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1
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 dépositionThe 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
2
Hall, Paul S. "Free and forced convection in earth's upper mantle /." View online ; access limited to URI, 2003. http://0-wwwlib.umi.com.helin.uri.edu/dissertations/dlnow/3115631.
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Park, Sewon. "Diurnal cycle of deep tropical convection." Thesis, Massachusetts Institute of Technology, 1992. http://hdl.handle.net/1721.1/54985.
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Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1992.Title as it appears in the M.I.T. Graduate List, Feb. 1992: Diurnal cycle of deep cloud cover in tropics.
Includes bibliographical references (leaf 53).
by Sewon Park.
M.S.
4
Kocha, Cécile. "Interactions entre poussières désertiques et convection profonde en Afrique de l'Ouest : Observations et modélisation à échelle convective." Phd thesis, Université Paul Sabatier - Toulouse III, 2011. http://tel.archives-ouvertes.fr/tel-00741943.
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Dans un contexte de réchauffement climatique, la région d'Afrique de l'Ouest, où les sécherresses sont dévastatrices pour les populations, montre les plus fortes incertitudes sur l'évolution des précipitations. C'est aussi la première source de poussières désertiques au monde dont la production est en augmentation par la sur-exploitation des sols. Or ces poussières désertiques, associées à des épidémies de méningite, absorbent et diffusent le flux solaire entraînant alors une modification du taux d'échauffement atmosphérique. En modifiant le bilan radiatif atmosphérique elles sont alors succeptible d'influencer de la turbulence de fine échelle aux circulations atmosphériques de grande échelle. Seulement les processus en jeu dans les interactions entre les poussières désertiques et l'atmosphère sont très variés, complexes, et constituent une grande source d'incertitude dans la prévision numérique. Afin d'appréhender ces processus, un cadre de modélisation à été développé permettant à la fois la résolution explicite de la convection, la résolution du cycle de vie des poussières et de leur impact radiatif, et la prise en compte de leurs interactions sur toute l'Afrique de l'Ouest à échelle mensuelle. Sur un cas extrême de tempête de poussières en saison sèche, la forte quantité de poussières soulevée par le front entraîne une amplification par eux de la signature de la tempête. La modélisation explicite des poussières améliore la prévisibilité de la tempête. De manière plus statistique, pendant la mise en place de la mousson, en juin, la fréquente présence de poussières participe activement à l'initiation du saut de mousson. D'une part, en affaiblissant la dépression thermique Saharienne, un des principaux moteurs de la mousson, mais aussi, d'autre part, en renforçant le déplacement vers le nord du coeur de la zone de convergence inter-tropicale et des jets. A plus fine échelle, la présence de poussières impacte la localisation et le cycle diurne des systèmes convectifs. Les poussières modifient alors la couverture nuageuse dont les impacts radiatifs sont plus forts que l'effet direct des poussières. La prise en compte des effets semi-directs des poussières sont donc essentiels pour la prévision en Afrique de l'Ouest.
5
Alverson, Keith D. 1965. "Topographic preconditioning of open ocean deep convection." Thesis, Massachusetts Institute of Technology, 1995. http://hdl.handle.net/1721.1/53023.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1995.Includes bibliographical references (leaves 143-146).
by Keith D. Alverson.
Ph.D.
6
Amiri, Khanmakani Hosein. "The visible consequences of rising convective streams in the Earth." Thesis, University of Newcastle Upon Tyne, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.336266.
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Boldi, Robert A. (Robert Arthur). "A model of ion chemistry of electrified convection." Thesis, Massachusetts Institute of Technology, 1992. http://hdl.handle.net/1721.1/51502.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1992.Includes bibliographical references (leaves 175-182) and index.
by Robert A. Boldi.
Ph.D.
8
Larson, Vincent Edwin 1970. "The effects of thermal radiation on dry convection." Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/54433.
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Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1999.Includes bibliographical references (p. 96-99).
This work seeks to improve understanding of atmospheres in radiative-convective equilibrium. We use two types of idealized, dry radiative-convective models. The first type of model resembles Rayleigh-Benard convection, except that thermal radiative transfer is included. This type of system can be investigated in the laboratory. The second type of model is a more faithful representation of the earth's atmosphere. In this model, the temperature at the upper boundary is left unspecified, unlike the case of Rayleigh-Benard convection. For these two types of radiative-convective models, we perform various theoretical and numerical analyses of the stability properties, and various analyses of the weakly nonlinear convecting state. We prove that in these models, convection arises as monotonically growing cells, not as an oscillatory instability. That is, we prove exchange of stabilities. We investigate the linear stability modes. We find that in most cases, the linear stability threshold can be described approximately or exactly in terms of a radiative Rayleigh number. The radiative Rayleigh numbers used are like the classical Rayleigh number but with modified temperature and thermal diffusivity scales. Inspection of the radiative Rayleigh numbers reveals how various external parameters, such as the net incoming solar radiation or infrared opacity, affect the stability properties. We use the energy method to find a threshold value of a stability parameter below which all disturbances to the radiative equilibrium state, regardless of magnitude, decay. For those radiative equilibrium states which have a linear temperature profile, the energy stability threshold coincides with the linear stability threshold, thereby ruling out the possibility of subcritical instabilities. When the temperature profile is nonlinear, the energy stability profile lies below the linear stability threshold. We study weakly nonlinear convection in the atmospheric radiative-convective model via the mean field approximation. In contrast to the stability threshold, the vertical convective heat flux in the weakly nonlinear convecting state turns out to be little affected by the values of viscosity, thermal diffusivity, or radiative damping. However, the convective heat flux is strongly affected by the net incoming solar radiation and the optical depth. We formulate scaling laws for vertical convective heat flux, vertical velocity, and temperature perturbations. These scales extend the Prandtl scales to higher altitudes.
by Vincent Edwin Larson.
Ph.D.
9
Bell, Peter Ian. "The effect of bumps on convection in the earth's core." Thesis, University of Newcastle Upon Tyne, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.335890.
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de, Lavaissiere de Lavergne Casimir. "Cessation of southern ocean deep convection under anthropogenic climate change." Thesis, McGill University, 2013. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=119766.
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In 1974, newly available satellite observations unveiled the presence of a giant ice-free area within the Antarctic ice pack, which persisted throughout the winter, and formed again in the next two winters. Subsequent research showed that deep convective overturning kept the waters ice-free, through the massive release of heat rising from the deep sea. While the polynya has aroused continued interest among climate scientists, it has not reappeared since 1976. Here we use model experiments to show that deep convection in the Southern Ocean, common in current generation climate models, is highly sensitive to anthropogenic forcing, and ceases in many models when forced by a high emissions climate change scenario. The slowdown in deep ventilation follows from the gradual freshening of polar surface waters, a trend which is borne out by observations over recent decades. Our results suggest that deep convection in the Southern Ocean will be less common in future, and may have already been significantly reduced compared to the pre-industrial period, with important consequences for ocean circulation and climate.En 1974, des observations satellite nouvellement disponibles révélèrent la présence d'une géante surface d'eau libre au sein de la glace de mer entourant l'Antarctique, qui persista tout au long de l'hiver et réapparut les deux hivers suivants. Les recherches qui suivirent montrèrent que les eaux étaient maintenues libres de glace par la convection profonde, permettant à une grande quantité de chaleur de remonter des profondeurs pour être ensuite libérée dans l'atmosphère. Si la polynya continue de susciter l'intérêt des climatologues, elle n'est cependant pas réapparue depuis 1976. Nous utilisons ici des expériences de modélisation pour montrer que la convection profonde dans l'Océan Austral, commune dans les modèles de climat actuels, est fortement sensible au forçage anthropique, et cesse dans beaucoup de modèles quand ceux-ci sont forcés par un scénario de fortes émissions. Le ralentissement de la ventilation profonde résulte de la baisse progressive de la salinité des eaux de surface, une tendance corroborée par les observations des dernières décennies. Nos résultats suggèrent que la convection profonde dans l'Océan Austral sera moins fréquente dans le futur, et a peut-être déjà été significativement affaiblie relativement à la période préindustrielle, avec d'importantes conséquences pour la circulation océanique et le climat.
11
Khaleque, Tania Sharmin. "Strongly variable viscosity flows in mantle convection." Thesis, University of Oxford, 2015. https://ora.ox.ac.uk/objects/uuid:851f7069-8164-4499-8afa-5a06141c5911.
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Convection in the Earth's mantle is a complicated phenomenon that causes various tectonic activities and affects mantle evolution on geologic time scales (billions of years). It is a subject as yet not fully understood. The early success of the high Rayleigh number constant viscosity theory was later tempered by the absence of plate motion when the viscosity is more realistically strongly temperature dependent. A similar problem arises if the equally strong pressure dependence of viscosity is considered, since the classical isothermal core convection theory would then imply a strongly variable mantle viscosity, which is inconsistent with results from postglacial rebound studies. We consider a mathematical model for Rayleigh-Bénard convection in a basally heated layer of a fluid whose viscosity depends strongly on both temperature and pressure, defined in an Arrhenius form. The model is solved numerically for extremely large viscosity variations across a unit aspect ratio cell, and steady solutions are obtained. To improve the efficiency of numerical computation, we introduce a modified viscosity law with a low temperature cut-off. We demonstrate that this simplification results in markedly improved numerical convergence without compromising accuracy. Continued numerical experiments suggest that narrow cells are preferred at extreme viscosity contrasts. We are then able to determine the asymptotic structure of the solution, and it agrees well with the numerical results. Beneath a stagnant lid, there is a vigorous convection in the upper part of the cell, and a more sluggish, higher viscosity flow in the lower part of the cell. We then offer some comments on the meaning and interpretation of these results for planetary mantle convection.
12
Conrad, Clinton Phillips 1971. "Effects of lithospheric strength on convection in the Earth's mantle." Thesis, Massachusetts Institute of Technology, 2000. http://hdl.handle.net/1721.1/53042.
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Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, February 2000.Includes bibliographical references (p. 235-244).
