sample size the issue of autocorrelation was addressed. High maximum temperatures, little to no precipitation, low average wind speeds at the surface, coupled with dominant anticyclones/high pressure and little moisture aloft were found to be associated with the 40 days during which Federal ozone exceedances occurred. The days with the most exceedance were Fridays (8) while the fewest were observed on Sundays (3). Like most summers, the greatest number of exceedance days occurred during the month of July (16). Precursors to ozone, and persistent ozone itself, also led to extended periods of high ozone. All of these factors, combined with emissions from vehicles and from industry, led to days on which the surface air quality may have been detrimental to human health.

This thesis examines characteristics of the tropical cold-point tropopause (CPT)and associated dynamic mechanisms using observations, idealized model, and generalcirculation model simulations. Fine-scale structure of the CPT is investigated usingnewly available high-resolution temperature data. Then, the dynamic mechanism ofthe CPT formation is rigorously tested and examined using a dry primitive equationmodel under the framework of transformed Eulerian-mean. Finally, the characteristicsof the CPT are examined in the state-of-art climate models that participated inthe Coupled Model Intercomparison Project Phase 5 (CMIP5). The current climatemodels' capability to reproduce the CPT features is evaluated and discussed.In the observational study, climatology, seasonal cycle, and intraseasonal variabilityof three CPT properties (temperature, pressure, and sharpness) are examinedusing high-resolution temperature profiles derived from COSMIC global positioningsystem (GPS) radio occultation (RO) measurements. The climatology obtained fromthe GPS RO data captures the characteristic structure of the CPT successfully. Thethree CPT properties show coherent seasonal cycle in the tropics that the CPT iscolder, higher (lower in pressure) and sharper in boreal winter compared to that inboreal summer. This seasonality is consistent with that of the large-scale tropopauseupwelling, which is largely driven by near-tropopause processes. The variability onintraseasonal timescales is more likely controlled by the tropical deep convectionand associated tropical waves. Particularly, intraseasonal variability of the CPTproperties shows clear signature of Kelvin waves, with a secondary contribution byMadden-Julian Oscillation in the deep tropics.The formation mechanism of the CPT is further investigated using a dry primitiveequation general circulation model with a simplified thermal forcing. The simpledynamic system reproduces a distinct cold level at the top of the tropical troposphere,which is analogous to the CPT in the real atmosphere. The modeled CPT is primarilymaintained by adiabatic cooling due to upwelling at the tropical tropopause,and synoptic-scale wave forcing in the subtropical lower stratosphere is found as theprimary driver of the upwelling. Further investigation on the evolution mechanismsuggests that the formation of the CPT is a part of balancing process between waveforcings and mean flow response in the tropics, and this is likely an important processthat decides the vertical extent of the subtropical jet in the lower stratosphere.The thermal characteristics of the CPT examined in CMIP5 models exhibitreasonable agreements with observations. Historical simulations successfully capturethe spatio-temporal structure of the CPT on annual and seasonal timescales. The interannualvariability associated with El Ni˜no Southern Oscillation and intraseasonalvariability associated with equatorial waves are also roughly reproduced. However,many of the models still have overall warm bias near the CPT and non-negligiblebiases in the amplitude of variability in intraseasonal to interannual timescales. Inthe future projections based on the Representative Concentration Pathway (RCP)8.5 scenario, the models predict robust warming both at the 100-hPa and zero-lapseratelevels (estimated CPT), but cooling at the 70-hPa level. A weakened seasonalcycle in the temperature is also found in most models at both the 100- and 70-hPalevels, which could have an important implication for the stratosphere-troposphereexchange and related climate variability. / Cette thèse examine les caractéristiques du point froid de la tropopause tropicale, "tropical