Ocean ambient noise is one of factors that can affect the performance of sonar and underwater communication system, it can degrade the performance of sonar system on listening or active detection, and also can affect the quality of underwater communication. Due to the variation of temperature and density in the ocean which make ambient noise has directionality. Beamforming can analyze the directionality of noise energy. Conventional beamforming is based on the assumption of plane wave sound field, so the energy from each angle is obtained by linear accumulation of every element. However plane wave assumption may not be satisfied because of the boundary interactions of sound propagation and energy attenuation of water column, therefore conventional beamforming may have poor beam resolution and SNR in applications. This research is to study of the influence of spatial coherence of ambient noise on beam resolution, and to improve the beam resolution by using the adaptive algorithm from the communication system theory. Firstly, simulations were performed to study spatial coherence between plane wave and non-plane wave in ambient noise, and the results were compared with beam resolution. This research also analyzes the influence of different conditions of noise spatial coherence on beamforming with ASIAEX data. The results showed that ambient noise has lower spatial coherence at high frequency, and the beamforming has poor beam resolution because of the lower spatial coherence in noise. Therefore, the adaptive beamforming were performed to improve the beam resolution, and compared with the conventional beamforming. The results showed that the highest improvement on beam resolution is 42.9 %, and increased SNR by 6 dB. On the other hand, the application of ASIAEX data show that, the highest improvement on beam resolution is 40.0 %, and increased SNR by 8 dB. The noise notch of ambient noise became more significant by increasing in beam resolution, and it also promoted the accuracy of analysis on noise directionality.

Synchrotron light source are accelerating research and development and fueling innovation in a wide range of research disciplines and industries worldwide. The third-generation synchrotron radiation facilities such as Advanced Photon Source (APS), produce ultra-brilliant x-rays using insertion devices consisting mainly of undulators, which provide exciting opportunities for advanced research into materials, earth science, life science, and medicine. Using high brightness x-ray radiation with high spatial coherence, unique coherence-based experiments are now becoming possible: coherence imaging techniques such as phase contrast imaging, holography, and tomography, are under intensive development, opening up a range of new areas of investigation. At the same time some useful optical elements used in the synchrotron radiation system have been created rapidly. Crucial to the development of all these fields is some knowledge of the spatial coherence of the light produced by these sources. In other words, the characterization of spatial coherence is a high priority. / The aim of this project is to develop a theoretical and experimental program to allow the measurement of the spatial coherence of synchrotron radiation. A technique to measure the spatial coherence of x-rays from undulators is presented. The essence of the coherence measurement technique is based on the interpretation of a complex diffraction pattern. We measure the spatial coherence function of a 7.9 keV x-ray beam from an undulator at a third-generation synchrotron (APS) using a sophisticated diffracting aperture known as a Uniformly Redundant Array (URA). The URA was also used to measure the spatial coherence function for soft x-rays at the APS. When a traditional Young’s double-slit experiment is used to test the degree of coherence, the separations of the two-slit have to be changed repeatedly to full map the spatial coherence function. The URA is a complex aperture consisting of many slits, (or, for a two-dimensional array, pinholes), organized such that all possible slit separations occur, and do so with exactly the same frequency. One might regard the URA as able to simultaneously perform many Young’s experiments a precisely equal number of times across the full range of slit separations permitted by the overall size of the URA. Therefore one experiment using a one-dimensional (1D) URA can perform the equivalent of multiple double-slit experiments. The diffraction theory developed in this thesis a convenient theoretical basis for interpreting this diffraction pattern.

