Les ondes internes sont produites par suite de l’équilibre dynamique entre les forces de flottabilité et la gravité quand une particule de fluide est déplacée verticalement dans un milieu stratifié stable. Les systèmes géophysiques tels que océan et l’atmosphère sont naturellement stratifiés et donc favorables à la propagation des ondes internes. En outre, ces deux environnements stockent une grande quantité de particules tant dans leur intérieur que sur les bords. Par conséquent, les ondes internes et les particules vont inévitablement interagir dans ces systèmes. Au cours de ce travail, des expériences exploratoires sont réalisées pour étudier le transport par érosion des particules, généré par les ondes internes. Afin de déterminer un seuil de transport, les propriétés particulières des réflexions d’ondes internes («réflexion critique ») sont utilisées pour augmenter l’intensité du champ d’ondes à la surface de réflexion. Une méthode a été développée en collaboration avec une équipe de traitement du signal pour améliorer la détermination des composantes de l’onde impliquées dans une réflexion quasi critique. Cela nous a permis de comparer nos résultats expérimentaux avec une théorie de la réflexion critique, montrant un bon accord et permettant d’extrapoler ces résultats à des expériences au-delà de la nôtre et à des conditions océaniques. Nous avons aussi étudié l’interaction des ondes internes avec une colonne de particules en sédimentation. Deux effets principaux ont été observés : la colonne oscille autour d’une position d’équilibre, et elle est déplacée dans son ensemble. La direction du déplacement de la colonne est expliquée par le calcul de l’effet de la dérive Lagrangienne produite pour des ondes. Cet effet pourrait également expliquer la dépendance en fréquence du déplacement. / Internal waves are produced as a consequence of the dynamic balance between buoyancy and gravity forces when a particle of fluid is vertically displaced in a stably stratified environment. Geophysical systems such as ocean and atmosphere are naturally stratified and therefore suitable for internal waves propagation. Furthermore, these two environments stock a vast amount of particles at their boundaries and in their bulk. Therefore, internal waves and particles will inexorably interact in these systems. In this work, exploratory experiments are performed to study wave generated erosive transport of particles. In order to determine a transport threshold, the peculiar properties of internal waves (“critical reflection”) are employed to increase the intensity of the wave field at the boundaries. A method was developed in collaboration with a signal processing team to improve the determination of the wave components involved in near-critical reflection. This method enabled us to compare our experimental results with a theory of critical reflection, showing good agreement and allowing to extrapolate these results to experiments beyond ours and to oceanic conditions. In addition, we study the interaction of internal waves with a column of particles in sedimentation. Two main effects are observed: the column oscillates around an equilibrium position, and it is displaced as a whole. The direction of the displacement of the column is explained by computing the effect of the Lagrangian drift of the waves. This effect could also explain the frequency dependence of the displacement.

