Les nuages jouent un rôle important dans le cycle de l'eau et de l'énergie au sein de l'atmosphère. De plus, ils représentent l'une des principales sources d'incertitudes dans les projections des modèles de climat en raison notamment de la difficulté à paramétrer les processus qui leurs sont associés ainsi que leurs interactions avec l'environnement. Dans cette thèse nous étudions les nuages de mi-niveau qui ont été beaucoup moins étudiés que les nuages bas et les nuages hauts, en se focalisant sur l’Afrique de l'Ouest. L'Afrique de l'Ouest se caractérise par une forte saisonnalité des précipitations survenant au Sahel de juin à septembre durant la période dite de mousson. Cette période coïncide également avec le maximum annuel de la couverture nuageuse. Au travers du déploiement de la station mobile ARM (Atmospheric Radiation Measurement) durant une année en 2006 à Niamey (Niger), Bouniol et al. (2012) ont documenté les différents types de nuages observés pendant la mousson et ont montré la présence récurrente de nuages vers 6 km d'altitude dont l’impact radiatif est important dans les domaines du visible et de l'infrarouge. Dans le prolongement de cette étude, l'objectif de cette thèse est donc d'analyser plus en détails ces nuages de mi-niveau en documentant leur occurrence, leur cycle diurne et leurs caractéristiques macro- et microphysiques. Nous analysons également l'environnement thermodynamique dans lequel ces nuages sont observés ainsi que leurs effets radiatifs. D’autre part, en analysant les simulations effectuées dans le cadre du projet CMIP5, Roehrig et al. (2013) ont montré une sous-estimation de ce type de nuages dans les modèles de climat. Nous documentons plus précisément ici comment les modèles de climat et les modèles à aire limitée simulent ces nuages de mi-niveau. Afin de documenter ces nuages, des observations obtenues à partir d'instruments de télédétection active déployés sur deux sites sols : Niamey au Sahel et Bordj Badji Mokhtar au Sahara ont été combinées avec les données satellites de CloudSat et CALIPSO. Ces observations ont révélé une occurrence de ces nuages tout au long de l'année avec une prédominance durant la période de mousson. Ces nuages sont majoritairement observés dans le Sud et l'Ouest de l'Afrique de l'Ouest mais s’étendent jusqu’au cœur du Sahara. Leur présence dans cette zone désertique pourrait s’expliquer par la dynamique de la dépression thermique saharienne (Saharan Heat Low). Ces nuages sont généralement fins (la plupart ont une épaisseur inférieure à 1000 m) et sont principalement composés d'eau liquide. Une méthode de clustering appliquée à ces données nous a permis d'identifier trois types de nuages : le premier avec des bases plus basses, le deuxième avec des bases plus hautes et le dernier avec de plus fortes épaisseurs. Les radiosondages et les mesures de rayonnement nous ont permis de déterminer la stratification thermodynamique dans laquelle ces nuages sont observés ainsi que d’estimer leur impact radiatif. On observe généralement des inversions de température potentielle au sommet des nuages des deux premières familles. Dans les modèles de climat, nous avons mis en évidence une forte dispersion des occurrences des nuages de mi-niveau en termes de fréquence, de position et d’extension sur la verticale et de cycle saisonnier. L'analyse des simulations régionales indique aussi une influence de la résolution spatiale et de la paramétrisation de la convection sur la modélisation des nuages de mi-niveau simulés au Sahel et sur le Sahara. / Clouds have an important impact on the water and energy fluxes within the atmosphere. They also represent one of the main sources of uncertainties of climate models projections as a consequence of the difficulty to parametrize their associated processes as well as their interactions with their environment. In this thesis mid-level clouds are studied. Such clouds have been much less studied than low clouds and high clouds and the focus is on the West Africa. West Africa is characterized by a strong seasonality in precipitation that occur in the Sahel from June to September named the monsoon season. This period also coincides with the annual