Dissertations / Theses: 'Processus convectifs'

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Wang, Di. "What controls the 3D distribution of water vapor isotopic composition in East Asia?" Electronic Thesis or Diss., Sorbonne université, 2023. https://accesdistant.sorbonne-universite.fr/login?url=https://theses-intra.sorbonne-universite.fr/2023SORUS567.pdf.

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Contrairement aux carottes de glace polaires, les variations isotopiques dans les carottes de glace tibétaines compliquent l'interprétation des signaux de température. L'une des principales raisons est que dans les régions de mousson aux latitudes basses et moyennes, les isotopes de l'eau sont influencés par des processus convectifs et nuageux. Il est nécessaire de mieux comprendre le comportement des isotopes de l'eau et les contrôles dynamiques impliqués dans la transpiration et la convection de l'humidité. Des observations in situ à grande échelle et des profils verticaux des isotopes de la vapeur pendant la convection seraient utiles, mais ces données sont rares. Le but de cette thèse a été de documenter les variations horizontales, verticales et temporelles de la mousson d'Asie de l'Est. D’abord, afin de documenter les variations horizontales près de la surface et saisonnières de la composition isotopique stable de l’eau en Chine, nous avons effectué des observations in situ des isotopes de la vapeur d'eau sur une vaste région de la Chine (plus de 10 000 km) pendant les saisons de pré-mousson et de mousson, à l'aide d'un système de mesure des isotopes de la vapeur d'eau nouvellement conçu et installé sur un véhicule. Nous avons constaté que les variations spatiales de la vapeur δ18O sont principalement contrôlées par la distillation de Rayleigh le long des trajectoires des masses d'air pendant la saison de pré-mousson, et sont fortement influencées par différentes sources d'humidité, les processus de recyclage continental et la convection le long du transport de l'humidité pendant la saison de la mousson. Ces résultats donnent un aperçu de la distribution spatiale et de la variabilité saisonnière de la composition isotopique de l'eau en Asie de l'Est et de leurs facteurs de contrôle, et soulignent la nécessité d'interpréter les enregistrements proxy dans le contexte du système régional et des sources d'humidité. Ensuite, pour mieux comprendre les processus physiques qui contrôlent la distribution verticale des isotopes de la vapeur et sa variabilité intra-saisonnière et saisonnière, nous avons observé les profils verticaux des isotopes de la vapeur de l'atmosphère jusqu'à la haute troposphère (de la surface du sol à 3856 m jusqu'à 11000 m d'altitude) de juin à octobre dans le sud-est du plateau tibétain à l'aide d'un drone spécialement conçu à cet effet. Pour l’échantillonnage, nous avons utilisé des sacs d’air, mais avons rencontré des problèmes de perméabilité généralement associés à ces sacs. Pour corriger ces problèmes, nous avons développé un modèle de diffusion et en avons calibré les paramètres avec des expériences de laboratoire. Ceci nous a permis de documenter pour la première fois que la distribution verticale des isotopes de la vapeur d'eau atmosphérique pendant toute la période de la mousson jusqu'à la haute troposphère, avec une résolution verticale et une gamme d'altitudes sans précédent. Nous constatons que les profils verticaux de la composition isotopique de la vapeur d'eau reflètent une combinaison de processus à grande échelle, en particulier la convection profonde et le recyclage continental le long des trajectoires, et les processus convectifs locaux, en particulier le détrainement convectif et la sublimation des cristaux de glace. Les variations saisonnières et intra-saisonnières observées sont généralement cohérentes verticalement, en raison du fort mélange convectif vertical et du détrainement convectif local de la vapeur provenant des basses couches, et reflètent les variations de convection profonde le long des trajectoires
Unlike polar ice core records, the isotope variations in Tibetan ice cores challenge the interpretation of temperature signals. One of the main reasons is that in monsoon regions at low and middle latitudes, water isotopes are influenced by convective and cloud processes. A deeper understanding of water isotope behavior and the dynamical controls involved in moisture transpiration and convection is needed. Large-scale in-situ observations and vertical profiles of vapor isotopes during convection would be helpful. However, such data are rare. The aim of this thesis was to document horizontal, vertical, and temporal variations in the isotopic composition of water in East Asian monsoon region. First, to document the horizontal and seasonal variations of water isotopes near the surface across China, we made in-situ observations of near-surface vapor isotopes over a large region (over 10000 km) across China in both pre-monsoon and monsoon seasons, using a newly-designed vehicle-based vapor isotope monitoring system. We found that the observed spatial variations of vapor δ18O are mainly controlled by Rayleigh distillation along air mass trajectories during the pre-monsoon period, but are significantly influenced by different moisture sources, continental recycling processes, and convection along moisture transport during the monsoon period. These results provide an overview of the spatial distribution and seasonal variability of water isotopic composition in East Asia and their controlling factors and emphasize the need to interpret proxy records in the context of the regional system and moisture sources. Second, to better understand the physical processes that control the vertical distribution of vapor isotopes and its intra-seasonal and seasonal variability, we observed the vertical profiles of atmosphere vapor isotopes up to the upper troposphere (from the ground surface at 3856m up to 11000m a.s.l.) from June to October in the southeastern Tibetan Plateau using a specially-designed unmanned-aerial-vehicle (UAV) system. For the sampling, we chose to carry air bags on UAVs as a portable sampling device, but encountered the permeability problem commonly associated with these bags. To corrected for this problem, we developed a diffusion model with diffusion parameters calibrated through laboratory experiments. This allows us to document for the first time the vertical distribution of atmospheric water vapor isotopes across the entire monsoon period up to the upper troposphere, boasting an unprecedented vertical resolution and altitude range. We find that the vertical profiles of water vapor isotopic composition reflect a combination of large-scale processes, in particular deep convection and continental recycling along trajectories, and local convective processes, in particular convective detrainment, and sublimation of ice crystals. The observed seasonal and intra-seasonal variations are generally vertically coherent, due to the strong vertical convective mixing and local convective detrainment of vapor originating from the low levels, and are mainly due to deep convection along trajectories

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CHAUMAT, LAURE. "Etude experimentale des processus de condensation dans les nuages convectifs : elargissement des spectres et distribution spatiale des gouttelettes." Clermont-Ferrand 2, 1999. http://www.theses.fr/1999CLF21118.

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La formation rapide des precipitations dans les cumulus est un phenomene encore mal explique. Plusieurs hypotheses ont ete avancees dont la contribution de noyau ultra geants, une efficacite de collison entre gouttelettes renforcee et l'elargissement des spectres de gouttelettes. Notre etude est consacree plus particulierement a la caracterisation des distributions dimensionnelles de gouttelettes dans les regions de nuage non melangees, a leur elargissement et a l'analyse de la distribution spatiale des gouttelettes a micro-echelle, source potentielle d'elargissement. De recentes mesures in-situ montrent que les spectres de gouttelettes dans les cellules adiabatiques sont plus larges que ceux prevus par la theorie classique de croissance par diffusion de vapeur. Une telle particularite implique que le champ de sursaturation n'est pas uniforme dans les ascendances non melangees. Un grand echantillon d'observations effectuees dans de telles regions est analyse et l'elargissement spectral est represente par la fonction de densite de probabilite de l'integrale de la sursaturation. Il est montre que la dispersion dans cette fonction augmente lineairement avec la valeur moyenne, et une parametrisation simple de l'elargissement des spectres est proposee. Cette variabilite dans l'integrale de la sursaturation peut provenir des heterogeneites dans la distribution spatiale des gouttelettes. Des mesures a tres fine echelle sont analysees pour caracteriser la statistique du comptage des particules. Celle-ci montre que les gouttelettes sont distribuees aleatoirement dans l'espace. Les effets des heterogeneites resultantes sur l'elargissement spectral sont estimes avec un modele conceptuel qui prouve qu'ils ne sont significatifs que pour des valeurs irrealistes du temps de vie des heterogeneites. De recentes simulations numeriques des effets inertiels des gouttelettes dans les structures de micro-echelle de la turbulence sont alors testees. Il est montre que le modele qui produit un elargissement significatif genere des repartitions spatiales de gouttelettes dont les proprietes statistiques sont irrealistes. Nos resultats indiquent que la concentration preferentielle des gouttelettes ne contribue pas a l'elargissement des spectres, du moins tel qu'il est decrit actuellement par les modeles numeriques.

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Burnet, Frédéric. "Validation des mesures aeroportees de la microphysique nuageuse et etude des processus d'entrainement-melange dans les nuages convectifs." Toulouse 3, 1999. http://www.theses.fr/1999TOU30097.

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Le declenchement rapide des precipitations dans les nuages convectifs est encore mal explique. La theorie de la condensation predit en effet des distributions dimensionnelles de gouttelettes etroites alors que la formation des embryons de precipitation necessite des spectres larges avec de grosses gouttelettes. La mesure in situ des distributions dimensionnelles de gouttelettes est essentielle pour mieux comprendre la microphysique nuageuse. La campagne de mesures scms etait ainsi dediee a l'etude de la formation des precipitations dans les petits cumulus chauds (sans phase glace). La premiere partie de ce manuscrit est consacree a la validation des mesures collectees pendant scms, ou de nouveaux instruments etaient utilises. La principale difficulte est qu'il n'existe pas de standard absolu pour etalonner les instruments. Notre methodologie de validation s'appuie sur la comparaison des resultats obtenus dans des conditions variees, afin d'etudier et d'identifier les limites des differents instruments bases sur des principes de mesure differents (compteur de particules, integrateur, capteur a fil chaud). Dans la seconde partie de ce travail, nous utilisons les mesures ainsi validees pour etudier la formation des grosses gouttelettes a partir des processus de melange entre l'air nuageux et son environnement plus sec. Nous developpons une nouvelle methode d'analyse pour caracteriser la nature statistique du melange, homogene ou heterogene. Il est montre que le concept heterogene fournit une description satisfaisante de ses proprietes statistiques. Nous nous appuyons sur ce resultat pour construire un modele simple d'evolution des spectres de gouttelettes dans les cellules melangees, a concentration constante. Les conditions qui favorisent la formation des gouttes suradiabatiques sont identifiees. Les resultats du modele sont ensuite confrontes aux mesures.

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CAILLAULT, KARINE. "Processus physiques impliques dans la dynamique et thermodynamique aux differentes echelles de mouvement des systemes convectifs observes lors de toga-coare." Paris 7, 1998. http://www.theses.fr/1998PA077027.

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La convection profonde joue un role essentiel dans la circulation de l'atmosphere en redistribuant l'energie tropicale excedentaire vers les couches superieures de l'atmosphere et en favorisant les mouvements horizontaux de grande echelle. Une des manifestations de la convection profonde se presente sous la forme des systemes convectifs de mesoechelle (mcs) responsables d'une grande partie des precipitations terrestres. L'objectif de ce travail est l'etude du mcs du 12 decembre 1992 observe durant l'experience toga-coare realisee sur le reservoir d'eaux chaudes du pacifique ouest, zone cle dans la dynamique climatique. Au cours de cette etude, trois echelles de mouvement ont ete distinguees. L'echelle synoptique constitue l'environnement de la perturbation. La part des forcages dynamiques de cette echelle sur le cycle de vie du mcs etudie a ete quantifiee a partir des sorties du modele du cepmmt. La moyenne echelle correspond en general a l'echelle du phenomene (quelques centaines de kilometres carres) et permet d'avoir une vue d'ensemble des processus dynamiques et thermodynamiques gouvernant le systeme convectif et de mettre en evidence les liens existant entre celui-ci et son environnement. Les donnees utilisees pour acceder a cette echelle de mouvement sont issues de radars aeroportes. Elles ont ete cumulees sur des temps assez longs de facon a couvrir une zone d'echantillonnage la plus etendue possible. La comprehension des mecanismes de formation et d'evolution du systeme passe par l'analyse des donnees radar a l'echelle convective. Les domaines de restitution plus petits couvrent des zones distinctes du systeme telles que parties convectives et stratiformes permettant la description precise de la circulation d'echelle convective. Une etude des interactions entre les differentes echelles de mouvement a egalement ete menee ainsi qu'une comparaison des resultats de travaux anterieurs sur les mcs avec ceux issus de cette etude.