L’imagerie échographique a connu un fort développement ces dernières années. Elle possède une cadence d’imagerie allant jusqu’à plusieurs milliers d’images par secondes notamment avec l’émergence des méthodes dites ultrarapides. Il s’agit donc de la modalité la plus adaptée pour des applications cardiaques permettant non seulement la reconstruction d’images mais également l’extraction de paramètres pour la caractérisation tissulaire, comme l’anisotropie locale présente dans le cœur. En effet, cet arrangement fibreux peut être modifié dans le cas de pathologies cardiaques. L’objectif de ce travail de doctorat est le développement d’une méthode d’extraction par imagerie ultrasonore 3D de l’orientation locale d’un milieu anisotrope. Cette approche doit permettre l’imagerie avec un large champ de vue pour être appliquée en imagerie cardiaque. Enfin, la validation de la chaîne de traitement est nécessaire. Pour répondre à ces problématiques, plusieurs solutions ont été proposées. Tout d’abord, l’orientation locale a été évaluée grâce à une méthode de cohérence spatiale permettant l’estimation de l’orientation dans un plan parallèle à la surface de la sonde. Une fois mise au point et validée, cette stratégie a été étendue afin d’extraire l’orientation locale en 3D et non uniquement l’angle dans un plan. Enfin, l’étude de différents types de transmissions a également été effectuée dans le but d’élargir le champ de vue imagé. Toutes ces méthodes originales ont été appliquées et validées sur des données acquises sur fantôme et in vivo. Ainsi, la détermination de l’orientation locale d’un milieu anisotrope a tout d’abord été réalisée sur un fantôme monodirectionnel puis sur le biceps d’un volontaire. Pour cela, un système expérimental constitué de quatre échographes de recherche a été élaboré grâce à la mise en commun d’équipements de CREATIS et du LabTAU, un autre laboratoire Lyonnais, afin d’acquérir des données en 3D. Ces travaux ont ainsi permis l’extension au cas d’une orientation non parallèle à la surface de la sonde d’un milieu anisotrope ainsi qu’une amélioration en termes de taille de champ de vue de la méthode existante. La validation de toute la chaîne de traitement a été réalisée. L’application au tissu cardiaque in vivo s’inscrit dans les directes perspectives des travaux / Ultrasound imaging has strongly developed in recent years. It reaches now a frame rates of several thousand images per second, thanks to the emergence of ultrafast imaging. It is therefore the most suitable modality for cardiac applications. Not only does it allow the reconstruction of images, it also enables the extraction of parameters for tissue characterization, such as local anisotropy inside the heart muscle. Indeed, this fibrous layout can be modified in the case of cardiac pathologies. The aim of this doctoral work is the development of a method to extract the local orientation of an anisotropic environment by 3D ultrasound imaging. This approach should allow imaging with a wide field a view to be applied in cardiac imaging. Finally, the validation of the processing chain is necessary. To address these issues, several solutions have been proposed. First, the local orientation was evaluated using a spatial coherence method. It allowed assessing the orientation of fibres in a plane parallel to the surface of the probe. Once developed and validated, this strategy was extended to extract the local orientation in 3D and not only the angle in a plane. Finally, the study of different types of transmissions was also carried out in order to widen the imaged field of view. All these original methods have been applied and validated on phantom and in vivo data: the determination of the local orientation of an anisotropic environment was first performed on a monodirectional phantom and then on the biceps of a volunteer. For this purpose, an experimental system consisting of four research ultrasound scanners was developed thanks to the sharing of equipment from CREATIS and LabTAU, another laboratory in Lyon, in order to acquire 3D data. This work has thus made it possible both to extend an anisotropic environment to the case of an orientation not parallel to the surface of the probe and to improve the size of the field of view of the existing method. The validation of the entire processing chain has been completed. Applying this method to in vivo cardiac tissue is directly part of future studies

No description available.

