1. Titan’s methane cycle in the TitanWRF general circulation model
Claire E. Newman
Yuan Lian, Mark I. Richardson and Christopher Lee
Ashima Research
Anthony D. Toigo
APL
Work was supported by NASA’s OPR program and the NASA Astrobiology Institute, and all simulations were conducted on NASA’s High End Computing facility at NASA Ames.

2. Overview The TitanWRF General Circulation Model (GCM)
Using a GCM as a global, integrated retrieval tool
Example: stratospheric superrotation in TitanWRF
TitanWRF’s methane cloud scheme and an example of one possible methane cycle produced
North-south asymmetry of polar methane in TitanWRF
Cloud movies
Conclusions and further work

3. The TitanWRF GCM 3D atmospheric model from surface to ~400km
Includes thermal and gravitational tides, seasonally and diurnally-varying solar forcing, and full radiative transfer
Simulates observed magnitude of stratospheric superrotation [Newman et al., 2011]
Includes a simple methane cloud scheme with latent heat effects and finite surface methane

4. A GCM as a global retrieval tool
A GCM is the encapsulation of what we think we know and a collection of other hypotheses to be tested
If a GCM doesn’t match observations it’s either missing or incorrectly representing (e.g., incorrect parameters; inadequate complexity) a physical process that’s actually present
The more disparate the observations the better: it’s highly unlikely that a GCM will be able to match them all if a physical process is missing or inadequately represented
‘Tuning’ a GCM = retrieving quantities with a real physical meaning (e.g. thermal inertia of surface; total methane mass)

5. Example: stratospheric superrotation
TitanWRF produces realistic amounts of stratospheric superrotation (see movie next and at end)
We find that low latitudes receive ‘kicks’ of eastward angular momentum from the strong winter jet, during infrequent wave-driven ‘transfer events’ [Newman et al., 2011]
We (and others) have found that to produce stratospheric superrotation we must limit the amount of horizontal dissipation / diffusion imposed in the model
Note this has a real physical meaning: horizontal diffusion is used to represent sub-grid scale mixing, but too much appears to ‘mix away’ the smaller perturbations that develop into the large-scale waves responsible for superrotation

6. Next slide: zonal mean zonal wind movie
Zonal mean zonal winds predicted by TitanWRF, from the surface to ~400km over a period of ~3 Titan years

8. Methane cloud scheme
Methane is advected as a tracer in the atmosphere, and tracked at the surface (surface methane = initial surface methane + precipitation – evaporation)
Surface evaporation occurs if lowest atmospheric layer is sub-saturated, provided surface methane is present
Condensation occurs when the atmosphere is saturated
Condensate falls to surface as precipitation, unless re-evaporates in sub-saturated layers en route

9. Moist convection & latent heat effects
Latent heat is released / used when atmospheric methane condenses / re-evaporates
δT due to moist processes is limited to a maximum rate => condensation and evaporation are limited also
Vertical diffusion scheme mixes methane mmr and temperature following phase changes
Evaporation of surface methane also cools the surface
delta T of 0.01K/s has been used thus far

10. Looking at two Titan years of model output:
Latitude (degrees north)
Planetocentric solar longitude (Ls)

11. One Titan year
Latitude (degrees north)
Planetocentric solar longitude (Ls)

12. Latitude (degrees north)
Northern spring equinox
Northern fall equinox
Northern spring equinox
Latitude (degrees north)
Planetocentric solar longitude (Ls)

13. Latitude (degrees north)
Huygens
Today
Ls 90°
Ls 180° (Nov 1995)
Ls 270° (Oct 2002)
Ls 0° (Aug 2009)
Ls 90° (May 2017)
Ls 180°
Ls 270°
Ls 0°
Latitude (degrees north)
Today April 4th is Ls 32.3°
Planetocentric solar longitude (Ls)

14. One possible methane cycle with latent heating on
Surface temperature (K)
Column mass of methane
Near-surface methane abundance
Peak vertical velocity in troposphere
Planetocentric solar longitude (Ls)
Planetocentric solar longitude (Ls)

15. One possible methane cycle with latent heating on
Peak vertical velocity in troposphere
Integrated column cloud mass
Precipitation at surface
Surface evaporation
Planetocentric solar longitude (Ls)
Planetocentric solar longitude (Ls)

