1. General Circulation Modelling on Triton and Pluto
F. Forget (N. Descamps)
LMD, ISPL Paris

2. General Circulation Models  GCMs  Global Climate models Designed 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

3. 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

4. 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

5. 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 !

6. Numerical algorithm adapted from Mars4 3 2 1
Numerical algorithm adapted from Mars
(Forget et al. 2006, Granada abstracts)

7. Experience with Mars CO2 polar caps

8. MARS : Detection of Argon enrichment due to CO2 condensation by Mars Oddyssey Gamma Ray Spectrometer (GRS) (mean Ar mixing ratio in 75°S-90°S)
Sprague et al. 2004

9. Mars polar night

12. Sublimation of CO2 ice and snow on Mars
1 km

14. Formation of “Spider” in the “criptic” region (Piqueux et al
Formation of “Spider” in the “criptic” region (Piqueux et al Kieffer et al. 2006)

15. Formation of “Spider” in the “criptic” region (Piqueux et al. 2003)

16. Some results with Triton’ GCM

17. 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 don’t know well:
Surface frost distribution (N2, CO, CH4, …)

18. 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
Surface N2 condensation
Turbulence
Internal heat flux
Subsurface conduction (13 layers)

19. 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
Tmin
Tmean
Tmax
Period = 1 triton day
Diurnal cycle
Period = 15 Earth years

20. Case # 1 Triton totally covered by CO2 frost
Flux top = W m-2
Ice emissivity = 0.6
Thermal inertia = 293 SI
Flux géothermique = 0.06 W m-2
Albedo :
0.85
0.6
90°S °S °N °N

21. Case # 1 Triton totally covered by CO2 frost
With condensation effect
No condensation effect

22. Case # 1 Triton totally covered by CO2 frost
Retro-super-rotation

23. Case # 1 Triton totally covered by CO2 frost
Plume
( ~8km)
Wind
streaks
Plumes
(z~8km)
60°S
40°S
surface
20°S

24. Case # 2 Triton Unfrosted Equatorial band (from Ingersoll 1990)
Flux top = W m-2
Ice emissivity = 0.6
Thermal inertia = 293 SI
Flux géothermique = 0.06 W m-2
Albedo :
0.85
0.6
90°S °S °N °N

25. Case # 2 Triton Unfrosted Equatorial band

26. Case # 2 Triton Unfrosted Equatorial band

27. Case # 2 Triton Unfrosted Equatorial band
Plume
( ~8km)
Wind
streaks
Plume
( ~8km)
Plumes
(z~8km)
Wind
streaks
Plumes
(z~8km)
60°S
40°S
surface
60°S
40°S
surface
20°S
20°S

28. Case # 3 Frost Free Southern hemisphere (« Dark cap model », Hansen and Paige, 1992)
Flux top = W m-2
Ice emissivity = 0.53
Thermal inertia =
Flux géothermique = 0.06 W m-2
Albedo :
0.8
0.6
90°S °N °N

29. Case # 3 Frost Free Southern hemisphere (« Dark cap model », Hansen and Paige, 1992)

30. Case # 3 Frost Free Southern hemisphere (« Dark cap model », Hansen and Paige, 1992)

31. Case # 3 Frost Free Southern hemisphere (« Dark cap model », Hansen and Paige, 1992)
Plume
( ~8km)
Wind
streaks
Plumes
(z~8km)
60°S
40°S
surface
20°S

32. 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…

33. New calculation of Triton seasonal variations required to compute cap evolution…
(Forget et al. 2000)

34. Example :
«Dark cap model »
Suggested by Hansen and Paige (1991)
 Fail to explain pressure increase

35. Example :
«Dark cap model »
Suggested by Hansen and Paige (1991)
 Fail to explain pressure increase

36. 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)

37. 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

38. 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…