In shallow water environments, sound propagation experiences multiple interactions with the surface/bottom interfaces, with hydrodynamic disturbances such as internal waves, and with tides and fronts. It is thus very difficult to make satisfactory predictions of sound propagation in shallow water. Given that many of the ocean characteristics can be modeled as stochastic processes, the statistical measure, spatial coherence, is consequently an important quantity. Spatial coherence provides valuable information for array performance predictions. However, for the case of long-range, low frequency propagation, studies of spatial coherence influenced by various environmental parameters are limited insofar as having the appropriate environmental data with which to model and interpret the results. The comprehensive Asian Seas International Experiment 2001 (ASIAEX01) examined acoustic propagation and scattering in shallow water. Environmental oceanographic data were taken simultaneously with the acoustic data. ASIAEX01 provided a unique data set which enabled separate study of the characteristics of the oceanographic features and their influence on long range sound propagation. In this thesis, the environmental descriptors considered include sediment sound speed and attenuation, background internal waves, episodic non-linear internal waves, and air-sea interface conditions. Using this environmental data, the acoustic data are analyzed to show the characteristics of spatial coherence in a shallow water waveguide. It is shown that spatial coherence can be used as an inversion parameter to extract geoacoustic information for the seabed. Environmental phenomena including internal waves and wind-generated surface waves are also studied. The spatial and temporal variations in the sound field induced by them are presented. In addition, a tank experiment is presented which simulates propagation in a shallow water waveguide over a short range. Based on the data model comparison results, the model proposed here is effective in addressing the major environmental effects on sound propagation in shallow water.

L’imagerie sans lentille a élargi le champ d’applications de l’imagerie aux sources cohérentes de courte longueur d’onde dans le domaine XUV, pour lequel les systèmes optiques pour l’imagerie ne sont pas facilement disponibles. En outre, les sources pulsées ultra brèves XUV et X basées sur la génération d’harmoniques laser d’ordre élevé (HHG) et les lasers à électrons libres (FEL) offrent une très bonne résolution temporelle (femto 10-15s - atto 10-18s). Ce sont donc les outils indispensables pour suivre les dynamiques ultrarapides à l’échelle nanométrique. Il est donc nécessaire de disposer de techniques d’imagerie en un tir unique pour profiter pleinement des capacités de ces sources XUV. Les techniques d’imagerie sans lentille sont basées sur la mesure directe du champ électromagnétique diffracté lors de l’interaction de la source avec l’échantillon. La diffraction est liée à la transmittance de l’objet mais aussi à la cohérence spatiale de la source et à son front d’onde. La caractérisation en un tir unique de ces propriétés permet l’amélioration de la résolution de la reconstruction de l’objet.Les résultats de cette thèse sont présentés en deux parties dans ce manuscrit. La première partie est consacrée à la caractérisation des sources XUV et la deuxième au développement de nouvelles techniques d’imagerie multidimensionnelle. Nous présentons différentes applications de la mesure du front d’onde en un tir unique des sources XUV. Les résultats sont le produit de différentes campagnes expérimentales, sur des sources HHG et les FEL LCLS (Stanford) et FERMI (Trieste). Nous présentons également une nouvelle méthode pour la caractérisation en simple tir de la cohérence spatiale qui ne nécessite pas la connaissance de la distribution d’intensité du faisceau incident. De plus, nous présentons une nouvelle technique d’imagerie basée sur l’holographie par transformée de Fourier pour améliorer la résolution dans la reconstruction de l’objet dans le cas de l’utilisation d’une source partialement cohérente.La deuxième partie est consacrée à deux techniques d’imagerie multidimensionnelle développées pendant cette thèse. Une nouvelle technique d’imagerie 3D en simple tir, facile à implémenter et réduisant fortement la dose de rayonnement reçu par l’échantillon, est présentée. Différents schémas expérimentaux pour la génération de deux sources XUV synchronisées pour cette technique d’imagerie stéréographique 3D sont proposés. D’autre part, nous présentons une technique holographique compatible avec une source de large bande spectrale. Deux applications sont envisagées. La première est l’imagerie ultrarapide résolue spectralement, la deuxième est l’imagerie attoseconde. A