Cette thèse s’intéresse aux ondes de gravité et à la turbulence dans la région de la tropopause tropicale (TTL pour tropical tropopause layer, entre 14 et 18 km d’altitude), et à leurs impacts sur les cirrus.Dans un premier temps, les fluctuations de température et de vent vertical induites dans la TTL par les ondes de gravité sont quantifiées et caractérisées à partir de mesures provenant de vols de ballons stratosphériques longue durée. Les perturbations observées sont comparées aux champs de fluctuations résolues par différents modèles atmosphériques globaux. À la lumière des observations, différentes méthodes de paramétrisation des fluctuations de température sont discutées.Dans un second temps, l’influence des ondes équatoriales et de gravité sur la microphysique des cirrus est étudiée. On considère d’abord l’impact des ondes de gravité de haute fréquence sur la nucléation des cristaux de glace. La question du rôle des anomalies de vent vertical induites par les ondes de basse fréquence sur le transport de la glace est ensuite abordée et son impact quantifié à l’aide d’observations in situ. Enfin, on étudie la formation et l’évolution d’un cirrus de grande échelle à l’aide de simulations numériques. Parmi les différents processus en jeu (radiatifs,...), on montre l’importance d’une onde équatoriale de grande échelle dans la structuration et l’évolution du champ nuageux.Dans une dernière partie, les fluctuations de vents de petite échelle dans la TTL, interprétées comme de la turbulence, sont étudiées à partirdes observations avion de la campagne ATTREX au-dessus de l’océan Pacifique. Leur impact sur le transport vertical de différents traceurs est quantifié. Il est inférieur à l’impact de l’upwelling équatorial de grande échelle mais néanmoins significatif. / Atmospheric waves and turbulence and their impacts on cirrus clouds in the Tropical Tropopause Layer (TTL, 14-18 km altitude) are studied using in situ observations, numerical simulations and theoretical approaches.First, long-duration stratospheric balloon measurements are used to analyze Lagrangian temperature and vertical wind fluctuations induced by gravity waves at the tropical tropopause. The amplitude and intermittency of wave fluctuations are assessed, and the observations are compared with resolved wave fluctuations in atmospheric models. Methods to parameterize Lagrangian temperature fluctuations are then discussed.Then, some impacts of waves on cirrus clouds microphysics are examined. We first consider the influence of high frequency gravity waves on the ice nucleation process. Next, we explore the interplay between ice crystal sedimentation and advection by the wind perturbations induced by low frequency waves. At last, we use numerical simulations to investigate the formation of a large-scale cirrus in the TTL. We demonstrate the role of large-scale equatorial waves and quantify the relevance of different processes (dynamics, radiative heating,...) in the cloud evolution.Finally, small-scale wind fluctuations, interpreted as turbulent bursts, are characterized using aircraft measurements from the ATTREX campaign in the tropical Pacific. The impact of the fluctuations on vertical mixing and on the TTL tracer budget is quantified. The vertical transport induced by turbulent mixing is found to be smaller than that induced by mean tropical upwelling, but nonetheless significant.

Internal gravity waves play a fundamental role in the dynamics of stably stratified regions of the atmosphere and ocean. In addition to the radiation of momentum and energy remote from generation sites, internal waves drive vertical transport of heat and mass through the ocean by wave breaking and the mixing subsequently produced. Identifying regions where internal gravity waves contribute to ocean mixing and quantifying this mixing are therefore important for accurate climate and weather predictions. Field studies report significantly enhanced measurements of turbulence near 'rough' ocean topography compared with those recorded in the ocean interior or near more gradually varying topography (e.g. Toole et al. 1997, J. Geophys. Res. 102). Such observations suggest that interaction of waves with rough topography may act to skew wave energy spectra to high wavenumbers and hence promote wave breaking and fluid mixing. This thesis examines the high wavenumber scatter and spatial partitioning of wave energy at 'rough' topography containing features that are of similar scales to those characterising incident waves. The research presented here includes laboratory experiments using synthetic schlieren and PIV to visualise two-dimensional wavefields produced by small amplitude oscillations of cylinders within linear salt-water stratifications. Interactions of wavefields with planar slopes and smoothly varying sinusoidal topography are compared with those with square-wave, sawtooth and pseudo knife-edge profiles, which have discontinuous slopes. Far-field structures of scattered wavefields are compared with linear analytical models. Scatter to high wavenumbers is found to be controlled predominantly by the relative slopes and characterising length scales of the incident wavefield and topography, as well as the shape and aspect ratio of the topographic profile. Wave energy becomes highly focused and the spectra skewed to higher wavenumbers by 'critical' regions, where the topographic slope is comparable with the slope of the incident wave energy vector, and at sharp corners, where topographic slope is not defined. Contrary to linear geometric ray tracing predictions (Longuet-Higgins 1969, J. Fluid Mech. 37), a significant back-scattered field can be achieved in near-critical conditions as well as a forward scattered wavefield in supercritical conditions, where the slope of the boundary is steeper than that of the incident wave. Results suggest that interaction with rough benthic topography could efficiently convert wave energy to higher wavenumbers and promote fluid mixing in such ocean regions.