Convection in Earth's mantle is driven largely by horizontal density gradients that form when cold, dense, mantle lithosphere descends into the mantle interior, either through subduction for plate-scale flow, or as localized convective instability beneath lithospheric plates. The deformation associated with these processes is resisted by the extreme temperature-dependence of the lithosphere's strength. Ways in which lithosphere deformation affects convection in the mantle are examined here, by comparing both theory and the results of numerical experiments. Convective instability at the base of a cold thermal boundary layer with temperature and strain-rate-dependent viscosity is investigated by defining a quantity, termed here the "available buoyancy," that takes into account the tradeoff between cold temperatures both promoting and resisting convective instability. This quantity can be used to determine approximately whether, and how fast, convective instability grows. Horizontal shortening is also included, which tends to increase gravitational instability, allowing up to 60% of the mantle lithosphere to be convectively removed. The subsequent influx of hot, buoyant, asthenosphere could cause rapid surface uplift. For plate-scale flow, subduction zone deformation may resist convection. This possibility is studied here using a regional finite element model of subduction. This model shows that for sufficiently strong lithosphere, convection is resisted more by the bending deformation of a subducting plate than by shearing of the underlying mantle. Such behavior can be explained by a variation of boundary layer theory that includes an analytic expression for the energy required to bend a viscous plate. For the mantle, the bending resistance should control plate velocities if the effective lithosphere viscosity is greater than about 1023 Pa s. This produces a reasonable distribution of plate velocities for Earth and may reconcile models for its thermal evolution with surface heat flow observations. These results are verified using a new method for implementing subduction that parameterizes plate bending within a small region of a mantle-scale convection model. This model also shows that small-scale convection, by removing the basal part of the oceanic lithosphere, can decrease the bending resistance and thus may be an essential aspect of plate tectonics on Earth.
by Clinton Phillips Conrad.
Ph.D.
13
Kárason, Hrafnkell 1970. "Constraints on mantle convection from seismic tomography and flow modeling." Thesis, Massachusetts Institute of Technology, 2002. http://hdl.handle.net/1721.1/8059.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2002.Includes bibliographical references (p. 205-224).
In this thesis I combine high resolution seismic tomography and realistic flow modeling to constrain mantle convection. The bulk of the data used in the tomographic imaging are millions of P, pP and pwP travel time residuals, read from high frequency seismic records. The distribution of earthquakes and stations results in uneven data coverage and to improve the model I use core phases (PKP, Pdiff) for the deep mantle and surface reflected phases (PP) for the shallow mantle. Since narrow rays are not adequate for low frequency measurements, I construct broad 3-D sensitivity kernels to relate some of the added data to earth's structure. Furthermore, the parameterization of the tomographic model is adapted to data density and the model shows, among other details, the varying style of subduction in the shallow mantle and complex flow patterns around the transition zone between the upper and lower mantle. I develop a novel and efficient method of modeling buoyancy driven mantle flow in spherical geometry. Here, the linear Stokes equation is solved using a Green's function approach and 3-D surfaces, representing the boundaries of dense material, such as subducting slabs, are tracked through time.
by Hrafnkell Kárason.
(cont.) I describe the basis of the approach, represent tests, and investigate the thickening and slowing of subducting slabs for different viscosity contrasts between the upper and lower mantle. I compare seismic tomography, flow modeling and reconstructed plate motions associated with the collision of India to mainland Asia, in particular, the south and south-west motion of the Sunda block. This way I can evaluate subduction scenarios and make quantitative comparison between tomography and plate reconstructions. I conclude that a viscosity increase of -200 in the lower mantle, resulting in -10 times higher viscosity than previously estimated, best fits the observations. Juxtaposing the tomography, the flow models and inversion tests using the flow models as input, indicates that the results are robust.
Ph.D.
14
Park, Young-Gyu. "Rotating convection driven by differential bottom heating and its application." Thesis, Massachusetts Institute of Technology, 1996. http://hdl.handle.net/1721.1/55050.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1996.Includes bibliographical references (leaves 133-137).
by Young-Gyu Park.
Ph.D.
15
Beucler, Tom(Tom George). "Interaction between water vapor, radiation and convection in the tropics." Thesis, Massachusetts Institute of Technology, 2019. https://hdl.handle.net/1721.1/121758.
Full textAbstract:
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2019Cataloged from PDF version of thesis.
Includes bibliographical references (pages 227-251).
The spatiotemporal variability of water vapor near the Equator remains poorly understood because convective organization simultaneously spans the cloud scale (~ 10km) and the planetary scale (~ 10, 000km). Spatiotemporal variability of tropical water vapor may result from internal instabilities of the atmosphere, arising from the interaction between water vapor, radiation and convection. The present work leverages the instability of radiative-convective equilibrium, the most fundamental state of the tropical atmosphere, to connect convective organization in cloud-permitting models with the observed variability of water vapor through common physical mechanisms. First, we propose a simple theory that explains when instability of radiative-convective equilibrium may occur: If the total atmospheric cooling decreases with column water vapor, then radiative-convective equilibrium may be unstable to the growth of moist and dry perturbations.
Secondly, we combine a linear response framework with the weak temperature gradient approximation to analyze the interaction between convection, radiation and water vapor at each level of the atmosphere. We find that convection may interact with radiation to trigger the growth of mid-tropospheric water vapor anomalies by transporting water vapor to the upper troposphere, where it can prevent lower-tropospheric water vapor from radiatively cooling to space. Thirdly, we turn to the spatial organization of water vapor anomalies and relate the evolution of the size of moist and dry regions to diabatic fluxes in twenty cloud-permitting simulations on large domains. Longwave radiation from ice clouds aggregates convection at larger scales, shortwave radiation aggregates convection at smaller scales, and surface enthalpy fluxes smooth out water vapor anomalies through their enthalpy disequilibrium component.
Finally, we relate the transient zonal variability of precipitable water to convective-aggregation mechanisms in realistic models and observations of the atmosphere. Radiative fluxes generate transient water vapor structures of planetary scales, while surface enthalpy fluxes and horizontal energy transport act to smooth out these structures, suggesting parallels between observations and idealized simulations of aggregated convection.
by Tom Beucler.
Ph. D.
Ph.D. Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences
16
Zaranek, Sarah Ellen. "Roles of convection in the evolution of planetary interiors and terrestrial lithospheres /." View online version; access limited to Brown University users, 2005. http://wwwlib.umi.com/dissertations/fullcit/3174708.
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Yuan, Huaiyu. "Western United States lithosphere-asthenosphere interaction Modern day small scale convection, plume and ancient lithospheric heterogeneity /." Laramie, Wyo. : University of Wyoming, 2007. http://proquest.umi.com/pqdweb?did=1400957251&sid=1&Fmt=2&clientId=18949&RQT=309&VName=PQD.
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Hsung, Jenwa. "Does the theory of parameterized conveciton apply to layered mantle convection?" Thesis, Massachusetts Institute of Technology, 2000. http://hdl.handle.net/1721.1/114097.
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Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2000.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 70-71).
There are numerous models for convection of the Earth's mantle, the end cases of which are whole-mantle convection and layered convection. Heat flow is an important consideration in the evalution of these models. Simple thermal evolution models based on boundary layer theory have in the past been used to look at these models. However, insufficient attention has been paid to how well the theory applies. This was particularly uncertain for the case of layered convection with a radiogenically enriched lower mantle. I modified the finite-element code ConMan to include exponentially decaying internal heating so that the radiogenic isotopes in the lower layer would be accurately represented, and compared the experimental results of a one-layer case and a two-layer case to the theoretical solutions for those cases from boundary layer theory. It turns out that boundary layer theory does indeed seem to be accurate for the case of a two-layered convecting system with a radiogenic lower layer that produces exponentially decaying internal heating.
by Jenwa Hsung.
S.B.
19
Bhushan, Vikas. "Modeling convection in the Greenland Sea." Thesis, Massachusetts Institute of Technology, 1998. http://hdl.handle.net/1721.1/58537.
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Thesis (S.M.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1998.Includes bibliographical references (leaves 155-161).
A detailed examination of the development of a deep convection event observed in the Greenland Sea in 1988-89 is carried out through a combination of modeling, scale estimates, and data analysis. We develop a prognostic one-dimensional mixed layer model which is coupled to a thermodynamic ice model. Our model contains a representation of the lowest order boundary layer dynamics and adjustable coupling strengths between the mixed layer, ice, and atmosphere. We find that the model evolution is not very sensitive to the strength of the coupling between the ice and the mixed layer sufficiently far away from the limits of zero and infinite coupling; we interpret this result in physical terms. Further, we derive an analytical expression which provides a scale estimate of the rate of salinification of the mixed layer during the ice-covered preconditioning period as a function of the rate of ice advection. We also derive an estimate for the rate of the mixed layer deepening which includes ice effects. Based on these scale estimates and model simulations, we confirm that brine rejection and advection of ice out of the convection area were essential ingredients during the preconditioning process. We also demonstrate that an observed rise in the air temperature starting in late December 1988 followed by a period of moderately cold ~ -10*C temperatures was key to the development of the observed convection event. Finally, we show that haline driven deep convection underneath an ice cover is possible, but unlikely to occur in the Greenland Sea. On the basis of these results, we develop a coherent picture of the evolution of the convection process which is more detailed than that presented in any previous work. We also comment on the likelihood that deep convection occurred in the Greenland Sea in the past two decades from an examination of historical data, and relate these findings to what is known about the inter-annual variability of convective activity in the Greenland Sea
by Vikas Bhushan.
S.M.
20
Wing, Allison A. "Physical mechanisms controlling self-aggregation of convection in idealized numerical modeling simulations." Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/90606.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2014.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 141-146).
The ubiquity of cloud clusters and their role in modulating radiative cooling and the moisture distribution underlines the importance of understanding how and why tropical convection organizes. In this work, the fundamental mechanism underlying the self-aggregation of convection is explored using a cloud resolving model. The objective is to identify and quantify the interactions between the environment and the convection that allow the convection to spontaneously organize into a single cluster. Specifically, the System for Atmospheric Modeling is used to perform 3-d cloud system resolving simulations of radiative-convective equilibrium in a non-rotating framework, with interactive radiation and surface fluxes and fixed sea surface temperature. Self-aggregation only occurs at sea surface temperatures above a certain threshold. As the system evolves to an aggregated state, there are large changes to domain averaged quantities important to climate, such as radiative fluxes and moisture. Notably, self-aggregation begins as a dry patch that expands, eventually forcing all the convection into a single clump. Thus, when examining the initiation of self-aggregation, we focus on processes that can amplify this initial dry patch. Sensitivity tests suggest that wind-dependent surface fluxes and interactive longwave radiative fluxes are important for permitting self-aggregation. A novel method is introduced to quantify the magnitudes of the various feedbacks that control self-aggregation within the framework of the budget for the spatial variance of column - integrated frozen moist static energy. The absorption of shortwave radiation by atmospheric water vapor is found to be a key positive feedback in the evolution of aggregation. In addition, there is a positive wind speed - surface flux feedback whose role is to counteract a negative air-sea enthalpy disequilibrium - surface flux feedback. The longwave radiation - water vapor feedback transitions from positive to negative in the early and intermediate stages of aggregation. The long-wave radiation - cloud feedback is the dominant positive feedback that maintains the aggregated state once it develops. Importantly, the mechanisms that maintain the aggregated state are distinct from those that instigate the evolution of self-aggregation. These results and those of a companion study suggest that the temperature dependence of self-aggregation enters through the longwave feedback term.
by Allison A. Wing.