la fin du manuscrit des conclusions générales du travail accompli pendant la thèse, ainsi que des perspectives sont présentées. / Lensless imaging techniques have broadened imaging applications to coherent sources in the short wavelength XUV domain, where optical systems to create an image are still not readily available. Furthermore, high harmonic generation sources (HHG) and free electron lasers (FEL) have the advantage of providing short temporal resolutions (atto 10-18s - femto 10-15s), opening the way towards ultrafast time resolved nanoscale imaging. Single shot imaging techniques are then highly important to exploit the shortest temporal resolution that can be reached with XUV sources. Lensless imaging is based on the direct measurement of the electric field diffracted by the sample. The diffraction pattern depends on the object transmittance but also on the source spatial coherence and wavefront. Single shot characterization of those properties thus leads to an improvement of the resolution of the object reconstruction.The results presented in this thesis are divided in two parts; the first one is focused on the characterization of the sources and the second on the development of new multidimensional imaging techniques. We will present different applications of single shot wavefront sensing of XUV sources. The results presented are the product of different experimental campaigns performed during this thesis using HH sources and FEL facilities at LCLS (Stanford) and FERMI (Trieste). Furthermore, a new method for single shot characterization of the spatial coherence that does not require the simultaneous measurement of the intensity distribution is presented. Additionally, we present a new holographic technique to improve the resolution of the object reconstruction when a partially coherent source is used.The second part is dedicated to two new multidimensional imaging techniques developed during the thesis. A new tri-dimensional imaging technique that is single shot, easy to implement and that lowers drastically the X-ray dose received by the sample, is presented. Different experimental setups for the generation of two synchronized XUV sources suitable for this ultrafast single shot 3D stereo imaging technique are presented. In addition, we present a holographic technique to extend imaging using a broadband source towards spectrally resolved single shot imaging and attosecond applications. Finally, we present the general conclusions from the work done during the thesis, together with the perspectives drawn from this work.

Peu d'études ont été menées sur l'influence de la stéréoscopie sur la perception d'un mixage audio au cinéma. Les témoignages de mixeurs ou les articles scientifiques montrent pourtant une grande diversité d'opinions à ce sujet. Certains estiment que cette influence est négligeable, d'autres affirment qu'il faut totalement revoir notre conception de la bande-son, aussi bien au niveau du mixage que de la diffusion. Une première série d'expériences s'est intéressée à la perception des sons d'ambiance. 8 séquences, dans leurs versions stéréoscopiques (3D-s) et non-stéréoscopiques (2D), ont été diffusées dans un cinéma à des sujets avec plusieurs mixages différents. Pour chaque présentation, les sujets devaient évaluer à quel point le mixage proposé leur paraissait trop frontal ou au contraire trop « surround », le but étant de mettre en évidence une éventuelle influence de la stéréoscopie sur la perception de la balance frontal/surround d'un mixage audio. Les résultats obtenus ont rejoint ceux d'une expérience préliminaire menée dans un auditorium de mixage, où les sujets se trouvaient en situation de mixeur et devaient eux-mêmes régler la balance frontal/surround : l'influence de la stéréoscopie était faible et n'apparaissait que pour quelques séquences. Des études ont ensuite été menées sur la perception des objets sonores tels que dialogues et effets. Une quatrième expérience s'est intéressée à l'effet ventriloque en élévation : lorsque l'on présente à un sujet des stimuli audio et visuel temporellement coïncidents mais spatialement disparates, les sujets perçoivent parfois le stimulus sonore au même endroit que le stimulus visuel. On appelle ce phénomène l’effet ventriloque car il rappelle l'illusion créée par le ventriloque lorsque sa voix semble plutôt provenir de sa marionnette que de sa propre bouche. Ce phénomène a été très largement étudié dans le plan horizontal, et dans une moindre mesure en distance. Par contre, très peu d'études se sont intéressées à l'élévation. Dans cette expérience, nous avons présenté à des sujets des séquences audiovisuelles montrant un homme en train de parler. Sa voix pouvait être reproduite sur différents haut-parleurs, qui créaient des disparités plus ou moins grandes en azimut et en