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Cette thèse vise à montrer rapport d'un lidar multispectral, en particulier en ajoutant des longueurs d'onde dans le proche infrarouge proche, pour la caractérisation des aérosols. En effet par rapport à un lidar mono-longueur d'onde, l'information contenue dans les profils multispectraux permet de remonter aux propriétés microphysiques des aérosols (distribution en aille et composition). Pour cela un simulateur de signaux lidar multispectraux a été adapté à notre étude afin de pouvoir développer et tester deux méthodes permettant de retrouver les propriétés microphysiques des aérosols le long de la ligne e visée à partir de signaux lidar synthétiques. La première méthode, basée sur l'inversion des signaux lidar, permet de retrouver la répartition en taille des aérosols et donc d'en déduire notamment leur concentration et leur rayon modal. Cette méthode nécessite des informations a priori sur les aérosols. Un bilan d'erreur a été réalisé en introduisant des incertitudes sur ces paramètres a priori et montre que les résultats obtenus sur la concentration et le rayon modal sont précis (respectivement 16% et 17% d'erreur). Cette méthode présente l'avantage de ne pas nécessiter d'étalonnage absolu de l'instrument. La deuxième méthode est basée sur la minimisation de l'écart entre des signaux simulés et les signaux que l'on étudie. Même si la précision obtenue sur la répartition en taille retrouvée est plus faible (35% et 40 % d'erreur sur la concentration t le rayon modal) et que la constante d'étalonnage de l'instrument doit être connue, cette méthode a l'avantage de retrouver la composition des aérosols dans 74 % des cas. / The purpose of this thesis is to show the contribution of a multispectral Iidar for the characterisation of aerosols, in particular hen wavelengths in near infrared are added. Indeed, compared with a mono-wavelength Iidar, the information contained in multispectral profiles allow to retrieve the microphysical properties of aerosols (particule size distribution and composition). To this end, we adapted a multispectral Iidar signal simulator to our study in order to develop and test two methods which objective is to obtain the microphysical properties of aerosol along the line-of-sight from synthetic lidar signals. The first method, based on the inversion of lidar signals, enables to find the length distribution of aerosols and therefore to educe their concentration and their modal radius. This method requires a priori information about the aerosols. An error budget was made by introducing uncertainties on the a priori parameters. It shows that the results obtained regarding the concentration and modal radius are accurate (respectively 16% and 17% uncertainty). The advantage of this method is that it does not require absolute calibration of the instrument. The principle of the second method is to minimize the difference between the studied and the simulated signals. Even if the accuracy on the size distribution is lower (35% and 40% on the concentration and modal radius) and the calibration constant of the instrument has to be known, this method has the advantage to find the concentration of the aerosols in 74% of the cases. Finally, the first method was validated on real data, coming from a collaboration with the RSLab (Barcelona), by comparing ur results with those obtained by this team (7% difference on the modal radius).

This dissertation focuses on the global investigation of optically thin cirrus cloud optical thickness (tau) and microphysical properties, such as, effective particle size (D_(eff)) and ice crystal habits (shapes), based on the global satellite observations and fast radiative transfer models (RTMs). In the first part, we develop two computationally efficient RTMs simulating satellite observations under cloudy-sky conditions in the visible/shortwave infrared (VIS/SWIR) and thermal inferred (IR) spectral regions, respectively. To mitigate the computational burden associated with absorption, thermal emission and multiple scattering, we generate pre-computed lookup tables (LUTs) using two rigorous models, i.e., the line-by-line radiative transfer model (LBLRTM) and the discrete ordinates radiative transfer model (DISORT). The second part introduces two methods (i.e., VIS/SWIR- and IR-based methods) to retrieve tau and D_(eff) from satellite observations in corresponding spectral regions of the two RTMs. We discuss the advantages and weakness of the two methods by estimating the impacts from different error sources on the retrievals through sensitivity studies. Finally, we develop a new method to infer the scattering phase functions of optically thin cirrus clouds in a water vapor absorption channel (1.38-µm). We estimate the ice crystal habits and surface structures by comparing the inferred scattering phase functions and numerically simulated phase functions calculated using idealized habits.