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Coutris, Pierre. "Analyse des propriétés dimensionnelles et massiques des cristaux de glace pour l’étude des processus microphysiques dans les systèmes convectifs à méso-échelle." Thesis, Université Clermont Auvergne‎ (2017-2020), 2019. http://www.theses.fr/2019CLFAC007/document.

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L’étude des propriétés et processus microphysiques caractérisant la phase glace permet de mieux définir le rôle des nuages dans le cycle de l’eau et sur bilan radiatif de l’atmosphère. Les modèles atmosphériques et les codes d’inversion des données de télédétection utilisent des paramétrisations établies à partir de mesures in situ. Ces mesures servent également des besoins industriels en lien avec la problématique du givrage en aéronautique. L’étude présentée se base sur les données de deux campagnes aéroportées réalisées dans le cadre de la collaboration internationale HAIC-HIWC, ciblant les zones à fort contenu en glace que l’on peut observe rau sein des systèmes convectifs à méso-échelle (MCS) tropicaux. Sur la question des relations « masse-diamètre » (m - D) d’abord, une nouvelle approche est présentée. Basée sur la résolution d’un problème inverse, elle permet de restituer la masse des cristaux à partir de mesures colocalisées classiques en s’affranchissant de la traditionnelle hypothèse de loi puissance, et montre que cette dernière ne permet pas de représenter correctement les propriétés massiques de populations de cristaux hétérogènes (morphologie et tailles différentes) typiques des MCS. La variabilité horizontale des distributions de tailles permet d’étudier le vieillissement de l’enclume d’un point de vue microphysique et de souligner le rôle essentiel du processus d’agrégation dans l’élimination des petits cristaux apportés dans la haute troposphère par la convection profonde et dans la formation d’agrégats supra-millimétriques, précurseurs glacés des précipitations stratiformes. Les relations m - D restituées permettent d’identifier des régimes microphysiques distincts et ouvre la voie aux développement d’une paramétrisation de la masse volumique des hydrométéores en fonction de critères environnementaux
The detailed characterization of ice cloud microphysics is key to understand their role in theEarth’s hydrological cycle and radiation budget. The developement of atmospheric models and remote sensingalgorithms relies on parametrisations derived from in situ measurements. These measurements are also usedby the aviation industry to handle the problem of ice crystal icing. This PhD work presents an analysis of themass and size properties of ice crystals observed in high ice water content areas embedded in tropical mesoscaleconvective systems (MCS) during two airborne field campaigns of the HAIC-HIWC international project.A new approach is developped to derive mass-size relationships (m - D) from size distributions and icewater contents. The retrieval is formulated as an inverse problem which waives the power law constraint, aclassical assumption that proves to be an oversimplification when applied to heterogeneous populations of iceparticules typical of MCS anvils.The horizontal variability of size distributions and the aging of MCS anvils is described in terms of microphysicalprocesses. The importance of the aggregation growth process is emphasized as it efficiently removessmall ice particles brought into the upper troposphere by deep convection and significantly contributes to theformation of large agregates, precusor of the stratiform precipitations. The analysis of mass properties revealsthat distinctive microphysical regimes may be identified from the m-D relationship retrieved in various conditions.It paves the way toward a statistical model of the effective density of ice particles as a function of environmentalparameters

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Deheuvels, Sébastien. "Apports de la sismologie des étoiles F et G à l'étude des cœurs convectifs." Phd thesis, Observatoire de Paris, 2010. http://tel.archives-ouvertes.fr/tel-00553197.

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La qualité des données sismiques du satellite CoRoT ouvre de nouveaux horizons dans l'étude de la structure interne et de l'évolution des étoiles. Mon travail de thèse a consisté à l'analyse et à l'interprétation des spectres d'oscillations de pulsateurs de type solaire observés au sol et depuis l'espace, en particulier avec le satellite CoRoT. L'objectif de l'analyse d'un spectre d'oscillations est de déterminer les paramètres des modes propres de l'étoile (fréquences propres, amplitudes, durées de vie). Au sein du DAT (groupe en charge de l'analyse des pulsateurs de type solaire observés avec CoRoT), j'ai contribué à adapter aux objets étudiés une méthode utilisée avec succès pour extraire les paramètres des modes du Soleil. J'ai appliqué cette méthode à l'analyse des cinq premiers pulsateurs de type solaire observés avec CoRoT. Un soin particulier a été consacré à l'étude de la significativité des pics détectés, qui a permis de conforter la détection de modes mixtes dans le spectre de l'étoile HD 49385. Dans le cadre de mon travail de modélisation et d'interprétation, j'ai recherché les informations qu'apportent les paramètres sismiques sur la structure du cœur de certaines des étoiles analysées. Ces étoiles possèdent (ou ont possédé) un cœur convectif, dont les caractéristiques dépendent des processus de transport des éléments chimiques (e.g. l'overshooting), aujourd'hui mal décrits par les modèles théoriques. J'ai modélisé trois pulsateurs de type solaire de masses et de stades évolutifs différents, dont les paramètres sismiques permettent de sonder le cœur et donc de contraindre observationnellement les processus de transport. En particulier, la détection dans l'étoile HD 49385 de modes mixtes en croisement évité m'a amené à étudier l'apport de ce type de mode à la compréhension de la structure du cœur.

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Legoix, Léonard. "Étude expérimentale et modélisation de mélangeurs convectifs : agitation de poudres de différentes coulabilités." Thesis, Ecole nationale des Mines d'Albi-Carmaux, 2016. http://www.theses.fr/2016EMAC0020/document.

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Les étapes de mélange sont souvent délicates à appréhender, car il subsiste encore des lacunes sur les lois dynamiques qui régissent ces opérations. De ce fait, la prédiction de l’homogénéité d’un mélange de poudres nécessite encore de nombreux essais. Dans ce travail de thèse, nous nous attachons à développer une méthodologie qui permet de contribuer au développement de modèles prédictifs dans les mélangeurs de poudres tout en mettant en évidence des invariants possibles pour les changements d’échelle sur ces procédés. Ainsi nous avons étudié l’agitation de poudres, avec différentes résistances à l’écoulement, dans un mélangeur convectif planétaire de type Triaxe® d’une contenance de 48 L. Des mesures de propriétés rhéologiques à l’échelle du laboratoire (rhéomètre FT4, voluménomètre) sont effectuées afin de mieux comprendre le comportement des poudres à l’échelle du mélangeur. Un mélangeur convectif prototype a été conçu dans le cadre de cette thèse. Cet appareil polyvalent constitué d’une cuve cylindrique transparente et agité par un mobile constitué de quatre pales, permet de visualiser les régimes et les mécanismes d’écoulement tout en effectuant des mesures rhéologiques. Deux régimes d’écoulement ont été identifiés (roulement, cataracte), ainsi que trois mécanismes (convection, diffusion et avalanche). Ces mécanismes ont permis d’établir un modèle stochastique, dont les paramètres ont été évalués expérimentalement pour une poudre à écoulement libre et pour une poudre cohésive
Mixing systems are usually difficult to understand, because there is a lack of knowledge concerning dynamic laws ruling these operations. Thus, nowadays, several tests are needed to predict properly the homogeneity of a powder mix. Throughout this PhD work, a method is developed to build predictive models for powder mixers and to bring out possible invariants for scale switching on these processes. Thus the stirring of powders is studied using different flow resistances within a 48L capacity Triaxe®, a convective planetary mixer. Rheological properties measurements are done at labscale (FT4 rheometer, volumenometer) for a better understanding of powder behavior at a wider mixer scale. A prototype blender has been built for this work. This polyvalent device, made of four blades and of a transparent vessel, allows to observe flow regimes and mechanisms, and to do rheological measurements. Two flow regimes have been identified (rolling, cataracting) and three flow mechanisms (convection, diffusion and avalanching). These mechanisms allowed to do stochastic modelling, for which parameters have been evaluated with experiments for free-flowing and cohesive powders

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Besson, Lucas. "Processus physiques responsables de l'établissement et de la variabilité de la mousson africaine." Paris 6, 2009. https://tel.archives-ouvertes.fr/tel-00420498.

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La Mousson Africaine est un système complexe, qui présente des variations interannuelles très importantes. Ces variations se traduisent par des fluctuations considérables des précipitations. Avoir une meilleure compréhension du système complexe qu'est la mousson peut permettre de mieux cerner les mécanismes qui influencent la production des précipitations sur le Sahel. La première partie de ce travail consiste en une comparaison entre deux saisons de mousson aux régimes de précipitations différents afin d'extraire les caractéristiques communes et différentes, pour les appliquer à l’étude de la saison de mousson de la campagne AMMA. Cette étude est complétée par une compréhension des processus dynamiques et thermodynamiques qui sont à l'origine du déclenchement de l'Onset de la mousson, et qui permettent l'installation des conditions favorables au développement des précipitations sur le Sahel. Le second volet est basé sur les mécanismes physiques qui entrent en compte dans le cycle de vie des lignes de grains sur le Sahel. Enfin, la dernière partie traite de l'impact des systèmes de méso-échelle sur leur environnement en terme de bilan de chaleur et d'humidité.

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Sánchez, Martín Xavier. "Processos convectius en un sistema aquàtic natural: l'estany de Banyoles." Doctoral thesis, Universitat de Girona, 2008. http://hdl.handle.net/10803/7810.

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En aquesta tesi s'aborda l'estudi de la turbulència convectiva, a partir d'un treball experimental realitzat a l'estany de Banyoles. Més concretament, els objectius s'emmarquen en dues categories diferents. En la primera, es pretén sistematitzar el marc teòric-conceptual de la turbulència i més concretament de la turbulència convectiva, el qual es troba molt dispers a la bibliografia i sovint enfocat des de perspectives prou diverses. Dins aquest primer objectiu també es pretén sistematitzar tot un conjunt d'eines experimentals de tractament de dades que ens permetran obtenir els valors de les velocitats de dissipació de l'energia cinètica turbulenta i de la variància de temperatura de forma automàtica y obtenir els fluxos turbulents. També volem abordar la caracterització del les escales turbulentes, pel que treballarem amb l'escala de Thorpe i utilitzarem l'anàlisi d'ondetes del senyal de microstructura tèrmica. Dins la segona categoria d'objectius, aquests aplicats directament a l'estany de Banyoles, es pretén aplicar els conceptes desenvolupats al primer objectiu en dos sistemes convectius, un de tipus plomall i l'altre del tipus multidifusiu. Dins aquest context, en els dos primers capítols es presenta teòricament com s'obtenen els fluxos turbulents i s'introdueixen els fenòmens convectius, i en el tercer i quart capítols es descriuen tot un conjunt d'eines d'anàlisi experimental que ens serviran per a l'estudi del cas concret de la convecció al fons de l'estany de Banyoles.

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Gouze, Philippe. "Modélisation des transferts de masse liés aux circulations dans les aquifères sédimentaires. Application à l'aquifère du dogger du Bassin de Paris et aux écoulements thermo-convectifs dans les réservoirs gréseux." Paris 6, 1993. http://www.theses.fr/1993PA066105.

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Au cours de la diagenese, les transformations mineralogiques induites par les mouvements du fluide sont susceptibles de modifier les caracteristiques geophysiques des aquiferes et des reservoirs sedimentaires. Cette these a pour but (i) de contribuer a la modelisation mathematique de ces processus et (ii) d'appliquer ces modeles a deux cas d'interet geologique : l'aquifere du dogger du bassin de paris et les reservoirs greseux affectes par des circulations thermo-convectives

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Besson, Lucas. "Processus Physiques Responsables de l'Etablissement et de la Variabilité de la Mousson Africaine." Phd thesis, Université Pierre et Marie Curie - Paris VI, 2009. http://tel.archives-ouvertes.fr/tel-00420498.