Ph. D.
21
Dennedy-Frank, P. James (Peter James). "Low-degree convection with melting and application to the Martian northern hemisphere." Thesis, Massachusetts Institute of Technology, 2006. http://hdl.handle.net/1721.1/37983.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2006.Includes bibliographical references (leaves 57-64).
I investigate the hypothesis that the young and smooth surface of the Martian northern hemisphere is due to volcanic resurfacing driven by degree-one convection. I implement a batch melting process in a finite element convection model and run numerical experiments to quantify the melt fraction, timing of melting, and timing of the onset of degree-one convection. All models include a stratified viscosity to induce degree-one flow. To assure that the model's result is robust I vary the model's initial conditions, core-mantle boundary temperature and radius, and the thickness of the lithospheric lid. Long-wavelength convection is a consistent result of the viscosity stratification, and degree-one occurs in one third of the numerical experiments. I compare the melt fraction and onset of degree-one convection to the geological evidence from Martian orbiters, rovers, and meteorites. Good agreement is found between the numerical models and geological evidence, so this model suggests that volcanism driven by degree-one convection may play a significant role in the young age of the northern hemisphere of Mars.
by P. James Dennedy-Frank.
S.M.
22
Singh, Martin Simran. "The response of moist convection and the atmospheric general circulation to climate warming." Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/90683.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2014.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 169-177).
In this thesis, the response of atmospheric circulations to changes in surface temperature is investigated. Both cloud-scale and planetary-scale circulations are considered, and a number of different theoretical and numerical tools are employed. First, mechanisms maintaining the thermal stratification of a convecting atmosphere are examined based on simulations of radiative-convective equilibrium (RCE) over a wide range of surface temperatures and radiosonde observations of the tropical troposphere. It is argued that cloud entrainment plays a role in determining the lapse rate of convecting regions of the atmosphere, and that this may explain the large increase in convective available potential energy (CAPE) with warming found in the simulations. Increases in updraft velocity are also found in the RCE simulations as the atmosphere warms. The magnitude of these increases is explained through a conceptual model based on an entraining plume. The results imply that the increase in CAPE identified above is important in determining the behavior of the simulated cloud updrafts in a warming atmosphere. The precipitation distribution in RCE is also examined. The scaling of precipitation extremes with temperature is found to be amplified by increases in the mean fall speed of hydrometeors as the atmosphere warms. The hydrometeor fall speed affects precipitation rates by altering a measure of the efficiency with which condensation within the column is converted to precipitation at the surface. The potential relevance of this mechanism for convective precipitation extremes on Earth is discussed. Finally, the large-scale atmospheric circulation response to increasing surface temperature is investigated through a novel transformation of the governing equations. The transformation allows for an upward shift of the circulation with warming, and it is applied to a hierarchy of numerical simulations, including simulations with both idealized and comprehensive general circulation models. The transformation is found to reproduce some of the features of the simulated circulation responses to warming in the middle and upper troposphere, and it has potential applications in understanding the source of inter-model scatter in climate model simulations of global warming.
by Martin Simran Singh.
Ph. D.
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Ewen, Susan A. "A study of amplitude equations governing thermal convection models of the earth's fluid core." Thesis, University of Newcastle Upon Tyne, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.358894.
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Cronin, Timothy (Timothy Wallace). "Land-atmosphere interaction and radiative-convective equilibrium." Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/90605.
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Thesis: Ph. D. in Climate Physics and Chemistry, Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2014.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.
Cataloged from student-submitted PDF version of thesis.
Includes bibliographical references (pages 161-171).
I present work on several topics related to land-atmosphere interaction and radiative-convective equilibrium: the first two research chapters invoke ideas related to land-atmosphere interaction to better understand radiative-convective equilibrium; the last two research chapters use the framework of radiative-convective equilibrium to better understand land-atmosphere interaction. First, I calculate how averaging the incident solar radiation can lead to biases in idealized climate models. I derive an expression for the absorption-weighted solar zenith angle, which minimizes the bias in mean absorbed shortwave radiation, and I find that it is closely matched by the insolation-weighted zenith angle. Common use of daytime-weighted zenith angle likely leads to high biases in albedo by ~3%. Second, I explore the time scales of approach to radiative-convective equilibrium with both a simple linearized two-variable model, and a single-column model with full physics. I show that there is a long time scale of approach to radiative-convective equilibrium that is order hundreds of days even when the surface heat capacity vanishes. The impact of water vapor on the effective atmospheric heat capacity can more than double this time scale for warm temperatures and low surface heat capacities. Third, I develop an analytic theory for the sensitivity of near-surface temperature to properties of the land surface. I show that the theory compares well against a simple numerical model of the coupled boundary layer-surface system, as well as a more complex two-column model, and discuss application of the theory to questions of how changes in land use or ecosystem function may affect climate change. Finally, I find that the diurnal cycle of convection is important for the spatial distribution of rainfall in idealized simulations of radiative-convective equilibrium with a cloud-resolving model. In a region that is partly an island and mostly ocean, precipitation over the island falls primarily in a regular, strong, afternoon thunderstorm, with a time-mean rainfall rate more than double the domain average. I explore mechanisms for this island rainfall enhancement, investigate the importance of island size for my results, and find that the upper troposphere warms with the inclusion of an island, which may have implications for the large-scale tropical circulation.
by Timothy Cronin.
Ph. D. in Climate Physics and Chemistry
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Jernigan, Jonathan. "Mathematical Modeling of Convective Heat Transfer in Mammoth Cave." TopSCHOLAR®, 1997. http://digitalcommons.wku.edu/theses/787.
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Around two centuries ago, changes were made to the entrances of Mammoth Cave and its passages. Today the Historic Entrance to Mammoth Cave is enlarged and the passage just beyond the entrance known as Houchins' Narrows has been cleared of rubble and filled with sediments. These enlargements have resulted in an increase in airflow throughout the Historic Section of the cave causing environmental conditions such as air temperature and airflow to fluctuate. These fluctuations have negatively impacted inhabitants and contents of the cave system. To restore natural conditions within the cave, Science and Resource Management personnel at Mammoth Cave National Park have been collecting large data sets on atmospheric conditions inside the cave. The author has access to data from eight sites within the cave. In this thesis, the author provides a brief introduction to the effects of the increase in airflow as well as a short discussion of the data gathered by Science and Resource Management. The author then proposes a natural cause for airflow (i.e., convection) in Mammoth Cave, constructs empirical models with this as the underlying driving force, and uses atmospheric data to verify the validity of the claim of convection as the force driving airflow in Mammoth Cave. Data from the site in Houchins' Narrows is used to predict atmospheric data at other locations in the cave. The author concludes this thesis with time series analysis on data from Houchins' Narrows.
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Bouffard, Mathieu. "Double-diffusive thermochemical convection in the liquid layers of planetary interiors : a first numerical exploration with a particle- in-cell method." Thesis, Lyon, 2017. http://www.theses.fr/2017LYSEN063/document.
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De nombreux corps du système solaire possèdent des enveloppes liquides internes, comme par exemple les noyaux métalliques des planètes telluriques et les océans profonds des satellites de glace de Jupiter et Saturne, dans lesquelles se produisent des courants de convection. La modélisation de la dynamique de ces enveloppes est cruciale pour comprendre la génération des champs magnétiques planétaires (pour les noyaux) et pour mieux déterminer l’habitabilité potentielle des satellites joviens. La convection dans ces enveloppes est généralement produite par la combinaison d’au moins deux sources de flottabilité : une source thermique et une source solutale. Une telle situation est plus complexe qu’un régime de convection purement thermique ou purement solutale, d’une part en raison de l’existence d’un couplage thermochimique lorsqu’un processus de fusion ou de cristallisation se produit à l’une des frontières de l’enveloppe, et d’autre part à cause de la forte différence de diffusivité moléculaire entre les champs thermique et compositionnel qui permet potentiellement le développement d’instabilités double-diffusives. Classiquement, ces complexités ont été ignorées dans les simulations numériques de la dynamo terrestre ; les champs thermique et compositionnel ayant été combinés en une seule variable nommée « codensité ». Cette approche est sans doute simpliste mais permet d’esquiver une difficulté technique liée à la description du champ compositionnel dont la très faible diffusivité nécessite de recourir à des méthodes numériques adaptées. Cette thèse présente d’abord l’implémentation d’une méthode semi-Lagrangienne du type « particle-in-cell » dans un code de dynamo pré-existant, permettant ainsi de traiter de manière plus réaliste le champ de composition dans les enveloppes liquides internes des planètes. Les optimisations réalisées sont détaillées ainsi que les résultats de tests sur des cas de benchmark qui valident cet outil. Une comparaison avec des méthodes Eulériennes est également présentée. Une première exploration de la physique de la convection compositionnelle et thermochimique en rotation dans la limite d’un nombre de Prandtl compositionnel infini est ensuite conduite dans le contexte du noyau liquide terrestre. Il est montré que la dynamique convective est très différente de celle de la convection thermique pure. Notamment, les matériaux légers injectés à la frontière graine/noyau liquide sont capables d’atteindre la frontière noyau/manteau et de s’y accumuler pour former une couche chimiquement stratifiée, dont l’existence a été évoquée théoriquement mais qui n’a jamais pu être produite dans de précédentes simulations. Enfin, la dynamique double-diffusive des couches stratifiées est également discutée, et de premières simulations de « salt fingers » sont présentéesNumerous planetary bodies contain internal liquid layers in which convective currents are generated by the combination of buoyancy sources of thermal and compositional origin. The strong difference between the thermal and chemical molecular diffusivities and the possibility of thermo-chemical coupling at melting or freezing boundaries create a convective regime that is much more complex than pure thermal convection, partly due to the potential occurrence of double-diffusive instabilities. Traditionally, numerical simulations have modeled the dynamics of the liquid part of planetary cores in a more simplistic way by neglecting the diffusivity difference and combining both fields into one single variable, an approximation that is convenient but maybe not relevant. However, distinguishing both fields and dealing with a large or infinite diffusivity ratio makes it compulsory to use numerical methods that minimize numerical diffusion as much as possible. In this thesis, I adapted a semi-Lagrangian particle-in-cell method into a pre-existing dynamo code to describe the weakly diffusive compositional field. I optimized the code for massively parallel computing and validated it on two different benchmarks. I compared the particle-in-cell method to Eulerian schemes and showed that its advantages extend beyond its lower numerical dissipation. Using this new tool, I performed first numerical simulations of rotating pure compositional and thermochemical convection in the limit of null chemical diffusivity. I explored the physics of pure compositional convection and addressed questions related to the existence and the dynamics of a stratified layer below the Earth’s core mantle boundary. In particular, I showed that the stratification could potentially be of chemical origin and proposed some mechanisms to explain its formation. In the case of a thermally stratified layer, I performed a scaling analysis of fingering instabilities, wrote the first steps of a linear stability analysis and ran a few simulations of fingering instabilities in the rotating case. The potential effects of the magnetic field and the coupling of thermochemical boundary conditions in planetary cores are finally discussed in this thesis
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Våge, Kjetil. "Circulation and convection in the Irminger Sea." Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/58395.