maximum of the cloud cover. Taking advantage of the one-year ARM Mobile Facility (AMF) deployment in 2006 in Niamey (Niger), Bouniol et al. (2012) documented the distinct cloud types observed during the monsoon and showed a frequent occurrence of clouds around 6 km height with an important radiative impact in the surface short-wave and long-wave domains. In the continuity of this study, the aim of this thesis is therefore to analyse in more details these mid-level clouds by documenting their occurrence, their diurnal cycle as well as their macro- and microphysical characteristics. We also analyse the thermodynamical environment in which these clouds are observed and their radiative effects. In a process-oriented evaluation of CMIP5 climate models, Roehrig et al. (2013) showed an underestimation of mid-level clouds. We document more precisely here how climate models and limited-area models simulate these mid-level clouds. To document those clouds, we combine observational data from active remote sensing instruments deployed at the two groundbased sites : Niamey in the Sahel and Bordj Badji Mokhtar in the Sahara, with merged CloudSatCALIPSO satellite. These observations reveal an occurrence of those clouds throughout the year with a predominance around the monsoon season. These clouds are preferentially observed in the Southern and Western part of West Africa but extend to the heart of the Sahara. Their presence in this desert zone may be explained by the dynamics of the Saharan Heat Low. Those clouds are usually quite thin (most of them are less than 1000 m deep) and mainly composed of liquid water. A clustering method applied to this data allows us to identify three different types of clouds : one with lower bases, one with higher bases and another with larger thicknesses. Radiosondes and radiation measurements allowed us to determine the thermodynamical stratification in which these clouds are observed as well as to estimate their radiative impact. Potential temperature inversions are generally observed at the top of the clouds of the first two families. In the climate models, we showed a strong dispersion of the occurrences of mid-level clouds in terms of frequency, location and vertical extension and seasonal cycle. Analysis of regional simulations also indicates an influence of spatial resolution and of the convection parametrization on the model ability in simulating mid-level clouds in the Sahel and in the Sahara.

No description available.

The three dimensional Monte Carlo radiation transport model McArtim is extended to account for the simulation of the propagation of polarized radiation and the inelastic rotational Raman scattering which is the cause of the so called Ring effect. From the achieved and now sufficient precision of the calculated Ring effect new opportunities in optical absorption spectroscopy arise. In the calculation the method of importance sampling (IS) is applied. Thereby one obtains from an ensemble of Monte Carlo photon trajectories an intensity accounting for the elastic aerosol particle-, Cabannes- and the inelastic rotational Raman scattering (RRS) and simultaneously an intensity, for which Rayleigh scattering is treated as an elastic scattering process. By combining both intensities one obtains the so called filling-in (FI, which quantifies the filling-in of Fraunhofer lines) as a measure for the strength of the Ring effect with the same relative precision as the intensities. The validation of the polarized radiometric quantities and the Ring effect is made by comparison with partially published results of other radiation transport models. Furthermore the concept of discretisation of the optical domain into grid cells is extended by making grid cells arbitrarily joining into so called clusters, i.e. grid cell aggregates. Therewith the program is able to calculate derivatives of radiometrically or spectroscopically accessible quantities, namely the intensities at certain locations in the atmospheric radiation field and the light path integrals of trace gas concentrations associated thereto, i.e. the product of the DOAS (differential optical absorption spectroscopy) method, with respect to