cold-point tropopause" (CPT) et les mécanismes dynamiques associés en utilisant des observations, un modèle idéalisé ainsi qu'on model de circulation globale. La fine structure du CPT est évaluée en utilisant de nouvelles mesures de températures à haute résolution. Ensuite, les mécanismes dynamiques de la formation du CPT sont testées de manière rigoureuse et examinées en utilisant un model sec basé sur les équations primitives dans le cadre du "Transformed Eulerian Mean". Finalement, les caractéristiques du CPT sont examinées avec les modèles climatiques de pointe du "Coupled Model Intercomparions Project Phase 5" (CMIP5). La capacité actuelle de ces modèles de reproduire les caractéristiques du CPT sont par la suite évaluées.Dans l'étude observationnelle, la climatologie, le cycle saisonnier et la variabilité intra-saisonnière de trois propriétés du CPT (température, pression et finesse) sont examinées en utilisant des profils de température haute-résolution dérivés du system de mesure par radio occultation gps COSMIC. La climatologie obtenue par COSMIC capture la structure caractéristique du CPT avec succès. Les trois propriétés du CPT montrent une évolution saisonnière cohérente dans les tropiques : le CPT est plus haut, plus froid, et plus fin lors de l'hiver boréal compare à l'été boréal. Cette saisonnalité est consistante avec les mouvements verticaux de large échelle qui sont principalement générées par des procédés locaux à la tropopause. La variabilité intra saisonnière est plus vraisemblablement contrôlée par la convection tropicale profonde et les ondes tropicales associées. Particulièrement, la variabilité saisonnière du CPT montre clairement une signature d'onde Kelvin, avec une contribution secondaire de la Madden-Julian Oscillation dans les tropiques. Les mécanismes de formation du CPT sont par la suite évalués en utilisant un model basé sur les équations primitives avec un forçage thermal simplifie. Le système dynamique simple reproduit un niveau froid distinct au sommet de la troposphère tropicale, qui est analogue au CPT de l'atmosphère terrestre. Le CPT modelé est principalement maintenu par de refroidissement adiabatique par ascension à la tropopause tropicale. Le forçage par onde synoptiques dans la basse stratosphère subtropicale est le mécanisme dominant régissant ce refroidissement par ascension. Une évaluation plus poussée suggère que la formation du CPT fait partie d'un processus de stabilisation entre le forçage d'onde et la réponse de la circulation moyenne dans les tropiques et que ce processus est vraisemblablement important pour décider de l'étendue verticale du courant jet subtropical dans la basse stratosphère.Les caractéristiques thermales du CPT observées dans les modèles CMIP5 démontrent une correspondance raisonnable avec les observations. Les simulations historiques capturent avec succès la structure spatio-temporelle du CPT sur des bases annuelles et saisonnières. La variabilité interannuelle associée avec le « El Nino-Southern Oscillation» et la variabilité intra saisonnière associée avec les ondes équatoriales sont reproduites avec succès. Cependant, plusieurs des modèles présentent une erreur chaude près du CPT et des erreurs non-négligeables dans l'amplitude de la variabilité intra saisonnière et interannuelle. Dans la projection future basée dur le scenario « Representative Concentration Pathway » (RCP) 8.5, les modèles prédisent un réchauffement au niveau du CPT, mais un refroidissement à 70 hPa. Un cycle saisonnier de températures plus faible est aussi observé dans la plupart des modèles au niveau de 100 et 70 hPa, ce qui pourrait avoir des implications importantes pour les échanges stratosphère troposphère et la variabilité climatique associée.

To a first approximation hurricanes can be considered as axisymmetric vor- tices. However, asymmetries often accompany the symmetric circulation. Spiral rainbands, elliptical eyewalls, polygonal eyewalls, secondary eyewalls, mesovortices, and deep rotating convective clouds are examples of such asym- metric structures. The contribution of these asymmetries to the intensity and structural changes of hurricanes are not well understood.In this thesis, a suite of numerical models (both two dimensional [2D] and three dimensional [3D]) along with a series of diagnostic tools (Empirical Normal Mode [ENM] method, linear eigenmode analysis, wave activities con- servations laws, Eliassen-Palm [EP] flux calculations, and eddy kinetic energy budget calculations), are used to study the underlying dynamics associated with: the lifetime of elliptical eyewalls (Chapter 2), the genesis of secondary eyewalls (Chapter 2,3), the origin of polygonal eyewalls and mesovortices and their contribution to the rapid intensification process (Chapter 4), and finally the role of asymmetric convective bursts to the intensification of developing hurricanes (Chapter 5). In Chapter 2, it is shown that the evolution and the lifetime of a 2D el- liptical eyewall may be controlled by the inviscid damping of sheared vortex Rossby waves (VRWs) or the decay of an excited quasimode. In the context of 2D vortex flows, it is also shown that secondary rings of enhanced vorticity (secondary eyewalls) can form by the radiation of VRWs that result from the release of barotropic instability, and the process of wave-mean flow in- teractions. In Chapter 3, the importance of VRWs to the secondary eyewall genesis is verified in a successful numerical simulation of hurricane Wilma (2005) that developed a secondary eyewall. In Chapter 4, an analysis of the simulated Wilma during the rapid intensification stage is performed to show that the observed polygonal eyewall in Wilma is the result of counterpropa- gating unstable VRWs. The analysis also indicates that the polygonal eyewall and mesovortices can assist in the rapid intensification process. Finally, in Chapter 5 it is shown that the response of a weak vortex to asymmetric con- vective bursts is dominated by the excitation of damped sheared VRWs that act to accelerate the symmetric flow through the radial transport of angular momentum. / D'une façon approximative, les ouragans peuvent être considérés comme des vortex axisymétriques. Cependant, des asymétries accompagnent souvent la circulation symétrique. Des bandes de pluie en spirale, des mésovortex et de profonds et tourbillonnants orages convectifs sont des exemples de tels structures. Le mur autour de l'oeil de l'ouragan (ou simplement le mur) peut aussi prendre des formes elliptiques ou polygonales. Parfois même, la présence de murs secondaires est observée. La contribution de ces asymétries à l'intensité et aux changements structuraux des ouragans sont mal compris. Dans cette thèse, nous utilisons plusieurs modèles en deux et trois dimensions (2D et 3D) ainsi que des outils de diagnostiques (incluant la méthode des "Empirical Normal Mode" [ENM], l'analyse des "eigenmode" linéaires, les lois dictant la conservation des activités ondulatoires, le calcul des flux d'Eliassen-Palm [EP] ainsi que les calculs des budgets d'énergie cinétique) afin d'étudier les effets dynamiques associées à: la dure de vie des murs elliptiques (chapitre 2), la genèse des murs secondaires (chapitres 2 et 3), l'origine des murs polygonaux et des mésovortex ainsi que leur contribution associé à l'intensification des ouragans (chapitre 4) et, finalement, le rôle des pulsations convectives aussi associés à l'intensification des ouragans en développement (chapitre 5). Dans le chapitre 2, nous démontrons que l'évolution et la durée de vie de murs elliptiques en 2D pourrait être contrôlés par l'amortissement inviscide d'ondes de Rossby tourbillonnantes (VRWs) ou de la dissipation d'un quasi-mode préalablement excité. Dans des simulations en 2D, il est aussi démontré que des anneaux secondaires de vorticité prononcée (des murs secondaires) peuvent être formés par la radiation de VRWs qui résultent de la libération d'instabilité barotropique ainsi que des interactions avec l'écoulement moyen. Dans le chapitre 3, l'importance des VRWs à la genèse des murs secondaires est vérifiée dans une successions de simulations numériques de l'ouragan Wilma (2005) qui a développé un mur secondaire. Dans le chapitre 4, l'analyse de la simulation de Wilma durant le stade d'intensification rapide est utilisée pour dmontrer que le mur de forme polygonale observé durant Wilma est le résultat de VRWs instables se propageant en sens contraire. Notre analyse indique aussi que les murs polygonaux et les mésovortex peuvent contribuer au processus d'intensification rapide des ouragans. Finalement, dans le chapitre 5, il est démontré que la réponse d'un vortex de faible intensité aux pulsations convectives asymétries est dominée par l'excitation de VRWs amortis et cisaillés qui accélêrent l'écoulement symétrique par le transport de moment angulaire.