This thesis is concerned with the role of internal gravity waves (IGWs) in the stratospheric dynamics and variability demonstrating the effect of spatiotemporal distribution of their activity on the stratospheric dynamics and transport. The first part introduces a theoretical overview of the most recent as well as classical approaches used for description of the wave-mean interaction in the middle atmosphere. Methodology for an IGW analysis from the GPS radio occultation density data is described in the next chapter and the advantages of utilization of density data are listed. The third chapter presents results describing the peculiar dynamics and anomalous IGW activity in the Eastern Asia/Northern Pacific region. An important part is dedicated to a discussion of accuracy limits and usability of different IGW activity proxies. The possible impact of the localized IGW activity is investigated using a mechanistic middle and upper atmosphere model in the last chapter. Sensitivity simulations are used to demonstrate an important role of the spatial distribution of IGW activity for a formation of planetary waves and for the longitudinal variability of the Brewer-Dobson circulation. Implications for the middle atmospheric and climate change research are discussed along with consequences for parameterizations of...

Internal gravity waves (GWs) are an important component of the atmospheric dynamics, significantly affecting the middle atmosphere by momentum and energy transport and deposition. In order to be able to improve global circulation models, in which the majority of the GW spectrum is not resolved, it is necessary to quantify their effects as precise as possible. We study GWs in a high-resolution simulation of the WRF model around Southern Andes, Antarctic Peninsula and South Georgia Island. We analyse a Gaussian high-pass filter method for separation of GWs from the basic flow. To overcome an observed problem of dependence of the method on a cutoff parameter, we propose an improved method that determines the parameter at each time step from the horizontal kinetic energy spectrum. The differences between the methods are further examined using the horizontal kinetic energy spectrum, vertical potential energy spectrum and forcing to the divergence equation evaluated by the active wind method, which is a recent theory-based method that divides the flow into a balanced flow and a perturbation field. The results suggest that the high-pass filter method does not produce correct results for time periods with strong wave activity.

Le rayonnement atmosphérique appelé nightglow est un phénomène se produisant à haute altitude (environ 90 km). Il consiste en l’émission d’un rayonnement suite à la désexcitation de certaines molécules et atomes (OH, Na, O2 et O). Il se répartit sur une large gamme spectrale, en particulier dans l’infrarouge et se propage jusqu’au niveau du sol. Le rayonnement nightglow constitue un marqueur important pour la haute atmosphère, permettant de remonter à la température, mais également à de nombreux phénomènes dynamiques comme les marées atmosphériques ou les ondes de gravité. Sa propagation au niveau du sol permet l’éclairage de scène terrestre et ainsi la vision nocturne à l’aide de caméras proche infrarouge. Afin de mieux connaître les fluctuations de ces émissions en fonction du temps à différentes échelles et en différents lieux sur la planète, la thèse s’est axée sur une étude observationnelle et une étude théorique. L’étude observationnelle a produit une climatologie à grande échelle par l’extraction du rayonnement issu des données de l’instrument GOMOS. Les campagnes de mesures réalisées au sol ont quant à elles mis en avant certains aspects dynamiques importants comme les marées et les ondes de gravité. Pour reproduire le rayonnement nightglow, il a été nécessaire de modéliser les réactions chimiques des nombreuses espèces présentes à haute altitude, le chauffage, la photodissociation de certaines molécules par le rayonnement solaire et la propagation du rayonnement vers le sol. Certains processus dynamiques ont été inclus comme la diffusion moléculaire, la diffusion turbulente et une paramétrisation des marées. Enfin, les résultats du modèle sont comparés aux observations satellitaires ainsi qu’au niveau du sol et des tests de sensibilité sont effectués pour estimer la réponse du rayonnement aux différents modules du modèle. / The nightglow is an atmospheric radiation which occurs at high altitude (around 90 km). It comes from the desexcitation of specific molecules and atoms (OH, Na, O2 and O). It spreads over a wide spectral band, especially in the infrared and propagates to the ground level. The nightglow emission is an important mark for the high atmosphere, as it allows the retrieval of the temperature and many dynamic processes such as atmospheric tides or gravity waves. Its propagation to the ground level allows the illumination of terrestrial scene and therefore the night vision with the use of near infrared cameras. In order to have a better knowledge of the emission fluctuations as a function of time for various scales and at various locations, the work is focused on an observational and theoretical study. The observational study produced large scale climatology with the extraction of nightglow emission from GOMOS data. On the other hand, ground measurements highlighted some dynamical aspects such as tides and gravity waves. To model the nightglow emission, it has been necessary to take into account the chemical reactions of the species available at high altitude, the heating, the photodissociation process and the propagation of the emission to the ground. Selected dynamical processes have been included, such as the molecular and turbulent diffusion, and a tide parameterization. Finally, the results of the model are compared to the satellite and ground observations and sensitivity tests are run to estimate the response of the emission to the various modules of the model.