16. 3 Titan years:
Huygens
Today
Huygens
Today
Huygens
Today
Jan:
Jan 1st 2003 Ls 272.7; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;
Planetocentric solar longitude (Ls)

17. 3 Titan years:
Huygens
Today
Huygens
Today
Huygens
Today
Jan:
Large cloud outbursts at the south pole in summer
Jan 1st 2003 Ls 272.7; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;
Planetocentric solar longitude (Ls)

18. 3 Titan years:
Huygens
Today
Huygens
Today
Huygens
Today
Jan:
Clouds (with occasional rain) follow the ITCZ as it crosses the equator in northern spring
Jan 1st 2003 Ls 272.7; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;
Note year-to-year differences
Planetocentric solar longitude (Ls)

19. 3 Titan years:
Huygens
Today
Huygens
Today
Huygens
Today
Jan:
Appear more extended in latitude than in the south
Large cloud outbursts at the north pole in its summer
Jan 1st 2003 Ls 272.7; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;
Planetocentric solar longitude (Ls)

20. 3 Titan years:
Huygens
Today
Huygens
Today
Huygens
Today
Jan:
Far fewer clouds as the ITCZ crosses the equator again in southern spring
Jan 1st 2003 Ls 272.7; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;
Again, note year-to-year differences
Planetocentric solar longitude (Ls)

21. 3 Titan years:
Huygens
Today
Huygens
Today
Huygens
Today
Jan:
Some cloud activity at the poles before the ‘main events’; more at north than south
Jan 1st 2003 Ls 272.7; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;
Planetocentric solar longitude (Ls)

22. So what is the net effect on surface methane?
Red = surface methane increase > 70°N
Blue = surface methane increase > 70°N
Green = surface methane decrease outside polar regions
Net gain in northern polar surface methane
Change in surface mass
Titan years

23. So what is the net effect on surface methane?
Red = surface methane increase > 70°N
Blue = surface methane increase > 70°N
Green = surface methane decrease outside polar regions
Note: remaining non-polar surface methane now resides in atmosphere
Note: results shown previously came from here
Change in surface mass
Titan years

24. So what is the net effect on surface methane?
Red = surface methane increase > 70°N
Blue = surface methane increase > 70°N
Green = surface methane decrease outside polar regions
Net gain in NORTHERN polar surface methane
Change in surface mass
Titan years

25. What happens if we reverse perihelion (so it now occurs during northern summer instead)?

26. What happens if we reverse perihelion (so it now occurs during northern summer instead)?
Red = surface methane increase > 70°N
Blue = surface methane increase > 70°N
Green = surface methane decrease outside polar regions
Net gain in SOUTHERN polar surface methane
Change in surface mass
Titan years

27. Why?
There is increased methane transport into high latitudes by the tropospheric circulation in spring/summer
Rainout to surface over this period (increasing surface methane) is balanced by increased evaporation (decreasing surface methane); timings vary annually even in steady state
Summer not containing perihelion (currently northern) is longer and cooler => more precipitation and less evaporation => gains more surface methane
Both similarities and differences to Schneider et al. [2012]

28. Next slide: methane cloud map movie
Integrated column mass of ice ‘cloud’ in troposphere
Actually, integrated column mass of methane ice that condenses out in all tropospheric layers and falls to lower layers – does not subtract that which re-evaporates before reaching the surface
Following slide: zonal mean methane cloud movie
Zonal mean of methane ‘clouds’
Actually, zonal mean condensation (in yellow / bright green) and evaporation (blue / purple) in units of mass mixing ratio

31. Conclusions
TitanWRF has a simple methane cycle scheme with latent heat effects and a finite methane inventory (i.e., surface can dry)
The tropical surface dries out and high latitude surface moistens
For present day (warmer southern summer) we predict more surface methane in northern high latitudes at steady state
With timing of perihelion reversed (warmer northern summer) we predict more surface methane in southern high latitudes

32. Ongoing and future work
Now performing detailed comparisons between methane cycle observations and GCM predictions using steady state results
How can we improve the realism of our steady state results?
Vary physical parameters:
Surface thermal inertia (uniform or global map)
Maximum δT per second allowed due to latent heat
Total methane inventory
Etc.
Add / modify representations of processes:
More complex clouds (e.g. entrainment effects; microphysics)
Sub-surface diffusion of methane
Treat solar insolation properly (discussed in Lora’s talk yesterday)