Investigating Titan’s Surface-Atmosphere Interactions with a General Circulation Model

1. Investigating Titan’s Surface-Atmosphere Interactions with a General Circulation Model Claire E. Newman, Mark I. Richardson and Yuan Lian Ashima Research, Pasadena, CA

2. Why a General Circulation Model (GCM)? • Atmospheric processes involve non-linear interactions and feedbacks => related phenomena can be complex, and hard to predict using theory or simple models • To fully understand surface phenomena linked to the atmosphere, we must model the full 3D circulation • Titan GCMs have uncertainties in physical processes / parameters (e.g., strength of sub-grid scale mixing) and boundary conditions (e.g., surface properties) • We can use comparison between GCM predictions and observations to constrain and refine the GCM, and thus improve our knowledge and understanding of real Titan

3. Surface Phenomena Examined Here • The observed seasonal variation in surface methane, and estimated evaporation/precipitation rates • These depend on atmospheric transport, cloud processes, and the atmosphere-surface exchange of heat and methane • We use a simple methane cycle scheme in our GCM, TitanWRF: • Surface evaporation • Atmospheric transport (advection and mixing) • Atmospheric condensation followed by instant deposition [unless sub-saturated layers exist below, in which case sufficient condensate is re-evaporated to saturate them] • Surface precipitation of any condensate that remains • Latent heating effects and finite surface methane abundances

4. Shortwave heating into the surface as a function of latitude and time of year (Ls) Newman et al. Icarus 2013, in review

5. Titan has a tilt, timing of perihelion, and seasons that are in many ways similar to Earth Present day perihelion Newman et al. Icarus 2013, in review

6. Infinite surface methaneNo latent heating effects

7. Surface temperature as a function of latitude and time of year Newman et al. Icarus 2013, in review

8. Tropospheric mass streamfunctions Northern summer (Ls=90-120°) Northern spring (Ls=0-30°) Northern autumn (Ls=180-210°) Northern winter (Ls=270-300°) Newman et al. Icarus 2013, in review

9. Tropospheric mass streamfunctions Northern summer (Ls=90-120°) Northern spring (Ls=0-30°) Northern autumn (Ls=180-210°) Northern winter (Ls=270-300°) Newman et al. Icarus 2013, in review

10. Peak upwelling speed in the troposphere Newman et al. Icarus 2013, in review

11. Peak upwelling speed in the troposphere Peak low and mid-latitude upwelling follows the approximate path of the Inter-tropical Convergence Zone (ITCZ) where main Hadley cell branches converge and rise Newman et al. Icarus 2013, in review

12. Peak upwelling speed in the troposphere Even stronger upwelling at high latitudes in late winter through into early summer Newman et al. Icarus 2013, in review

13. Peak upwelling speed in the troposphere Even stronger upwelling at high latitudes in late winter through into early summer Polar cells producing strong high latitude upwelling are greatly enhanced in TitanWRF with strong stratospheric superrotation Newman et al. Icarus 2013, in review

14. Meridional divergence in the lowest layer Newman et al. Icarus 2013, in review

15. Meridional divergence in the lowest layer Newman et al. Icarus 2013, in review

16. Methane distribution in the troposphere Column abundance (in kg m-2) Lowest layer mole fraction Newman et al. Icarus 2013, in review

17. Methane distribution in the troposphere Column abundance (in kg m-2) Lowest layer mole fraction Strong upwelling in late spring depletes near-surface methane Newman et al. Icarus 2013, in review

18. 3 Titan years of evaporation and precipitation Evaporation rate (in mm per Earth hour) Precipitation rate (in mm per Earth hour) Newman et al. Icarus 2013, in review

19. 3 Titan years of evaporation and precipitation Evaporation rate (in mm per Earth hour) Peak evaporation in late spring through early summer Precipitation rate (in mm per Earth hour) Newman et al. Icarus 2013, in review

20. Resultant change in surface methane over time Change (in mm) from value at start of simulation (~6 Titan years earlier) Newman et al. Icarus 2013, in review

21. Infinite surface methane simulations indicated that low and mid-latitudes tended to dry out, while high latitudes tended to gain methane [as in Mitchell 2008]Re-ran simulation starting with finite, globally-uniform surface methane cover equivalent to ~11m depth

22. Finite surface methaneStill no latent heating effectsResults shown for years after simulation reached steady state=> no long-term change in surface methane distribution

23. 3 Titan years of evaporation and precipitation Evaporation rate (in mm per Earth hour) Precipitation rate (in mm per Earth hour) Newman et al. Icarus 2013, in review

24. 3 Titan years of evaporation and precipitation Evaporation rate (in mm per Earth hour) Precipitation rate (in mm per Earth hour) Condensation (cloud) still occurs in low and mid-latitudes, but all the condensate re-evaporates before reaching the surface Newman et al. Icarus 2013, in review

25. 3 Titan years of surface methane (in mm) Newman et al. Icarus 2013, in review

