University Documents

PhD Thesis
General circulation and photochemistry in Titan's atmosphere
Defense: June 14, 2000 at CESR, Univ. Paul Sabatier, Toulouse, France
For twenty years, the atmosphere of Titan has been intensively observed and studied. A lot of questions about this complex system still need to be adressed, especially about the interactions between microphysics of the haze, photochemistry and atmospheric dynamics. To better understand the atmosphere of Titan before the arrival of the Cassini-Huygens mission, we developed a two-dimensional photochemical general circulation model. A three-dimensional description of the ultraviolet flux was used to get a realistic ultraviolet field at every latitude and season, and an up-to-date review of the hydrocarbon photochemistry was done. We introduced the transport by meridional winds and horizontal transient eddies in a two-dimensional (latitude-altitude) photochemical and transport model, which, along with a purely dynamic study and a sensitivity study, demonstrated the role of dynamical transport on the latitudinal variations of the stratospheric composition, as observed by the Voyager I mission. Comparison between the different observations and our results is fairely good, but the uncertainties on chemical data remain a main source of difficulties. We investigated the radiative retroaction of the variations of the composition on the temperature field. The coupled version of the general circulation model will be a powerful tool to investigate the new data obtained with the Cassini-Huygens mission.
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Habilitation à Diriger des Recherches
Venus et Titan, étude d'atmosphères en superrotation
Defense: July 5, 2013 at LMD, UPMC, Paris, France
Among the Solar System, Venus and Titan (Saturn's satellite) share a peculiar atmospheric phenomenon: their dens atmosphere rotate globally much faster than the solid body below it. This is called the superrotation. In this document, I describe the studies I have done on these atmosphere during the past ten years, as well as the perspectives for my research in the years to come.
To interpret the large datasets being harvested by the Cassini-Huygens and Venus-Express missions, I participated to the development of efficient numerical tools: the General Circulation Models (GCM). Used to study Earth's climate, these GCMs are now adapted to study other similar atmospheres, such as Venus's and Titan's. The GCMs developed at LMD are recognized as leaders in the field.
My work has been organized around two main axes: first, the analysis of interactions that play a crucial role in the behavior of these atmospheric systems; then the in-depth analysis of the mechanisms that control superrotation. In these atmospheres, several sub-systems are interacting in a deeply coupled way, through radiative transfer and transport: atmospheric dynamics, haze and clouds (microphysics), and gas composition (photochemistry). Using GCMs that include all these couplings, it is possible to demonstrate how the circulation modifies the distribution of opacity sources, which in return modifies the thermal structure and the circulation. Modeling a superrotation atmosphere has often been a difficult problem. It is extremely sensitive to the details of the GCM dynamical core, as we have illustrated with an intercomparison study of most of the Venus GCMs developed around the world, forced with identical physical parameters. One hypothesis to understand the wide range of results obtained is the crucial impact of angular momentum conservation in GCMs. An in-depth analysis of this conservation in the two Venus GCMs I have been working with shows how it is difficult to guarantee. Despite these difficulties, the analysis of the circulation modeled with the LMD Venus and Titan GCM allows to understand the mechanisms behind the superrotation. Angular momentum distribution is mainly controled by the mean meridional circulation and by horizontal barotropic waves. However, seasons on Titan must be taken into account, as well as thermal tides on Venus, that redistribute angular momentum vertically above the clouds.
The perspectives for my research in the coming years are organized along several directions: improving the parametrizations of small-scale processes (planetary boundary layer, convection, gravity waves), extending vertically the GCMs into the lower thermosphere, and adapting new dynamical cores that will help to better describe polar regions.
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