Implementing a nonlinear gravity wave (GW) parameterization into a mechanistic middle and upper atmosphere model, which extends to the lower thermosphere (160 km), we study the response of the atmosphere in terms of the circulation patterns, temperature distribution, and migrating terdiurnal solar tide activity to the upward propagating smallscale internal GWs originating in the lower atmosphere. We perform three test simulations for the Northern Hemisphere winter conditions in order to assess the effects of variations in the initial GWspectrum on the climatology and tidal patterns of the mesosphere and lower thermosphere. We find that the overall strength of the source level momentum flux has a relatively small impact on the zonal mean climatology. The tails of the GW source level spectrum, however, are crucial for the lower thermosphere climatology. With respect to the terdiurnal tide, we find a strong dependence of tidal amplitude on the induced GW drag, generally being larger when GW drag is increased.

Implementing a nonlinear gravity wave (GW) parameterization into a mechanistic middle and upper atmosphere model, which extends to the lower thermosphere (160 km), we study the response of the atmosphere in terms of the circulation patterns, temperature distribution, and migrating terdiurnal solar tide activity to the upward propagating small scale internal GWs originating in the lower atmosphere. We perform three test simulations for the Northern Hemisphere winter conditions in order to assess the effects of variations in the initial GW spectrum on the climatology and tidal patterns of the mesosphere and lower thermosphere. We find that the overall strength of the source level momentum flux has a relatively small impact on the zonal mean climatology. The tails of the GW source level spectrum, however, are crucial for the lower thermosphere climatology. With respect to the terdiurnal tide, we find a strong dependence of tidal amplitude on the induced GW drag, generally being larger when GW drag is increased.

The present work combines experimental and numerical analyses to improve current understanding of turbulence in stably stratified flows. An extensive literature review is presented on the mechanisms governing turbulence under stratified conditions, with a special focus on the Richardson number parameter, as it is often adopted as a switch to turn turbulence modelling on/off. Anisotropization of turbulence is investigated, as it is found to be an important mechanism for turbulence survival at any Richardson number, but usually overlooked in turbulence parameterizations. For this purpose, an experimental dataset previously collected over an Alpine glacier is used, with a focus on the anisotropy of the Reynolds stress tensor, as the scientific community has recently shown improvements in the description of the atmospheric surface layer by taking this aspect into account. Different sources leading stresses to deviate from the isotropic limit are explored, as well as energy exchanges across scales and between kinetic and potential reservoirs, in order to identify the main processes that should be included in turbulence parameterizations to properly represent anisotropic turbulence under stable conditions. High-resolution numerical simulations are then performed with the Weather Research and Forecasting (WRF) model to evaluate different PBL parameterizations in reproducing specific stable atmospheric conditions developing over complex terrain, and their influence on the local circulation. For this purpose, two wintertime case studies in a basin-like area of an Alpine valley are investigated. Both are fair-weather episodes with weak synoptic forcing and well-developed diurnal local circulations, differing by the thermal stratification in the basin. In particular, the influence of thermal stratification on the outbreak of a valley-exit wind coming from a tributary valley is investigated, and the influence of such type of flows on turbulence anisotropy in stably stratified conditions is discussed for future investigations.