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La Mousson Ouest Africaine est un système complexe, qui présente des variations interannuelles très importantes. Ces variations se traduisent par les fluctuations considérables des précipitations sur le continent. Avoir une meilleure compréhension du système complexe qu'est la mousson africaine peut permettre de mieux cerner les mécanismes qui influencent la production des précipitations sur le Sahel. La première partie de ce travail de thèse consiste en une comparaison entre deux saisons de mousson aux régimes de précipitations différents afin d'extraire les caractéristiques communes et différentes, pour les appliquer à l'étude de la saison de mousson durant laquelle s'est déroulée la campagne AMMA. Cette étude est complétée par une compréhension des processus dynamiques et thermodynamiques qui sont à l'origine du déclenchement de l'Onset de la mousson, et qui permettent l'installation des conditions favorables au développement des précipitations sur le Sahel. Le second volet est basé sur les mécanismes physiques qui entrent en compte dans le cycle de vie des lignes de grains sur le Sahel. Enfin, la dernière partie traite de l'impact des systèmes de méso - échelle sur leur environnement en terme de bilan de chaleur et d'humidité dans le quadrilatère sud de l'expérience AMMA.

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Hirt, Mirjam [Verfasser], and George C. [Akademischer Betreuer] Craig. "Convective initiation - relevant processes and their representation in convection-permitting models / Mirjam Hirt ; Betreuer: George C. Craig." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2020. http://d-nb.info/1213659043/34.

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Morin, Julien. "Processus dynamos dans les étoiles entièrement convectives." Phd thesis, Université Paul Sabatier - Toulouse III, 2009. http://tel.archives-ouvertes.fr/tel-00480428.

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Dans les étoiles de type solaire la génération de champ magnétique par effet dynamo se concentre dans la tachocline, une fine zone de fort cisaillement à l'interface entre le cœur radiatif et l'enveloppe convective. En dessous de 0.35 masse solaire, les étoiles de la séquence principale sont entièrement convectives et ne possèdent donc pas de tachocline. Or certaines de ces étoiles sont très actives, et des champs magnétiques très intenses ont été mesurés. La dynamo de ces objets doit donc reposer sur des processus physiques différents de ceux à l'œuvre dans le Soleil. En dépit des avancées théoriques récentes, la dynamo des étoiles entièrement convective reste mal comprise. La partie observationnelle de ce travail a consisté en l'étude d'un échantillon d'étoiles situées de part et d'autre de la limite entièrement convective à partir d'observations spectropolarimétriques et de la technique d'imagerie Zeeman-Doppler. Cela a permis d'analyser comment les paramètres stellaires, période de rotation et masse, influent sur le champ magnétique à grande échelle. Un changement brutal de topologie magnétique des naines M est mis en évidence à proximité de la limite entièrement convective. Un comportement inattendu est également détecté en dessous de 0.2 masse solaire : des étoiles de paramètres stellaires quasi-identiques présentent des topologies magnétiques radicalement différentes. Ce travail observationnel est doublé d'une approche numérique : des simulations MHD 3D «star-in-a-box» visent à mieux comprendre les divergences qui existent entre les premières simulations et les observations.

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14

張明恩 and Ming-yan William Cheung. "Circulant preconditioners for convection diffusion equation." Thesis, The University of Hong Kong (Pokfulam, Hong Kong), 2001. http://hub.hku.hk/bib/B31224180.

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Cheung, Ming-yan William. "Circulant preconditioners for convection diffusion equation." Hong Kong : University of Hong Kong, 2001. http://sunzi.lib.hku.hk/hkuto/record.jsp?B23316743.

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16

Hunter, Kim R. "Macroscopic convection in the thin-film processor." Thesis, Virginia Tech, 1988. http://hdl.handle.net/10919/45162.

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The thesis explores the proposal that macroscopic fluid convection in thin-film processors may be adequately represented by simple linear deterministic models. In addition, it examines the suggestion that the models themselves provide a useful tool in the search for a generalizable 'intrinsic' process heat transfer film coefficient, i.e., one that includes the effects of axial dispersion of the process fluid. Such a parameter would be helpful in the design and scale up of thin-film equipment.

The following approach was used to investigate this proposal: first, experimental fluid residence time distributions were obtained t over a range of operating conditions, using an industrial pilot plant thin -film processor. The experimental data were used to select an appropriate linear fluid flow model for the process. The model parameters were evaluated over this range using frequency response techniques. These models were subsequently incorporated into a numerical heat transfer simulation of the thin -film processor. Careful matching of the pilot plant transient temperature responses to those predicted by the simulation yielded the sought after intrinsic (dispersion corrected) heat transfer film coefficients for the processor.
Master of Science

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17

Al-Maskari, Juma. "Processes of convection and airflow over the Hajar Mountains." Thesis, University of Leeds, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.432311.

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18

Rostami, Masoud. "Dynamical influence of diabatic processes upon developing instabilities of Earth and planetary jets and vortices." Thesis, Paris 6, 2017. http://www.theses.fr/2017PA066186.

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Le but de la thèse est de comprendre l'influence dynamique des effets diabatiques, comme la convection humide, sur les instabilités des vortex atmosphériques terrestres et planétaires. Un modèle verticalement intégré, avec les paramétrisations type relaxation des transitions de phase et de dégagement de la chaleur latente, le modèle de St-Venant avec la convection humide, a été utilisé. La version précédente du modèle a été améliorée pour inclure l'eau précipitable, sa vaporisation et son entrainement. L'approche consiste en 1) analyse détaillée de stabilité des profils idéalisés, ou extraits des données, des vortex, 2) étude de saturation non-linéaire des instabilités à l'aide de schéma numérique de haute résolution aux volumes finis. Les résultats principaux de la thèse sont : 1. Démonstration et quantification d'une forte influence des effets humides sur les instabilités des vortex synoptiques, y compris asymétrie cyclone-anticyclone des vortex de faible intensité aux latitudes moyennes, et de l'intensification des vortex type cyclones tropicaux, avec formation des nuages caractéristiques. 2. Explication de l'origine dynamique de l'hexagone au pôle Nord de Saturne, et de l'absence de structure similaire au pôle Sud, en termes d'instabilité du système vortex polaire - jet circumpolaire, et sa saturation non-linéaire. 3. Explication de la structure observée du vortex polaire hivernal sur Mars en termes d'instabilité et sa saturation en présence de réchauffement /refroidissement radiatif et de déposition de CO2 (transition de phase gaz - solide). Une nouvelle paramétrisation simple a été proposée pour ce processus, incluant l'influence des noyaux de déposition
The thesis is devoted to understanding dynamical influence of diabatic effects, like moist convection, on instabilities of vortices in Earth and planetary atmospheres. A vertically integrated atmospheric model with relaxational parameterisation of phase transitions and related heat release, and with convective fluxes included in mass and momentum equations, the moist-convective rotating shallow water model, was used for this purpose. The previous version of the model was improved to include precipitable water and its vaporisation and entrainment. The approach consists in 1)detailed stability analysis of idealised, or extracted from the data, vortex profiles, 2)study of nonlinear saturation of the instabilities with the help of finite-volume high-resolution numerical code. The main results of the thesis are: 1. Demonstration and quantification of strong influence of moist effects upon instabilities of synoptic vortices, including cyclone-anticyclone asymmetry of mid-latitude vortices of weak intensity, and intensification of tropical-cyclone like vortices with formation of typical cloud patterns. 2. Explanation of the dynamical origin of the Saturn's North Polar hexagon, and of the lack of similar structure at the South Pole, in terms of instability of the coupled polar vortex and circumpolar jet, and their nonlinear saturation.3. Explanation of the observed structure of Mars' winter polar vortex in terms of instability of the latter, and its saturation in the presence of radiative heating/cooling and CO2 deposition (gas-solid phase transition). A new simple parameterisation of the latter process, including the influence of deposition nuclei, was developed in the thesis

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Takahashi, Ken. "Processes controlling the mean tropical Pacific precipitation pattern /." Thesis, Connect to this title online; UW restricted, 2006. http://hdl.handle.net/1773/10069.

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Lamazouade, André. "Modélisation du processus de croissance cristalline de Bridgman par une méthode enthalpique." Aix-Marseille 2, 2000. http://www.theses.fr/2000AIX22049.

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21

Pagalthivarthi, Krishnan V. "Thermosolutal convection and related transport processes in binary alloy solidification." Diss., Georgia Institute of Technology, 1988. http://hdl.handle.net/1853/18245.

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22

Sharpe, Michael Anthony. "On convection and stability of some welding and solidification processes." Thesis, University of Southampton, 2000. https://eprints.soton.ac.uk/50624/.

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In this thesis a variety of problems are considered, the first of which is associated with the welding process. Experiments indicate that the shape of a weld pool is influenced by convection in the liquid metal. In recent years it has been shown that this convection is crucially affected by the Marangoni (or thermocapillary) force. Recently Craine and Belgrove ([30], [7]) have developed a two-dimensional, axisymmetric model which includes the Marangoni force on the free surface of a semi-infinite region of liquid steel when a point source of current and heat is incident on the free surface. An asymptotic solution to this problem is obtained in this thesis, and the surface tension gradient with respect to temperature, dj/dT, a parameter which is crucial to the magnitude of the Marangoni force, is found to affect every coefficient in the leading and first order asymptotic expansions. In various theoretical and experimental models purely poloidal flow bifurcates to a rotating flow. To investigate this possibility for our flow a linear stability analysis is performed on a numerically obtained poloidal solution for the flow and temperature distribution in a hemisphere (a model first derived in [7]). For the azimuthal stability mode m = 0 the equation governing the linear stability of the rotating motion is found to decouple from the corresponding poloidal equations. The poloidal and azimuthal stability equations both become unstable at different critical currents dependent on the sign and magnitude of d^/dT. An investigation of the eigenvectors indicates the onset of instability near to the point source. For the upper modes instability occurs only when m = 1 and in a very small region of parameter space. In the second part of this thesis a freezing sphere problem with flow is used to compare a sharp interface Stefan model and a diffuse interface phase-field model. Firstly a Stefan model that includes a disparity between the density of the solid and liquid phases is derived and solved numerically. This model is compared with a recent phase-field model with flow, derived by Anderson et al. in [2]. In this thesis the one-dimensional isotropic version of Anderson's model is obtained in spherical polar coordinates and using certain simplifications when the dimensionless thickness of the interface £5 is vanishingly small a leading order asymptotic expression reproduces the Stefan model with flow. The phase-field model is subsequently modified and solved numerically, and the results are compared with the sharp interface model. Close agreement is observed between these models when es < 0.01.

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23

Chen, Yin-Heng. "Study of solidification, shrinkage and natural convection in casting processes /." The Ohio State University, 1990. http://rave.ohiolink.edu/etdc/view?acc_num=osu1487676847114631.

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24

Chow, Peter M. Y. "Control volume unstructured mesh procedure for convection-diffusion solidification processes." Thesis, University of Greenwich, 1993. http://gala.gre.ac.uk/6133/.