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Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2010.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 131-149).
Aspects of the circulation and convection in the Irminger Sea are investigated using a variety of in-situ, satellite, and atmospheric reanalysis products. Westerly Greenland tip jet events are intense, small-scale wind phenomena located east of Cape Farewell, and are important to circulation and convection in the Irminger Sea. A climatology of such events was used to investigate their evolution and mechanism of generation. The air parcels constituting the tip jet are shown to have a continental origin, and to exhibit a characteristic deflection and acceleration around southern Greenland. The events are almost invariably accompanied both by a notable coherence of the lower-level tip jet with an overlying upper-level jet stream, and by a surface cyclone located in the lee (east) of Greenland. It is argued that the tip jet arises from the interplay of the synopticscale flow evolution and the perturbing effects of Greenland's topography upon the flow. The Irminger Gyre is a narrow, cyclonic recirculation confined to the southwest Irminger Sea. While the gyre's existence has been previously documented, relatively little is known about its specific features or variability. The mean strength of the gyre's circulation between 1991 and 2007 was 6.8 ± 1.8 Sv. It intensified at a rate of 4.3 Sv per decade over the observed period despite declining atmospheric forcing. Examination of the temporal evolution of the LSW layer thickness across the Irminger Basin suggests that local convection formed LSW during the early 1990s within the Irminger Gyre. In contrast, LSW appeared outside of the gyre in the eastern part of the Irminger Sea with a time lag of 2-3 years, consistent with transit from a remote source in the Labrador Sea. In the winter of 2007-08 deep convection returned to both the Labrador and Irminger seas following years of shallow overturning. The transition to a convective state took place abruptly, without going through a preconditioning phase, which is contrary to general expectations. Changes in the hemispheric air temperature, tracks of storms, flux of freshwater to the Labrador Sea, and distribution of pack ice all conspired to enhance the air-sea heat flux, resulting in the deep overturning.
Kjetil Våge.
Ph.D.
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Puster, Peter. "The characterization of seismic earth structures and numerical mantle convection experiments using two-point correlation functions." Thesis, Massachusetts Institute of Technology, 1995. http://hdl.handle.net/1721.1/55028.
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Bister, Marja Helena. "Development of tropical cyclones from mesoscale convective systems." Thesis, Massachusetts Institute of Technology, 1996. http://hdl.handle.net/1721.1/57851.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1996.Includes bibliographical references (p. 109-112).
by Marja Helena Bister.
Ph.D.
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Cetola, Jeffrey David. "The Role of Terrain and Convection on Microfront Formation Leading to Severe Low-Level Turbulence." NCSU, 2003. http://www.lib.ncsu.edu/theses/available/etd-09022003-233653/.
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Two low-level convectively-induced turbulence (CIT) events east of the Appalachian Mountains are investigated utilizing observations, satellite, radar, and numerical simulations. Both events had an inordinate amount of low-level turbulence reported, but one event had more than twice as many severe or greater reports. The events were compared?to include the 72 hours leading up to the turbulence reports?and similarities and differences at the various scales from the synoptic to meso-alpha, meso-beta, meso-gamma, and microscale are noted. The case of weaker turbulence featured a meridional wave pattern with ridging over the East Coast and a single upper-level jet closely coupled with the large-scale frontal system. The stronger turbulence case possessed a zonal wave pattern with a vortex over eastern Canada and both a polar jet and subtropical jet. These differences are reflected in the low-level temperature and potential vorticity patterns and affected the hydraulic structures as well?with the stronger turbulence environment more prone to a blocking-type regime. Hydrostatic mountain waves were observed for both events. Stronger cross-mountain flow combined with a strong low-level leeside inversion resulted in a more vigorous mountain wave with a stronger downstream isentropic upfold (mid-level cold pool) in the stronger turbulence event. This mid-level cold pool was deformed by the large-scale jet resulting in a mid-level cold front (downstream from the surface cold front), surface pressure rises to the lee of the Allegheny Mountains, and ultimately a surface cold surge (edgewave) that merges with warm air from the south. The phasing of the mid-level cold pool and the convergence with the northerly cold surge and southerly warm air results in kata-frontogenesis and cellular convection that transits the severe turbulence location in space and time. Convection in the weaker turbulence case was lineal in structure and tied to the large-scale cold anafront. Vorticity, enstrophy, turbulent kinetic energy, and Richardson number analyses indicated maxima were lineal in structure and upstream from the convection in the weaker case and arc-like in appearance and downstream from convection in the stronger case. A turbulence index was formulated based on three-dimensional vorticity (enstrophy), vertical wind shear, and static stability.
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French, Adam James. "The Initiation and Evolution of Multiple Modes of Convection Within a Meso-Alpha Scale Region." NCSU, 2007. http://www.lib.ncsu.edu/theses/available/etd-10192007-180611/.
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On 30 March 2006 a convective episode occurred featuring isolated supercells, a mesoscale convective system (MCS) with parallel stratiform (PS) precipitation, and an MCS with leading stratiform (LS) precipitation. These three distinct convective modes occurred simulataneously across the same region in eastern Kansas. Multi-modal events are especially challenging for forecasters given the wide range of severe weather threats that accompany the different modes. In order to better understand the mechanisms that govern such events, this study examined the 30 March 2006 episode through a combination of an observation-based case study and numerical simulations. From the results of this study we conclude that, for this event, localized environmental variations were largely responsible for the eventual convective mode, with the method of storm initiation having only limited effects. The resultant mode was very sensitive to both the environmental thermodynamic and shear profiles, as variations in either led to different convective modes within the numerical simulations. Finally, we conclude that while the individual modes each developed within an environment distinctly favorable for that mode, they were able to persist in close proximity to one another due to a "middle ground" environment permissive of all three. Strong vertical shear and moderate instability led to the development of supercells in western Oklahoma and similarly strong shear oriented parallel to a surface dryline coupled with dry air in the middle and upper levels led to the development of the PS linear MCS in central Kansas. Meanwhile, moderate wind shear coupled with high instability and strong linear forcing led to the development of the LS MCS in eastern Kansas. Without this linear forcing, the moderate shear environment was supportive of both linear and isolated supercell modes, resulting in the storms that moved into this region maintaining their original organization.
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Robe, Françoise R. (Françoise Romain). "Sea, sun, and shear : a recipe for precipitating convection, tropical rainbands, and hurricane spiral arms." Thesis, Massachusetts Institute of Technology, 1996. http://hdl.handle.net/1721.1/17416.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1996.Includes bibliographical references (p. 235-242).
by Françoise R. Robe.
Ph.D.
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Vidal, Valérie. "Interaction des différentes échelles de convection dans le manteau terrestre." Phd thesis, Institut de physique du globe de paris - IPGP, 2004. http://tel.archives-ouvertes.fr/tel-00009127.
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Deux approches nous ont permis d'améliorer la compréhension des modes de convection dans le manteau terrestre. Nous avons étudié expérimentalement les mouvements de fluides visqueux dans une cuve chauffée latéralement et refroidie par le dessus. La convection à petite échelle qui se développe par déstabilisation de la couche limite thermique froide, analogue de la lithosphère océanique, s'organise en présence de l'écoulement cisaillant de la tectonique des plaques. Elle prend alors la forme de rouleaux stationnaires ou de panaches, deux régimes possibles dans le cas de la Terre. L'interaction d'un panache avec la lithosphère a quant à elle été étudiée grâce à une analyse précise des observables géophysiques associées au point chaud d'Hawai'i. L'évolution temporelle des volumes du bombement et des édifices volcaniques met en évidence une importante augmentation de l'activité de ce dernier depuis 30 Ma. Le taux de production magmatique en est le meilleur indicateur.
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Cordery, Matthew Jean. "Mantle convection, melt migration and the generation of basalts at mid-ocean ridges." Thesis, Massachusetts Institute of Technology, 1991. http://hdl.handle.net/1721.1/52936.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, and Woods Hole Oceanographic Institute, 1991.Includes bibliographical references (leaves 183-190).
by Matthew Jean Cordery.
Ph.D.
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Soderholm, Brett. "The evolution of convective storms initiated by an isolated mountain range." Thesis, McGill University, 2013. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=119510.