Rainfall is one of the central features of Earth’s climate. Understanding the physical mechanisms that control it has deep social impacts on water and food security. In this thesis, we use a series of idealized single column models to reveal mechanisms driving steady-state precipitation both in the tropics and in the global mean. These mechanisms yield a deeper understanding of precipitation in model outputs (Chapter 1), observations (Chapter 2), and projections for a warming climate (Chapter 3). Chapter 1 centers around model development. We use the single column model version of NCAR’s Community Earth System Model (CESM) to better understand its simulation of tropical rainfall under various representations of radiation, convection, and circulation. Using a variety of existing methods – the weak temperature gradient (WTG), damped gravity wave (DGW), and spectral weak temperature gradient (SWTG) method – we parameterize the column’s large-scale dynamics and consider the response of steady-state tropical precipitation to changes in relative sea surface temperature (SST). Radiative cooling is either specified or interactive, and the convective parameterization is run using two different values of a parameter that controls the degree of convective inhibition (CIN) required to cap a convective plume. Under all three methods, circulation strength is decreased when greater CIN is required, that is, when convection is allowed to occur more easily. This effect is shown to come from increased static stability in the column’s reference radiative-convective equilibrium profile and results in decreased rainfall over warm SSTs. This argument can be extended to aquaplanet simulations in CESM, which show that the warmest regions in the tropics rain less when greater CIN is required to cap a convective plume. This suggests that the parameter in CESM which controls the degree of convective inhibition significantly affects the strength of the model’s intertropical convergence zone (ITCZ). In Chapter 2, we use a similar set of idealized models to better understand the observed climatology of tropical rainfall. The distribution of climatological rainfall over tropical oceans can be thought of as primarily the result of two mechanisms: conditional instability in the free troposphere and convergence in the boundary layer. We modify the SWTG method to assess the relative influence of these mechanisms. In its original configuration, the SWTG method applies the weak temperature gradient approximation to the full depth of the troposphere without consideration of the stronger horizontal temperature and pressure gradients in the planetary boundary layer (PBL). To account for convergence in the PBL induced by these stronger pressure gradients, we modify the SWTG method to include an externally-specified vertical mass flux at the PBL top. When forced using the climatological SST and 850 hPa vertical velocity taken from observation-based reanalysis data, the Forced SWTG method reproduces most features of the observed annual mean tropical rainfall climatology. Its predictions remain largely unchanged when it is forced by a spatially uniform SST field. Insofar as the boundary layer convergence field can be interpreted as an external forcing on the column, this would indicate that it controls the precipitation field. However, local column stability likely also plays a role in determining PBL convergence, so this method does not fully untangle the causality behind the climatological precipitation field. In Chapter 3, we shift our perspective from column dynamics to column radiative transfer. Global mean rainfall is known to be constrained by the atmosphere's column-integrated radiative cooling. However, the surface temperature dependence of this radiative constraint on mean rainfall, and the mechanisms which set it, are not well understood. We present a simple spectral model for changes in the clear-sky column-integrated radiative cooling with surface warming. We find that surface warming increases column-integrated radiative cooling – and thus mean rainfall – by decreasing atmospheric transmission in spectral regions with significant longwave emission, that is, by closing the water vapor window. Water vapor's spectroscopy implies a hydrological sensitivity whose magnitude is roughly set by surface Planck emission, and which peaks near tropical surface temperatures. We also examine the role of carbon dioxide and shortwave heating, which primarily act to mute the hydrological response to warming. We validate our findings using line-by-line calculations. Overall, we demonstrate that idealized frameworks, such as those provided by single column models, can elucidate mechanisms controlling tropical and global-mean precipitation. However, the relevance of these results to more complex simulations and observations is tempered by the extent to which our simplifying assumptions neglect important physics.