Introduction: Tropical Cyclones (TCs) inflict considerable damage to life and property every year. A major problem is that residents often hesitate to follow evacuation orders when the early warning messages are perceived as inaccurate or uninformative. The root problem is that providing accurate early forecasts can be difficult, especially in countries with less economic and technical means. Aim: The aim of the thesis is to investigate how cyclone early warning systems can be technically improved. This means, first, identifying problems associated with the current cyclone early warning systems, and second, investigating if biologically based Artificial Neural Networks (ANNs) are feasible to solve some of the identified problems. Method: First, for evaluating the efficiency of cyclone early warning systems, Bangladesh was selected as study area, where a questionnaire survey and an in-depth interview were administered. Second, a review of currently operational TC track forecasting techniques was conducted to gain a better understanding of various techniques’ prediction performance, data requirements, and computational resource requirements. Third, a technique using biologically based ANNs was developed to produce TC track and intensity forecasts. Systematic testing was used to find optimal values for simulation parameters, such as feature-detector receptive field size, the mixture of unsupervised and supervised learning, and learning rate schedule. Five types of 2D data were used for training. The networks were tested on two types of novel data, to assess their generalization performance. Results: A major problem that is identified in the thesis is that the meteorologists at the Bangladesh Meteorological Department are currently not capable of providing accurate TC forecasts. This is an important contributing factor to residents’ reluctance to evacuate. To address this issue, an ANN-based TC track and intensity forecasting technique was developed that can produce early and accurate forecasts, uses freely available satellite images, and does not require extensive computational resources to run. Bidirectional connections, combined supervised and unsupervised learning, and a deep hierarchical structure assists the parallel extraction of useful features from five types of 2D data. The trained networks were tested on two types of novel data: First, tests were performed with novel data covering the end of the lifecycle of trained cyclones; for these test data, the forecasts produced by the networks were correct in 91-100% of the cases. Second, the networks were tested with data of a novel TC; in this case, the networks performed with between 30% and 45% accuracy (for intensity forecasts). Conclusions: The ANN technique developed in this thesis could, with further extensions and up-scaling, using additional types of input images of a greater number of TCs, improve the efficiency of cyclone early warning systems in countries with less economic and technical means. The thesis work also creates opportunities for further research, where biologically based ANNs can be employed for general-purpose weather forecasting, as well as for forecasting other severe weather phenomena, such as thunderstorms.

Recent studies have found that equatorial Atlantic sea surface temperature (SST) variability may be influencing tropical Indian Ocean climate (Kucharski 2009, Wang 2009). Due to the economic and social impact of tropical cyclones, it is important to investigate how an Atlantic-Indian Ocean connection may be affecting tropical cyclone behavior in the southern Indian Ocean. In this study, the International Best Track Archive for Climate Stewardship (IBTrAC) tropical cyclone database is used to derive metrics of tropical cyclone behavior, which are then compared with indices of tropical Atlantic SST variability representing Atlantic Niño, and Benguela Niño events. Changes in tropical Atlantic SSTs are found to coincide with significant differences in tropical cyclone activity for portions of the southern Indian Ocean. In addition, for these same regions, tropical Atlantic SST variability is associated with changes in large-scale atmospheric conditions, including steering flow, low level vorticity, and humidity, typically associated with tropical cyclogenesis, and tropical cyclone track. The changes in steering flow related to both indices of Atlantic SST anomaly are reproduced by an atmospheric model. The changes in steering flow are also found to be linked to changes in TC translational velocity, and TC tracking. These findings indicate a possible link between tropical Atlantic conditions and cyclone activity in the Indian Ocean mediated through a teleconnection between tropical Atlantic SSTs and large scale atmospheric conditions over the southern Indian Ocean. The teleconnection related to the Benguela Niño region of SST variability was found to consist of a Rossby wave initiated off the coast of South America, and propagating into the Indian Ocean, thereby influencing several atmospheric variables, including steering flow. The teleconnection related to the Atlantic Niño region of SST variability was not well defined, although there was some evidence of a Walker circulation anomaly extending from the equatorial Atlantic over the continent of Africa, and influencing SIO steering flow. / Graduation date: 2012