optical properties of aerosols and gases in connected spatial regions. The first and second order derivatives are validated through so called self-consistency tests. These derivatives allow the inversion of three dimensional tracegas and aerosol concentration profiles and pave the way down to 3D optical scattered light tomography. If such tomographic inversion scheme is based solely on spectral intensitites the available second order derivatives allows the consideration of the curvature in the cost function and therefore allows implementation of efficient optimisation algorithms. The influence of the instrument function on the spectra is analysed in order to mathematically assess the potential of DOAS to a sufficient degree. It turns out that the detailed knowledge of the instrument function is required for an advanced spectral analysis. Concludingly the mathematical separability of narrow band signatures of absorption and the Ring effect from the relatively broad band influence of the elastic scattering processes on the spectra is demonstrated which corresponds exactly to the DOAS principle. In that procedure the differential signal is obtained by approximately 4 orders of magnitude faster then by the separate modelling with and without narrow band structures. Thereby the fusion of the separated steps DOAS spectral analysis and subsequent radiation transport modeling becomes computationally feasible.:1.1. Radiation Transport Modeling and Atmospheric State Inversion 1.2. Vector RTE Solution Methods 1.3. Scope of the Thesis 1.4. Outline of the Thesis 2.1. General Structure 2.1.1. Chemical Composition of the Gas Phase 2.1.2. The Troposphere, Temperature and Pressure Vertical Structure 2.1.3. The Stratosphere 2.2. Aerosols and Clouds 2.2.1. Classification and Morphology 2.2.2. Water Related Particle Growth and Shrinking Processes 2.2.3. Size Spectra and Modes 3.1. Electromagnetic Waves 3.1.1. Maxwell\''s Equations 3.1.2. Measurement of Electromagnetic Waves 3.1.3. Polarization State of EM Waves 3.1.4. Stokes Vectors 3.2. Scattering and Absorption of EM Waves by Molecules and Particles 3.2.1. General Description of Scattering and Coordinate Systems 3.2.2. Molecular Scattering 3.2.3. Molecular Absorption Processes and Electronic Molecular States 3.2.4. Scattering On Spherical Particles - Mie Theory 3.3. Mathematical Description of Radiation Transport 3.3.1. Radiance and Irradiance 3.3.2. Absorption, Scattering and Extinction Coefficients 3.3.3. Optical Thickness and Transmission 3.3.4. Scattering 3.3.5. Incident (Ir)Radiance 3.3.6. The Black Surface Single Scattering Approximation 3.3.7. Radiative Transfer Equations 4.1. General Monte Carlo Methods 4.1.1. Numerical Integration 4.1.2. Importance Sampling and Zero Variance Estimates 4.1.3. Optimal Sampling 4.1.4. Sampling from Arbitrary Distributions 4.2. Path Generation or Collision Density Estimation 4.2.1. Discretization of the Optical Domain into Cells and Clusters 4.2.2. RTE Integral Form 4.2.3. Formal Solution of the IRTE 4.2.4. Overview on Monte Carlo RTE Solution Algorithms 4.2.5. Crude Monte Carlo 4.2.6. Sequential Importance Sampling (SIS) or Path Generation 4.3. Importance Sampling in Monte Carlo SIS Radiative Transfer 4.3.1. Weights for Alternate Kernels 4.3.2. Weights in the Calculation of RTE Functional Estimates 4.3.3. Application of IS to Mie Phase Functions Scatter Angle Sampling 5.1. Radiances, Intensities and the Reciprocity Theorem 5.1.1. Scalar Radiance Estimates 5.1.2. Backward Monte Carlo Scalar Radiance 5.1.3. Vector Radiances 5.2. Radiance Derivatives 5.2.1. Variables for Radiance Derivatives 5.3. Validation of Functionals 5.3.1. Validation of Vector Radiances 5.3.2. Validation of Radiance Derivatives 6.1. A Simply Structured Instrument Forward Model 6.2. Pure Atmospheric Spectra and Absorption 6.2.1. Direct Light Spectra 6.2.2. Scattered Sun Light Spectra 6.3. (D)OAS from the Perspective of Radiative Transfer Modeling 6.3.1. (Rest) Signatures of Weakly Absorbing Gases 6.3.2. Spectroscopic Measurements and Standard DOAS 6.4. DOAS Analysis Summary 6.4.1. DSCD Retrieval 6.4.2. Inversion 7.1. RRS-Modified RTE 7.1.1. RRS Cross Sections for Scattering out and into a