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This thesis focuses on the study of Titan's atmosphere ans its seasonal variations, with a Global Climate Model (GCM). The longevity of the Cassini-Huygens mission having made it possible to map the atmosphere for half a Titan-year, many seasonal phenomena have been highlighted, concerning the evolution of temperature in the polar stratospheric regions, the role of trace compounds, or the polar clouds at high altitude. A global understanding of these mechanisms is achieved through tools such as GCMs. As the IPSL Titan GCM preexisting before this thesis did not satisfactorily address these problems, as it suffered from a poor representation of the temperature in the stratosphere, we set up a new radiative transfer scheme. This revision extends to the entire coupling with photochemical and microphysical modules. A method is developed to take into account the opacities variations of trace compounds induced by seasonal changes. Since the preexisting microphysical model has many limitations, a new one is being implemented. Fully coupled dynamic-physical-photochemical-microphysical simulations can thus be performed. They enable the study of the influence of the couplings on the thermal structure and atmospheric dynamics. Sepcial attention is brought to bear on the stratospheric polar regions where the mechanisms at work in the circulation reversal are compared to Cassini data. Finally, the seasonal evolution of the compounds is discussed and the role of couplings is analyzed., Cette thèse porte sur l'étude de l'atmosphère de Titan et ses variations saisonnières, avec un modèle de climat global (GCM). La longévité de la mission Cassini-Huygens ayant permis de cartographier l'atmosphère pendant une demi-année titanienne, de nombreux phénomènes saisonniers ont été mis en évidence, concernant l'évolution de la température dans les régions stratosphériques polaires, le rôle des composés traces, ou les nuages polaires à haute altitude. Une compréhension de ces mécanismes passe par le biais d'outils comme les GCM. Le GCM Titan de l'IPSL existant avant ces travaux ne permettant pas d'aborder correctement ces problèmes, car souffrant d'une mauvaise représentation de la température dans la stratosphère, on met en place un nouveau transfert radiatif. Cette refonte s'étend à l'ensemble des couplages avec les modules photochimiques et microphysiques. On développe notamment une méthode pour prendre en compte les variations d'opacité des composés traces induites par les changements saisonniers - méthode potentiellement applicable à d'autres objets. Le modèle microphysique existant ayant de nombreuses limitations, on en implémente un nouveau. Des simulations entièrement couplées dynamique-physique-photochimie-microphysique peuvent ainsi être réalisées. Avec celles-ci on étudie l'influence des couplages sur la structure thermique et la dynamique atmosphérique. Une attention particulière est portée sur les régions polaires stratosphériques, où les mécanismes œuvrant dans le renversement de la circulation sont comparés aux observations de Cassini. Enfin, l'évolution saisonnière des composés est discutée et le rôle des couplages est analysé.

International audience; 1. Context The atmosphere of Titan is interesting by many aspects: it has the thickest atmosphere for a moon in the solar system, an atmosphere in superrotation in the stratosphere, an hemispheric asymmetry of temperature and an haze feedback of haze distribution on circulation between many others. There is another feature by which the atmosphere of Titan is unique, a strong decrease of the zonal wind between 60 and 100 km known as the "zonal wind collapse" (Fig-ure 1). The first measurement of this feature performed by ground-based radio-telescopes recording the Doppler Wind Experiment measurements of the carrier frequency during the Huygens descent [1]. The wind measured above 120 km was approximately of 100 m s −1. Then, below, the wind decreased to about few meters per seconds around 70 km before increasing again to 40 m s −1 at 60 km. 2. Our methodology 2.1 Principle Global Circulation Models (GCM) are powerful tools to study atmospheric circulations and have been employed to study the different planets of the solar system as well as Titan [2, 3, 4]. Although the different models are able to reproduce a realistic atmospheric circulation with superrotation, they fail to reproduce the observed zonal wind collapse characterized by a decrease towards only a few meters per second. We propose here to study for the first time this wind structure using turbulence-resolving model [5]. 2.2 Model description In order to investigate this peculiar wind feature we use the WRF compressible and non-hydrostatic dy-namical core to perform large-eddy simulation (LES) [6]. The timescale of the resolved turbulence is significantly smaller than the radiative timescale, comparable to one Titan year at this altitude [7], so no radiative Figure 1: Huygens temperature (K) and zonal wind profile (m s −1) between 50 and 100 km. processes are taken into account. The model is initialized using pressure, temperature and wind vertical profile as measured by the Huygens probe and shown in Figure 1. The atmospheric and planetary constants (gravity, heat capacity ...) within the model are set to Titan values. The horizontal grid spacing is 20 m spread over a 2 km-wide domain and the vertical grid features 300 levels from 60 to 90 km altitudes. 3. Wave generation Figure 2 displays the vertical wind (top) the associated vertical Eliassen-Palm flux (bottom) ρu w with ρ the density of the atmosphere and u and w the mean perturbation to the mean (domain-averaged) value of the zonal wind u and vertical wind w. The strong decrease of the zonal wind between 65 and 60 km causes a Kelvin-Helmholtz instability that leads to the generation of gravity waves. These waves propagates both towards the ground and towards the upper atmosphere. The dissipation of the wave engenders a momentum transfer to the flow and impacts the zonal wind.