Annually, tropical cyclones do tremendous damage and are agents of long-term coastal change. To test this idea of different tropical cyclones delivering consistent coastal change, a landform with such evolution is needed. One such landform is a spit. What contributions do tropical cyclones give toward the evolution of a spit, and do tropical cyclones give the same kinds of impacts? To determine if tropical cyclones have similar impacts, shoreline and volumetric change from four storms impacting Bolivar Peninsula are considered. Being a southwest-trended spit at a length of 33.5 kilometers, storm impacts are measured in the form of one dimensional shoreline and two dimensional volumetric change. These impacts are abstracted into shoreline change and volumetric change patterns. These patterns are identified and compared for differences between each storm and similarity among all storms. Results indicate that shoreline accretionary zones vary alongshore. Results from Hurricane Ike indicate an accretionary zone ten kilometers from the distal end. Shoreline change patterns for Hurricane Rita show an unstable accretionary zone at four kilometers from the distal end. Results for Tropical Storm Fay indicate an unstable accretionary zone that begins at the distal end and continues to the middle of the spit. In terms of similarity for shoreline change, all patterns from storms demonstrated erosion near Rollover Fish Pass. One dimensional volumetric change patterns were entirely erosive for Hurricanes Rita and Ike, and Tropical Storm Fay had by small zones of accretion near the distal portion of the spit. Tropical Storm Josephine demonstrated an accretion zone between the middle and distal portion of the spit. Results from two dimensional volumetric change patterns suggest a threshold for inland penetration. Tropical Storm Fay showed a ten to twenty meter wide pattern of erosion around five kilometers from the distal end and near the proximal end of the spit, and Hurricane Rita demonstrated a twenty meter wide pattern of erosion near the distal end. Hurricane Ike had erosive penetration of up to 200 meters around fifteen kilometers from the distal end. Results suggest that certain storms reinforce the standard spit growth model, and others work against it.

The direct and indirect effects of aerosols on the hurricane ‘Katrina’ have been investigated using the WRF model with a two-moment bulk microphysical scheme and modified Goddard shortwave radiation scheme. Simulations of the hurricane ‘Katrina’ are conducted under the three aerosol scenarios: 1) the clean case with an aerosol number concentration of 200 cm-1, 2) the polluted case with a number concentration of 1000 cm-1, and 3) the aerosol radiative effects (AR) case with same aerosol concentration as polluted case but with a modified shortwave radiation scheme. The polluted and AR cases have much larger amounts of cloud water and water vapor in troposphere, and the increased cloud water can freeze to produce ice water paths. A tropical cyclone in dirty and dusty air has active rainbands outside the eyewall due to aerosol indirect effects. The aerosol direct effect can lead to the suppressing of convection and weakening of updraft intensity by warming the troposphere and cooling the surface temperature. However, these thermal changes in atmosphere are concerned with the enhanced amounts of cloud hydrometeors and modification of downdraft and corresponding the low level winds in rainband regions. Thus, the AR case can produce the enhanced precipitation even in the weakest hurricane. When comparing the model performance between aerosol indirect and direct effect by ensemble experiments, the adjustment time of the circulation due to modification of the aerosol radiative forcing by aerosol layers may take a longer time than the hurricane lifetime, and the results from the simulated hurricane show that it is more sensitive to aerosol indirect effects which are related to the cloud microphysics process changes. From this aerosol study, we can suggest that aerosols can influence the cloudiness, precipitation, and intensity of hurricanes significantly, and there may be different results in the meso-scale convective clouds cases. The hurricane system is a large and complex convective system with enormous heating energy and moistures. Moreover, relationships between various hydrometeors in hurricane systems are difficult to isolate and thus, it needs further study with more realistic cloud microphysical processes, aerosol distributions, and parameterizations.