Wavelength 7.1.2. Modification of the RTE Loss and Source Terms 7.2. Intensity Estimates Considering Rotational Raman Scattering 7.2.1. RRS in the Path Sampling Procedure 7.2.2. Adjoint RRS Correction Weights 7.2.3. Local Estimates of Intensities with RRS 7.2.4. Intensity Estimates 7.3. Ring Spectra 7.3.1. Elastic Biasing of the Local Estimates 7.3.2. Cumulative Weights and Local Estimates 7.3.3. Test of the Elastic Biasing 7.4. Validation 7.4.1. Comparison to an Analytic Single Scattering Code 7.4.2. Single Scattering Model Including Rotational Raman Scattering 7.4.3. Multiple Scattering Model Comparison 7.4.4. Comparison with A Measurement 7.4.5. Validation of Approximate Methods For Ring Effect Modeling 7.5. Summary and Discussion 8.1. Status and Summary 8.1.1. Ring-Effect and Absorption Corrected Radiances 8.1.2. Derivatives of Radiometric Quantities Accessible Through Spectroscopy 8.1.3. Polarization 8.1.4. Time Integrated Sensitivities for 3D UV/vis/NIR Remote Sensing 8.2. Outlook A.1. Zero Variance Estimates A.2. Free Path Length Sampling in a Homogeneous Medium A.3. Cumulative Differential Scatter Cross Sections A.3.1. Cardanic formulas A.3.2. Rayleigh and Raman Phase Functions A.3.3. Henyey-Greenstein Model A.3.4. Legendre Polynomial Phase Function Model A.3.5. Table Methods A.4. Greens Function in the Derivation of the IRTE A.5. Source Code For Stokes Vector Transformation Plot B.1. 1st Order Derivatives B.2. 2nd Order Derivatives B.3. Hessian of Integrals Depending on Many Variables C.1. Slit Function f Derivatives C.2. Signal Sn Derivatives C.3. Chi Square Spline Fitting C.3.1. Constrained Non-Linear Least Square Problem C.3.2. Spline Fitting C.3.3. Jacobians and Hessian

No description available.

Depuis les années quarante, la consommation énergétique mondiale n'a cessé d'augmenter. Cette énergie étant majoritairement d'origine fossile, il en résulte une augmentation globale de température terrestre. De ce fait, il est devenu urgent de réduire les émissions de gaz à effet de serre pour stopper le changement climatique. Dans ce contexte, le développement de la production d'électricité à partir d'énergie solaire concentrée par voie thermodynamique est une solution prometteuse. Les efforts de recherche visent à rendre cette technologie plus efficace et plus compétitive économiquement. Dans ce but, ce manuscrit présente une méthode de conception optimale pour les centrales solaires à récepteur central. Elle tire parti des méthodes développées depuis de nombreuses années par le groupe de recherche StaRWest, regroupant notamment des chercheurs des laboratoires RAPSODEE (Albi), LAPLACE (Toulouse) et PROMES (Odeillo). Couplant des algorithmes de Monte Carlo à hautes performances et des algorithmes stochastiques d'optimisation, le code de calcul implémentant cette méthode permet la conception et l'optimisation d'installations solaires. Il est utilisé pour mettre en évidence les potentialités d'un type de centrales à récepteur central peu répandu : les centrales à réflecteur secondaire, également appelées centrales de type "beam down". / Since the early 40's, world energy consumption has grown steadly. While this energy mainly came from fossil fuel, its use has included an increase in temperatures. It has become urgent to reduce greenhouse gas emissions to halt climate change. In this context, the development of concentrated solar power (CSP) is a promising solution. The scientific community related to this topic has to focus on efficiency enhancement and economic competitiveness of CSP technologies. To this end, this thesis aims at providing an optimal design method applied to central receiver power plants. It takes advantage of methods developed over many years by the research group StaRWest. Both RAPSODEE (Albi), LAPLACE (Toulouse) and PROMES (Odeillo) researchers take an active part in this group. Coupling high performance Monte Carlo algorithms and stochastic optimization methods, the code we developed allows an optimal design of concentrated solar systems. This code is used to highlight the potential of an uncommon type of central receiver plants: reflective towers, also called "beam down" central receiver systems.