élévation entre le son et l'image. Pour chaque présentation, les sujets devaient indiquer si la voix semblait ou non provenir de la même direction que la bouche de l'acteur. Les résultats ont montré que l'effet ventriloque était très efficace en élévation, ce qui suggère qu'il n'est peut-être pas nécessaire de rechercher la cohérence audiovisuelle en élévation au cinéma. / Few psychoacoustic studies have been carried out about the influence of stereoscopy on the sound mixing of movies. Yet very different opinions can be found in the cinema industry and in scientific papers. Some argue that sound needs to be mixed differently for stereoscopic movies while others pretend that this influence is negligible.A first set of experiments was conducted, which focused on the perception of ambience. Eight sequences - in their stereoscopic (s-3D) and non-stereoscopic (2D) versions, with several different sound mixes - were presented to subjects. For each presentation, subjects had to judge to what extent the mix sounded frontal or “surround.” The goal was to verify whether stereoscopy had an influence on the perception of the front/surround balance of ambience. Results showed that this influence was weak, which was consistent with a preliminary experiment conducted in a mixing auditorium where subjects had to mix the front/surround balance of several sequences themselves.Studies were then conducted on the perception of sound objects such as dialogs or on-screen effects. A fourth experiment focused on ventriloquism in elevation: when presented with a spatially discordant auditory-visual stimulus, subjects sometimes perceive the sound and the visual stimuli as coming from the same location. Such a phenomenon is often referred to as ventriloquism, because it evokes the illusion created by a ventriloquist when his voice seems to emanate from his puppet rather than from his mouth. While this effect has been extensively examined in the horizontal plane and to a lesser extent in distance, few psychoacoustic studies have focused on elevation. In this experiment, sequences of a man talking were presented to subjects. His voice could be reproduced on different loudspeakers, which created disparities in both azimuth and elevation between the sound and the visual stimuli. For each presentation, subjects had to indicate whether or not the voice seemed to emanate from the mouth of the actor. Ventriloquism was found to be highly effective in elevation, which suggests that audiovisual coherence in elevation might be unnecessary in theaters.

Most of Eastern Africa has arid and semi-arid climate with high space-time variability in rainfall. The droughts are very common in this region, and often persist for several years, preceded or followed by extreme floods. Most of the livelihoods and socio-economic activities however remain rain-dependent leading to severe negative impacts during the periods of occurrence of climate extremes. It has been noted that one extreme event was capable of reversing national economic growth made over a period of several years. Thus no sustainable development can be attained in eastern Africa without effective mainstreaming of climate information in the development policies, plans and programmes. Many past studies in the region have focused on rainfall variability at seasonal, annual and decadal scales. Very little work has been done at intraseasonal timescale that is paramount to most agricultural applications. This study aims at filling this research gap, by investigating the structure of rainfall season in terms of the distribution of wet and dry spells and how this distribution varies in space and time at interannual time scale over Equatorial Eastern Africa. Prediction models for use in the early warning systems aimed at climate risk reduction were finally developed. The specific objectives of the study include, delineate and diagnose the some aspects of the distribution of the wet and dry spells at interannual timescale; investigate the linkages between the aspects of the distribution of wet and dry spells identified and dominant large scale climate fields that drive the global climate; and assess the predictability of the various aspects of wet and dry spells for the improvement of the use in the early warning systems of the region.Several datasets spanning a period of 40 years (1961 - 2000) were used. The data included gauged daily rainfall amount for the three Eastern Africa countries namely Kenya, Uganda, and Tanzania; Hadley Centre Sea Surface Temperature (SST); re-analysis data and radiosonde observations from Nairobi (Kenya) and Bangui (Central Africa Republic) upper air stations. The indices of El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole and SST gradients which constituted the predefined predictors were also used [...]