26. The steady state surface methane distribution Surface methane changes over the entire simulation Green line shows surface methane decrease equatorward of 60° Red line shows surface methane increase north of 60° N In steady state, north pole has ~50% more methane than south Blue line shows surface methane increase south of 60° S Titan years from start Newman et al. Icarus 2013, in review

27. The steady state surface methane distribution Evaporation, precipitation and net change over the last 3 Titan years Evaporation N of 60° N Precipitation N of 60° N Net N of 60° N Evaporation S of 60° S Precipitation S of 60° S Net S of 60° S Planetocentric solar longitude, Ls (deg) Newman et al. Icarus 2013, in review

28. Results did not include evaporative cooling at the surface, or evaporative cooling / condensational heating in the atmosphere – i.e., there were no latent heat effects

29. Finite surface methaneAnd latent heating effects

30. 3 Titan years of evaporation and precipitation Evaporation rate (in mm per Earth hour) Precipitation rate (in mm per Earth hour) Newman et al. Icarus 2013, in review

31. 3 Titan years of condensation and precipitation Column-integrated methane condensate (in kg m-2 over 8 Titan days) Precipitation rate (in mm per Earth hour) Newman et al. Icarus 2013, in review

32. 3 Titan years of condensation and precipitation Column-integrated methane condensate (in kg m-2 over 8 Titan days) * * * * * * Precipitation rate (in mm per Earth hour) Times and locations of rainfall inferred from ISS observations * * * * * * Newman et al. Icarus 2013, in review

33. Peak upwelling speed in the troposphere Newman et al. Icarus 2013, in review

34. 3 Titan years of condensation and precipitation Column-integrated methane condensate (in kg m-2 over 8 Titan days) Precipitation rate (in mm per Earth hour) Only the strongest upwelling and condensation events result in surface precipitation Newman et al. Icarus 2013, in review

35. 3 Titan years of surface methane (in mm) Newman et al. Icarus 2013, in review

36. Surface temperatures and evaporative cooling No evaporative cooling With evaporative cooling Evaporative cooling + higher surface thermal inertia Including latent heating modifies the surface heat balance and produces much larger Tsurf gradients (and lower mean Tsurf) than observed The (currently uniform) surface thermal inertia is one of many parameters that impact this => may perhaps be constrained by comparison between modeled and observed Tsurf Newman et al. Icarus 2013, in review

37. Impact of reversing timing of perihelion Present day timing (just after southern summer solstice) Green line shows surface methane decrease equatorward of 60° Red line shows surface methane increase north of 60° N North gains relative to south Blue line shows surface methane increase south of 60° S Cumulative mass change (kg) ‘Reversed’ timing (just after northern summer solstice) South gains relative to north Newman et al. Icarus 2013, in review

38. Methane cycle conclusions • TitanWRF shows partial match to several aspects, including evaporation rates and cloud and rainfall observations • We predict more surface methane in northern than in southern high latitudes, with this reversed when perihelion is switched from southern to northern summer • In TitanWRF, this is basically due to more high latitude evaporation during the warmer (perihelion) summer • Ongoing work involves the improved treatment of cloud physics and methane’s behavior at/below the surface, before using observations to constrain the model further

39. Surface Phenomena Examined Here • The observed seasonal variation in surface methane and estimated evaporation/precipitation rates • Observed dune characteristics • Dunes are the time-integrated result of a non- linear, threshold-dependent, wind-driven process (saltation) • We predict dune orientations using the Gross Bedform-Normal Transport (GBNT) approach • We predict the direction of dune migration / motion using the net (resultant) sand transport direction over a Titan year

40. Predicting Titan dune characteristics Predicting direction of dune motion in a given wind field: • Expect overall direction of motion / migration to be roughly the resultant transport direction R • Rat any given time depends on • the choice of threshold wind stress for saltation to occur, and • the sand flux formulation • Ris then summed over a full Titan year to give the long-term resultant transport direction

41. Predicting Titan dune characteristics Predicting dune orientations in a given wind field: • Basic concept: dunes form due to sand transport in both directions across bedform • => Dunes will be oriented such that the total transport across the dune crest is maximized • Total transport = |B|+|C| summed over entire year = Gross Bedform-Normal Transport • Also depends on threshold and sand flux formulation B C See Rubin & Hunter, Science, 1987 for a full description of the GBNT approach

42. Dune crest orientation & resultant transport direction predicted for a threshold of 0.014Pa and no topography • Very little eastward resultant transport (very few eastward-pointing black arrows) equatorward of 25° • => Almost all dune motion expected to be toward the west

43. But if we include topography into TitanWRF… An extrapolated ‘plausible’ global topography map based on Cassini Radar data (pre Lorenz et al.’s 2013 map, which we will use next) [data provided by Karl Mitchell and Jeff Andrews-Hanna]

44. 30°N 0° 30°S from Lorenz and Radebaugh

45. Dune conclusions • Interaction between dunes and topographic obstacles hypothesized to imply largely eastward dune movement • Topography + higher thresholds can produce more predicted eastward resultant transport • However, predicted dune crest orientations are less consistent with observations for these conditions • Predicted dune morphology (based on relationship between predicted orientation and resultant transport direction) is also less consistent with dunes observed