ÉMERGÉANTES : a new Global Climate Model

1. ÉMERGÉANTES: a new Global Climate Model to study the dynamics of Saturn’s stratosphere – and beyond Aymeric SPIGA aymeric.spiga@upmc.fr (1), Sandrine GUERLET (1), Mélody SYLVESTRE (1,2), Thierry FOUCHET (2), Ehouarn MILLOUR (1), Robin WORDSWORTH (3), Jérémy LECONTE (1), François FORGET (1) Laboratoire de Météorologie Dynamique,France (2) Laboratoire d’Études Spatiales et d’Instrumentation en Astrophysique, France (3) University of Chicago, USA … WITH NUMEROUS APPLICATIONS The aim of this project is to study in detail the atmospheric circulation of giant planets by resolving atmospheric circulations in their stratosphere (and, possibly, in the future, the coupling between their troposphere and stratosphere). It will serve as a new tool to address fundamental questions in geophysical fluid dynamics, explore the giant planets circulation patterns, and better interpret current and future observations. This new GCM will first be focused on reproducing Saturn's climate (sample 3D results are shown above), following the harvest of observations obtained by the Cassini mission. We plan to also study Jupiter in the future, both in the frame of future missions (Juno, JUICE) and a comparative planetology approach with Saturn. Another area of fruitful application of our model is extrasolar planets, such as “hot Jupiters”, that act as natural laboratories to broaden our knowledge of atmospheric dynamics in extreme environments. FROM OBSERVATIONS … Temperature maps and hydrocarbons’ distribution in the Saturn’s stratosphere were obtained by the CIRS spectrometer on board the Cassini spacecraft. These new observations are in contradiction with existing photochemical and radiative models. Enigmatic temperature anomalies were unveiled, reminiscent of the Earth’s Quasi-Biennal Oscillation [Figure 1]. Observed hydrocarbons distributions hint at interhemispheric seasonal circulations, with possible impact by rings’ shadow [Figure 2]. Mysterious “beacons” [Fletcher et al. 2011] following the 2010 Great White Spot (a powerful convective storm) are also yet to be explained. … TO BUILDING A SATURN GCM … Cassini observations therefore suggest that a rich and active atmospheric dynamics is at play in Saturn’s stratosphere. This motivated the development of a new Global Climate Model (GCM) for gas giants. This new model is based on the LMDz dynamical core [Figure 3, Hourdin et al., 2006, 2012], which has been successfully adapted to terrestrial planets and moons: the Earth, Mars, Venus, Titan, Triton/Pluton. For Saturn simulations, we use correlated-k radiative transfer [Figure 4, Wordsworth et al., 2010] to account for stratospheric radiative species (CH4,C2H6,C2H2). We account for CIA continuum for H2-H2 and H2-He [Wordsworth et al., 2012]. The model includes energy-conserving vertical mixing parameterizations [Leconte et al., 2013]. We are currently working on setting lower boundaries (internal heat flux, tropospheric jets) constrained by observations, as well as absorption by aerosols in the stratosphere. We plan to optimize radiative transfer computations, and to parameterize seasonal variations of rings’ shadow. [Figure 1] An equatorial oscillation in Saturn’s middle atmosphere, Fouchet et al., Nature 2008 [<< Figure 3] Schematics of the LMDz dynamical core (credit: Fairhead/Hourdin) [Figure 4 >>] Preliminary tests of Saturn GCM radiative transfer [Figure 2] Vertical and meridional distribution of ethane and acethylene in Saturn’s stratosphere from CIRS/Cassini limb observations, Guerlet et al., Icarus 2010