Este trabalho, sendo pioneiro no Brasil, teve o intuito de efetuar uma investigação das nuvens cirrus na região Metropolitana de São Paulo (23,33ºS / 46,44ºW), SP, através do sistema MSP-Lidar para o período de Junho à Julho de 2007. Durante este período, foi verificada uma ocorrência de cirrus de aproximadamente 54% sobre o total de medidas efetuadas pelo sistema Lidar. Medidas com Lidar nos forneceram uma alta resolução espacial e temporal destas nuvens, permitindo assim caracterizá-las e classificá-las de acordo com as suas propriedades macro- e microfísicas. Para obter tais parâmetros, uma metodologia própria foi desenvolvida na recuperação dos dados de Lidar e uma robusta estatística foi aplicada para determinar as diferentes classes de cirrus. A metodologia adotada se resumiu basicamente (a) na determinação de períodos estacionários (ou observações) durante a evolução temporal de detecção de cirrus, (b) determinação da base e topo através de um valor limiar para o cálculo das variáveis macrofísicas (altitudes, temperaturas, espessuras geométricas), (c) aplicação do método da transmitância para cada camada de nuvem e a determinação das variáveis microfísicas (profundidade óptica e razão de Lidar). Neste processo, a razão de Lidar é calculada iterativamente até que haja a convergência da mesma. Análises estatísticas de multivariáveis foram efetuadas para a determinação das classes de cirrus. Estas classes são baseadas na espessura geométrica, altitude média e sua respectiva temperatura, a altitude relativa (diferença entre a altura da tropopausa e topo da nuvem) e a profundidade óptica. O uso sucessivo da Análise de Componentes Principais (PCA), do Método de Cluster Hierárquico (MCH) e da Análise de Discriminantes (AD) permitiu a identificação de 4 classes. Vale ressaltar que tais métodos foram aplicados somente para os casos identificados como camadas únicas de nuvens, pois não se observou significativamente a ocorrência de nuvens com multicamadas. A origem de formação das classes de cirrus encontradas, embora apresentando propriedades macro- e microfísicas distintas, foi identificada basicamente como a mesma, isto é, provenientes da injeção de vapor dágua na atmosfera por meio de sistemas frontais e seu respectivo resfriamento para a formação dos cristais de gelo. O mesmo mecanismo de formação também é atribuído aos jatos subtropicais. Uma análise em relação ao perfil de temperatura e a comparação com a literatura mostrou que as cirrus classificadas apresentam possivelmente cristais em forma de placas e colunas hexagonais. As razões de lidar (RL) calculadas também estão de acordo com a literatura. / This pioneer work in Brazil, aimed at investigating cirrus clouds in the metropolitan region of São Paulo (23.33 ºS / 46.44 ºW), SP, observed by the MSP-Lidar system in June and July 2007. During this period, cirrus clouds were observed during approximately 54% of the time of all Lidar measurements available. The Lidar provided measurements with high spatial and temporal resolution measurements of these clouds that allowed characterizing and classifying them according to their macro-and microphysical properties. For such parameters, a unique methodology was developed for the Lidar data retrieval and a robust statistic was applied to determine the different classes of cirrus. The following steps were adopted to characterize the observations: (a) the determination of stationary periods (or observations) during the time evolution of cirrus detection, (b) determination of the base and top of clouds through a so called threshold value to derive the macrophysical variables (altitude, temperature, geometrical thickness), (c) the application of the transmittance method for each layer and the determination of cloud microphysical variables (optical depth and Lidar ratio). In this process, the Lidar ratio is calculated iteratively until a convergence of this value is achieved. Multivariate statistical analyses were performed to determine the classes of cirrus. These classes are based on geometric thickness, average altitude and the respective temperature, relative altitude (difference between tropopause height and cloud top) and optical depth. The successive use of Principal Component Analysis (PCA), Hierarchical Clustering Method (HCM) and Discriminant Analysis (DA) allowed the identification of four classes of cirrus. It is important to point out here that such methods were applied only to cases identified as single layers of clouds, due to the rare occurrence of multilayered clouds. The origin of formation for the four cirrus classes, though they have distinct macro-and microphysical properties, was found to be basically the same, i.e., from the injection of water vapor in the atmosphere provided by frontal systems, followed by the cooling process to form ice crystals. The same formation mechanism is also attributed to the subtropical jet. An analysis of the temperature profile and comparison with the literature showed that the cirrus crystals possibly have the form of hexagonal plates and columns. The Lidar Ratio (LR) was also found to be in accordance with the literature.

