The thesis consists of three projects. Each of these projects is a diagnostic study of the interaction between strong convective events and the background atmosphere. In all projects, we use a satellite rainfall dataset to identify strong rain events. We then use radiosonde soundings to generate composite anomaly patterns of meteorological variables about the strong rain events. In Project 1, we examine temperature, relative humidity, and divergence anomalies about strong convective events in the Western Tropical Pacific. A low-level convergence coupled to a midlevel divergence develops prior to peak rainfall. A midlevel convergence coupled to a low-level divergence develops after peak rainfall. Strong surface cold pools develop in response to high rainfall. Observations were compared to models and reanalyses. In general, models and reanalyses do not fully represent the timing, strength, and altitude of the mid-level convergence and divergence features. The surface cold anomaly is also underestimated in models. These discrepancies suggest that the mesoscale downward transport of mid-level air into the boundary layer in models may be too weak. In Project 2, we investigate the impact of convection on the background distribution of a chemical tracer (ozone). Negative ozone anomalies and higher frequency of midlevel cloud tops occur in a layer between 3 and 8 km prior to peak rainfall. Negative ozone anomalies in the upper troposphere develop in response to high rainfall. Chemistry transport model simulations also exhibit negative ozone anomalies at upper and midlevels. However, the ozone anomalies in the model are symmetric about peak rainfall and are more persistent than observations. In Project 3, we identify regional variations in the interaction between convection and the background atmosphere. In all four regions, deep convection imposes cooling in the lower and warming in the upper troposphere. In mid-latitudes, convection is associated with stronger anomalies in surface pressure, geopotential height, and CAPE. Over land, a low-level warm anomaly develops prior to peak rainfall and the surface cold pool that develops during peak rainfall is more persistent. The PV generated prior to peak rainfall, is advected towards the surface after peak rainfall and may play a role in hurricane genesis.

Cette étude a pour objectif d'analyser le rôle des ondes de Rossby dans la variabilité des courants-jets troposphériques des moyennes latitudes à l'aide d'un modèle numérique idéalisé. Elle s'intéresse aux mécanismes dynamiques responsables de la persistance des principaux modes de variabilité : celui de déplacement méridien et celui de pulsation d'amplitude. Le premier est souvent le principal mode de variabilité du fait de sa grande persistance causée par une rétroaction positive des ondes de Rossby.Deux nouveaux types de rétroactions négatives ont été mis en évidence à une échelle de temps plus courte que cette rétroaction classique et dont le mécanisme dépend de la nature des ondes impliquées. Ces différents mécanismes sont aussi retrouvés et ainsi validés dans le contexte plus réaliste des données de réanalyse.Une réflexion sur les conséquences du changement climatique est aussi proposée via l'étude de sensibilité menée sur trois paramètres clés du modèle : le gradient méridien de température, conduisant à un mode de déplacement plus persistant dans le futur, la position moyenne du jet, conduisant à un mode moins persistant pour des jets plus proches du pôle, et enfin la friction dans les basses couches de l'atmosphère, aussi étudiée car étant un paramètre plutôt difficile à évaluer et présentant une forte disparité entre les modèles.Cette thèse a donc permis de mettre en évidence deux nouveaux mécanismes de rétroaction des ondes sur les courants-jets et de développer des diagnostics théoriques qui pourront être plus amplement testés et appliqués dans d'autres contextes, particulièrement des réanalyses et des simulations de climat réalistes. / This study investigate the impact of Rossby waves on the tropospheric midlatitude eddy-driven jets using an idealized numerical model. It focuses on the dynamical mechanisms driving the persistence of the main modes of variability: a shifting mode and a pulsing mode. The shifting mode is often found to be the leading mode of variability due to an enhanced persistence caused by a positive feedback of Rossby waves. Two new kinds of negative feedbacks have been found for a shorter time-scale than the more classical feedback which mechanism depends on wave properties. These new mechanisms have also been found in the more realistic set up of reanalysis. The impact of climate change is also investigated using the sensitivity analysis of the model to three key parameters : the meridional gradient of temperature, which leads to longer lasting phases of the shifting mode in the future, the jet mean position, which leads to less persistent shifting mode for poleward shifted jets, and frictional damping, because it is a parameter difficult to tune and which varies between numerical models. In conclusion, two new feedback mechanisms acting on eddy-driven jets variability have been found and theoretical diagnostics have been developed and could be used to probe more realistic data such as future climate simulations and reanalysis.

The infrared atmospheric background modeling can be considered as one of the important key factor to develop a successful target detection technique. During the infrared atmospheric background modeling, defining the input parameters of the atmospheric profile are very important for the calculations of the absorption, emission and scattering effects of the atmosphere. The main objective of this thesis is to find the answer for the question &ldquo / is it possible to determine the &ldquo / effective&rdquo / height range for the sea level midlatitude clear weather conditions in the three special wavelength bands, 1-3 &amp / #61549 / m, 3-5 &amp / #61549 / m, 8-12 &amp / #61549 / m ?&rdquo / . The answer is important for three cases. These are to overcome the difficulties of finding all the parameters of the new atmospheric profile, to determine the dominant height range for the midlatitude region, and to shorten the time of the calculations of infrared background processes. In this study, it has been shown that it should be possible to determine the effective height range for sea level midlatitude clear weather conditions in the three special wavelength bands. As a result of this study it is shown that a new atmosphere model can be constructed more easily by overcoming the difficulties of finding all the parameters of the new atmospheric profile for the sea level clear weather midlatitude regions in a short time respectively. In this study the infrared radiation flux below 5 % difference between the whole, 100 km, and the effective height ranges is accepted.