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While significant attention has been given to understanding the initiation mechanisms of convective storms over mountainous terrain, far less has been given to the factors controlling their subsequent evolution. Here we perform an observational and numerical investigation of the evolution of convective storms initiated by the Black Hills mountains of South Dakota. These Hills are preferentially located within the United States to access moist, unstable air, and are thus a local hot-spot for convection initiation. Applying a convective-celltracking algorithm to 53 observed events initiated by the Black Hills revealed three types of storm evolution: short-lived, short-track cells; long-lived, short-trackcells; and long-lived, long-track cells. Analysis of the background wind profiles during each event revealed modest differences amongst the storm types, which were tested using quasi-idealized, convection-permitting numerical simulations. The track-lengths and durations of convective cells produced in these simulations were consistent with those in our observed events, demonstrating that these modest differences in background wind profile could indeed largely explain a convective storm's evolution.Bien que beaucoup d'attention a été accordée à la compréhension des mécanismes d'initiation des tempêtes convectives sur un terrain montagneux, beaucoup moins a été mis sur les facteurs qui contrôlent leur évolution ultérieure. Ici, nous effectuons une enquête observationelle et numérique des tempêtes initiées par les montagnes Black Hills du South Dakota. Ces montagnes initient beaucoup de convection grace à leur accès à l'air humide et instable. L'application d'un algorithme suivant les pistes de 53 tempêtes initiés par les Black Hills a révélé trois types d'évolution: tempête de courte-piste, courte-durée; tempête de courte-piste, longue-durée; et tempête de longue-piste, longue-durée. Un analysis des profils de vents pendant chaque événement a révélé de légères differences entre les types de tempêtes, qui ont été testés par des simulations numériques. Les longueurs de piste et les durées des tempêtes produites dans ces simulations étaient similaires à celles des événements observés, ce qui démontre que ces légères différences dans les profils de vent pourraient en effet expliquer l'évolution de chaque type de tempête.
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Agard, John Vincent. "Dependence of continental severe convective instability on climatological environmental conditions." Thesis, Massachusetts Institute of Technology, 2017. http://hdl.handle.net/1721.1/113799.
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Thesis: Ph. D. in Atmospheric Science, Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2017.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 115-119).
Most of Earth's strongest atmospheric convection occurs over the continents, where potential energy is stored over time in metastable vertical profiles, only to be released rapidly by severe storms. In contrast to equilibrium-state convection in tropical ocean regions, there is a relative paucity of research exploring the climate dynamics of continental deep convection. This work makes a contribution to rectifying that deficiency by exploring the physical mechanisms by which convective available potential energy (CAPE) is generated, and their dependence on climatological properties of the Earth's environment. First, a budget of the time tendency of CAPE is used to examine the buildup of CAPE in advance of severe convective storm events in North America via case studies using reanalysis data. Contributions to extreme peak CAPE from relative advection of air masses, diabatic heating of the surface boundary layer, and radiative cooling of the free troposphere are computed. In all 8 cases studied, CAPE buildup is found to be driven primarily by fluxes of heat and moisture from the surface into the boundary layer on sub-diurnal time scales, and not by radiative cooling or the relative advection of air masses at low and upper levels over multiple days. This result is then further explored using an idealized two-dimensional continental framework using a minimal numerical model. Experiments in both 2-column and 40- column configurations demonstrate a link between discontinuities in surface moisture and high levels of transient CAPE. Surface entropy flux is once again found to be the primary driver of peak CAPE buildup within the model. Finally, a thermodynamic constraint on CAPE in continental environments is established using an idealized, one-dimensional model. This theoretical model incorporates the physical principle of CAPE generation identified using reanalysis and two-dimensional modeling by considering a dry adiabatic column that comes into contact with a moist land surface. A system of equations is derived to describe the evolution of the ensuing surface boundary layer. From these, the maximum value of transient CAPE in the column can be found for any particular combination of surface temperature and moisture. It is demonstrated that, for a given range of surface temperatures, the value of peak CAPE scales with the Clausius-Clapeyron relation.
by John Vincent Agard.
Ph. D. in Atmospheric Science
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Chavas, Daniel Robert. "Tropical cyclone size in observations and in radiative-convective equilibrium." Thesis, Massachusetts Institute of Technology, 2013. http://hdl.handle.net/1721.1/82308.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2013.Cataloged from PDF version of thesis.
Includes bibliographical references (p. 147-154).
Tropical cyclone size remains an unsolved problem in tropical meteorology, yet size plays a significant role in the damage caused by tropical cyclones due to wind, storm surge, and inland freshwater flooding. This work explores size, defined as the radius of vanishing wind, in observations and at equilibrium in an idealized numerical model. First, a climatology of size is created from the QuikSCAT database of near-surface wind vectors for the years 1999-2008. Globally, the distribution of the outer radius is found to be log-normal, with statistically significant variation across ocean basins, but with minimal correlation with various dynamic and thermodynamic parameters. Second, the sensitivity of the structure of a numerically-simulated axisymmetric tropical cyclone at statistical equilibrium to the set of relevant model, initial, and environmental external parameters is explored. The analysis is performed in a highly-idealized state of radiative-convective equilibrium (RCE). The non-dimensional equilibrium radial wind profile is found to be modulated primarily by a single nondimensional parameter given by the ratio of the storm radial length scale to the parameterized eddy radial length scale. The relevant storm length scale is shown to be the ratio of the potential intensity to the Coriolis parameter, matching the prediction for the "natural" storm length scale in prevailing axisymmetric tropical cyclone theory. The outer storm circulation is further modulated by a second non-dimensional parameter that represents the non-dimensional Ekman suction rate. Third, size is explored in three-dimensional "tropical cyclone world" simulations, with preliminary results confirming the relevant length scale obtained in axisymmetry. Ultimately, the results of the equilibrium storm analysis are insufficient to explain the observed distribution of tropical cyclone size, but they provide the first steps toward a more fundamental understanding of the dynamics of size.
by Daniel Robert Chavas.
Ph.D.
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Xu, Kuanman. "Vertical structure and the convective characteristics of the tropical atmosphere." Thesis, Massachusetts Institute of Technology, 1987. http://hdl.handle.net/1721.1/57925.
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Louis-Napoléon, Aurélie. "Modélisation d'instabilités gravitaires au sein de la croûte partiellement fondue par une méthode de Volume-Of-Fluid." Thesis, Toulouse, INPT, 2020. http://www.theses.fr/2020INPT0124.
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Les instabilités gravitaires au sein des racines orogéniques partiellement fondues peuvent être à l'origine du développement de dômes de migmatites. Non seulement ces dômes peuvent concentrer des matériaux d’intérêt économique, mais une meilleure compréhension de leur formation présente un intérêt fort dans la connaissance de l’évolution et de la différenciation de la croûte terrestre. Bien que les forces horizontales peuvent être impliquées dans la formation de dômes, l’objectif de cette thèse consistait à regarder uniquement l’influence des forces verticales en étudiant la convection et le diapirisme. Pour cela, nous avions le choix entre deux codes, basés sur la méthode numérique de Volume-Of-Fluid (VOF): JADIM (code de l'IMFT) et OpenFOAM (open-source). Nous avons dans un premier temps montré que la méthode VOF est adaptée à la modélisation des instabilités gravitaires en comparant les résultats obtenus avec i) ceux de la littérature concernant les instabilités de Rayleigh-Taylor et de Rayleigh-Bénard, ii) ceux donnés par le code de géodynamique ASPECT. Nous avons trouvé que JADIM et OpenFOAM fournissent des résultats en bon accord avec ceux de la littérature et conservent mieux la masse qu'ASPECT. Nous avons choisi OpenFOAM pour la suite de ce travail car il est plus rapide que JADIM. Ensuite, nous avons appliqué la méthode VOF au contexte de formation des dômes de migmatites, en considérant plus particulièrement le cas de l'île de Naxos (Grèce). Ces dômes se sont formés entre 24 et 16 millions d'années et présentent une structure imbriquée de sous- dômes de 2 km dans un dôme majeur de 10 km. Des datations sur zircon ont montré que les roches ont subi une alternance de température d'une période de 2 Ma. Ces dômes sont intéressants car ils se sont formés récemment dans l'histoire de la Terre dans une zone orogénique, ils sont donc mieux préservés que les dômes plus anciens. Nous avons cherché à montrer que ces structures peuvent résulter d'épisodes convectif et diapirique. Nous avons d'abord considéré la croûte chaude comme un système constitué de trois couches horizontales. Ce système ne permet pas de former des dômes imbriqués par convection et diapirisme. En effet, les dômes formés au cours de la simulation sont détruits par la convection. Nous avons donc complexifié le système. Nous avons pris en compte le chauffage interne, la fusion partielle, la dépendance des rhéologies sur la température et le taux de déformation, et avons ajouté des hétérogénéités compositionnelles (inclusions). Tous ces éléments ont fait l'objet de tests paramétriques et nous permettent finalement de proposer deux scénarios pour expliquer la formation des dômes de Naxos : le premier fait émerger des diapirs composés d'inclusions au dessus de cellules convectives locales, formées lors de la ségrégation des inclusions lourdes et légères, tandis que le second scénario permet aux inclusions de s'élever pendant la disparition de la cellule convective globale pour former des dômes lors du refroidissement du système. Toutefois, ces scénarios n'excluent pas le rôle de forces latérales extérieures à la fin de l'orogène pour expliquer l'exhumation des dômes. Plus généralement, nous avons trouvé que les caractéristiques des dômes (taille et homogénéité) sont déterminées par leur mode de formation (présence d'inclusions, de convection ...). L'analyse dimensionnelle des systèmes avec fusion partielle nous a permis de distinguer plusieurs régimes convectifs et de déterminer les paramètres nécessaires à la sédimentation (vers le haut et vers la bas) des inclusions lors d'une convection crustaleGravity instabilities within partially molten orogenic roots may form migmatite domes. Not only can these domes concentrate materials of economic interest, but they also bring a better understanding of the evolution and differentiation of Earth's crust. Although horizontal forces can be involved in the formation of domes, the objective of this thesis was to look only at the influence of vertical forces by studying convection and diapirism. We had at our disposal two numerical codes based on Volume-Of-Fluid methods (VOF): JADIM (IMFT code) and OpenFOAM (open-source). We first showed that the VOF method is suitable for the modeling of gravity instabilities. Therefore we compared the results obtained with JADIM and OpenFOAM with i) theoretical, experimental and numerical results from the literature of RayleighTaylor and Rayleigh-Bénard instabilities and with ii) the results of the geodynamic code ASPECT . We found that JADIM and OpenFOAM provide results in good agreement with those in the literature and conserve mass better than ASPECT. We chose OpenFOAM for the rest of this work because it is faster than JADIM. Then, we applied the VOF method to the context of the migmatite domes from Naxos island (Greece). These domes were formed between 24 and 16 Ma and feature nested structures of 2 km sub-domes in a 10 km major dome. Zircon dating has shown that the rocks have undergone a temperature alternation with a 2 Ma period. These domes are interesting because they are recent and thus better preserved than older domes. We have tried to show that they can result from a combination of convective and diapiric episodes. We first considered the hot orogenic crust as a system made of 3 horizontal layers with temperature dependent viscosity. We showed that both in 2D and 3D, we cannot reproduce nested domes: the simulated domes are destroyed by convection when it appears. Therefore we made the system more complex. We took into account internal heating and partial melting which allow to achieve low viscosities. In addition, we added the strain-rate dependency of the rheology, as well as compositional heterogeneities (inclusions). All these factors were subjected to parametric tests, which finally allow us to propose two scenarios explaining the formation of the Naxos domes. The first scenario takes into account a nonlinear rheology and heavy and light inclusions, making it possible to generate local convection: convection and diapirism are then simultaneous, and diapirs emerge above the convective cells. The second scenario takes into account partial melting, but the progressive cooling of the system is the key mechanism that allows light inclusions to rise and form domes. However, both these scenarios do not exclude the role of external lateral forces at the end of the orogen on the remaining exhumation. More generally, we have found that the characteristics of the domes (size and homogeneity) are determined by their mode of formation (presence of inclusions, convection ...) The dimensional analysis of systems with partial melting allows us to distinguish several convective regimes and to determine the parameters necessary for the sedimentation (upwards and downwards) of inclusions during crustal convection
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DeCroix, David Scot. "Large-eddy Simulations of the Convective and Evening Transition Planetary Boundary Layers." NCSU, 2001. http://www.lib.ncsu.edu/theses/available/etd-20010319-182404.