Since water vapor plays an important role in the atmosphere, many different instruments capable of measuring quantities of vapor are available to atmospheric scientists. One such instrument is the microwave radiometer that can measure low-resolution profiles of water vapor. However, in the case of boundary layer and convective initiation studies, the resolution requirements by the scientific community are more demanding than what the instrumentation is currently capable of providing. Therefore, measurements techniques have been developed to help bridge this gap. One of these measurement techniques is tomography. The goal of this technique is to combine several similar instruments providing measurements at low ranging resolution or of integrated values, in such a way that they provide information on crossing paths, thereby making it possible to provide measurements at higher resolution. This technique has been used in meteorology to measure fields of water vapor and clouds. In this study, we examined the performance of the tomographic retrieval of a 2D water vapor field using ground-based microwave radiometry. Since microwave tomography requires the radiometers to scan near the horizon in order to provide measurements of water vapor near the surface, we first examined the biases that occur when scanning at these low elevations. The biases may be caused by the antenna beam width, the channel bandwidth, and the propagation models used in the instrument model. It was found that the errors in antenna beam width are larger that those of channel bandwidth. Then, we evaluated the information content and the accuracy of the microwave radiometer tomographic retrieval. The impact of different instrument characteristics (number of frequencies and elevation angles) and of different instrument set-ups (number and spacing between instrument) on the retrieval's solution was assessed. Although the use of multiple instruments in a tomographic set-up did improve the information content of the retrieval, the accuracy of the field was limited by the passive nature of the radiometric measurement, especially in the case of very variable water vapor fields. Finally, we investigated the impact of adding a Raman lidar to the microwave tomographic retrieval. This was done with the hope that including an active remote sensor would help the retrieval in variable fields. Three different ways to include the Raman lidar were tried. The simplest, and best way, to include the Raman lidar was to combine the observed vertical profile with the microwave radiometers in the retrieval. Here again, adding the Raman lidar improved the information content of the tomographic retrieval, but it did not significantly improve the accuracy of the retrieved field compared to the radiometer only retrieval. Therefore, although the tomographic technique did improve the information content and the accuracy of the solution in slow varying fields was good, the results in highly variable fields were disappointing. / Comme la vapeur d'eau joue un rôle important dans l'atmosphère, plusieurs instruments pouvant mesurer des quantités de vapeur sont disponibles aux scientifiques. Un tel instrument est le radiomètre à microondes qui peut fournir des données sur les profils de vapeur d'eau à basse résolution. Par contre, dans le cas d'études portant sur la couche limite ou sur l'initiation de la convection, la résolution requise par la communauté scientifique est au-delà des capacités courantes des instruments. Par conséquent, des techniques de mesure ont été développées pour combler le fossé. Une de ces techniques de mesure est la tomographie. Le but de cette technique est de combiner plusieurs instruments du même genre, qui fournissent des mesures à faible résolution en portée ou des mesures intégrées, de sorte qu'ils fournissent des informations sur des trajets qui se croisent. Ce qui permet de fournir des mesures à plus haute résolution. Cette technique a été utilisée en météorologie pour mesurer des champs de vapeur d'eau et de nuages.Dans cette étude, nous avons examiné la performance du recouvrement tomographique d'un champ de vapeur d'eau 2D en utilisant la radiométrie microonde à partir du sol. Vu que la tomographie microondes exige aux radiomètres de balayer près du sol afin de fournir des mesures de la vapeur d'eau en surface, nous avons commencé par examiner les biais qui se produisent lors du balayage à ces basses altitudes. Les biais peuvent être causées par la largeur du faisceau d'antenne, la largeur des canaux et les modèles de propagation utilisés dans le modèle de l'instrument. Il a été constaté que les biais causés par la largeur du faisceau sont plus importants que ceux causés par la largeur des canaux. Ensuite, nous avons évalué le contenu de l'information et de l'exactitude du recouvrement tomographique avec les radiomètres microondes. L'impact des différentes caractéristiques des instruments (nombre de fréquences et les angles d'élévation) et de la configuration des instruments (le nombre d'instrument et l'espacement entre eux) sur la solution tomographique a été évaluée. Bien que l'utilisation de plusieurs instruments dans une configuration tomographique a amélioré le contenu de l'information du recouvrement, la précision du champ a été limitée par la nature passive de la mesure radiométrique, en particulier dans le cas des champs de vapeur d'eau très variables.Enfin, nous avons étudié l'impact de l'ajout d'un lidar Raman au recouvrement tomographique microondes. Ceci a été fait avec l'espoir que l'ajout un instrument de télédétection actif aiderait au recouvrement des champs variables. Trois différentes façons d'inclure le lidar Raman ont été essayées. La façon la plus simple, et la meilleure, d'inclure le lidar Raman était de combiner le profil vertical observé par le lidar avec les données des radiomètres microondes dans le recouvrement. L'ajout du lidar Raman a amélioré le contenu de l'information du recouvrement tomographique. Cependant, ceci n'a pas amélioré la précision du champ recouvré de manière significative par rapport au recouvrement qui utilise seulement les radiomètres. Par conséquent, bien que la technique tomographique a amélioré le contenu de l'information et que la précision de la solution du recouvrement dans le cas des champs lisses est bonne, les résultats dans des champs variables sont décevants.