Este trabalho, sendo pioneiro no Brasil, teve o intuito de efetuar uma investigação das nuvens cirrus na região Metropolitana de São Paulo (23,33ºS / 46,44ºW), SP, através do sistema MSP-Lidar para o período de Junho à Julho de 2007. Durante este período, foi verificada uma ocorrência de cirrus de aproximadamente 54% sobre o total de medidas efetuadas pelo sistema Lidar. Medidas com Lidar nos forneceram uma alta resolução espacial e temporal destas nuvens, permitindo assim caracterizá-las e classificá-las de acordo com as suas propriedades macro- e microfísicas. Para obter tais parâmetros, uma metodologia própria foi desenvolvida na recuperação dos dados de Lidar e uma robusta estatística foi aplicada para determinar as diferentes classes de cirrus. A metodologia adotada se resumiu basicamente (a) na determinação de períodos estacionários (ou observações) durante a evolução temporal de detecção de cirrus, (b) determinação da base e topo através de um valor limiar para o cálculo das variáveis macrofísicas (altitudes, temperaturas, espessuras geométricas), (c) aplicação do método da transmitância para cada camada de nuvem e a determinação das variáveis microfísicas (profundidade óptica e razão de Lidar). Neste processo, a razão de Lidar é calculada iterativamente até que haja a convergência da mesma. Análises estatísticas de multivariáveis foram efetuadas para a determinação das classes de cirrus. Estas classes são baseadas na espessura geométrica, altitude média e sua respectiva temperatura, a altitude relativa (diferença entre a altura da tropopausa e topo da nuvem) e a profundidade óptica. O uso sucessivo da Análise de Componentes Principais (PCA), do Método de Cluster Hierárquico (MCH) e da Análise de Discriminantes (AD) permitiu a identificação de 4 classes. Vale ressaltar que tais métodos foram aplicados somente para os casos identificados como camadas únicas de nuvens, pois não se observou significativamente a ocorrência de nuvens com multicamadas. A origem de formação das classes de cirrus encontradas, embora apresentando propriedades macro- e microfísicas distintas, foi identificada basicamente como a mesma, isto é, provenientes da injeção de vapor dágua na atmosfera por meio de sistemas frontais e seu respectivo resfriamento para a formação dos cristais de gelo. O mesmo mecanismo de formação também é atribuído aos jatos subtropicais. Uma análise em relação ao perfil de temperatura e a comparação com a literatura mostrou que as cirrus classificadas apresentam possivelmente cristais em forma de placas e colunas hexagonais. As razões de lidar (RL) calculadas também estão de acordo com a literatura. / This pioneer work in Brazil, aimed at investigating cirrus clouds in the metropolitan region of São Paulo (23.33 ºS / 46.44 ºW), SP, observed by the MSP-Lidar system in June and July 2007. During this period, cirrus clouds were observed during approximately 54% of the time of all Lidar measurements available. The Lidar provided measurements with high spatial and temporal resolution measurements of these clouds that allowed characterizing and classifying them according to their macro-and microphysical properties. For such parameters, a unique methodology was developed for the Lidar data retrieval and a robust statistic was applied to determine the different classes of cirrus. The following steps were adopted to characterize the observations: (a) the determination of stationary periods (or observations) during the time evolution of cirrus detection, (b) determination of the base and top of clouds through a so called threshold value to derive the macrophysical variables (altitude, temperature, geometrical thickness), (c) the application of the transmittance method for each layer and the determination of cloud microphysical variables (optical depth and Lidar ratio). In this process, the Lidar ratio is calculated iteratively until a convergence of this value is achieved. Multivariate statistical analyses were performed to determine the classes of cirrus. These classes are based on geometric thickness, average altitude and the respective temperature, relative altitude (difference between tropopause height and cloud top) and optical depth. The successive use of Principal Component Analysis (PCA), Hierarchical Clustering Method (HCM) and Discriminant Analysis (DA) allowed the identification of four classes of cirrus. It is important to point out here that such methods were applied only to cases identified as single layers of clouds, due to the rare occurrence of multilayered clouds. The origin of formation for the four cirrus classes, though they have distinct macro-and microphysical properties, was found to be basically the same, i.e., from the injection of water vapor in the atmosphere provided by frontal systems, followed by the cooling process to form ice crystals. The same formation mechanism is also attributed to the subtropical jet. An analysis of the temperature profile and comparison with the literature showed that the cirrus crystals possibly have the form of hexagonal plates and columns. The Lidar Ratio (LR) was also found to be in accordance with the literature.

