Global Climate Modeling of Saturn's Troposphere and ...
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Global Climate Modeling of Saturn’s Troposphere and Comparative Analysis with Cassini Observations A. Spiga and S. Guerlet // LMD (Sorbonne Université / CNRS), Paris, France [[email protected]] Context The Cassini mission unveiled the intense and diverse activity in Saturn’s troposphere and strato- sphere: banded jets 1 , eddies and waves 2 , vortices 3 , equatorial oscillations 4 . To better understand those phenomena, we built a new Global Climate Model [GCM] for Saturn 5,6 , named the Saturn DYNAMICO GCM, by combining a radiative-seasonal model 5 tai- lored for Saturn to a hydrodynamical solver 7 based on a icosahedral grid suitable for massively-parallel ar- chitectures. Here we compare Cassini observations to high-resolution multi-annual GCM simulations of Sat- urn’s troposphere and lower stratosphere. The study of Saturn’s equatorial oscillations in the upper strato- sphere is detailed in Bardet et al. (this conf.). Thermal structure Our Saturn DYNAMICO GCM produces a satisfactory meridional and vertical thermal structure, and seasonal variability thereof, compared to Cassini CIRS measurements (Figure 1). The radiative-convective transition, between the neutral profile in the bulk of the troposphere and the stable profile in the upper troposphere and lower stratosphere, occurs around 500 - 600 mbar as is observed 8 . The thermal signatures in equilibrium with jets (thermal wind ∂ T/∂y ↔ ∂ u/∂z) are of similar amplitude between modeling and observa- tions, although the localization (i.e. latitude) of those thermal signatures is not compliant between models and observations, echoing the discrepancies in latitude between the observed and modeled jet structures. Equatorial waves Fourier analysis on the tem- perature field simulated by our Saturn GCM at the tropopause is shown in Figure 2. Spectral mapping enables to evidence Rossby and Kelvin waves in the symmetric component accross the equator (and Yanai Rossby-gravity waves in the antisymmetric compo- nent, not shown). The Rossby wavenumber-2 signal in the tropics and mid-latitudes of Saturn’s tropopause (130 mbar) detected by Voyager 10 is present in our our Saturn GCM DYNAMICO simulations. Our GCM re- sults also indicate that other tropical Rossby modes (wavenumbers 3, 4, 5, . . . ) are likely to be significant Figure 1: Latitude-pressure section of zonal-mean temper- ature T [top] from Cassini/CIRS 2015 nadir retrievals 9 vs. [middle] from our Saturn DYNAMICO GCM in the 10th simulated year, Ls ∼ 70 ◦ . [Bottom] Meridional profiles of upper-troposphere T at two opposite solstices, Cassini (crosses, CIRS nadir 8,9 ) vs. GCM simulations (lines). within Saturn’s tropics. This is compliant with Cassini CIRS observations 4 which show a complex structure at a pressure level of 150 mbar. The presence of both eastward and westward vertically-propagating equato- rial waves in our Saturn DYNAMICO GCM simula- tions is crucial to explain equatorial oscillations. EPSC Abstracts Vol. 13, EPSC-DPS2019-758-1, 2019 EPSC-DPS Joint Meeting 2019 c Author(s) 2019
Transcript of Global Climate Modeling of Saturn's Troposphere and ...
Global Climate Modeling of Saturn’s Troposphere andComparative Analysis with Cassini Observations
A. Spiga and S. Guerlet // LMD (Sorbonne Université / CNRS), Paris, France[[email protected]]
Context The Cassini mission unveiled the intenseand diverse activity in Saturn’s troposphere and strato-sphere: banded jets1, eddies and waves2, vortices3,equatorial oscillations4. To better understand thosephenomena, we built a new Global Climate Model[GCM] for Saturn5,6, named the Saturn DYNAMICOGCM, by combining a radiative-seasonal model5 tai-lored for Saturn to a hydrodynamical solver7 based ona icosahedral grid suitable for massively-parallel ar-chitectures. Here we compare Cassini observations tohigh-resolution multi-annual GCM simulations of Sat-urn’s troposphere and lower stratosphere. The studyof Saturn’s equatorial oscillations in the upper strato-sphere is detailed in Bardet et al. (this conf.).
Thermal structure Our Saturn DYNAMICOGCM produces a satisfactory meridional and verticalthermal structure, and seasonal variability thereof,compared to Cassini CIRS measurements (Figure 1).The radiative-convective transition, between theneutral profile in the bulk of the troposphere andthe stable profile in the upper troposphere and lowerstratosphere, occurs around 500 − 600 mbar as isobserved8. The thermal signatures in equilibriumwith jets (thermal wind ∂T/∂y ↔ ∂u/∂z) are ofsimilar amplitude between modeling and observa-tions, although the localization (i.e. latitude) of thosethermal signatures is not compliant between modelsand observations, echoing the discrepancies in latitudebetween the observed and modeled jet structures.
