This thesis examines the co-occurrence of wintertime low temperature extremes (cold spells) over North America and wet or windy extremes over Europe. Prolonged (≅ 5 days) North American cold spells can be associated with both upstream and downstream anomalous large-scale atmospheric flows, the latter modulating extreme weather occurrences over Europe. The approximate co-occurrence of European wet or windy extremes with North American cold spells is temporally and spatially dependent on the location of cold spells. We identify three broad regional clusters of cold spells:  Central Canada: cold spells are predominantly preceded by Iberian precipitation extremes. Eastern United States: occurrences of both wind and precipitation extremes in Iberia before and after the cold spell. East Canada: cold spells are predominantly followed by wind extremes over the British Isles and Northern Europe. We show that cold spells over these three regions are associated with distinct storm track and jet stream anomalies over the North Atlantic. Iberia experiences a higher number of extratropical cyclones during Central Canada cold spells due to an equatorward displacement of the jet. However, the propagation of extratropical cyclones over Europe is limited due to a Scandinavian block-type configuration over Northern Europe. Eastern United States cold spells show a similar configuration to Central Canada. However, this configuration does not show the Scandinavian block-type feature, resulting in a higher number of extratropical cyclones affecting Iberia and Southern Europe. Eastern Canada cold spells are associated with an extended and accelerated jet over Northern Europe. As a result, the UK, France and Northern Europe experience more intense extratropical cyclones on average, with the accelerated jet resulting in stronger extratropical cyclones throughout the North Atlantic in general. These results evidence the existence of a systematic statistical and dynamical connection between North American cold spells and European wet or windy extremes.

The QuikSCAT scatterometer provides global daily coverage of oceanic near-surface vector winds. Recently, algorithms have been developed to enhance the spatial resolution of QuikSCAT winds from 25~km to 2.5~km posting. These ultra-high resolution winds are used, in comparison with standard L2B data product winds, to observe and track tropical cyclones. Resolution enhanced winds are found to provide additional storm structure such as inner core size and structure and the presence of multiple eyewalls compared with standard resolution winds. The 2.5~km winds are also able to observe storms nearer to the shore than 25~km winds. An analysis of circulation center locatability with each resolution wind field is performed. Center fixes with enhanced resolution winds are nearer the National Hurricane Center best-track positions than are standard resolution center fixes. A data and image set of every tropical cyclone worldwide observed by Seawinds on QuikSCAT or SeaWinds on ADEOS II from 1999 through 2005 is generated and made available to the scientific community at http://scp.byu.edu.

Storm induced flooding presents a multifaceted threat to coastal communities across the world.With climate change and sea level rise this danger is expected to increase. As coastal communities become exposed to more frequent and more severe flooding, the need for protective measures will increase. To know how to optimally protect against coastal flooding requires an understanding of future flood risk, storms, and storm surge. These are challenging to estimate due to many sources of uncertainty. In this thesis I present a methodology to forecast this future flood risk. I combine multiple computational, physics and statistical models to accurately describe the fluid dynamics of flooding, the cyclones that drive surge, and how climate change will influence these different components in the future. These computational models must be fast so that they can be embedded into an optimization framework that makes many evaluations. To find an optimal protective measure I employ stochastic and derivative free optimization methods. A complete study is conducted on New York City and optimal protective strategies are found for minimizing the total cost from storm surge subject to different budget constraints.

No description available.

No description available.

No description available.

<p dir="ltr">Tornadoes have caused billions of dollars in damage and are one of the leading causes of weather-related deaths in the United States each year. Recent studies have suggested spatial shifts in tornado activity, though the reason is unclear. Extratropical cyclones (ETCs), which are strongly associated with the jet stream, are known to produce an environment favorable for tornadoes in their warm sector. However, little recent research has been done on the spatiotemporal relationship between tornadoes and ETCs, so there is a poor understanding of whether or not the changes in tornado activity are affected by ETC patterns. ERA5 reanalysis, ETC tracking, and historical tornado data from 1980-2022 are used to analyze the climatology of tornadoes relative to ETCs. We found that 73% of F/EF1+ tornadoes occurred within 2000km of an ETC and are likely associated with the ETC. Most of those tornadoes occurred near the median position around 465km away from and to the southeast of the ETC center. Of those tornadoes, 68% occurred in large outbreaks of 6 or more tornadoes, where most tornadoes formed closer to and to the southeast of an ETC track as compared to small outbreak and isolated tornadoes. The spatial and relative distributions were similar across all intensity levels, though stronger tornadoes tended to have more tornadoes directly to the southeast of an ETC. Seasonal variances in tornadoes strongly corresponded with seasonal changes in the jet stream. Summer tornadoes occurred in northern portions of the US when the jet stream shifts poleward. The jet stream and ETCs are also weakest in the summer, and the weakest association was found in summer tornadoes based on their distribution relative to ETCs being more uniform towards the northeast and north-southeast directions. Winter tornadoes occurred in more southern portions of the US when the jet stream shifts equatorward, and they had a stronger association with most of the tornadoes occurring to the southeast and closer to the ETC center, aligning with a strong ETC and jet stream in the winter. Finally, tornadoes and ETCs had strong spatial covariance and showed similar linear trends, including a similar rate of change in the eastward shift, providing strong evidence that a shift in ETCs may be driving the shift in tornadoes. Furthering our understanding of the relationship between tornadoes and ETCs will help to better predict how tornadoes will change in the future based on changes in ETCs.</p>