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The research work presented herein addresses the unstructured mesh problem in finite volume (FV) or control volume (CV) method used in numerical simulations. The modelling work conducted is in context of solidification for casting processes. The control volume-unstructured mesh (CV-UM) method can be categorised into two approaches, a vertex-centred and a cell-centred approach. The classification of the approach is based on the relationship between the control volume and the unstructured mesh. The vertex-centred is naturally unstructured and has been used successfully in fluid flow and heat transfer calculations. The cell-centred on the other hand has always been associated with structured (quadrilateral) meshes, this has been extended to handle unstructured mesh in the current work and is called the irregular control volume (ICV) method. Both approaches have been studied for solidification by conduction only, using several standard phase change test cases and one with experimental data from the casting industry. The result of this work is reported and their suitability for solidification addressed. For the ICV method, the extension to solve the full convective-diffusive solidification was undertaken, these are primarily the fluid flow and energy equations solved using the well known SIMPLE algorithm. One spin-off from the ICV is the appearance of "highorder cell" control volumes, control volumes with more than the standard four cell faces in two-dimensions. The high-order cell technique is exhibiting the same characteristics as high-order schemes used in standard CV method, when applied to standard CFD test cases. The one current drawback for the technique is the generation of these high-ordercells, currently no fully- or semi-automatic mesh generation is available. This prevented further study of the technique and used in the solidification test cases, where in one, experimental data is available for the phase change fronts. This was carried Out using quadrilateral meshes, but solved using the unstructured approach of the ICV. The predicted solution is in qualitative agreement with experiment. The second convective-diffusive solidification problem is the first to demonstrate the CV-UM integrated framework by solving two major casting components simultaneously, the solidification (the work undertaken in this research) and the residual stress for deformation. This is still an on going research work, where refinement and validation are required and further integration of casting processes, such as mould filling, are necessary to complete the various stages of the shape casting process. This kind of integrated simulation requires huge amount of computations, it will take days for traditional scalar computers to do one prediction. Vector and parallel machines offer ways in which to bring down the computing times to a level that is in hours instead of days. To utilise machines with vector and parallel capability efficiently, the algorithm of the model process need to be mapped onto such architectures for it to take full advantage of the computing powers. The solidification algorithm in threedimensions has been vectorised and a speed-up of five is possible. This was part of a collective study into mapping algorithms Onto vector and parallel computers, where it emerged that the ideal computing architecture is a network of processors each with its own vector capabilities.

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25

Veber, Maicon Eirolico. "Desenvolvimento de atividades convectivas sobre a região nordeste do Brasil, organizadas pela extremidade frontal." Universidade Federal de Alagoas, 2011. http://repositorio.ufal.br/handle/riufal/891.

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Coordenação de Aperfeiçoamento de Pessoal de Nível Superior
Este trabalho tem como objetivo estudar as influências frontais sobre a Região Nordeste do Brasil. Procurando-se encontrar quais fatores são responsáveis pela formação de processos convectivos sobre esta Região, organizados pela extremidade frontal sobre o Atlântico Tropical Sul (ATS). Buscando-se, dessa forma, estabelecer um padrão de circulação para esses casos, o que poderá facilitar a previsão desses eventos. Assim, foi realizada a análise de 103 eventos de sistemas frontais, que influenciaram as condições de tempo sobre a Região Nordeste do Brasil, para o período de dez anos (2000-2009). Os casos foram divididos em dois grupos (G), de acordo com desenvolvimento ou não da convecção. G1- atividades convectivas sobre a região Nordeste do Brasil, organizadas pela extremidade frontal sobre o Atlântico Tropical Sul (ATS); G2 - banda de nebulosidade pouco ativa sobre a Região Nordeste do Brasil, organizada pela extremidade frontal, sobre o ATS. As frentes foram identificadas, utilizando-se métodos sinóticos clássicos e imagens de satélite. Foram utilizados dados de reanálise do modelo global do NCEP/NCAR, imagens de satélite do canal infravermelho e dados de radiossondagens. Após uma comparação entre os dois grupos foram definidos oito fatores dinâmicos (F) que influenciaram na formação de atividades convectivas associadas à extremidade frontal. Sendo esses fatores em baixos níveis: F1 Zona de Convergência Intertropical (ZCIT); F2 fluxo bifurcado sobre o Atlântico e convergência sobre o Nordeste; F3 borda oeste da Alta Subtropical do Atlântico Sul (ASAS). Em níveis médios: F4 convergência e cavado sobre o NEB; F5 cavado com eixo NW-SE; F6 cavado ciclone do HN. Em altos níveis: F7 cavado em 200 hPa; F8 Interação entre a AB e o VCAN. Foram analisados perfis termodinâmicos SkewT-LogP e os índices de instabilidade CAPE, K, TT e LI para avaliar o potencial de desenvolvimento convectivo sobre o NEB, para os eventos do G1 e G2. Durante o período de estudo foram verificados 68 casos do G1 e 35 casos do G2. Os casos do G1 foram encontrados durante todas as estações do ano, porém de forma muito mais frequente durante o verão austral e, muito raro durante o inverno, com apenas um caso. Em 11 oportunidades, esses casos estiveram relacionados com eventos de ZCAS. Os eventos do G2 foram mais frequentes durante o inverno austral e não foram observados durante o verão austral. Os fatores (F) para níveis inferiores foram encontrados com frequências semelhantes sendo que F1, F2 e F3 estiveram presentes respectivamente em 45, 50 e 43% dos casos estudados. Para níveis médios, o fator mais frequente foi o F4, presente em cerca de 60% dos casos. Para altos níveis, em 75% dos casos foi encontrada a interação entre a AB e o VCAN (F8). Com isso temos como padrões de circulação em maior frequência as interações F1/F4/F8; F2/F4/F8 e F3/F4/F8, em 25, 22 e 25%, respectivamente. A análise termodinâmica mostrou padrões semelhantes para os casos do G1 e do G2. Os índices de instabilidade CAPE e LI mostraram um forte potencial convectivo (com valores de CAPE acima de 1500 J/Kg) em 88% dos casos do G1 e 63% dos casos do G2, para 12 horas anteriores a atuação da zona frontal sobre o nordeste. Os índices K e TT não mostraram potencial convectivo para a quase totalidade dos casos do G1 e G2. Portanto, os processos convectivos organizados pela extremidade frontal ocorrem preferencialmente quando se tem confluência em níveis médios, e forte difluência em altos níveis. Tendo em níveis baixos convergência no escoamento e/ou atuação conjunta ao sistema frontal da ZCIT. A avaliação somente de índices de instabilidade não se mostrou eficiente para determinar os processos convectivos associados à zona frontal. Sendo os mecanismos dinâmicos preponderantes para o desenvolvimento desses processos.

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26

Gérard, Thomas. "Theoretical study of spatiotemporal dynamics resulting from reaction-diffusion-convection processes." Doctoral thesis, Universite Libre de Bruxelles, 2011. http://hdl.handle.net/2013/ULB-DIPOT:oai:dipot.ulb.ac.be:2013/209861.

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Dans les réacteurs industriels ou dans la nature, l'écoulement de fluides peut être couplé à des réactions chimiques. Dans de nombreux cas, il en résulte l'apparition de structures complexes dont les propriétés dépendent entre autres de la géométrie du système.

Dans ce contexte, le but de notre thèse a été d'étudier de manière théorique et sur des modèles réaction-diffusion-convection simples les propriétés de dynamiques spatio-temporelles résultant du couplage chimie-hydrodynamique.

Nous nous sommes focalisés sur les instabilités hydrodynamiques de digitation visqueuse et de densité qui apparaissent respectivement lorsqu'un fluide dense est placé au-dessus d'un fluide moins dense dans le champ de gravité et lorsqu'un fluide visqueux est déplacé par un fluide moins visqueux dans un milieu poreux.

En particulier, nous avons étudié les problèmes suivants:

- L'influence d'une réaction chimique de type A + B → C sur la digitation visqueuse. Nous avons montré que les structures formées lors de cette instabilité varient selon que le réactif A est injecté dans le réactif B ou vice-versa si ces réactifs n'ont pas un coefficient de diffusion ou une concentration initiale identiques.

- Le rôle de pertes de chaleur par les parois du réacteur dans le cadre de la digitation de densité de fronts autocatalytiques exothermiques. Nous avons caractérisé les conditions de stabilité de fronts en fonction des pertes de chaleur et expliqué l'apparition de zones anormalement chaudes lors de cette instabilité.

- L'influence de l'inhomogénéité du milieu sur la digitation de densité de solutions réactives ou non. Nous avons montré que les variations spatiales de perméabilité d'un milieu poreux peuvent figer ou faire osciller la structure de digitation dans certaines conditions.

- L'influence d'un champ électrique transverse sur l'instabilité diffusive et la digitation de densité de fronts autocatalytiques. Il a été montré que cette interaction peut donner lieu à des nouvelles structures et changer les propriétés du front.

En conclusion, nous avons montré que le couplage entre réactions chimiques et mouvements hydrodynamiques est capable de générer de nouvelles structures spatio-temporelles dont les propriétés dépendent entre autres des conditions imposées au système.

/

In industrial reactors or in nature, fluid flows can be coupled to chemical reactions. In many cases, the result is the emergence of complex structures whose properties depend among others on the geometry of the system.

In this context, the purpose of our thesis was to study theoretically using simple models of reaction-diffusion-convection, the properties of dynamics resulting from the coupling between chemistry and hydrodynamics.

We focused on the hydrodynamic instabilities of viscous and density fingering that occur respectively when a dense fluid is placed above a less dense one in the gravity field and when a viscous fluid is displaced by a less viscous fluid in a porous medium.

In particular, we studied the following issues:

- The influence of a chemical reaction type A + B → C on viscous fingering. We have shown that the fingering patterns observed during this instability depends on whether the reactant A is injected into the reactant B or vice versa if they do not have identical diffusion coefficients or initial concentrations.

- The role of heat losses through the reactor walls on the density fingering of exothermic autocatalytic fronts. We have characterized the conditions of stability of fronts depending on heat losses and explained the appearance of unusually hot areas during this instability.

- The influence of the inhomogeneity of the medium on the density fingering of reactive solutions or not. We have shown that spatial variations of permeability of a porous medium may freeze or generate oscillating fingering pattern under certain conditions.

- The influence of a transverse electric field on the Rayleigh-Taylor and diffusive instabilities of autocatalytic fronts. It was shown that this interaction may lead to new structures and may change the properties of the front.

In conclusion, we showed that the coupling between chemical reactions and hydrodynamic motions can generate new space-time structures whose properties depend among others, on the conditions imposed on the system.
Doctorat en Sciences
info:eu-repo/semantics/nonPublished

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MacTavish, Flora Pamela. "The application of adaptive mesh techniques to convective processes in oceanography." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/11616.

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Idealised numerical simulations of two oceanographic processes, salt finger formation and the restratification phase of open ocean deep convection, are considered. These processes are modelled using the Imperial College Ocean Model (Fluidity-ICOM). This is a finite element code with the novel capability to perform mesh adaptivity. The mesh is triangular/tetrahedral and can be unstructured. If mesh adaptivity is used, all the fields are periodically interpolated onto a new mesh that has been optimised from the previous mesh. The new mesh is designed to represent one or more of the solution fields as accurately as possible by putting more nodes in regions where the Hessians of the fields being adapted to are higher. The first process to be presented is the formation of salt fingers in double diffusive convection. A secondary instability is observed to form. Due to the unstructured mesh, the fingers start off slightly different lengths from each other and this difference is observed to grow with time. A new set of simulations are run in which the secondary instability is initialised from a perturbation in the initial condition. These results are used to compare between fixed and adaptive mesh results. Evidence is obtained to show that adaptive mesh is able to produce the same results as the fixed mesh with fewer computational nodes because the resolution is used in the regions where is it most needed. The second process is the restratification after open ocean deep convection. In order to run ocean scale simulations the model must be able to accurately represent geostrophic and hydrostatic balance on a high aspect ratio domain. In order to do this with an unstructured mesh it is shown that it is necessary to constrain the nodes to be aligned in the vertical. This type of mesh is known as a 2+1 mesh and it can be adapted in both the vertical and the horizontal in order to resolve the solution fields more accurately. The model is able to reproduce previous results for a simple restratification test case using mesh adaptivity. The representation of balance is investigated using different types of mesh and different finite element shape functions. A more complex restratification test case in which baroclinic eddies form is then examined. The results obtained are compared to other models with different numerical schemes. Fixed and adaptive results are compared. These results demonstrate that Fluidity-ICOM is able to represent balance and model relatively complex processes on ocean scale, high aspect ratio domains whilst using mesh adaptivity.

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Albrecht, Rachel Ifanger. "Eletrificação dos sistemas precipitantes na região Amazônica: processos físicos e dinâmicos do desenvolvimento de tempestades." Universidade de São Paulo, 2008. http://www.teses.usp.br/teses/disponiveis/14/14133/tde-24102008-154430/.