La prévision de l'intensité des cyclones tropicaux est aujourd'hui un enjeu scientifique majeur. Parmi de nombreux facteurs multi-échelle, l'impact de la microphysique nuageuse et des aérosols sur les variations d'intensité a été récemment mis en évidence. Cette problématique a motivé l'évaluation du schéma microphysique à 2-moments LIMA en milieu tropical et le développement d'un couplage avec le schéma d'aérosols ORILAM au sein du modèle atmosphérique Meso-NH. L'intérêt de ce développement numérique est d'inclure l'émission des aérosols marins en fonction des vents cycloniques et des paramètres océaniques. L'application de ce couplage aérosols-microphysique à la simulation du cyclone tropical Dumile (2013) montre que le modèle couplé tend à améliorer la représentation de l'intensité, la trajectoire, la structure microphysique du cyclone tropical et les précipitations associées, en comparaison avec les observations. La production secondaire des cristaux de glace est également un thème de recherche actif en microphysique nuageuse. Ainsi, une paramétrisation du processus de rupture collisionnelle de la glace a été implémentée dans le schéma microphysique LIMA. L'impact de ce processus a été testé sur le développement d'un orage des moyennes latitudes et sur le cyclone tropical Dumile. Les deux cas d'étude ont des réponses similaires vis-à-vis de ce processus : une augmentation de la concentration et de la masse des cristaux de glace et une diminution des cumuls de précipitations. La poursuite de ces travaux pourrait permettre de déterminer si ce processus de formation secondaire peut améliorer la modélisation de la couverture cirriforme des cyclones tropicaux. / Intensity forecast of tropical cyclones is a major scientific issue. Among many factors, the impact of cloud microphysics and aerosols on intensity variations has been recently underlined. This issue motivated the evaluation of the 2-moment microphysical scheme LIMA in a tropical context and the development of a coupling with the aerosol scheme ORILAM into the atmospheric model Meso-NH. The interest of this numerical development is to represent the emission of sea salt aerosols depending on cyclonic winds and oceanic parameters. The application of this aerosols-microphysics coupling to the simulation of tropical cyclone Dumile (2013) shows that the coupled model tends to improve the representation of the intensity, the track, the microphysical structure of the tropical cyclone and the associated precipitation, when comparing with observations. The secondary production of ice crystals is also an active research topic in cloud microphysics. A parameterization of the collisional ice break-up process is thus implemented into the microphysical scheme LIMA. The impact of this process has been analyzed on a mid-latitude storm and on tropical cyclone Dumile. Both case studies display similar results regarding this process: an increase of ice crystals concentration and mass, and a decrease of precipitation. The continuation of this work could allow to determine if this process of secondary formation could improve the cirrus modelling in tropical cyclones.

No description available.

Wavelike spiral asymmetries are an intriguing aspect of Tropical Cyclone dynamics. Previous work hypothesized that some of them are Vortex Rossby Waves propagating on the radial gradient of mean–flow relative vorticity. In the Intermittently Forced Vortex Rossby Wave theory, intermittent convection near the eyewall wind maximum excites them so that they propagate wave energy outward and converge angular momentum inward. The waves’ energy is absorbed as the perturbation vorticity becomes filamented near the outer critical radii where their Doppler–shifted frequencies and radial group velocities approaches zero. This process may initiate outer wind maxima by weakening the mean–flow just inward from the critical radius. The waves are confined to a relatively narrow annular waveguide because of their slow tangential phase velocity and the narrow interval between the Rossby wave cut–off frequency, where the radial wavenumber is locally zero, and the zero frequency, where it is locally infinite.

Climate modeling and tropical cyclone forecasting are two significant is- sues that are continuously being improved upon for more accurate weather forecasting and preparedness. In this thesis, we have studied three climate models and formulated a new model with a view to determine the outgoing longwave radiation (OLR) budget at the top of the atmosphere (TOA) as ob- served by the National Oceanic and Atmospheric Administration’s (NOAA) satellite based Advanced Very High Resolution Radiometer (AVHRR). In 2006, Karnauskas proposed the African meridional OLR as an Atlantic hur- ricane predictor, the relation was further proven in 2016 by Karnauskas and Li. Here we have considered a similar study for all other tropical cyclone basins.