Depuis une vingtaine d'années, la qualité de la prévision de la trajectoire des cyclones tropicaux a fortement progressé mais peu d'améliorations ont été apportées à la prévision de l'intensité. Cette thèse s'intéresse aux changements d'intensité des cyclones tropicaux du sud-ouest de l'océan Indien sous l'angle original de l'activité électrique.Une première étude climatologique s'appuyant sur les données du réseau de détection d'éclairs World Wide Lightning Location Network (WWLLN) a permis de valider la qualité de ces données et de montrer que les cyclones tropicaux pouvaient être une source importante de production d'éclairs à l'échelle du bassin. À partir de ce résultat, une étude ciblée a été menée en considérant la spécificité du bassin et trois régions ont été définis (océan ouvert, région à proximité de la côte est de Madagascar et Canal du Mozambique). L'activité électrique des cyclones tropicaux dépend de la région où ils se trouvent ainsi que de leur stade d'intensité. Les éclairs semblent être un marqueur des phases d'intensification et d'affaiblissement dans certains situations. La troisième étape de cette thèse a consisté à simuler de manière idéalisée un cyclone tropical mature et a tenté d'expliquer les processus physiques à l'origine de l'activité électrique. Le modèle reproduit bien le comportement sporadique des éclairs habituellement observé. Des bilans montrent que ce cyclone présente trois phases caractéristiques, chacune associée à un comportement dynamique, microphysique et électrique différent. Enfin, une deuxième étude purement numérique a tenté d'identifier des estimateurs de l'activité électrique. Pourcelà deux approches ont été adoptées : une étude globale et une étude par cellule. Dans les deux cas, les meilleurs estimateurs sont la masse totale de graupel, le volume d'updraft et le produit des flux des masses de glace précipitante et non précipitante. / Tropical cyclone track forecast has improved over the past two decades but little improvement have been done in intensity changes forecast. This thesis focuses on the intensity changes of tropical cyclones in the southwest Indian ocean through the original aspect of lightning activity. A climatology of lightning activity in the southwest Indian ocean using data from the World Wide Lightning Location Network (WWLLN) is first proposed. Results showed that the WWLLN was able to locate and capture the lightning activity in the basin and that tropical cyclones can be considered as a major source of lightning flashes in some oceanic parts of this region. From this result, study of lightning activity in tropical cyclones of the southwest Indian ocean has been done considering the basin configuration. Thus, three regions were defined: open ocean, region near the eastern coast of Madagascar and the Mozambique Channel. The location and intensity of lightning activity depend on the region and the intensity stage. Lightning flashes seem to be a proxy of intensification or weakening under some conditions. The third step of this thesis was to simulate in an idealized framework a mature tropical cyclone. The model was first able to reproduce the sporadic behavior of lightning activity as observed. Budgets have shown that the presence of three distinctive phases characterized by different dynamical, microphysical and electrical behaviors. Finally, another purely numerical work tried to identify some proxies of lightning activity according two approaches : a global analysis anda per-cell analysis. In both cases, the best proxies are the total graupel mass, the updraft volume and the product of precipitating and non-precipitating ice mass fluxes.

Indiana University-Purdue University Indianapolis (IUPUI)

Cette thèse apporte une meilleure compréhension des interactions océan-atmosphère au sein d'évènements extrêmes que sont les cyclones tropicaux. L'étude de la réponse océanique aux cyclones et de sa rétroaction a permis de souligner l'importance de la dynamique océanique et atmosphérique et tend à contredire les estimations extrêmes faites précédemment à partir de modèles théoriques simplifiés. L'impact des cyclones sur le climat est probablement surestimé dans les études qui négligent les processus d'advection et la ré-émergence d'anomalies océaniques en surface durant l'hiver. La rétroaction négative du refroidissement de surface induit par les cyclones est également surestimée dans les études théoriques en raison des fortes hypothèses faites sur les échelles de temps impliquées dans le processus d'intensification des cyclones. De même, la structure océanique à grande et moyenne échelle est souvent négligée (par exemple dans les indices de cyclogenèse), alors qu'elle module fortement les mécanismes de couplage. Enfin, l'utilisation de modèles à méso-échelle et de simulations à long terme produisant un grand nombre d'événements est essentielle afin de séparer un mécanisme robuste d'effets anecdotiques.