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Os sistemas convectivos da região Amazônica possuem características microfísicas peculiares, que variam de um caráter convectivo marítimo (estação chuvosa) a continental (estação de transição seca-chuvosa). Essas características modulam a eletrificação desses sistemas, porém ainda não se sabe quais são os processos dominantes que intensificam o número de descargas elétricas de uma estação para outra: efeito dos aerossóis, termodinâmico, grande-escala ou topografia? Para responder à essa pergunta, o objetivo deste trabalho foi identificar e quantificar a importância de cada um desses efeitos na eletrificação dos sistemas convectivos da Amazônia. A metodologia foi baseada em análises de dados observacionais do experimento de campo DRYTOWET e em um modelo numérico com parametrizações de transferências de cargas e descargas elétricas. A análise do ciclo anual das descargas elétricas do tipo nuvem-solo (CGs) mostrou que a atividade elétrica dos sistemas precipitantes da região sudoeste da Amazônia aumenta durante a transição da estação seca para a estação chuvosa (Agosto a Setembro), associada aos sistemas convectivos com maior desenvolvimento vertical que acontecem nesse período. Com o estabelecimento da estação chuvosa (Novembro a Março), o número de CGs diminui porém a atividade elétrica ainda se mantêm. A porcentagem desses totais de CGs que tinham polaridade positiva (+CGs) tem média de 12% durante todo o ano, aumentando drasticamente para até 25% em Setembro, durante a transição entre as estações secas e chuvosa. Esse aumento da %+CGs ocorreu simultaneamente ao aumento da poluição atmosférica provocada pela queima de biomassa das pastagens realizada pelos fazendeiros locais, que as preparam para a agricultura e pecuária durante o início das primeiras chuvas. Por outro lado, o aumento da %+CGs das tempestades também ocorreu preferencialmente sobre a área de pastagem do estado de Rondônia. Através da análise de dados de radar dos sistemas precipitantes que ocorreram durante o experimento DRYTOWET, foi constatado que as tempestades positivas (tempestades que produzem mais de 50% de +CGs em 50% de seu tempo de vida) se formaram em ambientes mais secos e com alturas do nível de convecção por levantamento (NCL, altura da base da nuvem) maiores do que as demais tempestades (tempestades negativas), durante todo o experimento mas com maiores diferenças durante o final da estação seca (Setembro-Outubro). Com altura da base da nuvem mais elevada, a espessura da camada quente (ECQ - base da nuvem até a isoterma de 0oC) diminui, aumentando assim a velocidade das correntes ascendentes através de um melhor processamento da energia potencial disponível para convecção (CAPE) devido a um menor entranhamento. O aumento da velocidade das correntes ascendentes dentro da nuvem resulta em tempestades mais profundas e mais intensas. O efeito do aumento do NCL é uma característica das regiões com vegetação de pastagem, onde a razão entre o calor sensível e latente na superfície é maior do que as áreas florestadas, aumentando a altura da camada limite planetária. As diferenças de concentração total e distribuição de tamanho dos aerossóis devido ao aumento da poluição durante a transição entre as estações seca e chuvosa não foram conclusivas quanto a um possível efeito na distribuição de hidrometeoros das tempestade positivas e negativas, uma vez que o ciclo diurno da concentração dos aerossóis acompanha o ciclo diurno da camada limite planetária, que também regula o efeito da ECQ. Simulações numéricas com um modelo 1D de nuvem, acoplado à parametrizações de transferências de cargas elétricas entre hidrometeoros e raios, mostraram que a estrutura termodinâmica da atmosfera foi a maior responsável pela eletrificação das tempestades simuladas, aumentando a velocidade das correntes ascendentes. O efeito do aumento do número de aerossóis, que inibe da fase quente da nuvem e conseqüentemente fortalece a da fase fria da nuvem fornecendo mais vapor e gotículas de nuvem para essa região, provocou a diminuição da quantidade de granizo nas tempestades simuladas e o aumento de partículas agregadas menores, como os flocos de neve e graupel, diminuindo a freqüência de raios.
Amazonian convective systems have unique microphysical characteristics, varying from a maritime convective behavior (rainy season) to a continental behavior (wet-dry transition season). These characteristics modulate the electrification of these systems, however it is still not well understood which are the dominant processes that intensify the frequency of lightning from one season to another: aerosol effect, thermodynamics, large-scale variability, landscape or topography? To answer this question, the objective of this study was to identify and quantify the importance of each one of these effects on the electrification of convective systems over the Amazon. The methodology was based on the analysis of observational data from the field experiment DRYTOWET and a numerical model with charge transfer parameterizations and lightning discharges. The cloud-to-ground (CG) lightning discharges annual cycle presented that the electrical activity of the southwestern Amazonian precipitating systems increased during the transition between the dry and wet seasons (August to September), in association with the convective systems deepening. With the establishment of the wet season (November to March), the number of CGs decreased but the electrical activity continued. The mean annual percentage of cloud-to-ground lightning of positive polarity (+CGs) was 12%, increasing drastically to 25% in September during the transition between the dry and wet seasons. This percentage of +CGs raise happened simultaneously with the increase in the atmospheric pollution due to the pasture biomass burning, held by local farmers to prepare the soil for agriculture and livestock during the begging of first rains. On the other hand, the increase in %+CGs also occurred preferentially over pasture areas of Rondonia state. Through the analysis of radar precipitating systems that occurred during the field campaign DRYTOWET, it was noted that positive thunderstorms (storms that produced more than 50% of +CGs over 50% of their life time) were initiated in drier and higher lift condensation levels (cloud base height) environments than other storms (negative thunderstorms) during all the field experiment, especially in the end of the dry season (September-October). A higher cloud base height is associated with a shallower warm cloud depth (cloud base height to the 0oC isotherm) and consequently less entrainment, increasing the updrafts due to a more efficient processing of the convective available potential energy (CAPE). This increase in updrafts inside the clouds results in deeper and stronger thunderstorms. The higher cloud base heights is a characteristic from pasture regions, where the ratio between sensible and latent heats at surface is greater than forested areas, which increases the top of the planetary boundary layer. The differences in the aerosol total concentration and size distribution, due to the increase in the atmospheric pollution during the transition between the dry and wet seasons, were inconclusive in a possible aerosol effect in the strength of positive and negative thunderstorms, once the aerosol concentration diurnal cycle follows the cycle of the planetary boundary layer, that also regulates the warm cloud depth effect. Numerical simulation of an 1D cloud model, coupled with charge transfer between the hydrometeors, showed that the thermodynamic structure was the main responsible feature for cloud electrification, increasing the updraft velocities. The pollution effect was masked in simulations, which inhibit the warm precipitation and consequently strength the the mixed and cold regions of the cloud, was responsible for an increase in the number of smaller aggregated particles, like snow flakes and graupel, decreasing the electrification and lightning frequency.

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29

Ohlberger, Mario. "A posteriori error estimates and adaptive methods for convection dominated transport processes." [S.l. : s.n.], 2001. http://deposit.ddb.de/cgi-bin/dokserv?idn=961616245.

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30

Sheldon, Luke. "The role of deep moist convective processes in western boundary currents-troposphere coupling." Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/25755.

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Previous studies have suggested that atmospheric deep convection is occurring above the oceans western boundary currents. To investigate the occurrence of deep (ocean surface to troposphere) convection in mid-latitudes, diagnostics of upright and slantwise convective instability are created and used in the ERA-interim re-analysis dataset. These diagnostics find that deep convection is restricted to the western boundary currents in winter. However, over the ocean currents deep convection can occur up to 50% of the winter. As these ocean currents, including the Gulf Stream and Kuroshio, are warm and carry large quantities of heat polewards they could influence the frequency of deep convection. A simple calculation demonstrates that decadal ocean variability can change the atmospheric stability by 80% in the core of the Kuroshio and Gulf Stream. The deep convective instability occurs in synoptic systems that pass over the ocean currents. Specifically, instability occurs on the atmospheric fronts in the extra-tropical cyclones. To investigate the convective instability and related circulation at atmospheric fronts a composite of all atmospheric fronts in the Gulf Stream region is produced. The front composites show that where slantwise instability occurred at a front an associated increase in frontal ascent throughout the troposphere and precipitation is also found. An investigation into the possible mechanisms that produced the increased ascent at slantwise unstable fronts concludes that a possible cause is a coarse resolution of slantwise circulations in the ERA-interim dataset. This increase in deep ascent associated with slantwise instability could explain, at least partially, the deep ascent found by previous studies over the Gulf Stream. This is consistent with the finding that the deep ascent in the Gulf Stream annual mean is a product of extreme events. The occurrence of deep slantwise instability and the associated circulations are also resolution dependent. A comparison between 25 km, 60 km and 135 km grid-spacing atmospheric model runs are made using the UPSCALE project dataset. The 25 km model runs indicate greater deep slantwise and upright convective instability. The 25-km set-up is also the only one where increased deep ascent at slantwise unstable fronts is observed.

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31

Oliveira, Fernando Pereira de. "Processos convectivos na iniciação de linhas de instabilidade na costa norte do Brasil." Instituto Nacional de Pesquisas Espaciais (INPE), 2017. http://urlib.net/sid.inpe.br/mtc-m21b/2017/04.26.18.41.

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Condições atmosféricas associadas à iniciação e o desenvolvimento de Linhas de Instabilidade (LIs) na costa norte do Brasil (CNB) foram identificadas para o mês de março de 10 anos (2004-2013). Para uma dada variável atmosférica, a metodologia consistiu em comparar a média dos casos mais intensos de LI (CLI+) com a média dos casos de convecção costeira mais fraca (CC-). Foram analisadas variáveis atmosféricas contidas nos perfis verticais sobre a região do Centro de Lançamento de Alcântara (CLA) para os horários das 0600, 1200, 1800 e 2400 UTC (0000 UTC do dia seguinte), extraídos da reanálise ERA-Interim. Além disso, avaliaram-se as condições antecedentes à iniciação da LI através de radiossondagens realizadas diariamente às 1200 UTC sobre o CLA. Com o foco nas condições que apresentaram diferenças significativas entre as categorias CLI+ e CC-, para ambos os conjuntos de dados, constatou-se que as seguintes características associadas à LI: maior conteúdo de umidade na camada 850-500 hPa; maior ascendência em níveis médios (500 hPa); maior convergência de umidade em baixos níveis (925 hPa). Essas condições são parte de um padrão de grande escala que abrange toda a CNB. Para complementar essas análises, foram realizados experimentos numéricos utilizando o modelo regional WRF para dois casos, um de LI e outro com ausência de convecção (NOC) sobre a CNB. Os resultados obtidos indicaram que a umidade na camada 850-500 hPa se apresenta como principal fator associado à iniciação da LI, uma vez que ao se prescrever a umidade nessa camada para magnitudes similares à observada no evento de LI, há iniciação da LI mesmo na simulação do evento de NOC. Enquanto que ao se prescrever a umidade para magnitudes similares à observada no evento de NOC, o modelo não se mostra capaz de iniciar núcleos convectivos intensos, descaracterizando a atuação da LI. Os resultados não mostraram a conexão esperada entre circulação de brisa e LI. Além disso, indicaram que maiores magnitudes da temperatura da superfície do mar induzem maiores taxas de evaporação sobre a região oceânica, que aumentam o transporte de umidade para níveis mais altos e advecção para a região continental.
The atmospheric conditions related to squall-line (SL) initiation in the late afternoon or early evening over the northern coast of Brazil (NCB) were obtained for a specific month, March. For a specific atmospheric variable, the methodology relays on the comparison between the average of the more intense SL cases (SLC+) and the less intense costal convection (CC-). For the central area of the NCB, the vertical profiles for SLC+ and CC- for different times of the day (0600, 1200, 1800 and 2400 UTC, next day 0000 UTC), computed from ERA-Interim data spanning10 years (200413), were compared to radiosonde data at 1200 UTC. By focusing on the significant differences for both datasets, it was found that the conditions that are favorable to SL initiation in March are: moister layer between 850 and500 hPa, intense convergence at low-level (925 hPa) and higher upward motion at mid-levels (500 hPa). These regional conditions are part of a larger-scale pattern. These analyses were supplemented with numerical experiments conducted with WRF model for two different cases, one without the formation of deep convection (NOC) over the NCB and the other corresponding to SLC. The results corroborated the previous findings and highlight that the moister conditions over the NCB are the most important atmospheric conditions associated to SL initiation, since the relative humidity prescribed in the initial and boundary conditions to values similarly to those observed in the SLC (NOC) event, results in SL (NOC) initiation even in those simulations for the NOC (SL) event. Furthermore, the results did not show the expected relation between the sea breeze circulation and SL initiation. Besides that showed that larger sea surface temperature cause a raise in evaporation rate over oceanic region, then raise the humidity transport to higher levels and the humidity advection to the continent.