Full textAbstract:
Large-eddy simulation (LES) is a very useful tool in computationalfluid dynamics. The LES model allows one to solve a filtered set of theNavier-Stokes equations, thereby explicitly resolving scales of motionlarger than the discretization or grid size. Those motions smaller thanthe grid size are parameterized using a so-called subgrid scale model.
In this series of papers, we will use the TASS LES model, originallya cloud model, which has been modified to simulate planetary boundarylayer turbulence. We will first introduce the LES model and a newgrid-nesting method for the LES. Then we will present simulations ofthe convective planetary boundary layer, and then use the LES to studythe decay of convective planetary boundary layer turbulence to a stablystratified state.
The LES model has been modified to include a grid nesting capability.Grid meshes of higher resolution may be embedded within the LES enablingone to resolve smaller scales of motion (turbulence) than would bepossible by using a single grid mesh. The grid nesting methodology isdescribed in detail in Chapter 2.
In Chapter 3, the nested-grid LES will be applied to thesimulation of the convective planetary boundary layer. We will usea total of three grid meshes to increase the resolution in the surfacelayer, allowing a detailed analysis of the turbulence near the surface ofthe earth.
In Chapter 4, we will focus on applying Rayleigh Benardconvection criteria, using a linearized perturbation method,to the surface layer of a CBL produced by large-eddy simulation.Similarities and differences will be discussed between the LESproduced surface layer and classical Rayleigh-Benard convection theory.
In Chapter 5, using a large-eddy simulation model, we willexamine in detail the turbulent kinetic energy (TKE) budget during theevening transition. The simulation will be performed in order to compareto observations gathered at the Dallas-Fort Worth International Airport,Fort-Worth, TX. during September and October 1997.
In Chapter 6 the decay of planetary boundary layerturbulence during the evening transition will be studied. In previousstudies of the decay of turbulence, the effects of mean winds and shearsdue to pressure gradient on the turbulence decay was not considered.We propose to examine the effects of increasing geostrophic wind onthe convective boundary layer and its transition or decay to a stablecondition. Finally, the overall conclusions of each chapter will bepresented.
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Hufford, Gwyneth Ellin. "Parameterization of convection in a rotating stratified ocean : comparison of numerical and laboratory experiments with theory." Thesis, Massachusetts Institute of Technology, 1994. http://hdl.handle.net/1721.1/59632.
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Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, and Woods Hole Oceanographic Institution, 1995.Includes bibliographical references (leaves 70-73).
by Gwyneth Ellin Hufford.
M.S.
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Chandra, Arunchandra. "The turbulent structure of the clear and cloud-topped convective boundary layer over land." Thesis, McGill University, 2013. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=114422.
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Over land, the daytime evolution of the convective boundary layer (CBL) is driven by the strong surface forcing evolution. Recent modeling studies have demonstrated that models cannot capture the observed evolution because of difficulty to represent turbulent processes in CBL and shallow cumulus clouds. The purpose of this thesis is to advance our understanding of the structure of clear and cloudy boundary layer over land by providing observational evidence using long-term dataset. Specifically, it focuses on documenting the turbulent structure and properties of CBL and FWC using observations at the U.S. Department of Energy Atmospheric Radiation Measurement Program (ARM) Southern Great Plains (SGP) Climate Research Facility. First, Doppler velocity measurements from insects occupying the lowest 2 km of the boundary layer during summer months are used to map the vertical velocity component in the CBL. The observations cover four summer periods (2004–08) and are classified into cloudy and clear boundary layer conditions. A conditional sampling method is applied to the original Doppler velocity data set to extract coherent vertical velocity structures and to examine plume dimension and its contribution to the total turbulent transport. Profiles of vertical velocity variance, skewness, and mass flux are estimated to study the daytime evolution of the convective boundary layer during these conditions. The properties of summer time FWC clouds are analyzed using the long data record (14–year) of ground-based MMCR (Millimeter Wavelength Cloud Radar) observations at the ARM facility at the SGP site to document the macroscopic and dynamical properties of FWC clouds. Doppler velocities are processed for lower reflectivity thresholds that contain small cloud droplets having insignificant terminal velocities; thus Doppler velocities used as tracers of air motion. A fuzzy-logic based algorithmis developed to eliminate insect radar echoes in the boundary layer that hinder our ability to develop representative cloud statistics. The refined data set is used to document composite daytime evolution of cloud vertical velocity statistics, surface parameters and profiles of updraft and downdraft fractions, updraft and downdraft velocity, and updraft mass fluxes. Statistics on the cloud geometrical properties such as, cloud thickness, cloud chord length, cloud spacing and aspect ratios are calculated on the cloud scale. Lastly, some of the updraft aspects in existing mass-flux parameterizations are tested using the recent observations from Doppler lidar deployed since Midlatitude Continental Convective Cloud Experiment (MC3E). The updraft aspects from two existing mass-flux schemes are evaluated with the Doppler lidar vertical velocity observations to test its applicability over land. The vertical velocity observations in the subcloud layer are analyzed for various cloud conditions by separately decomposing into clear and cloudy regions, and for various cloud fractions to investigate the role of clouds on the subcloud structure.Sur les continents, l'évolution diurne de la couche limite convective (CLC) est entrainée par l'évolution des forçages de surface. Les modélisations récentes ont démontré que les modèles ne peuvent pas capturer l'évolution observée en raison des difficultés pour représenter les processus turbulents dans les nuages cumulus de la CLC et peu profonde. Le but de cette thèse est de faire progresser notre compréhension de la structure de la couche limite claire et nuageuse sur les terres en fournissant des données d'observation à long terme et en utilisant un ensemble de données. Plus précisément, elle met l'accent sur la documentation de la structure turbulente et les propriétés de la CLC et des cumulus de beau temps (CBT) à l'aide d'observation acquise au site de recherches « Southern Great Plains » (SGP) du programme « Atmospheric Radiation Measurement » (ARM) du Département américain de l'énergie. Premièrement, les mesures de vitesse Doppler des insectes, qui occupent les 2 km au bas de la couche limite pendant les mois d'été, sont utilisés pour cartographier la composante de vitesse verticale dans la CLC. Les observations portent sur quatre périodes estivales (2004 - 08) et sont classées en conditions de couche limite claire et nuageuse. Un procédé d'échantillonnage conditionnel est appliqué aux données d'origine de vitesse Doppler pour extraire les structures cohérentes de vitesse verticale et pour examiner la dimension du panache et sa contribution au transport turbulent total. Les profils de la variance et de l'asymétrie de la vitesse verticale et du flux de masse sont estimés pour étudier l'évolution diurne de la CLC au cours de ces conditions. Les propriétés des nuages CBT d'été sont analysées en utilisant le long jeu de données d'observations (14 années) à partir du sol de MMCR (Radar de nuages à longueur d'onde millimétrique) au laboratoire SGP de ARM afin de documenter les propriétés macroscopiques et dynamiques des nuages CBT. Les vitesses Doppler sont traitées pour des seuils inférieurs de réflectivité qui contiennent de petites gouttelettes de nuages ayant des vitesses terminales négligeables; ainsi les vitesses Doppler sont utilisées comme des traceurs de mouvements de l'air. Un algorithme de logique floue a été développé pour éliminer les échos radar d'insectes dans la couche limite qui entravent notre capacité d'élaborer des statistiques représentatives des nuages. L'ensemble de données raffineés est utilisé pour documenter l'évolution diurne compose des statistiques de vitesse verticale des nuages, les paramètres de surface et des profils des fractions de courant ascendant et descendant, la vitesse des courants ascendant et descendants, et les flux de masse ascendant. Les statistiques sur les propriétés géométriques des nuages telles que, l'épaisseur des nuages, la longueur de corde des nuages, l'espacement des nuages et les rapports d'aspect sont calculés sur l'échelle nuageuse. Enfin, quelques uns des aspects des courants ascendants dans les paramétrisations existantes du flux de masse sont testées en utilisant les observations récentes du lidar Doppler déployé depuis l'expérience « Midlatitude Continental Convective Cloud Experiment » (MC3E). Les caractéristiques des courants ascendants de deux schéma existants de flux de masse sont évalués avec les observations de vitesse verticale du lidar Doppler pour tester son applicabilité sur les terres. Les observations de vitesse verticale dans la couche sous les nuages sont analysées pour différentes conditions nuageuses, en décomposant séparément les régions claires et nuageuses, et pour différentes fractions de nuages pour étudier le rôle des nuages sur la structure sous-nuageuse.
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Robinson, Elizabeth M. "The effect of a shallow low viscosity zone on mantle convection and its expression at the surface of the earth." Thesis, Massachusetts Institute of Technology, 1987. http://hdl.handle.net/1721.1/58496.
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Thesis (Ph. D.)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1987.Includes bibliographical references (v.2, leaves 309-317).