The work presented in the thesis focuses on systematically documenting the multi scale nature of the temporal persistence and spatial coherence of tropical rainfall. There are three parts to the thesis: The first two parts utilize satellite-retrieved rainfall at multiple observational resolutions to characterize the space-time organization of rain; the third part assesses the ability of state-of-the-art coupled models to reproduce some of the observed features. In the first part of the study, which focuses on the temporal persistence of rain, we analyze the Tropical Rainfall Measurement Mission (TRMM) satellite-based observations to compare and contrast wet and dry spell characteristics over the tropics (30 S-30 N). Defining a wet (dry) spell as the number of consecutive rainy (nonrainy) days, we find that the distributions of wet spells (independent of spatial resolution) exhibit universality in the following sense. While both ocean and land regions with high seasonal rainfall accumulation (humid regions) show a predominance of 2-4 day wet spells, those regions with low seasonal rainfall accumulation (arid regions) exhibit a wet spell duration distribution that is essentially exponential in nature, with a peak at 1 day. The behaviour that we observed for wet spells is reversed for dry spell distributions. The total rainfall accumulated in each wet spell has also been analyzed, and we find that the major contribution to seasonal rainfall for arid regions comes from very short length wet spells; however, for humid regions, this contribution comes from wet spells of duration as long as 30 days. An exhaustive sensitivity study of factors that can potentially affect the wet and dry spell characteristics (e.g., resolution) shows that our findings are robust. We also explore the role of chance in determining the 2-4 day mode, as well as the inuence of organized convection in separating reality from chance. The second part deals with the spatial coherence of tropical rain. We take two different approaches, namely, a global and local view. The global view attempts to quantify the con-ventional view of rain, i.e., the dominance of the intertropical convergence zone (ITCZ), while the local view tries to answer the question: if it rains, how far is the influence felt in zonal and meridional directions? In both approaches, the classical e-folding length for spatial decorrelation is used as a measure of spatial coherence. The major finding in the global view approach is that, at short timescales of accumulation (daily to pentad to even monthly), rain over the Equator shows the most dominant zonal scale. It is only at larger timescales of accumulation (seasonal or annual) that the dominance of ITCZ around 7 N is evident. In addition, we also find a semi-log linearity between the spatial scales, seen from afar, and timescale of accumulation, with a break in linearity around typical synoptic timescales of 5-10 days. The local view quantifies the dominance of the zonal scale in the tropical ocean convergence zones, with an anisotropy value (ratio of zonal to meridional scales) of 3-4. Over land, on the other hand, the zonal and meridional scales are comparable in magnitude, suggesting that rain tends to be mostly isotropic over continental regions. This latter finding holds true, irrespective of the spatial and temporal resolutions at which rain is observed. Interestingly, the anisotropy over ocean, while invariant with spatial resolution, is found to be a function of temporal resolution: from a value of 3-4 at daily timescale, it decreases to around 1.5 at 3-hourly resolution, suggesting that perhaps rain fundamentally might be isotropic in nature at an event scale. The final part analyses a few models from the suite of Coupled Model Intercomparison Project (CMIP5) models, to evaluate their ability to reproduce some of these aforementioned features. For all the strong biases that models are known to have, some of the observed features are captured well by the models. Specifically, on the temporal persistence front, the observed 2-4 day mode of wet (dry) spells of rain over humid (arid) regions is also seen in models. The overestimation of longer duration wet spells appears to be the primary cause of a positive bias in the number of rainy days from the models. In general, the tendency of models to not stop raining results in lower and higher number of shorter and longer duration wet spells, respectively, and consequently an overall reduction in dry spells of all durations. On the spatial coherence front, the main finding from the global view approach is that the observed semi-log linearity of the zonal spatial scale of rainfall as a function of timescale of accumulation is strikingly well-reproduced by the models. Even more remarkable is that the models are able to mimic the break in this linearity around 5 days (typical synoptic scale). What the models fail to do prominently is the transition of the dominance of equatorial rain at smaller timescales of accumulation to the dominance of ITCZ at around 7 N at higher timescales of accumulation. Based on the local view approach, we find that, in general, even though the zonal and meridional scales are overestimated, the observed isotropy of continental rain is captured very well by the models. Over the oceans, the success is less prominent, especially with the core of the ITCZ showing much larger ratios than those observed. Thus, the models seem to be able to reproduce the anisotropy for the wrong reasons, and the proposed anisotropy ratio could be a useful metric in further diagnosis of climate models.