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32

Patel, Mayur K. "On the false-diffusion problem in the numerical modelling of convection-diffusion processes." Thesis, University of Greenwich, 1986. http://gala.gre.ac.uk/8697/.

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This thesis is concerned with the classification and evaluation of various numerical schemes that are available for computing solutions for fluid-flow problems, and secondly, with the development of an improved numerical discretisation scheme of the finite-volume type for solving steady-state differential equations for recirculating flows with and without sources. In an effort to evaluate the performance of the various numerical schemes available, some standard test cases were used. The relative merits of the schemes were assessed by means of one-dimensional laminar flows and two-dimensional laminar and turbulent flows, with and without sources. Furthermore, Taylor series expansion analysis was also utilised to examine the limitations that were present. The outcome of this first part of the work was a set of conclusions, concerning the accuracy of the numerous schemes tests, vis-a-vis their stability, ease of implementation, and computational costs. It is hoped that these conclusions can be used by `computational fluid-dynamics' practitioners in deciding on an optimum choice of scheme for their particular problem. From the understanding gained during the first part of the study, and in an effort to combine the attributes of a successful discretisation scheme, eg positive coefficients. conservation and the elimination of 'false-diffusion', a new flow-oriented finite-volume numerical scheme was devised and applied to several test cases in order to evaluate its performance. The novel approach in formulating the new CUPID* scheme (for Corner UPw^nDing) underlines the idea of focussing attention at the control-volume corners rather than at the control-volume cell-faces. In two-dimensions, this leads to an eight neighbour influence for the central grid point value, depending on the flow-directions at the corners of the control-volume. In the formulation of the new scheme, false-diffusion is considered from a pragmatic perspective, with emphasis on physics rather than on strict mathematical considerations such as the order of discretisation, etc. The accuracy of the UPSTREAM scheme (for JJPwind in STREAMIines) indicates that although it is formally only first-order accurate, it considerably reduces 'false-diffusion'. Scalar transport calculations (without sources) show that the UPSTREAM scheme predicts bounded solutions which are more accurate than the upwind-difference scheme and the unbounded skew-upstream-difference scheme. Furthermore, for laminar and turbulent flow calculations, improved results are obtained when compared with the performances of the other schemes. The advantage of the UPSTREAM-difference scheme is that all the influence coefficients are always positive and thus the coefficient matrices are suitable for iterative solution procedures. Finally, the stability and convergence characteristics are similar to those of the upwind-difference scheme, eg converged solutions are guaranteed. What cannot be guaranteed, however, is the conservatism of the scheme and it is recommended that future work should be directed towards improving that disadvantage.

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33

Ware, Antony Frank. "A spectral Lagrange-Galerkin method for convection-dominated diffusion equations." Thesis, University of Oxford, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.302896.

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34

Bengtsson, Lisa. "On the Convective-Scale Predictability of the Atmosphere." Doctoral thesis, Stockholms universitet, Meteorologiska institutionen (MISU), 2012. http://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-75195.

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A well-represented description of convection in weather and climate models is essential since convective clouds strongly influence the climate system. Convective processes interact with radiation, redistribute sensible and latent heat and momentum, and impact hydrological processes through precipitation. Depending on the models’ horizontal resolution, the representation of convection may look very different. However, the convective scales not resolved by the model are traditionally parameterized by an ensemble of non-interacting convective plumes within some area of uniform forcing, representing the “large scale”. A bulk representation of the mass-flux associated with the individual plumes in the defined area provide the statistical effect of moist convection on the atmosphere. Studying the characteristics of the ECMWF ensemble prediction system it is found that the control forecast of the ensemble system is not variable enough in order to yield a sufficient spread using an initial perturbation technique alone. Such insufficient variability may be addressed in the parameterizations of, for instance, cumulus convection where the sub-grid variability in space and time is traditionally neglected. Furthermore, horizontal transport due to gravity waves can act to organize deep convection into larger scale structures which can contribute to an upscale energy cascade. However, horizontal advection and numerical diffusion are the only ways through which adjacent model grid-boxes interact in the models. The impact of flow dependent horizontal diffusion on resolved deep convection is studied, and the organization of convective clusters is found very sensitive to the method of imposing horizontal diffusion. However, using numerical diffusion in order to represent lateral effects is undesirable. To address the above issues, a scheme using cellular automata in order to introduce lateral communication, memory and a stochastic representation of the statistical effects of cumulus convection is implemented in two numerical weather models. The behaviour of the scheme is studied in cases of organized convective squall-lines, and initial model runs show promising improvements.

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 4: Submitted.

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35

Provod, Miroslav. "Processes controlling the evolution of deep moist convection in the summertime West African Sahel." Thesis, University of Leeds, 2016. http://etheses.whiterose.ac.uk/13875/.

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Deep moist convection and mesoscale convective systems (MCSs) are integral to the West African monsoon and, as the main rain-producing mechanism in the Sahel, are essential to the livelihoods of millions. Current operational forecasting models struggle to predict rainfall with a good precision. It is therefore necessary to study the processes controlling deep moist convection in detail, in order to understand them better and to be able to evaluate simulations to identify errors for future model development. In this thesis properties of cold pools from Sahelian MCSs were characterised from surface observations. It was observed that early season cold pools were stronger and drier, likely due to drier mid-levels before the monsoon onset. The properties of observed cold pools were used to evaluate a Unified Model (UM) convection-permitting simulation. The comparison showed that simulated cold pools are generally weaker than observed. Cold pools and MCS structure were further investigated in two case studies. This enabled an analysis of MCS vertical structure. Processes controlling the diurnal cycle of convection were analysed using observations and UM simulations. This showed that while surface CAPE follows a diurnal cycle with a maximum in the afternoon and minimum in the early morning, elevated CAPE was found to have a nearly opposite cycle, due to advection of high equivalent potential temperature air overnight by the nocturnal low-level jet. In addition, the low-level jet provides low-level wind shear which balances the cold-pool related vorticity and helps to maintain the MCS until morning, when the jet decays and MCSs tend to dissipate. The jet also creates moisture flux convergence overnight, supporting MCSs. Finally, mechanisms underlying storm initiation and regeneration were analysed in UM simulations, showing the roles of soil moisture boundaries, pre-existing cold pools and bookend vortices.

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36

Berteli, Michele Nenhemy. "Estudo comparativo de processos de secagem de solidos granulados com e sem assistencia de microondas." [s.n.], 2005. http://repositorio.unicamp.br/jspui/handle/REPOSIP/255322.

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Orientador: Antonio Marsaioli Junior
Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia de Alimentos
Made available in DSpace on 2018-08-04T03:44:11Z (GMT). No. of bitstreams: 1 Berteli_MicheleNenhemy_D.pdf: 3298845 bytes, checksum: ea9c6612c92dcdbe419e972f4ca5e13b (MD5) Previous issue date: 2005
Doutorado
Doutor em Engenharia de Alimentos

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37

Rochetin, Nicolas. "Interactions entre processus de surface et convection profonde sur les continents tropicaux : représentation dans un modèle de climat." Paris 6, 2012. http://www.theses.fr/2012PA066280.

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Le but de cette thèse est de montrer que les mécanismes de couplage entre la convection profonde et son environnement local, au travers des processus sous nuageux et de surface, affectent son déclenchement, et par là non seulement le cycle diurne mais aussi la variabilité inter-diurne de la convection nuageuse. L'analyse de ces couplages à travers le prisme l'équilibre radiatif convectif, d'une part, et leur meilleure représentation par l'introduction d'une paramétrisation stochastique du déclenchement, d'autre part, sont exclusivement menées au moyen du GCM unicolonne du LMD (LMDZ). L'Equilibre Radiatif Convectif (ERC) nous permet d'identifier le signature climatique de la nouvelle paramétrisation. On pose en premier la question de savoir quelle est la sensibilité de la convection profonde aux conditions de surface à l'équilibre. On travaille d'abord dans un cadre idéalisé en Equilibre Radiatif Convectif et sans couplage avec le rayonnement et le sol (les conditions de surface sont prescrites en température et en coefficient d'évapo-transpiration). Ce cas très simplifié nous permet d'évaluer comment évolue le poids relatif de chaque processus sous-maille selon le partitionnement entre flux sensible et flux latent. Ensuite on se place dans un cadre plus réaliste, avec un ERC continental comportant un cycle diurne, un rayonnement et une surface couplés à l'atmosphère, et enfin une hydrologie simplifiée. Une analyse de sensibilité de la convection continentale à l'humidité de surface menées dans le SCM (Single Column Model) et comparée avec des données satellites montre un bon accord entre l'ERC continental et les observations. On se penche ensuite sur les interactions entre la convection profonde et les processus sous-nuageux aux échelles de temps courtes: en particulier sur la problématique de la transition vers la convection profonde sur continents. D'aprés l'analyse statistique d'une simulation LES d'un cas de déclenchement d'orage isolé sur sol semi-aride (cas AMMA), on propose une formulation stochastique du déclenchement. Cette dernière est ensuite intégrée, sous la forme d'une paramétrisation, au GCM du LMD (LMDZ) puis testée au travers de cas d'études 1D variés, sa valeur ajoutée par rapport à l'ancienne paramétrisation est aussi discutée. Enfin cette nouvelle paramétrisation est analysée dans le cadre de l'ERC continental, et on y retrouve le même effet sur la variabilité intra-diurne et inter-diurne de la convection que celui diagnostiqué avec les cas réalistes 1D et le modèle 3D. L'ERC contiendrait donc l'empreinte climatique de cette nouvelle paramétrisation
The purpose of this thesis is to show that the coupling mechanisms between deep convection and its local environment, through processes in clouds and surface affect its onset, and thus not only the diurnal cycle but also inter-diurnal variability of convection cloud. The analysis of these couplings through the prism radiative convective equilibrium, on the one hand, and their best representation by the introduction of a parameterization of the triggering stochastic, on the other hand, are conducted exclusively through the LMD GCM unicolonne (LMDZ). Radiative Convective Equilibrium (ERC) allows us to identify the signing of the new climate parameterization. We put the first issue of which is the sensitivity of deep convection conditions surface at equilibrium. We first worked in an idealized convective and radiative equilibrium without coupling with the radiation and soil (surface conditions are prescribed temperature coefficient and evapotranspiration). This very simplified case allows us to assess how changing the relative weight of each subgrid process according to the partitioning between sensible flux and latent flux. Then we place ourselves in a more realistic, with a continental ERC with a diurnal cycle, radiation and a surface coupled to the atmosphere, and finally a simplified hydrology. A sensitivity analysis of convection in continental surface moisture carried in SCM (Single Column Model) and compared with satellite data shows good agreement between the ERC Continental and observations. It then examines the interactions between deep convection and processes sub-cloud to short time scales: in particular the problem of transition to deep convection over continents. According to the statistical analysis of a simulation LES of a case of isolated thunderstorm trigger on semi-arid land (if AMMA) we propose a stochastic formulation of the outbreak. The latter is then integrated in the form of a parameterization, the LMD GCM (LMDZ) and then tested through case studies of various 1D, its added value compared to the previous parameterization is also discussed. Finally this new parameterization is analyzed within the framework of the ERC Continental, and we find the same effect on the intra-day and inter-diurnal convection than diagnosed realistic case with 1D and 3D model. The ERC would then include the footprint of the new parameterization

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38

Andersson, Andreas. "Air-sea exchange of O2 and CO2 : Processes controlling the transfer efficiency." Doctoral thesis, Uppsala universitet, Luft-, vatten och landskapslära, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-314166.