Many features of the oceanic plates cannot be explained by conductive cooling with age. A number of these anomalies require additional convective thermal sources at depths below the plate: mid-plate swells, the evolution of fracture zones, the mean depth and heat flow relationships with age and the observation of small scale (150-250 km) geoid and topography anomalies in the Central Pacific and Indian oceans. Convective models are presented of the formation and evolution of these features. In particular, the effect of a shallow low viscosity layer in the uppermost mantle on mantle flow and its geoid, topography, gravity and heat flow expression is explored. A simple numerical model is employed of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region. Finite element calculations have been used to determine the effects of (1) the viscosity contrast between the two fluid layers, (2) the thickness of the low viscosity zone, (3) the thickness of the conducting lid, and (4) the Rayleigh number of the fluid based on the viscosity of the lower layer. A model simple for mid-plate swells is that they are the surface expression of a convection cell driven by a heat flux from below. The low viscosity zone causes the top boundary layer of the convection cell to thin and, at high viscosity contrasts and Rayleigh numbers, it can cause the boundary layer to go unstable. The low viscosity zone also mitigates the transmission of normal stress to the conducting lid so that the topography and geoid anomalies decrease. The geoid anomaly decreases faster than the topography anomaly, however, so that the depth of compensation can appear to be well within the conducting lid. Because the boundary layer is thinned, the elastic plate thickness also decreases and, since the low viscosity allows the fluid to flow faster in the top layer, the uplift time decreases as well. We have compared the results of this modeling to data at the Hawaii, Bermuda, Cape Verde and Marquesas swells, and have found that it can reproduce their observed anomalies. The viscosity contrasts that are required range from 0.2-0.01, which are in agreement with other estimates of shallow viscosity variation in the upper mantle. Also, the estimated viscosity contrast decreases as the age of the swell increases. This trend is consistent with theoretical estimates of the variation of such a low viscosity zone with age. Fracture zones juxtapose segments of the oceanic plates of different ages and thermal structures. The flow induced by the horizontal temperature gradient at the fracture zone initially downwells immediately adjacent to the fracture zone on the older side, generating cells on either side of the plume. The time scale and characteristic wavelength of this flow depends initially on the viscosity near the largest temperature gradient in the fluid which, in our model, is the viscosity of the low viscosity layer. They therefore depend on both the Rayleigh number and the viscosity contrast between the layers. Eventually the flow extends throughout the box, and the time scales and the characteristic wavelengths of the flow depend on the thickness and viscosity of both layers. When the Rayleigh number based on the viscosity of the top la er, and the depth of both fluid layers, is less than 10 , the geoid anomalies of these flows are dominated by the convective signal. When this Rayleigh number exceeds 106, the geoid anomalies retain a step across the fracture zone out to large ages. We have compared our results to geoid anomalies over the Udintsev fracture zone, and have found that the predicted geoid anomalies, with high effective Rayleigh numbers, agree at longer wavelengths with the observed anomalies and can produce the observed geoid slope-age behaviour. We have also compared the calculated topographic steps to those predicted by the average depth-age relationships observed in the oceans. We have found that only with a low viscosity zone will the flow due to fracture zones not disturb the average depth versus age relationships. We have also applied the model to a numerical study of the effect of a low viscosity zone in the uppermost mantle on the onset and surface expression of convective instabilities in the cooling oceanic plates. We find that the onset and magnitude of the geoid, topography and heat flow anomalies produced by these instabilities are very sensitive to the viscosity contrast and the Rayleigh number, and that the thickness of the low viscosity zone is constrained by the wavelength of the observables. If the Rayleigh number of the low viscosity zone exceeds a critical value then the convection will be confined to the low viscosity zone for a period which depends on the viscosity contrast and the Rayleigh number. The small scale convection will eventually decay into longer wavelength convection which extends throughout the upper mantle, so that the small scale convective signal will eventually be succeeded by a longer wavelength signal. We compare our model to the small scale geoid and topography anomalies observed in the Southeast Pacific. The magnitude (0.50-0.80 m in geoid and 250 m in topography), early onset time (5-10 m.y.) and lifetime (over 40 m.y.) of these anomalies suggest a large viscosity contrast of greater than two orders of magnitude. The trend to longer wavelengths also suggests a high Rayleigh number of near or over 10 and their original 150-250 km wavelength indicates a low viscosity zone of 75- 125 km thickness. We have found that the presence of such small scale convection does not disturb the slope of the depth-age curve but elevates it by up to 250 m, and it is not until the onset of long wavelength convection that the depth-age curves radically depart from a cooling halfspace model. In the Pacific, the depth-age curve is slightly elevated in the region where small scale convection is observed and it does not depart from a halfspace cooling model until an age of 70 m.y.. Models that produce the small scale anomalies predict a departure time between 55 and 65 m.y.
by Elizabeth M. Robinson.
Ph.D.
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Lherminier, Pascale. "Convection profonde en Mer du Groenland: Etude expérimentale des phases de préconditionnement et de mélange." Phd thesis, Université Pierre et Marie Curie - Paris VI, 1998. http://tel.archives-ouvertes.fr/tel-00881646.
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La convection profonde en Mer du Groenland, source importante des eaux profondes de l'Atlantique Nord, fait actuellement l'objet de recherches approfondies. Nous présentons ici les principaux résultats obtenus au cours des hivers de 1993 et 1994 à partir de mesures de flotteurs isobares dérivant entre 250 et 850 m de profondeur et d'un vaste ensemble de données hydrologiques, météorologiques et glaciologiques. L'hiver 1994, peu rigoureux, ne donne lieu qu'à une convection semi-profonde dans un bassin libre de glace, où un tourbillon anticyclonique d'environ 40 km de diamètre apparaît comme un site privilégié du mélange convectif à l'ouest du gyre du Groenland. Il semble provenir d'une interaction entre de l'Eau Atlantique Modifiée advectée en surface et une poche froide d'Eau Arctique Intermédiaire située en dessous et issue d'événements convectifs antérieurs pouvant remonter à l'hiver précédent. L'analyse de la physionomie et de la stabilité spatiale et temporelle de ce tourbillon n'a pas permis de relier son origine et son évolution à des scénarios de type cascade directe et cascade inverse habituellement invoqués dans les phénomènes de convection plus profonde. En 1994, l'évolution de la couche mélangée a été plutôt dominée par une dynamique turbulente tridimensionnelle très dépendante du forçage en surface et de la stratification du bassin, elle-même fortement influencée par la présence de structures de méso-échelle. Corrélée aux flux air-mer les plus intenses de l'hiver 1994, la phase de mélange active est identifiée par un changement de régime dans les amplitudes et la signature en fréquence des vitesses verticales de l'eau w mesurées par les flotteurs. A l'aide d'un modèle non hydrostatique à haute résolution simulant le comportement de flotteurs isobares, nous avons montré que, pendant la phase de mélange, l'apparition d'une composante basse fréquence observée sur les vitesses w est vraisemblablement associée à l'organisation de cellules turbulentes (panaches) engendrées par les fortes et soudaines pertes de flottabilité en surface.
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Aury, Claudia Indira. "Convection et dégazage d'un système magmatique : le cas du lac de lave l'Erebus, Antarctique." Phd thesis, Université d'Orléans, 2012. http://tel.archives-ouvertes.fr/tel-00772305.
Full textAbstract:
Le phénomène de dégazage permanent observé sur le volcan Erebus s'accompagne d'une variation cyclique de la composition des gaz et du niveau de son lac de cratère que nous nous sommes proposé de modéliser en partant de l'hypothèse que ces fluctuations sont causées par l'arrivée de batch de magma naissant à faible profondeur, ascendant dans un conduit à travers duquel percole un flux continu de gaz dont l'origine est plus profonde. Nous avons tout d'abord montré par simulation numérique que la vigueur de la convection observée en surface ne pouvait être expliquée par la seule convection thermique d'un mélange liquide-cristaux. Si une alimentation continue en magma et gaz dans un système ouvert permet de simuler un comportement pulsatif de la surface, cet apport doit être suffisamment important pour que les changements de vitesse de surface ainsi générés puissent être appréciés. Le conduit doit avoir un diamètre suffisant large pour assurer la pérennité de la convection et maintenir le magma au-dessus de sa température de transition vitreuse. La présence de cristaux accélère la convection et améliore l'efficacité du transfert de chaleur entre les régions inférieures et supérieures du système magmatique ; ces cristaux se déposent dans le fond de la chambre pour former une couche de forte concentration d'une dizaine de mètres d'épaisseur. L'introduction de deux batch de magma d'origines différentes a permis de mettre en évidence combien leur composition et le comportement du lac de lave étaient sensibles à la température à laquelle ces batch sont générés. D'autre part, un batch ne contribuera au budget de dégazage dans des proportions consistantes avec les observations que si un seuil de flottabilité suffisant a été atteint. Dans le cas contraire, la migration d'une quantité de magma nécessaire à ce dégazage ne pourrait se faire que dans un conduit de très grand diamètre. Les paramètres physiques de la croute qui se forme lors du refroidissement de la surface du lac de lave, tels que son épaisseur et sa perméabilité, influent sur sa capacité à se déformer sous la pression qu'elle piège et à permettre un dégazage de type effusif. Cette pression conditionne à son tour la porosité du magma en surface et éventuellement le degré de dégazage du magma qui redescend vers la chambre magmatique. Cette étude nous a permis de mieux appréhender les mécanismes associés à une éruption de régime effusif et constitue une étape dans la compréhension de la transition à un régime explosif, préoccupation majeure des centres de surveillance volcaniques.
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Wittmann, Philipp. "Measurement of the convective heat transfer with wind : Developing and testing an Earth Scaled Atmospheric Temperature Sensor." Thesis, Luleå tekniska universitet, Institutionen för system- och rymdteknik, 2016. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-59864.