Au cours de l’automne 2012, un réseau d’observation très complémentaire a été déployé dans la région des Cévennes pour la période d’observation spéciale (SOP) du projet HyMeX. Ce réseau d’observation a été spécifiquement élaboré afin d’étudier la structure et l’hétérogénéité des précipitations et en particulier, l’impact du relief sur cette structure. Dans un premier temps, l’analyse de la distribution des gouttes de pluie (DSD) au sol et le long d’un profil vertical à partir des observations nous permet de décrire précisément la structure des précipitations le long d’un gradient topographique. Afin de comprendre l’influence du relief sur cette structure, nous nous concentrons sur les processus microphysique associés à la structure des précipitations. Pour ce faire, nous définissons trois régimes de pluie et étudions l’évolution verticale de la DSD le long du gradient topographique. Les variations en nombre ou en taille dans la DSD peuvent être associées à différents processus microphysiques ou dynamiques. Pour finir, nous estimons la capacité d’un modèle paramétré de l’atmosphère tel que WRF à représenter la structure des précipitations et les processus associés dans une zone de montagne. / During Fall 2012, a complementary observational network has been deployed in the Cévennes region (South of France) for the Special Observation Period (SOP) of the HyMeX project. This network has been specifically designed to study the structure and heterogeneity of precipitations and, in particulat, the impact of orography on this structure. Firstly, the analysis of the Drop Size Distribution (DSD) at the ground et along a vertical profile from ground observations allow us to describe precisely the rainfall structure along a topographical gradient. In order to understand the influence of a relief on this structure, we focus our study on the microphysical process associated with the structure of precipitations. To do so, we define three rainfall regime et study the vertical evolution of the DSD along the topographical gradient. Variations in number and size of the DSD can be associated with different microphysical or dynamical process. Finally, we estimate the capacity of a bulk atmospheric model such as WRF to represent the rainfall structure and associated mechanisms above a mountainous area.

Dans cette thèse, nous analysons les principales caractéristiques des aérosols atmosphériques sur un site urbain en Afrique de l’Ouest : la ville de Ouagadougou. Cette analyse est suivie de l’évaluation du forçage radiatif produit par cette population d’aérosols au sommet de l’atmosphère, dans la couche atmosphérique ainsi qu’à la surface terrestre. Une étude climatologique des propriétés optiques (épaisseurs optiques, exposant d’Angström, albédo de simple diffusion, facteur d’asymétrie) et microphysiques (distribution granulométrique, indice complexe de réfraction) a été effectuée sur la base des données de mesures et d’inversions photométriques du réseau AERONET. L’analyse de ces données a permis de définir à diverses échelles de temps les différentes variabilités des propriétés étudiées. Ces propriétés ont mis en exergue les effets combinés de l’activité anthropique, du cycle de production des poussières minérales d’origine saharienne, de la succession saisonnière et la dynamique du climat spécifiques à cette région de la terre. La composition du panache d’aérosols a également été déduite de l’analyse des données optiques, et les conclusions tirées se sont avérées être en bon accord avec des mesures chimiques effectuées au LSCE sur des échantillons obtenus par prélèvement manuel sur filtres. Le bilan radiatif a été évalué en utilisant le modèle GAME. Cette simulation a montré une forte corrélation du forçage avec la succession des saisons sèche et humide, avec des valeurs extrêmes au mois d’Août (humide) et de Mars (sec avec poussières maximales). Les résultats traduisent un refroidissement en surface pouvant atteindre -41 W/m 2 en Mars, un réchauffement de la couche atmosphérique qui va de 15 à 35 W/m 2 environ et un refroidissement au sommet de l’atmosphère compris entre -2 et -6 W/m 2 . La représentation du forçage calculé en fonction de l’albédo de simple diffusion a montré une tendance linéaire avec un coefficient de corrélation traduisant une fiabilité de nos résultats de l’ordre de 88%. La cohérence des valeurs simulées a été aussi montrée par une comparaison avec des résultats obtenus dans la région Ouest Africaine par d’autres chercheurs utilisant des techniques différentes. De même, la représentation de l’efficacité radiative simulée en fonction de celle inversée par AERONET a montré un très bon accord. / In this thesis, we analyze the main characteristics of atmospheric aerosols on an