General Circulation Modelling on Triton and Pluto

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General Circulation Modelling on Triton and Pluto

• F. Forget (N. Descamps)
• LMD, ISPL Paris

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General Circulation Models ? GCMs ? Global
Climate modelsDesigned to completely simulate
a terrestrial planet environment

• Used on Earth for all climate science (weather
  forecast, data assimilation climatology,
  chemistry, plaeoclimate, climate change,
  biosphere studies)
• Mars idem
• Titan idem
• Venus under developement
• Triton
• ? Pluto

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Triton and Pluto very thin atmospheres

• Triton 1.4 Pa , mostly N2, with some CH4 (in
  1989)
• 2-4 or 6 Pa today ?
• Pluto 1-5 Pa , N2, with a little CO, Ar, CH4
• The pressure is high enough to fully compute the
  dynamic with the primitive equation of
  meteorology
• (GCM thermosphere are used up to 10-8 Pa)
• BUT Specific processes related to the direct
  condensation/sublimation of a large part of the
  atmosphere

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Turbulent Mixing in the planetary boundary layer

• On Pluto and Triton effect of surface
  condensation sublimation
• We use Turbulent closure scheme based on Mellor
  and Yamada 2.5 parameterisation specially
  adapted to stable atmosphere (Forget et al. 1999)
  numerical algorithm to compute the
  condensation/sublimation effect (Forget et al.
  1998)

Condensation
Sublimation
Bare Ground
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• Surface condensation of N2
• Near Surface enrichment of other gases
• ?The near surface composition can strongly
  enriched with less volatile species (Ar ?)
• Affect near surface condensation temperature
• Density Induced convection
• Affect fluid dynamics, etc
• Very alien meteorology !

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4 3 2 1

• Numerical algorithm adapted from Mars
• (Forget et al. 2006, Granada abstracts)

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Experience with Mars CO2 polar caps
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MARS Detection of Argon enrichment due to CO2
condensationby Mars Oddyssey Gamma Ray
Spectrometer(GRS)(mean Ar mixing ratio in
75S-90S)
Sprague et al. 2004
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Mars polar night
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Sublimation of CO2 ice and snow on Mars
1 km
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Formation of Spider in the criptic region
(Piqueux et al. 2003Kieffer et al. 2006)
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Formation of Spider in the criptic region
(Piqueux et al. 2003)
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Some results with Triton GCM
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TRITON ATMOSPHERE

• Some things we know about Triton in 1989
• Surface dark streaks direction eastward
  surface wind in the southern hemisphere
• Geyser like Plumes
• Westward wing at 8 km in the southern hemisphere
• Tropopause around 8 km
• What we dont know well
• Surface frost distribution (N2, CO, CH4, )

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Triton General Circulation Model

• 1) Hydrodynamical code
• to compute large scale atmospheric motions
• and transport
• Grid point model
• Horizontal resolution 200 km (32x24)
• 15 vertical layers (5m, 20m, 50 km)
• 2) Physical parameterizations
• ? to force the dynamic
• to compute the details of
• the local climate

Flux from thermosphere
T(z)
Thermal conduction
Atm N2 condensation
Convection
Turbulence
Surface N2 condensation
Internal heat flux
Subsurface conduction (13 layers)
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• Triton free atmosphere processes hypothesis
• No radiative transfer below 40 km (Yelle et al.
• Conduction Temperature below 40 km insensitive
  to thermosphere variations (400 km)
• ?Constant flux from thermosphere

Example of temperature profiles
Amplitude of temperature variations
400 km 60 km 40 km
Tmean
Tmin
Tmax
Period 1 triton day
Diurnal cycle
Period 15 Earth years
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Case 1 Triton totally covered by CO2 frost

• Flux top 1.15 10-6 W m-2
• Ice emissivity 0.6
• Thermal inertia 293 SI
• Flux géothermique 0.06 W m-2
• Albedo

0.85
0.6
90S 20S 25N 90N
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Case 1 Triton totally covered by CO2 frost
With condensation effect
No condensation effect
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Case 1 Triton totally covered by CO2 frost
Retro-super-rotation
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Case 1 Triton totally covered by CO2 frost
Plume ( 8km)
Wind streaks
Plumes (z8km)
60S
40S
surface
20S
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Case 2 Triton Unfrosted Equatorial band(from
Ingersoll 1990)

• Flux top 1.15 10-6 W m-2
• Ice emissivity 0.6
• Thermal inertia 293 SI
• Flux géothermique 0.06 W m-2
• Albedo

0.85
0.6
90S 20S 25N 90N
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Case 2 Triton Unfrosted Equatorial band
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Case 2 Triton Unfrosted Equatorial band
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Case 2 Triton Unfrosted Equatorial band
Plume ( 8km)
Wind streaks
Plumes (z8km)
Plume ( 8km)
Wind streaks
Plumes (z8km)
60S
40S
surface
60S
40S
20S
surface
20S
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Case 3 Frost Free Southern hemisphere( Dark
cap model , Hansen and Paige, 1992)

• Flux top 1.15 10-6 W m-2
• Ice emissivity 0.53
• Thermal inertia
• Flux géothermique 0.06 W m-2
• Albedo

0.8
0.6
90S 30N 90N
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Case 3 Frost Free Southern hemisphere( Dark
cap model , Hansen and Paige, 1992)
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Case 3 Frost Free Southern hemisphere( Dark
cap model , Hansen and Paige, 1992)
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Case 3 Frost Free Southern hemisphere( Dark
cap model , Hansen and Paige, 1992)
Plume ( 8km)
Wind streaks
Plumes (z8km)
60S
40S
surface
20S
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Triton first findings from GCM

• General results consistent with voyager
  observations in 1989
• Tropopause around 8 km
• Wind streaks diection
• But Enigma prograde winds at plume location
• Most likely Atmospheric absorbent (CH4)
• Sensitivity to Frost locations
• ? Simplified GCM used to explore the frost
  distributions on Triton

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New calculation of Triton seasonal variations
required to compute cap evolution (Forget et al.
2000)
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Example Dark cap model Suggested by Hansen
and Paige (1991) ? Fail to explain pressure
increase
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Example Dark cap model Suggested by Hansen
and Paige (1991) ? Fail to explain pressure
increase
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High thermal inertia model (inspired by Spencer
and Moore 1992) Inertia 500 SI It works
! Show the sensitivity of the frost distribution
to topography (i.e. pressure, geothermal flux
assymetry)
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High thermal inertia model (inspired by Spencer
and Moore 1992) Inertia 500 SI It works
! Show the sensitivity of the frost distribution
to topography (i.e. pressure, geothermal flux
assymetry) ?Also true for Pluto
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Adapting Triton GCM to Pluto

• Require to add radiative transfer modelling with
  CH4 (Strobel et al. 1995)
• Solar heating at 3.3 et 2.3 µm (NLTE)
• NLTE emission at 7.6 µm ?
• LTE cooling by rotational lines (CO)
• Very interesting atmosphere !
• Role of hazes and clouds
• Like on Triton ?Much to learn from simplified GCM
  (EBL) with interactive caps topography
• To be continued