La plus grande partie de l’Afrique orientale présente un climat aride à semi-aride, et une forte variabilité spatio-temporelle des pluies. Les sécheresses sont courantes dans cette région, et persistent souvent sur plusieurs années consécutives, précédées ou suivies par des inondations majeures. La plupart des modes de vie et des activités socio-économiques restent néanmoins dépendants des précipitations, avec des effets extrêmement dommageables en périodes d’extrêmes climatiques. Il a été montré qu’un seul événement extrême suffisait à inverser la croissance économique nationale plusieurs années d’affilée. Aucun développement durable ne peut donc être réalisé en Afrique de l’Est sans une prise en compte effective de l’information climatique dans les politiques, les plans et les programmes de développement. De nombreuses études antérieures sur la région ont été consacrées à la variabilité des pluies aux échelles saisonnière, annuelle et décadaire. Peu de recherches ont porté sur l’échelle intra-saisonnière, qui est déterminante pour la plupart des applications agricoles. Cette étude vise à combler cette lacune, en examinant la structure de la saison des pluies en termes de répartition des épisodes secs et pluvieux, et la façon dont cette répartition varie dans l’espace et le temps, à l’échelle interannuelle à travers l’Afrique est-équatoriale. Des modèles de prévision destinés à être utilisés dans des systèmes d’alerte précoce, en vue d’atténuer les risques liés au climat, sont en outre développés. Les objectifs spécifiques de l’étude incluent : un diagnostic des différents aspects de la répartition des épisodes secs et pluvieux, dans leur variation interannuelle ; une analyse des relations entre les aspects de cette répartition ainsi identifiés et les principaux champs climatiques d’échelle large qui gouvernent le climat global ; une évaluation de la prévisibilité des différents aspects des épisodes secs et pluvieux pour l’amélioration des systèmes d’alerte précoce de la région.Plusieurs bases de données couvrant une période de 40 ans (1961-2000) ont été utilisées. Elles comprennent des séries de précipitations journalières mesurées par pluviomètre dans les trois pays d’Afrique orientale (Kenya, Ouganda, Tanzanie) ; les températures de surface marine (SST) du Hadley Centre ; des données de réanalyses et des stations de radiosondages de Nairobi (Kenya) et de Bangui (République Centrafricaine). Des indices d’El Niño-Oscillation Australe (ENSO), du Dipôle de l’Océan Indien et de gradients de SST, constituant des prédicteurs SST prédéfinis, ont été également utilisés [...] / Most of Eastern Africa has arid and semi-arid climate with high space-time variability in rainfall. The droughts are very common in this region, and often persist for several years, preceded or followed by extreme floods. Most of the livelihoods and socio-economic activities however remain rain-dependent leading to severe negative impacts during the periods of occurrence of climate extremes. It has been noted that one extreme event was capable of reversing national economic growth made over a period of several years. Thus no sustainable development can be attained in eastern Africa without effective mainstreaming of climate information in the development policies, plans and programmes. Many past studies in the region have focused on rainfall variability at seasonal, annual and decadal scales. Very little work has been done at intraseasonal timescale that is paramount to most agricultural applications. This study aims at filling this research gap, by investigating the structure of rainfall season in terms of the distribution of wet and dry spells and how this distribution varies in space and time at interannual time scale over Equatorial Eastern Africa. Prediction models for use in the early warning systems aimed at climate risk reduction were finally developed. The specific objectives of the study include, delineate and diagnose the some aspects of the distribution of the wet and dry spells at interannual timescale; investigate the linkages between the aspects of the distribution of wet and dry spells identified and dominant large scale climate fields that drive the global climate; and assess the predictability of the various aspects of wet and dry spells for the improvement of the use in the early warning systems of the region.Several datasets spanning a period of 40 years (1961 – 2000) were used. The data included gauged daily rainfall amount for the three Eastern Africa countries namely Kenya, Uganda, and Tanzania; Hadley Centre Sea Surface Temperature (SST); re-analysis data and radiosonde observations from Nairobi (Kenya) and Bangui (Central Africa Republic) upper air stations. The indices of El Niño-Southern Oscillation (ENSO), Indian Ocean Dipole and SST gradients which constituted the predefined predictors were also used [...]