Global climate modeling of Saturn's atmosphere: exploration of seasonal variability and stratospheric dynamics
Abstract
A leap forward on our knowledge of Saturn's stratosphere has resulted from the combination of orbital observations on board the Cassini spacecraft and state-of-the-art ground-based observations. Maps of temperature and hydrocarbons in Saturn's stratosphere revealed puzzling anomalies: equatorial oscillations with a period of about half a Saturn year, meridional circulations affecting the hydrocarbons' distribution, including possible effects of rings shadowing, "beacons" associated with the powerful 2010 Great White Spot. Those signatures, reminiscent of fundamental wave-driven phenomena in the Earth's middle atmosphere (e.g., Quasi-Biennal Oscillation, Brewer-Dobson circulation), cannot be reproduced by 1D photochemical and radiative models. This motivated us to develop a complete 3D General Circulation Model (GCM) for Saturn, based on the LMDz hydrodynamical core, to explore the circulation, seasonal variability, and wave activity in Saturn's atmosphere. In order to closely reproduce Saturn's radiative forcing, a particular emphasis was put in obtaining fast and accurate radiative transfer calculations. Our radiative model uses correlated-k distributions and spectral discretization tailored for Saturn's atmospheric composition (methane, ethane, acetylene). In addition to this, we include CIA absorption (hydrogen and helium), internal heat flux, ring shadowing, and aerosols. A systematic study is carried out on the sensitivity of the model to spectral discretization, spectroscopic databases, and aerosol scenarios (varying particle sizes, opacities and vertical structures). Temperature fields obtained with this new radiative equilibrium model are compared to that inferred from Cassini/CIRS observations. In the troposphere, our model reproduces the observed temperature knee caused by heating at the top of the tropospheric aerosol layer. In the lower stratosphere, the overall meridional gradient between the summer and the winter hemispheres agrees with observations except in the equatorial region, where the temperature structure is governed by the dynamical equatorial oscillation. In the upper stratosphere, our modeled temperature is 5-10K too low compared to measurements. This suggests that processes other than radiative heating/cooling by trace species control the temperature at low pressure levels. Finally, we will show GCM simulations coupling the 3D dynamical core to this radiative model, and discuss the large-scale stratospheric circulations driven by the radiative forcing. In the troposphere and lower stratosphere, zonal winds are relaxed towards the observed winds by Cassini. The emergence and propagation of waves in Saturn's stratosphere will be discussed, as well as eddy-mean flow interactions. Seasonal variations of those dynamical signatures will be investigated.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMSM21C2200S
- Keywords:
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- 5704 PLANETARY SCIENCES: FLUID PLANETS Atmospheres;
- 6275 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS Saturn;
- 5739 PLANETARY SCIENCES: FLUID PLANETS Meteorology;
- 0342 ATMOSPHERIC COMPOSITION AND STRUCTURE Middle atmosphere: energy deposition