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The HAbitability, Brine Irradiation and Temperature Package (HABIT) instrument of the ExoMars Surface Platform will investigate the present day habitability of Mars at the near surface environment. This instrument includes three Atmospheric Temperature Sensor's (ATS's) which are similar to the ones previously used on the Rover Environmental Monitoring Station (REMS) of the Mars Science Laboratory (MSL) rover, that has now been operating on Mars for more than four years. The ATS of REMS is only used to provide the air temperature, however on HABIT it will be used furthermore to provide information about winds and heat transfer at the surface of Mars. The retrieval method needs to be further investigated and validated. This master thesis is aimed at three goals: 1) the development and testing of an Earth Scaled Atmospheric Temperature Sensor (ESATS) to test the retrieval concept; 2) the validation with other Earth-based standard wind sensing technologies under outdoors uncontrolled conditions; and 3) the analysis of the existing observations of the ATS of REMS on Mars to get a better understanding of its expected future performance on HABIT once it operates on Mars. The ESATS is an up-scaled semi-autonomous prototype version of an ATS which consists of a rod of different size and material to those that are used on REMS and will be used on HABIT. The rod shall be heated from the base where it is attached to. The temperature profile shall be measured at three different measurement points. All these temperatures are different from the one of the atmosphere to which the rod is exposed to. The temperature profile along the rod changes depending on the air temperature, air density and the wind speed because of the convective heat transfer. A preliminar analysis is used to define what is the ideal length of the rod, and what is the material that is best adapted for this experimental prototype. Since the air density is needed to retrieve the wind speed, the pressure will be monitored as well. In parallel, a second wind measuring technique based on the dynamic pressure changes detected in a Pitot tube is used as control. The measuring campaign is subdivided in several stages: 1) The first part will take place in a laboratory, where the system is exposed to static conditions with no heating and no wind, which means that there is no forced convection caused by wind. In this setup all sensors are calibrated against one another and with help of a reference resistor the temperature sensors are also calibrated to 273.15K. Additionally different Operational Amplifiers (OpAmps) will be used to observe how the noise level is affecting the measurements, so that the best one will be used in the end. 2) Furthermore, the best position to place the intermediate temperature sensor is investigated by testing one of the rods with different locations of the middle temperature sensor. 3) Next, also within the laboratory environment, the different rods are used to obtain the temperature profile and retrieve the air temperature and heat transfer values, solving the equations that describe the heat transfer problem under static conditions. 4) The second part of the measurement campaign will take place outdoors, where the ESATS is exposed to forced convection due to wind. In this setting first the influence of the Sun on the system is measured, as it is important to know, if the measurement can be performed when the illumination conditions change. 5) Next, the system is tested with the 50cm rod in long term tests with the reference measurement of a commercial weather station (HOBO) next to it. With the data obtained the convective heat transfer method is used and the retrieved wind speed is compared to the one received from the HOBO. Finally, to get a better understanding of its expected future performance on HABIT once it operates on Mars, the data of the ATS of REMS is used to perform the wind speed retrieval for Mars and to compare it with the data received from the REMS wind sensor. It is only operating during daytime and has still difficulties to retrieve a precise wind speed. The measurement campaign has given several information about ATS in general. First it was decided to place the temperature sensor in the middle at 1/4 of the rod length, which is optimal for the retrieval process and which is also coincident with the one chosen for HABIT and REMS. The measurements in the laboratory are providing good and constant temperature profiles with the chosen setup which correspond with the one expected from the equations that describe the heat transfer problem in a long rod. On the other hand, it is not possible to calculate a valid ambient temperature for the short rods, which is because of an overheated boundary layer around the rods due to the heating. For this reason, it is recommended to use the longest rod in the lab. For outside testing the influence of the Sun could be confirmed and was affecting the measurements of the copper rod. During the time span where the prototype was in the Sun, it was not possible to get any reasonable results. The next measurement campaign was defined in a shadowed area with diffuse light only. Finally, the tests of exposure to dynamic changes over time are in excellent agreement with the ones provided by the HOBO station and can even give a better resolution and sensitivity to small changes of wind magnitude. This prototype has confirmed experimentally, that under Earth conditions, this method can be used to retrieve the wind speed. Finally, the Martian data of the REMS ATS are analyzed and the comparisons suggest that the method is sensitive to wind changes on Mars as well, and shows better time and magnitude resolution than the existing REMS wind sensor. This confirms that this method can be successfully used for the HABIT sensor. In this work the methodology that shall be used on HABIT to retrieve the convective heat transfer and wind on the surface of Mars is validated for Earth outdoors conditions. It is recommended to verify the obtained results with tests in a wind tunnel and to see how the system will behave with a higher heating and different rod materials. Furthermore, the setup should be tested in a way which makes it possible to determine the different directions of the wind.
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Chung, Kao-Shen. "Use of single Doppler radar observations in data assimilation at convective scale with model as a weak constraint." Thesis, McGill University, 2010. http://digitool.Library.McGill.CA:8881/R/?func=dbin-jump-full&object_id=92252.
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LE, BARS MICHAEL. "Convection thermique dans un fluide visqueux hétérogène : phénoménologie, lois d'échelle et applications aux systèmes terrestres." Phd thesis, Institut de physique du globe de paris - IPGP, 2003. http://tel.archives-ouvertes.fr/tel-00002530.
Full textAbstract:
Le manteau terrestre est hétérogène, mais les caractéristiques de ses réservoirs sont inconnues. Nous avons donc étudié un système simple, dans lequel deux fluides miscibles de densités et de viscosités différentes sont soumis à un contraste thermique déstabilisant. À faible nombre de flottabilité B (rapport entre la stratification chimique et l'anomalie thermique de densité), la convection se développe sur l'intégralité du système, tandis qu'à B plus fort, les fluides convectent séparément. Dans les deux cas, le mélange prend place progressivement. Plusieurs comportements transitoires sont cependant possibles, parmi lesquels le régime pulsatif où de grands dômes oscillent sur toute l'épaisseur du système. Sur Terre, notre modèle analogique suggère une évolution depuis un régime à deux couches vers un régime à une couche. Une dynamique pulsative serait susceptible de fournir une explication simple aux grandes pulsations géologiques enregistrées sur les planètes terrestres.
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Molina, Indira. "Convection et dégazage d'un système magmatique : le cas du lac de lave de l'Erebus, Antarctique." Phd thesis, Université d'Orléans, 2012. http://tel.archives-ouvertes.fr/tel-00767068.
Full textAbstract:
Le phénomène de dégazage permanent observé sur le volcan Erebus s'accompagne d'une variation cyclique de la composition des gaz et du niveau de son lac de cratère que nous nous sommes proposé de modéliser en partant de l'hypothèse que ces fluctuations sont causées par l'arrivée de batch de magma naissant à faible profondeur, ascendant dans un conduit à travers duquel percole un flux continu de gaz dont l'origine est plus profonde. Nous avons tout d'abord montré par simulation numérique que la vigueur de la convection observée en surface ne pouvait être expliquée par la seule convection thermique d'un mélange liquide-cristaux. Si une alimentation continue en magma et gaz dans un système ouvert permet de simuler un comportement pulsatif de la surface, cet apport doit être suffisamment important pour que les changements de vitesse de surface ainsi générés puissent être appréciés. Le conduit doit avoir un diamètre suffisant large pour assurer la pérennité de la convection et maintenir le magma au-dessus de sa température de transition vitreuse. La présence de cristaux accélère la convection et améliore l'efficacité du transfert de chaleur entre les régions inférieures et supérieures du système magmatique ; ces cristaux se déposent dans le fond de la chambre pour former une couche de forte concentration d'une dizaine de mètres d'épaisseur. L'introduction de deux batch de magma d'origines différentes a permis de mettre en évidence combien leur composition et le comportement du lac de lave étaient sensibles à la température à laquelle ces batch sont générés. D'autre part, un batch ne contribuera au budget de dégazage dans des proportions consistantes avec les observations que si un seuil de flottabilité suffisant a été atteint. Dans le cas contraire, la migration d'une quantité de magma nécessaire à ce dégazage ne pourrait se faire que dans un conduit de très grand diamètre. Les paramètres physiques de la croute qui se forme lors du refroidissement de la surface du lac de lave, tels que son épaisseur et sa perméabilité, influent sur sa capacité à se déformer sous la pression qu'elle piège et à permettre un dégazage de type effusif. Cette pression conditionne à son tour la porosité du magma en surface et éventuellement le degré de dégazage du magma qui redescend vers la chambre magmatique. Cette étude nous a permis de mieux appréhender les mécanismes associés à une éruption de régime effusif et constitue une étape dans la compréhension de la transition à un régime explosif, préoccupation majeure des centres de surveillance volcaniques.
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Kaspi, Yohai. "Turbulent convection in the anelastic rotating sphere : a model for the circulation on the giant planets." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/45780.
Full textAbstract:
Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2008.Includes bibliographical references (p. 207-221).
This thesis studies the dynamics of a rotating compressible gas sphere, driven by internal convection, as a model for the dynamics on the giant planets. We develop a new general circulation model for the Jovian atmosphere, based on the MITgcm dynamical core augmenting the nonhydrostatic model. The grid extends deep into the planet's interior allowing the model to compute the dynamics of a whole sphere of gas rather than a spherical shell (including the strong variations in gravity and the equation of state). Different from most previous 3D convection models, this model is anelastic rather than Boussinesq and thereby incorporates the full density variation of the planet. We show that the density gradients caused by convection drive the system away from an isentropic and therefore barotropic state as previously assumed, leading to significant baroclinic shear. This shear is concentrated mainly in the upper levels and associated with baroclinic compressibility effects. The interior flow organizes in large cyclonically rotating columnar eddies parallel to the rotation axis, which drive upgradient angular momentum eddy fluxes, generating the observed equatorial superrotation. Heat fluxes align with the axis of rotation, contributing to the observed flat meridional emission. We show the transition from weak convection cases with symmetric spiraling columnar modes similar to those found in previous analytic linear theory, to more turbulent cases which exhibit similar, though less regular and solely cyclonic, convection columns which manifest on the surface in the form of waves embedded within the superrotation. We develop a mechanical understanding of this system and scaling laws by studying simpler configurations and the dependence on physical properties such as the rotation period, bottom boundary location and forcing structure. These columnar cyclonic structures propagate eastward, driven by dynamics similar to that of a Rossby wave except that the restoring planetary vorticity gradient is in the opposite direction, due to the spherical geometry in the interior.
(cont.) We further study these interior dynamics using a simplified barotropic annulus model, which shows that the planetary vorticity radial variation causes the eddy angular momentum flux divergence, which drives the superrotating equatorial flow. In addition we study the interaction of the interior dynamics with a stable exterior weather layer, using a quasigeostrophic two layer channel model on a beta plane, where the columnar interior is therefore represented by a negative beta effect. We find that baroclinic instability of even a weak shear can drive strong, stable multiple zonal jets. For this model we find an analytic nonlinear solution, truncated to one growing mode, that exhibits a multiple jet meridional structure, driven by the nonlinear interaction between the eddies. Finally, given the density field from our 3D convection model we derive the high order gravitational spectra of Jupiter, which is a measurable quantity for the upcoming JUNO mission to Jupiter.
by Yohai Kaspi.
Ph.D.