Equatorial waves Fourier analysis on the tem-perature field simulated by our Saturn GCM at thetropopause is shown in Figure 2. Spectral mappingenables to evidence Rossby and Kelvin waves in thesymmetric component accross the equator (and YanaiRossby-gravity waves in the antisymmetric compo-nent, not shown). The Rossby wavenumber-2 signalin the tropics and mid-latitudes of Saturn’s tropopause(130 mbar) detected by Voyager10 is present in our ourSaturn GCM DYNAMICO simulations. Our GCM re-sults also indicate that other tropical Rossby modes(wavenumbers 3, 4, 5, . . . ) are likely to be significant
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Figure 1: Latitude-pressure section of zonal-mean temper-ature T [top] from Cassini/CIRS 2015 nadir retrievals 9 vs.[middle] from our Saturn DYNAMICO GCM in the 10thsimulated year, Ls ∼ 70◦. [Bottom] Meridional profilesof upper-troposphere T at two opposite solstices, Cassini(crosses, CIRS nadir 8,9) vs. GCM simulations (lines).
within Saturn’s tropics. This is compliant with CassiniCIRS observations4 which show a complex structureat a pressure level of 150 mbar. The presence of botheastward and westward vertically-propagating equato-rial waves in our Saturn DYNAMICO GCM simula-tions is crucial to explain equatorial oscillations.
EPSC AbstractsVol. 13, EPSC-DPS2019-758-1, 2019EPSC-DPS Joint Meeting 2019c© Author(s) 2019
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Figure 2: Spectral analysis of the equatorial waves pro-duced in our Saturn DYNAMICO GCM (in dynamicalsteady-state). The dispersion relations for equatorial wavesare superimposed as colored lines.
Eddy forcing of jets Cloud tracking with CassiniISS permits to calculate the conversion C per unitmass of eddy kinetic energy to zonal-mean kinetic en-ergy2. The positive conversion rates C simulated byour Saturn DYNAMICO GCM (Figure 3) indicate thatour model supports the conclusion that Saturn’s zonalbanded jets are, for a significant part, driven and main-tained by eddies in the weather layer. Our model pre-dicts C ∼ 0.5 × 10−5 m2 s−3 at the tropopause, anorder-of-magnitude match to the Cassini values, im-plying a typical timescale of replenishing the jets ofless than a Saturn year. Nevertheless, the conversionrate C increases with altitude in our GCM simulationcontrary to the Cassini observations. This probably in-dicates a lack of tropospheric eddy forcing e.g. latentheat release and convective motions associated withmoist processes11 in the current version of our GCM.
Perspectives Those results strengthen our SaturnDYNAMICO GCM as a useful tool to study Saturn’stropospheric circulations. Challenges remain to repro-duce Saturn’s powerful superrotating jet and hexagon-shaped circumpolar jet in the troposphere6. Our Sat-urn GCM is only a first step towards a GCM systemable to simulate all giant planet environnements.
Bibliography[1] F. M. Flasar, et al. Temperatures, Winds, and Composition in the Satur-
nian System. Science, 307:1247–1251, 2005.[2] A. D. Del Genio and J. M. Barbara. Constraints on Saturn’s tropo-
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0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00eddy-to-mean KE conversion rate per unit mass (10 5 W/kg)
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Figure 3: [Top] View of simulated tropospheric jets, ed-dies and vortices by potential vorticity mapping. [Bottom]Rate per unit mass C in m2 s−3 (or W kg−1) estimating theconversion of eddy kinetic energy to zonal-mean kinetic en-ergy. Vertical profiles of C are shown, anually-averaged andglobally-averaged (60◦S to N latitudes).
[4] S. Guerlet, et al. Equatorial Oscillation and Planetary Wave Activity inSaturn’s Stratosphere Through the Cassini Epoch. Journal of Geophys-ical Research (Planets), 123:246–261, 2018.
[5] S. Guerlet, et al. Global climate modeling of Saturn’s atmosphere.Part I: Evaluation of the radiative transfer model. Icarus, 238:110–124,2014.
[6] A. Spiga, et al. Global climate modeling of Saturn’s atmosphere. PartII: multi-annual high-resolution dynamical simulations. arXiv e-prints,2018.
[7] T. Dubos, et al. Dynamico-1.0, an icosahedral hydrostatic dynamicalcore designed for consistency and versatility. Geoscientific Model De-velopment, 8(10):3131–3150, 2015.
[8] L. N. Fletcher, et al. Characterising Saturn’s vertical temperature struc-ture from Cassini/CIRS. Icarus, 189:457–478, 2007.
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[11] Y. Lian and A. P. Showman. Generation of equatorial jets by large-scalelatent heating on the giant planets. Icarus, 207:373–393, 2010.
