Titan’s methane cycle in the TitanWRF general circulation model

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, andtracked 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

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

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

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

16. 3 Titan years: Huygens Huygens Today Huygens Today Today Jan: 2015 2020 2025 2005 2010 2015 2020 2025 2005 2010 2015 2020 2025 2005 2010 2015 Planetocentric solar longitude (Ls)

17. 3 Titan years: Huygens Huygens Today Huygens Today Today Jan: 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 Large cloud outbursts at the south pole in summer Planetocentric solar longitude (Ls)

18. 3 Titan years: Huygens Huygens Today Huygens Today Today Jan: 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 Clouds (with occasional rain) follow the ITCZ as it crosses the equator in northern spring Note year-to-year differences Planetocentric solar longitude (Ls)

19. 3 Titan years: Huygens Huygens Today Huygens Today Today Jan: 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 Appear more extended in latitude than in the south Large cloud outbursts at the north pole in its summer Planetocentric solar longitude (Ls)

20. 3 Titan years: Huygens Huygens Today Huygens Today Today Jan: 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 Far fewer clouds as the ITCZ crosses the equator again in southern spring Again, note year-to-year differences Planetocentric solar longitude (Ls)

21. 3 Titan years: Huygens Huygens Today Huygens Today Today Jan: 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 Some cloud activity at the poles before the ‘main events’; more at north than south 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) • Etc.