L’étude du transfert radiatif dans les gaz (atmosphères planétaires, chambres de combustion, etc.) se heurte à deux principales difficultés : les hétérogénéités et la dépendance spectrale des propriétés radiatives du milieu d’intérêt. Les travaux présentés dans ce manuscrit proposent, au travers d’une approche statistique du rayonnement, une solution à ces deux limites qui ne nécessite aucune approximation de modèle physique ou numérique. Cette approche conduira au développement d’algorithmes de Monte-Carlo considérés à l’heure actuelle comme méthodes de référence dans la communauté du transfert radiatif. La difficulté liée aux hétérogénéités du milieu participant sera traitée par une technique empruntée à d’autres disciplines de la physique du transport : les algorithmes à collisions nulles. Leur application au rayonnement consiste à ajouter arbitrairement aux évènements d’absorption et de diffusion, un troisième type de collision n’ayant aucun effet sur le transport de photons : les collisions nulles. Ainsi, le coefficient d’extinction résultant de ces trois types de collision pourra être assumé comme homogène. Ensuite, il sera montré comment cette même technique lève un second verrou permettant de repenser de façon statistique l’idée de coefficient d’absorption. Cela ouvrira la voie à des algorithmes de Monte-Carlo qui estiment directement une observable radiative à partir de paramètres de transitions répertoriés dans des bases de données spectroscopiques, sans avoir à précalculer rigoureusement le coefficient d’absorption. / Two major challenges are encountered when studying radiative transfer in gases (e.g. combustion chambers or planetary atmospheres): heterogeneity and spectral dependence of radiative properties. The work introduced in this manuscript, addresses this problem through a statistical approach of radiation that requires no model or numerical approximation. This approach leads to the development of Monte-Carlo methods, currently considered as reference solutions in the community of radiative transfer. The difficulty related to heterogeneity is handled by a technique borrowed from other fields of transport physics: null-collision algorithms. Their application to radiation consists in adding to the events of absorption and scattering a third arbitrary type of collision that has no effect on the photon transport. Thus, the extinction coefficient resulting from these three types of collisions can be assumed to be homogeneous. Then, it is shown how this very same technique opens the door to rethinking statistically the concept of absorption coefficient. This leads to Monte-Carlo algorithms that directly estimate radiative observables from transition parameters indexed in molecular spectroscopic databases, without the need of rigorously precomputing absorption coefficients.

Diese Arbeit betrachtet die Wechselwirkung von solarer Einstrahlung mit Wolken in der Atmosphäre. Diese wird insbesondere repräsentiert durch den Wolkenstrahlungseffekt. Hierbei wurde vor allem auf die Auswirkungen von kleinskaliger Variabilität von Wolken und Wolkenfeldern auf die Genauigkeit des Wolkenstrahlungseffektes am Oberrand der Atmosphäre und am Boden Rücksicht genommen. Mit einer Schliessungsstudie ist der modellierte Wolkenstrahlungseffekt mit Schiffsmessungen verglichen worden. Hierbei wurden die Wolkeneigenschaften in dem Modell durch Schiffs- und Satellitendaten als Eingangsdatensatz beschrieben. Ein Zugewinn in der Genauigkeit konnte durch die kombinierte Nutzung beider Datenquellen erzielt werden, konkret durch die Kombination des Flüssigwasserpfads aus Schiffsmessungen und des effektiven Radius aus Satellitenbeobachtungen. Durch die Schliessungsstudie sind zwei Probleme in der Auflösung kleinskaliger Bewölkung und deren Auswirkung auf abgeleitete Wolkeneigenschaften identifiziert worden, die im weiteren Verlauf der Arbeit genauer betrachtet wurden. Ein Vergleich zweier Methoden zur Erkennung des Bedeckungsgrades, jeweils eine vom Boden und eine vom Oberrand der Atmosphäre, hat insgesamt eine gute Übereinstimmung ergeben. Jedoch zeigten sich Abweichungen bei geringer Bedeckung. So wurde bei einem Bedeckungsgrad von ca. 40% in der Hälfte der Fälle den Satellitenbildpunkt als bewölkt klassifiziert. Diese Unsicherheiten in der Klassifikation konnten auf die abgeleitete reflektierte solare Einstrahlung übertragen werden. Für als unbewölkt erkannte, tatsächlich aber bewölkte, Bildpunkte wurde eine mittlere Überschätzung der reflektierte