Ces travaux visent à inscrire le monitoring des plages récifales de l'île de La Réunion dans la « Stratégie Nationale de Gestion Intégrée des Zones Côtières » (2012). Elle oriente les recherches vers une démarche labellisée sur le long terme qui se concrétise par la mise en place de protocoles de mesures normalisés déployés sur des sites ateliers. Le site de l'Ermitage devient, en 2012, le premier site atelier en zone tropicale et de type bioclastique labellisé à l'échelle nationale (AllENVI, puis INSU en 2014). La dynamique des plages d'arrière-récif demeure à ce jour peu étudiée à travers le monde. Les plages bioclastiques de La Réunion sont le produit de récifs coralliens décrits comme dégradés par les biologistes depuis les années 80. Elles sont confrontées à une très forte anthropisation. Des formes marquées d'érosion se lisent dans ces paysages littoraux. La révision des protocoles de suivi de la topographie des plages tout en exploitant les suivis historiques, vise à illustrer la pluralité des dynamiques en contexte d'arrière-récif. La diversité des processus et des échelles spatio-temporelles impliqués dans le fonctionnement hydro-sédimentaire des plages récifales nécessite la mise en œuvre de méthodes d'observation in situ adaptées, comparables et reproductibles. Dans cette étude nous exploitons surtout les suivis topographiques des plages à l'échelle évènementielle, saisonnière et pluriannuelle. Par l'analyse morphologique et volumétrique des séries de profils topographiques, la variabilité morphosédimentaire en zone intertidale et supratidale est décrite. La significativité de la mesure de la mobilité du trait de côte est également questionnée. / Long-term assessment of beach morphodynamic is a great challenge to understand future trajectories of these landforms. In France, in 2012, a national strategy for an integrated management of shoreline has been adopted. This one lead to creation of research labels (SOERE and SNO) which aim to sustain scientific researches dedicated to sandy beach morphodynamic in response to various hydrodynamics forcing. Shoreline monitoring on selected pilot sites is conducted within network where normalized protocols and high quality data are guaranteed at long-term scale. The French coastline also extend in tropical zone through its ultramarine territories. Tropical shoreline are among the most sensitive environment however they remain poorly studied. Since, 2012, Reunion island, a French department in Indian Ocean, integrated the SOERE network. Coral reef and coral beaches which preferentially developed on the western coast are frequently threatened by major swell event associated with tropical cyclones or long period swell. Since now, the beach trend evolution and morphogenic processes have not been quantified. However past monitoring exists which has consisted in topographic measurement along beach profile based on empirical protocols which produced biaised data. Since its integration within SOERE network, DGPS survey are now conducted along the different coral beach compartments of Reunion Island at seasonal and post-storm scale. This study aims to capitalize historical and DGPS beach topographic data in order to quantify long term and post-storm beach response and recovery from Cap Champagne to Trou d'Eau.

Les données sismologiques enregistrées sur les îles océaniques offrent l'opportunité d'analyser la houle via sa signature dans le "bruit" microsismique. Nous avons d'abord analysé les sources de bruit « secondaires », qui sont générées par l'interaction entre des vagues de même période dans une tempête, un cyclone ou par le phénomène de réflexion des vagues sur la côte. L'analyse des « microséismes secondaires », à l'échelle du bassin océanique, permet d'assurer un suivi spatio-temporel de la source qui les génère, même si elle est distante de plusieurs milliers de km des stations sismiques d'enregistrement. À plus long terme, leur étude permet d'assurer un suivi climatique global de l'activité des vagues dans une région donnée. Nous avons également étudié les sources de bruit « primaires », qui sont générées par l'interaction directe de la houle avec la côte. L'analyse des « microséismes primaires » permet de caractériser la houle localement par son amplitude, sa période et sa direction de propagation. Il est alors possible d'utiliser un capteur sismique comme substitut d'un houlographe. Dans le contexte des îles Éparses et de La Réunion, nous avons étudié plusieurs épisodes de houles extrêmes qui les touchent fréquemment et qui peuvent avoir d'importantes répercutions sociales ou environnementales. Enfin, en utilisant un réseau temporaire de stations sismologiques à La Réunion, nous avons analysé les variations du niveau de bruit microsismique pour caractériser l'impact des houles extrêmes sur les différentes façades de l'île. Cette étude permet d'identifier les sites les plus exposés aux vagues qui présentent un intérêt pour la récupération de cette énergie renouvelable encore trop sous-exploitée. / Seismic data recorded on oceanic islands can be used to analyze the swell through its signature in the microseismic noise. We first analysed the "secondary" noise sources, which are generated by the interaction of ocean waves with similar period within a storm, a cyclone or by the reflection phenomena off the coast. By analyzing secondary microseisms at the scale of the oceanic basin, we first performed a spatio-temporal tracking of the source, even localized thousands km off the recording seismic stations. Secondly, in the long-term, they can be used to follow the global climate change related to the ocean waves activity in a specific region. We also studied the "primary" seismic noise sources, which are created by the direct interaction of ocean waves with the coast. By analyzing these primary microseisms, we succeeded to characterize ocean waves locally in terms of amplitude, period, and, sometimes, direction of propagation. This showed that it is possible to use a seismic station as an ocean waves gauge to get precious swell data, particularly in remote and poorly instrumented areas. In the context of the Iles Éparses and of La Réunion Island, we studied a few extreme swells, which occur there frequently and can have strong social and environmental consequences. Finally, by using a temporary network of seismic stations installed in La Reunion, we studied the spatial variations of the seismic noise level across the island to characterize the swell impact on the different coasts. This study enables us to identify the most exposed spots to the swell, which may be attractive for generating renewable energy with this powerful resource yet underexploited.