urban site in West Africa: Ouagadougou. This analysis is followed by an assessment of the radiative forcing produced by this population of aerosols at the top of the atmosphere, in the atmospheric layer and at the Earth's surface. A climatological study of the optical properties (optical thickness, Angstrom exponent, single scattering albedo, asymmetry factor) and microphysical properties (particle size distribution, complex refractive index) was performed on basis of data obtained from photometric measurement and inversions of AERONET network. The analysis of these data allowed us to define the different variabilities of the properties studied at various time scales. These properties have highlighted the combined effects of human activity, the cycle production of mineral dust from the Sahara region, the seasonal succession and the climate dynamics known in this region of the earth. The composition of the aerosol plume was also deduced from the analysis of optical data, and the conclusions were found to be in good agreement with chemical analysis carried out at LSCE on samples obtained by sampling on filters at Ouagadougou. The radiation balance was assessed using the GAME model. This simulation showed a strong relationship of the radiative forcing with the succesion of wet and dry seasons, with extreme values in August (wet) and Mars (dry with maximum dust emissions). The results showed a cooling at the surface that can reach -41 W / m 2 in March, a warming of the atmosphere, ranging from 15 to 35 W / m 2 about and a cooling at the top of the atmosphere between -2 and -6 W / m 2 . The representation of the radiative forcing calculated as a function of the single scattering albedo showed a linear trend with a correlation coefficient reflecting relatively good reliability of our results (about 88%). The consistency of simulated values was also shown by a comparison with the results obtained in the West African region by other researchers using different techniques. Similarly, the representation of the radiative efficiency simulated as a function of the inverted AERONET one showed a very good agreement.

La formation des systèmes convectifs est un processus complexe qui s'étend de l'échelle synoptique, avec la mise en place de circulations favorisant la convection, à la micro-échelle, avec les processus de formation et de croissance des hydrométéores. C'est aux échelles les plus fines que se concentre cette thèse dont l'objectif est d'étudier l'apport d'une microphysique complexe sur l'occurrence et la morphologie d'évènements fortement précipitants. La microphysique évaluée est celle du schéma LIMA, de type bulk à deux moments, capable de prendre en compte l'évolution d'une population d'aérosols multimodale et le traitement pronostique de son interaction avec les nuages et les précipitations. Dans un premier temps, l'apport de la microphysique de LIMA est évalué en comparaison à la microphysique bulk à un moment du schéma ICE3, moins sophistiqué et actuellement opérationnel à Météo-France dans le modèle AROME. Afin de mesurer l'apport de ce nouveau schéma sur la simulation de cas fortement précipitants tels que ceux qui touchent régulièrement le sud-est de la France à l'automne, deux cas d'étude de la campagne HyMeX ont été simulés avec Meso-NH et comparés aux nombreuses observations disponibles. Si l'évaluation des cumuls de précipitations montre un impact modéré de l'un ou l'autre des schémas microphysiques, l'écart est plus marqué en terme de composition et de structure des systèmes convectifs : la microphysique à 2 moments développe une structure verticale plus réaliste et introduit plus de variabilité sur les champs microphysiques. L'évaluation a aussi identifié des biais dans le schéma, notamment une surestimation des diamètres de gouttes de pluie. Des pistes d'amélioration de la microphysique de LIMA ont alors été proposées et évaluées sur les mêmes cas. Des tests de sensibilité à l'initialisation de la population d'aérosols ont ensuite été menés. Il s'avère que les aérosols n'affectent pas uniquement les hydrométéores primaires (gouttelettes d'eau nuageuse et cristaux de glace) mais aussi les autres hydrométéores, engendrant des impacts sur le développement des systèmes convectifs simulés, en termes de composition nuageuse et de précipitations. Les simulations avec une population d'aérosols réaliste initialisée à partir des analyses CAMS ont montré un impact modéré sur les cumuls de précipitations mais une amélioration plus significative de l'évolution temporelle du système (intensification, dissipation) et de la composition nuageuse, réduisant le diamètre des gouttes de pluie sur les cas d'étude. / The triggering and growth of Convective systems is a complex process