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World oceans cover more than 70% of the earth surface and constitutes a major sink of atmospheric CO2. Two of the most important gases in the marine carbon cycling are O2 and CO2 and hence accurate descriptions of the air-sea gas exchange of these gases are crucial. Still there is a lack of knowledge of the relative importance of processes controlling the efficiency of the air-sea gas transfer. This is especially true for Arctic and high latitude seas were studies on air-sea gas exchange are few. By studying processes causing water-side turbulence, using gases of different solubility and various measurement techniques, more knowledge on the governing processes can be obtained. Here we present the very first air-sea fluxes of O2 using atmospheric eddy covariance measurements and investigate the dependence between the gas transfer velocity of O2 and turbulence generated by the mean wind. The instrument was found to suffer from the limited precision and time response, causing significant corrections on the O2 flux. After correcting for this, the O2 fluxes displays an anti-correlation with the air-sea fluxes of CO2 in agreement with the measured air-sea gradient of O2. The transfer velocities for O2 indicates a stronger wind dependence than other commonly used parameterizations of the transfer velocity for CO2 and O2, this especially for wind speeds > 5 m s-1 where the typical onset of wave breaking occur. During two winter months eddy covariance measurements were taken over a high Arctic fjord. The data revealed a significant enhancement of the gas transfer velocity for CO2 from water-side convection, generated by cooling of surface waters. The dependence between water-side convection and gas transfer velocity were found for winds as high as 9 m s-1, but were strongest for wind speeds< 7 m s-1. The data also showed on enhanced air-sea gas transfer of CO2 when conditions were unstable very close to neutral. This enhanced transfer were associated to increased contribution to the CO2 flux from downdraft of air with higher concentrations of CO2. The combined effect of water-side convection and turbulence generated by wind results in a very effective transfer, thus the air-sea gas exchange at these latitudes may be significantly underestimated.

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39

Blanchard, Nicolas. "Convection dans une bande transporteuse d'air chaud : caractérisation et impacts sur la dynamique d'altitude." Thesis, Toulouse 3, 2020. http://www.theses.fr/2020TOU30229.

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Les bandes transporteuses d'air chaud (WCB) sont des courants d'air ascendants dans les tempêtes des moyennes latitudes. Elles transportent l'air chaud et humide des tropiques aux hautes latitudes et sont le lieu de la plupart des processus diabatiques nuageux. Ceux-ci modifient la dynamique d'altitude en injectant des anomalies négatives de tourbillon potentiel (PV) dans le flux de sortie des WCB qui renforcent le guide d'onde régissant la circulation générale. Leur représentation est une source d'erreurs de prévision en aval, en particulier en Europe. Alors que les WCB sont généralement considérées comme des ascendances obliques lentes et continues, des études récentes ont révélé l'existence d'ascendances convectives rapides dans les WCB dont l'impact sur la dynamique d'altitude est encore débattu. La réduction des incertitudes de prévision a motivé la campagne de mesure NAWDEX en Atlantique nord en automne 2016. Au cours de la campagne, la région d'ascendance et le flux de sortie de la WCB de la tempête Stalactite ont été observées par un radar Doppler aéroporté et des dropsondes. Ces observations sont reproduites par une simulation à convection explicite réalisée avec le modèle Méso-NH grâce à laquelle une analyse inédite combinant approches eulériennes et lagrangiennes a permis de caractériser la complexité des ascendances dans la WCB. La région d'ascendance de la WCB est d'abord étudiée. Trois types d'ascendances convectives sont trouvés et se produisent de manière cohérente et organisée plutôt que sous forme de cellules isolées. Deux sont causés par la convection peu profonde liée à la dynamique du front froid et à celle d'un jet de basse couche. Le troisième est due à la convection à mi-niveau située sur le bord ouest de la WCB entre le jet de basse couche et le courant-jet d'altitude. C'est cette dernière qui alimente la branche anticyclonique de la WCB. Le flux de sortie de la WCB et son alimentation par la convection à mi-niveau sont ensuite étudiés. Une simulation supplémentaire est réalisée pour laquelle les échanges de chaleur issus des processus nuageux sont coupés afin de mettre en avant leur impact sur la dynamique d'altitude. La simulation de référence montre que la convection à mi-niveau crée diabatiquement des dipôles horizontaux de PV dont le pôle orienté en direction du courant-jet atteint des valeurs négatives. Ces dipôles de PV sont ensuite advectés par la dynamique de grande échelle et persistent dans le temps. Des bandes allongées de PV négatif se retrouvent ainsi dans le flux de sortie de la WCB dix heures après. La comparaison des deux simulations montre que ces bandes de PV négatif renforcent le gradient horizontal de PV à la tropopause, accélérant localement le courant-jet. Les résultats de la thèse montrent la complexité de la convection que l'on peut trouver dans une WCB. Ils suggèrent que la convection à mi-niveau impacte la dynamique d'altitude et que sa mauvaise représentation dans les modèles de grande échelle pourrait expliquer en partie les erreurs de prévision en aval
Warm air conveyor belts (WCB) are ascending air streams in mid-latitude storms. They transport warm and moist air from the tropics to the high latitudes and are the site of most cloud diabatic processes. These processes modify the upper-level dynamics by injecting negative potential vortex (PV) anomalies into the WCB outflow, which reinforce the waveguide governing the general circulation. Their representation is a source of forecast errors downstream, especially over Europe. While WCBs are predominantly considered as slow continuous slantwise ascents, recent studies have revealed the existence of fast convective ascents in WCBs whose impact on upper-level dynamics is still debated. The reduction of forecast uncertainties motivated the NAWDEX field campaign over the North Atlantic in autumn 2016. During the campaign, the ascent region and the outflow of the WCB of the Stalactite cyclone were observed by an airborne Doppler radar and dropsondes. These observations are reproduced by a convection-permitting simulation carried out with the Meso-NH model, thanks to which a novel analysis combining Eulerian and Lagrangian approaches made it possible to characterize the complexity of the ascents in the WCB. The WCB ascent region is first studied. Three types of convective ascents are found and occur in a coherent and organized manner rather than as isolated cells. Two are caused by shallow convection related to the dynamics of the cold front and that of a low level jet. The third is due to mid-level convection, located on the western edge of the WCB between the low-level jet and the upper jet stream. It is the latter that feeds the anticyclonic branch of the WCB. The outflow of the WCB and its mid-level convection feed are then studied. An additional simulation is carried out for which the heat exchanges resulting from the cloud processes are cut off in order to highlight their impact on the upper-level dynamics. The reference simulation shows that the mid-level convection diabatically creates horizontal PV dipoles whose pole oriented towards the jet stream reaches negative values. These PV dipoles are then advected by the large-scale dynamics and persist over time. Thus, elongated bands of negative PV are found in the WCB outflow ten hours later. Comparing the two simulations shows that these negative PV bands reinforce the horizontal PV gradient at the tropopause, locally accelerating the jet stream. The results of the thesis show the complexity of convection that can be found in a WCB. They suggest that mid-level convection impacts the altitude dynamics and that its misrepresentation in large-scale models could explain in part the forecast errors downstream

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Gianotti, Rebecca L. (Rebecca Louise). "Convective cloud and rainfall processes over the Maritime Continent : simulation and analysis of the diurnal cycle." Thesis, Massachusetts Institute of Technology, 2012. http://hdl.handle.net/1721.1/79488.

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Thesis (Ph. D. in the Field of Hydrology)--Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, February 2013.
"February 2013." Cataloged from PDF version of thesis.
Includes bibliographical references (p. 290-307).
The Maritime Continent experiences strong moist convection, which produces significant rainfall and drives large fluxes of heat and moisture to the upper troposphere. Despite the importance of these processes to global circulations, current predictions of climate change over this region are still highly uncertain, largely due to inadequate representation of the diurnally-varying processes related to convection. In this work, a coupled numerical model of the land-atmosphere system (RegCM3-IBIS) is used to investigate how more physically-realistic representations of these processes can be incorporated into large-scale climate models. In particular, this work improves simulations of convective-radiative feedbacks and the role of cumulus clouds in mediating the diurnal cycle of rainfall. Three key contributions are made to the development of RegCM3-IBIS. Two pieces of work relate directly to the formation and dissipation of convective clouds: a new representation of convective cloud cover, and a new parameterization of convective rainfall production. These formulations only contain parameters that can be directly quantified from observational data, are independent of model user choices such as domain size or resolution, and explicitly account for subgrid variability in cloud water content and nonlinearities in rainfall production. The third key piece of work introduces a new method for representation of cloud formation within the boundary layer. A comprehensive evaluation of the improved model was undertaken using a range of satellite-derived and ground-based datasets, including a new dataset from Singapore's Changi airport that documents diurnal variation of the local boundary layer height. The performance of RegCM3-IBIS with the new formulations is greatly improved across all evaluation metrics, including cloud cover, cloud liquid water, radiative fluxes and rainfall, indicating consistent improvement in physical realism throughout the simulation. This work demonstrates that: (1) moist convection strongly influences the near surface environment by mediating the incoming solar radiation and net radiation at the surface; (2) dissipation of convective cloud via rainfall plays an equally important role in the convective-radiative feedback as the formation of that cloud; and (3) over parts of the Maritime Continent, rainfall is a product of diurnally-varying convective processes that operate at small spatial scales, on the order of 1 km.
by Rebecca L. Gianotti.
Ph.D.in the Field of Hydrology

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Pilon, Romain. "Transport et élimination des radionucléides naturels par les processus nuageux : Mécanismes et représentation dans un modèle de climat." Paris 6, 2013. http://www.theses.fr/2013PA066151.

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Cette thèse porte sur l’étude de la répartition des radionucléides dans l’atmosphère et de sa variabilité spatiale et temporelle. Les radionucléides objets de cette étude sont ceux mesurés par les stations du système de surveillance international (IMS) de l’Organisation du traité d’interdiction complète des essais nucléaires, et plus particulièrement les deux radionucléides naturels, 7Be et 210Pb, fréquemment utilisés dans l’étude du transport atmosphérique. L’outil principal est le modèle de circulation générale LMDz. Grâce à leur sources situées dans la haute troposphère et la stratosphère pour le 7be et dans la basse troposphère pour 210Pb, ces traceurs sont utiles pour évaluer les processus qui le redistribuent verticalement. Les tropiques constituent la région dont les concentrations à la surface sont le plus mal simulées par le modèle LDMz. Ce désaccord entre mesures et simulations met en évidence le besoin d’une meilleure représentation du transport dans les systèmes convectifs et une meilleure compréhension des processus physiques représentés dans le modèle. Nous avons développé une paramétrisation du transport des radionucléides et de leur élimination par la condensation à l’intérieur des nuages convectifs profonds et par les précipitations. Elle suit le modèle convectif d’Emanuel et a été développée dans le modèle de climat LMDz. L’analyse des différents processus affectant la distribution des traceurs dans le modèle 1D montre comment les processus convectifs et de précipitations de grande échelle combinent leurs effets. Les descentes précipitantes de la convection profonde déplacent du 7Be à la surface et en déposent peu dans la troposphère moyenne, les pluies de grande échelle éliminent le 7Be de toute la troposphère
This thesis focuses on the distribution of radionuclides in the atmosphere and of its spatial and temporal variability. Randionuclides, studied in this thesis, are thus from the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty Organization, particularly the two natural radionuclides 7Be et 210Pb which are frequently uses for atmospheric transport study. The main tool is the LMDz climate model. Thanks to the high/low tropospheric radionuclides sources, these tracers are useful for the evaluation of processes which affect its distribution. Tropics are the region where radionuclides surface concentrations are the worst simulated by the LMDz model. The disagreement between observations nd simulations underlines the need to a better representation of transport in convective systems et a better understanding of physical processes in the model. We developed a parametrization of transport of radionuclide and its removal by incloud condensation and by precipitation by convective clouds. The scheme follows closely the Emanuel moist convection scheme and has been developed in LMDz climate model. The analysis of the different processes affecting tracers distribution in the unicolum version of LMDz show how convective and large-scale processes combine their effects to move vertically tracers. Convective precipitating downdrafts move down 7Be at the surface, with the help of the evaporation of convective and large-scale precipitation, whereas large-scale condensation removes the radionuclide from the whole troposphere

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PROTAT, ALAIN. "Processus d'interaction entre les differentes echelles de mouvement au sein du systeme convectif observe le 15 decembre lors de toga-coare." Paris 7, 1996. http://www.theses.fr/1996PA077329.

