Observation and modelling of giant planet's atmospheres – OMAGE

We analyze infrared spectroscopic observations from Cassini in order to infer the thermal structure and composition of Saturn's atmosphere. These results will also bring important constraints to the development of a new Saturn GCM. This type of model includes a dynamical core which resolves the hydrodynamical equations on a 3D spatial and temporal grid. It is coupled to physics packages which parametrize various processes such as radiative transfer, cloud microphysics, waves, etc.

Our Cassini data analyses have already revealed significant seasonal changes in temperature: the northern hemisphere has seen its temperature increase by 10K between winter and spring. In parallel, we have made progress in the validation and optimization of the radiative transfer module. An internal heat flux as well as boundary conditions for the winds at the cloud top have also been implemented. First 3D runs are being performed, and the resulting thermal structure is compared to observations.

We will apply this new GCM to the study of Saturn's atmospheric dynamics: how is the large-scale circulation triggered in the stratosphere? What is the role and importance of wave forcing compared to radiative forcing? What governs the polar vortices and the great storm observed in 2010? We will then adapt this GCM to model the circulation of Jupiter and gas exoplanets. This will help better understand the mechanisms underlying atmospheric phenomena observed in different planetary environments.

This work has been presented at several conferences:
European Geophysical Union (April 2013)
Chapman Conference From Earth to exoplanets (June 2013)
European Planetary Science Conference (September 2013)
Annual Meeting of the Division for Planetary Sciences (October 2013)

The scope of the OMAGE project is to better understand the dynamics of giant planet's atmospheres as a whole, by associating remote sensing observations to the development of a novel, state-of-the-art, General Circulation Model (GCM). Recent observational programs, both spatial and ground-based, have revealed the complexity of giant planet's middle atmospheres. In particular, for Saturn, maps of the temperature and of the distribution of trace species have been obtained by the Cassini spacecraft with unprecedented details. These maps exhibit puzzling anomalies which cannot be explained by current photochemical and radiative models (none of them includes dynamics), and which have been interpreted as signatures of large-scale or seasonal dynamical motions. However, as no model of Saturn's stratospheric circulation currently exists, these assumptions have not yet been tested, and Saturn’s global circulation remains weakly characterized. Furthermore, on Saturn and Jupiter, equatorial oscillations in the zonal wind and temperature field have recently been discovered and are reminiscent of the Earth's Quasi-Biennial Oscillation, a fundamental dynamical phenomenon. These oscillations thus appear to be a common dynamical phenomenon in very different planetary atmospheres.
In this project, we propose to study in detail the atmospheric circulation of giant planets by developing the first general circulation model of their stratospheres. It will serve as a new tool to address fundamental questions in geophysical fluid dynamics, explore the giant planets circulations and better interpret current and future observations. The development of the GCM for gas giants will be based on 1) our current knowledge of their physical characteristics, brought forward by recent, high-quality observations and 2) the expertise of the Laboratoire de Météorologie Dynamique (LMD) for developing GCMs of the atmospheres of Venus, Mars and Titan over the past two decades. This new GCM will first be focused on reproducing Saturn's climate, for which observational constraints are the more documented. Then, we will move forward to study Jupiter, which presents similar problematics. Several case studies of comparative planetology will be investigated. Finally, this model will be adapted to extrasolar planets such as « hot Jupiters » that act as natural laboratories for this new tool, to broaden our knowledge of atmospheric dynamics in extreme environments.
The first part of this project is focused on adapting the current LMDz GCM to Saturn. Several tasks are defined and include:
- Adapting the existing dynamical core to gas giants,
- Adapting the physics packages (radiative transfer, chemistry...) to Saturn's specific conditions (radiative times, composition...),
- Parameterizing boundary conditions of these gas planets, which are very different from terrestrial planets,
- Optimizing the efficiency of calculations, as this new model will be costly in computation time.
In parallel, we will contribute to the analysis of new Cassini data (in orbit around Saturn until 2017), in order to monitor seasonal variations in temperature and composition, that will bring mandatory constraints to the model. When the GCM is mature enough, we will investigate the steady state circulation that takes place on the planet. Our objectives will be:
- Characterizing the global and/or seasonal circulation cells,
- What is their efficiency to transport trace species and can we reproduce Saturn's observed distribution of hydrocarbons,
- Studying the role of the ring's shadows (that moves from one hemisphere to another with seasons) on seasonal forcing,
- Studying what triggers and governs the evolution of the equatorial oscillation,
- Explore wave activity and their role in the observed phenomena.
In the last part of the project, we will apply Saturn's GCM to other gas giants : first Jupiter, quite similar to Saturn, then extrasolar planets, characterized by their extreme conditions.