that extends from the synoptic scale, with the establishment of atmospheric circulations promoting convection, to the microscale, with the formation and growth processes of hydrometeors. This PhD focuses on these finest scales and investigates the contribution of complex cloud-microphysics to the occurrence and morphology of heavy precipitation events. The two-moment microphysical scheme LIMA evaluated in this study takes into account the evolution of a multimodal aerosol population and the prognostic treatment of its interaction with liquid and ice clouds and precipitation. First, the contribution of LIMA is evaluated in comparison to the ICE3 one-moment bulk microphysical scheme, which is less sophisticated and currently operational in the AROME model at Météo-France. In order to measure the performance of this new scheme, two case studies of the HyMeX campaign were simulated with the Meso-NH model and compared to a wide variety of available measurements. The assessment of cumulative precipitation shows a moderate impact of each of these microphysical schemes, but the difference is more pronounced in terms of convective systems composition and structure: the two moment microphysics develops a more realistic vertical structure and introduces more microphysical variability. The evaluation also identified biases in the scheme (such as an overestimation of rain drop diameters). Some improvements to the implementation of LIMA were proposed and evaluated on the same cases. Then, the scheme is used to perform a sensitivity test to the aerosol population on the same case studies. Tests on the concentration of idealized populations have shown that aerosols do not only affect primary production of cloud droplets and ice crystals but also precipitating hydrometeors, causing impacts on the development of simulated convective systems in terms of cloud composition and generated precipitation. Simulations based on a realistic aerosol population initialized from CAMS analyses also showed a moderate impact on cumulative precipitation, but a more significant improvement on the temporal evolution of the system (intensification, dissipation) and cloud composition, leading to a reduction of rain drop diameters in the studied cases.

Ce doctorat est consacré à la modélisation de l’abattage humide comme procédé d’assainissement de l’air. Les situations d’abattage humide étudiées concernent des particules d’aérosol de diamètre aérodynamique variant entre 1 nm et 100 µm capturées dans l’air par des gouttes d’eau de diamètre compris entre 80 µm et 600 µm (nombre de Reynolds de goutte dans la gamme [1 ; 100]). La modélisation de l’assainissement de l’air par abattage humide nécessite deux phases complémentaires. La première étape consiste à définir un noyau de capture calculant le débit d’aérosols capturés par une goutte dans une situation qui peut être complexe de par la grande variété de collectes différentes (brownienne, phorétique, électrostatique et inertielle). La deuxième étape repose sur la définition d’une approche de simulation numérique des phénomènes prenant place dans l’abattage humide. À cet effet, l’approche de simulation de l’abattage humide proposée comporte une modélisation RANS pour simuler l’écoulement de l’air, une approche lagrangienne donnant la trajectoire des gouttes d’eau et une méthode eulérienne permettant de suivre l’évolution du champ de concentration en particules d’aérosol. Ainsi, la capture de particules d’aérosol par des gouttes d’eau est modélisée via l’implémentation du noyau de capture précédemment défini dans un terme puits au sein du modèle Diffusion-Inertia de Zaichik et al., (2004) / This PhD-Thesis is dedicated to the numerical modeling of aerosol particles scavenging by drops. Investigated situations are about aerosol particles of aerodynamic diameter ranging from 1 nm to 100 µm captured in the air by water drops of diameter varying between 80 µm and 600 µm, with corresponding droplet Reynolds number ranging between 1 and 100. This air depollution modeling is achieved in two steps. The first step consists in obtaining a scavenging kernel predicting the flow rate of aerosol particles captured by a drop in a situation where several collection mechanisms may take place (Brownian, phoretic, electrostatic and inertial scavenging). The aim of the second step is to propose a numerical simulation modeling the scavenging phenomenon. To do so, the scavenging simulation includes a RANS modeling for the air flow, a Lagrangian approach for the drops and an Eulerian approach for the aerosol particles. Thus, aerosol scavenging by drops is modeled by implementing the collection kernel defined previously into a sink term in the Diffusion-Inertia model of Zaichik et al., (2004)