Au sein de l’hémisphère sud, l’Afrique australe et le sud-ouest de l’océan Indien forment l’une des trois zones préférentielles de développement des interactions entre les tropiques et les moyennes latitudes. Il s’agit de la Zone de Convergence Sud-Indienne (ZCSI) où se forment, principalement en été austral, des systèmes synoptiques (entre 3 et 5 jours) caractérisés par des bandes nuageuses orientées nord-ouest/sud-est : les Talwegs Tropicaux-Tempérés (TTT). Cette recherche doctorale vise à améliorer les connaissances liées aux TTT, avec une étude sur la dynamique associée à ces systèmes et une analyse de leur évolution possible au cours du XXIème siècle. La première partie de l’étude s’attache à identifier les conditions favorables à la formation et au développement des TTT. Une classification en régimes de temps est utilisée pour identifier les TTT d’une part, et les perturbations des moyennes latitudes d’autre part, permettant de mieux documenter la variabilité spatio-temporelle des TTT et les conditions de leur développement. Ces événements sont responsables d’environ 20% des précipitations saisonnières sud-africaines. Cette contribution augmente selon un gradient ouest-est. La comparaison entre les deux classifications confirme que les perturbations des moyennes latitudes sont une condition nécessaire pour le développement de TTT, mais non suffisante. Dans les tropiques, des advections d’humidité depuis l’océan Atlantique tropical associées à un excès d'énergie statique humide sur le canal du Mozambique forment les conditions supplémentaires favorables à la convection atmosphérique profonde à proximité du continent. La seconde partie étudie comment les TTT, les précipitations et de manière plus générale le climat d’Afrique australe, pourraient évoluer au cours du XXIème siècle sous l’effet du forçage radiatif associé aux émissions de gaz à effet de serre (GES). Cette approche est multi-modèle (huit modèles climatiques sont retenus de l’exercice 5 du GIEC) et multi-trajectoire (RCP 8.5 et 2.6). Les huit modèles restituent convenablement les caractéristiques actuelles du climat d’Afrique australe ainsi que les bandes nuageuses associées aux TTT, tant en termes de variabilité spatiale que de fréquence d’occurrences. Sur le XXIème siècle, ils divergent sur l’évolution des précipitations saisonnières (NDJF). En revanche, ils convergent sur l’augmentation des quantités précipitées par jour de pluie sur le sud-est de l’Afrique australe. Ces changements ne sont pas à relier à une évolution spatio-temporelle des TTT, leur structure spatiale, leur fréquence d’occurrence et leur contribution aux précipitations restent stationnaires tout au long du siècle, mais à des événements pluviogènes extrêmes plus fréquents et plus intenses. / In the Southern Hemisphere, Southern Africa and the south-west Indian Ocean are one of the three preferred regions where interactions between the tropics and midlatitudes develop. This is the South Indian Convergence Zone (SICZ), where northwest-southeast oriented cloud bands form at the synoptic scale (between 3 and 5 days). These bands are mainly found during the austral summer and are commonly referred to as tropical temperate troughs (TTTs). This research aims at improving our knowledge related to TTTs, with a study on the dynamics associated with these systems, and an analysis of their possible evolution during the 21st century.The first part of this thesis aims at identifying favorable conditions for the formation and the development of TTTs. Weather regimes analysis is used to identify TTTs on the one hand and mid-latitude perturbations on the other hand, allowing us to better document the spatial and temporal variability of TTTs together with background climate conditions. The events identified account for 20% of seasonal rainfall on average. Their contribution increases according to a west to east gradient. The comparison between these two classifications, partitioned using a k-means clustering, first confirms that midlatitude perturbations are a necessary condition for TTT development, but they are not sufficient. An excess of moist static energy over the Mozambique Channel partly supplied by advections from remote regions (mostly the southern Atlantic basin and the south-west Indian Ocean) form additional conditions favoring deep atmospheric convection over and near the Southern Africa. The second part investigates possible changes in precipitation, TTTs and more generally climate over Southern Africa during the 21st century in response to radiative forcing associated with greenhouse gas emissions (GHG). A multi-model (height climate models taken on the IPCC Fifth Assessment Report) and multi-scenario (RCP 8.5 and 2.6) approach is chosen. All models are skillful to reproduce Southern Africa current climate characteristics and cloud bands associated with TTTs, both in terms of spatial variability and frequency of occurrences. During the 21th century, there is no consensus between the models on the future evolution of seasonal rainfall (NDJF). However, all simulate an increase in the amounts precipitated by rainy day over the south-east part of southern Africa. These changes are not related to an evolution of TTTs: their spatial patterns, frequency of occurrences and contribution to rainfall remain stationary throughout the 21st century, but they associated with extreme rainfall events that become more frequent and more intense.