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L'etude des processus d'interaction entre les differentes echelles de mouvements au sein des systemes convectifs tropicaux constitue un des objectifs majeurs du programme international toga-coare (tropical ocean global atmosphere- coupled ocean atmosphere response experiment). Ce travail de these s'inscrit dans ce cadre, et a ete guide par les objectifs scientifiques suivants : (i) acceder aux differents champs dynamiques et thermodynamiques au sein du systeme convectif selectionne (15 decembre 1992) a grande echelle, mesoechelle, et echelle convective, afin de caracteriser les processus responsables de sa formation, de son evolution et de sa dissipation, (ii) comparer les resultats obtenus aux travaux theoriques, ainsi qu'aux resultats issus d'etudes effectuees par le passe, et (iii) diagnostiquer, pour le cas selectionne, les processus d'interaction entre les differentes echelles de mouvement. Cette etude repose sur l'utilisation de sources de donnees tres variees. Les analyses du cepmmt (centre europeen de prevision meteorologique a moyen terme), et les images du satellite japonais gms-4 sont utilisees pour decrire la grande echelle. Les donnees issues de radars doppler aeroportes sont utilisees pour acceder aux champs a moyenne echelle et echelle convective. Au cours de cette these, un important travail de pretraitement de ces donnees a ete effectue, et de nouvelles methodes d'analyse de ces donnees ont pu etre developpees. Apres s'etre forge les outils necessaires a la realisation des objectifs scientifiques enumeres precedemment, une analyse de l'ensemble des resultats obtenus a pu etre menee, permettant d'interpreter en terme de processus les champs dynamiques et thermodynamiques obtenus aux differentes echelles de mouvement. Il a ete montre entre autres que la formation, l'organisation, et la dissipation du systeme etaient totalement gouvernees par les circulations de grande echelle, et qu'inversement les mouvements convectifs avaient un effet retroactif sur l'environnement de grande echelle.

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Hachani, Lakhdar. "Etude de l'influence de la convection naturelle et forcée sur le processus de la solidification : cas d'un alliage métallique binaire." Phd thesis, Université de Grenoble, 2013. http://tel.archives-ouvertes.fr/tel-00949060.

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Ce travail se situé dans la perspective d'un contrôle de la structure de solidification des alliages métalliques sous l'effet de la convection naturelle et forcée afin d'améliorer à terme la maîtrise des microstructures de solidification grâce à un brassage électromagnétique efficace permettant d'avoir une homogénéisation du bain liquide qui par la suite peut améliorer la microstructure finale de l'alliage. La possibilité retenue dans ce travail est de réaliser ce brassage sans contact avec la solution liquide (alliage sous fusion) et sans pollution par d'autres éléments en utilisant un brassage par la force de Lorentz. L'objet de la thèse comporte une étude théorique à la fois expérimentale basée sur une installation expérimentale particulièrement documentée et instrumentée, développée au laboratoire SIMAP/EPM à Grenoble, nommée AFRODITE. Ce dispositif expérimental permet de fournir des données de quantitatives et qualitatives sur le processus de solidification des alliages métalliques. Ces données sont nécessaires à la contribution aux études menées sur la solidification des alliages métallique et enrichir la base des donnée des modèles numériques développés pour prédire les défauts liés à la solidification. L'alliage choisi dans notre travail est l'étain-plomb, vue sa basse température de fusion. Les expériences envisagées visent à étudier l'effet de deux modes de configuration dynamique sur la solidification de l'alliage Sn-Pb: la convection thermosolutale avec la variation de deux paramètres essentiels (la vitesse de refroidissement et la différence de température expérimentale) et la convection forcée par l'utilisation de plusieurs modes de brassage électromagnétique. Cette étude s'intéresse en particulier à la caractérisation des macrostructures et les défauts liés à la macroségrégation. L'originalité de l'étude vise à mesurer in situ les températures instantanées. Ceci nous a permis d'évaluer l'évolution du transfert thermique due à la convection naturelle/forcée, ainsi que leurs influence sur le processus de la solidification sous différents aspects. L'analyse post-mortem de l'alliage métallique, fournit la structure de solidification et la distribution des ségrégations à différentes échelles (mésoscopique et macroscopique).

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Khodayar, Pardo Samiro [Verfasser]. "High-resolution analysis of the initiation of deep convection forced by boundary-layer processes / Samiro Khodayar Pardo." Karlsruhe : KIT Scientific Publishing, 2015. http://www.ksp.kit.edu.

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Potter, Stephen Edward. "Modelling of three-dimensional transient conjugate convection-conduction-radiation heat transfer processes and turbulence in building spaces." Thesis, Northumbria University, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.245440.

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Deng, Liping. "Evaluating and understanding the role of convective processes in general circulation model simulations of the Madden-Julian Oscillation." [Ames, Iowa : Iowa State University], 2010. http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:3403790.

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Graffino, Giorgio. "A study of air-sea interaction processes on water mass formation and upwelling in the Mediterranean sea." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2015. http://amslaurea.unibo.it/8337/.

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Air-sea interactions are a key process in the forcing of the ocean circulation and the climate. Water Mass Formation is a phenomenon related to extreme air-sea exchanges and heavy heat losses by the water column, being capable to transfer water properties from the surface to great depth and constituting a fundamental component of the thermohaline circulation of the ocean. Wind-driven Coastal Upwelling, on the other hand, is capable to induce intense heat gain in the water column, making this phenomenon important for climate change; further, it can have a noticeable influence on many biological pelagic ecosystems mechanisms. To study some of the fundamental characteristics of Water Mass Formation and Coastal Upwelling phenomena in the Mediterranean Sea, physical reanalysis obtained from the Mediterranean Forecating System model have been used for the period ranging from 1987 to 2012. The first chapter of this dissertation gives the basic description of the Mediterranean Sea circulation, the MFS model implementation, and the air-sea interaction physics. In the second chapter, the problem of Water Mass Formation in the Mediterranean Sea is approached, also performing ad-hoc numerical simulations to study heat balance components. The third chapter considers the study of Mediterranean Coastal Upwelling in some particular areas (Sicily, Gulf of Lion, Aegean Sea) of the Mediterranean Basin, together with the introduction of a new Upwelling Index to characterize and predict upwelling features using only surface estimates of air-sea fluxes. Our conclusions are that latent heat flux is the driving air-sea heat balance component in the Water Mass Formation phenomenon, while sensible heat exchanges are fundamental in Coastal Upwelling process. It is shown that our upwelling index is capable to reproduce the vertical velocity patterns in Coastal Upwelling areas. Nondimensional Marshall numbers evaluations for the open-ocean convection process in the Gulf of Lion show that it is a fully turbulent, three-dimensional phenomenon.

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48

Dugast, Florian. "Optimisation topologique en convection thermique avec la méthode de Lattice Boltzmann." Thesis, Nantes, 2018. http://www.theses.fr/2018NANT4056/document.

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L’optimisation des transferts de chaleur est un enjeu de recherche important pour améliorer les performances des systèmes énergétiques, notamment des échangeurs de chaleur. Cette optimisation peut s’appuyer sur différents leviers, comme le choix du régime d’écoulement, ou la modification de la géométrie de l’échangeur. C’est sur ce deuxième angle que nous avons travaillé au cours de cette thèse, en proposant une méthode d’optimisation topologique. Le nombre de paramètres impliqués dans cette méthode est important (plusieurs milliers). Une méthode à gradient est donc employée. Le calcul du gradient de la fonction de coût en fonction des paramètres de design est facilité par l’emploi d’une méthode adjointe. Le problème direct est résolu à l’aide de la méthode de Lattice Boltzmann (LBM). L’implémentation aisée des conditions aux limites dans l’algorithme LBM est un bénéfice pour l’optimisation topologique. De plus, la formulation de la LBM est explicite et hautement parallélisable, notamment sur les cartes graphiques (GPU), utilisées au cours de cette thèse. Ensuite le domaine d’optimisation est composé de fluide et de solide. Leur distribution est définie par une fonction Level- Set (LSF). Cette fonction est continue et le contour zéro définit précisément l’interface fluide/solide. La méthode d’optimisation développée a été testée et validée pour différents objectifs (minimisation de la température moyenne, maximisation de l’échange de chaleur) et contraintes (limitation des pertes de charges, porosité fixée)
Heat transfer enhancement is an important research area to improve the efficiency of energy systems, especially for heat exchangers. There are different ways of optimizing such systems as the choice of flow regime or the modification on the fluid channels geometry. This thesis is focused on the latter option with the development of a topology optimization method. The number of design parameters involved in this technique is important (several thousands) so a gradient-based method is employed. The calculation of the cost function gradient with respect to the design variables is done with an adjoint-state method. The forward problem is solved with the Lattice Boltzmann Method (LBM). The simple implementation of the LBM boundary conditions is an interesting feature for topology optimization. The LBM algorithm is also highly parallelizable and GPU cards have been used in this thesis to obtain fast computational times. The efficiency of the LBM is important because the forward problem must be solved at each optimization step. Then, the optimization domain is composed of either solid of fluid elements. This material distribution is defined by a Level-Set Function (LSF). This is a continuous function in which the zero contour defines the fluid/solid interface, allowing an accurate description of both domains. The proposed optimization method has been tested for different objectives (minimization of the mean temperature, maximization of heat transfer rate) and constraints (pressure drops limitation, fixed porosity)

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Rousset, Clément. "Modélisation des échanges entre un gyre convectif et un courant de bord : application à la mer du Groenland." Paris 6, 2008. http://www.theses.fr/2008PA066658.

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Cette thèse est motivée par la nécessité de mieux comprendre et identifier les processus physiques mésoéchelles opérant dans les mers subpolaires qui contraignent la formation et la circulation des eaux denses. Dans ce but, des expériences numériques idéalisées sont réalisées. Dans une première partie, nous nous intéressons à la représentation du processus de restratification dans trois modèles : MICOM (isopycnal), OPA (z) et une version adiabatique dérivée de MICOM dans laquelle la physique de la couche de mélange est retirée. Nos résultats montrent qu’en l'absence de forçage, la géométrie de la couche de mélange dans MICOM empêche presque tout retrait de cette couche et limite fortement le niveau d’instabilité ainsi que les échanges de chaleur. Le comportement des deux autres modèles est bien plus satisfaisant, mais des différences persistent dans le développement de l’instabilité barocline et dans les échanges et les transformations des masses d’eau. Dans une seconde partie, le modèle MICOM dans sa version adiabatique est retenu pour étudier les processus mésoéchelles associés à la transformation des masses d’eau en mer du Groenland et leur export vers le passage Groenland-Ecosse, avec pour objectif de comprendre le lien entre la formation d’eau dense et son export. Nous montrons que la saisonnalité de la convection n’engendre pas de saisonnalité de l’export d’eau dense, que le seuil modifie les échanges au détroit et la circulation dans le bassin, et que la formation des masses d’eau denses est non seulement dépendante de la répartition spatiale du forçage mais aussi de la déstabilisation du courant de bord liée à la structure de la topographie.

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Milovac, Josipa [Verfasser], and Volker [Akademischer Betreuer] Wulfmeyer. "Studies of soil-vegetation-atmosphere feedback processes with WRF on the convection permitting scale / Josipa Milovac ; Betreuer: Volker Wulfmeyer." Hohenheim : Kommunikations-, Informations- und Medienzentrum der Universität Hohenheim, 2017. http://d-nb.info/113316644X/34.

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Dissertations / Theses on the topic 'Processus convectifs'

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