solare Einstrahlung von ca. 30 W/m−2 gefunden. Ebenfalls wurde der Einfluss der zeitlichen Variabilität in der solaren Einstrahlung auf die Bestimmung des Wolkenstrahlungseffektes einer Wolke untersucht. Hierfür wurde ein lineares Modell entwickelt und präsentiert, das die diffuse Einstrahlung mit dem Bedeckungsgrad in Zusammenhang stellt. Das Modell liefert zwei Koeffizienten, die die Variation der diffusen Einstrahlung durch eine Wolke unter der Annahme, dass die beobachtete Wolke den ganzen Himmel bedeckt, beschreiben. Dies ermöglicht einen direkten Vergleich des Wolkenstrahlungseffektes einer beobachteten Wolke mit Modellergebnissen und die Entkopplung von der zeitlich variablen direkten Einstrahlung.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012. / 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 (p. 221-236). / This thesis focuses on ocean circulation and atmospheric forcing in the Atlantic Ocean at the Last Glacial Maximum (LGM, 18-21 thousand years before present). Relative to the pre-industrial climate, LGM atmospheric CO₂ concentrations were about 90 ppm lower, ice sheets were much more extensive, and many regions experienced significantly colder temperatures. In this thesis a novel approach to dynamical reconstruction is applied to make estimates of LGM Atlantic Ocean state that are consistent with these proxy records and with known ocean dynamics. Ocean dynamics are described with the MIT General Circulation Model in an Atlantic configuration extending from 35°S to 75°N at 1° resolution. Six LGM proxy types are used to constrain the model: four compilations of near sea surface temperatures from the MARGO project, as well as benthic isotope records of [delta]¹⁸O and [delta]¹³C compiled by Marchal and Curry; 629 individual proxy records are used. To improve the fit of the model to the data, a least-squares fit is computed using an algorithm based on the model adjoint (the Lagrange multiplier methodology). The adjoint is used to compute improvements to uncertain initial and boundary conditions (the control variables). As compared to previous model-data syntheses of LGM ocean state, this thesis uses a significantly more realistic model of oceanic physics, and is the first to incorporate such a large number and diversity of proxy records. A major finding is that it is possible to find an ocean state that is consistent with all six LGM proxy compilations and with known ocean dynamics, given reasonable uncertainty estimates. Only relatively modest shifts from modern atmospheric forcing are required to fit the LGM data. The estimates presented herein succesfully reproduce regional shifts in conditions at the LGM that have been inferred from proxy records, but which have not been captured in the best available LGM coupled model simulations. In addition, LGM benthic [delta]¹⁸O and [delta]¹³C records are shown to be consistent with a shallow but robust Atlantic meridional overturning cell, although other circulations cannot be excluded. / by Holly Janine Dail. / Ph.D.

The present thesis is concerned with methods for determination and modeling of characteristic parameters of combustion of gaseous fuels. The focus is stressed on formation of nitrogen oxides and heat transfer from hot flue gases into combustion chamber’s walls. Experimental work, which is focused on testing of two burners with suppressed formation of nitrogen oxides, is an important part of the thesis. Its aim is to obtain data that is necessary for further processing and modeling. The work presents two methods that may be used in modeling of characteristic combustion parameters, namely the method based on statistical processing of data and the method based on computational fluid dynamics. The approaches are applied to two devices (burner with two-staged fuel supply, burner with two-staged air supply) with the objective to analyze their parameters. First approach covers detailed planning of burner test prior to its own carrying out (definition of the goal of experiment, choice of input factors and response, experimental plan) and subsequent statistical processing of experimental data. On the contrary, CFD approach offers simulations as an alternative option to traditional experimental methods. The simulation of combustion includes building of computational grid, setup of boundary conditions, turbulence model, heat transfer model and chemical kinetics. Results of simulations are compared with experimental measured data.

No description available.