general circulation modelling on triton and pluto f. forget (n. descamps) lmd, ispl paris
TRANSCRIPT
General Circulation Modelling on Triton and Pluto
F. Forget (N. Descamps)
LMD, ISPL Paris
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
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
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)
Bare Ground
Condensation Sublimation
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 !
• Numerical algorithm adapted from Mars
(Forget et al. 2006, Granada abstracts)
4
3
2
1
Experience with Mars CO2 polar caps
MARS : Detection of Argon enrichment due to CO2 condensationby Mars Oddyssey Gamma Ray Spectrometer
(GRS)(mean Ar mixing ratio in 75°S-90°S)
Sprague et al. 2004
Mars polar night
Sublimation of CO2 ice and snow on Mars
1 km
Formation of “Spider” in the “criptic” region
(Piqueux et al. 2003Kieffer et al. 2006)
Formation of “Spider” in the “criptic”
region (Piqueux et al. 2003)
Some results with Triton’ GCM
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, …)
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 T(z) Thermal conduction
Flux from thermosphere
Convection
Surface N2 condensation
Atm N2 condensation
TurbulenceInternal heat
fluxSubsurface conduction (13 layers)
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
400 km
60 km
40 km
Example of temperature profiles Amplitude of temperature variations
Tmin Tmean Tmax
Diurnal cycle
Period = 1 triton day
Period = 15 Earth
years
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
0.690°S 20°S 25°N 90°N
Case # 1 Triton totally covered by CO2 frost
With condensation effect
No condensation effect
Case # 1 Triton totally covered by CO2 frost
Retro-super-rotation
Plume( ~8km)
60°S40°S
20°S
Plumes(z~8km)
Wind streaks
surface
Case # 1 Triton totally covered by CO2 frost
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
0.690°S 20°S 25°N 90°N
Case # 2 Triton Unfrosted Equatorial band
Case # 2 Triton Unfrosted Equatorial band
Plume( ~8km)
60°S40°S
20°S
Plumes(z~8km)
Wind streaks
surface
Plume( ~8km)
60°S40°S
20°S
Plumes(z~8km)
Wind streaks
surface
Case # 2 Triton Unfrosted Equatorial band
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
0.690°S 30°N 90°N
Case # 3 Frost Free Southern hemisphere
(« Dark cap model », Hansen and Paige, 1992)
Case # 3 Frost Free Southern hemisphere(« Dark cap model », Hansen and Paige, 1992)
Case # 3 Frost Free Southern hemisphere(« Dark cap model », Hansen and Paige, 1992)
Plume( ~8km)
60°S40°S
20°S
Plumes(z~8km)
Wind streaks
surface
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…
New calculation of Triton seasonal variations required to compute cap evolution…
(Forget et al. 2000)
Example :«Dark cap model »Suggested by Hansen and Paige (1991)
Fail to explain pressure increase
Example :«Dark cap model »Suggested by Hansen and Paige (1991)
Fail to explain pressure increase
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)
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
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…