general circulation modelling on triton and pluto f. forget (n. descamps) lmd, ispl paris

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


With condensation effect

No condensation effect


Retro-super-rotation

Plume( ~8km)

60°S40°S

20°S

Plumes(z~8km)

Wind streaks

surface


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

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)


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

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…

general circulation modelling on triton and pluto f. forget (n. descamps) lmd, ispl paris

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