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.
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
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
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)
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
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
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 reevaporates in sub-saturated layers en route
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
Looking at two Titan years of model output:
Planetocentric solar longitude (Ls)
One Titan year
Planetocentric solar longitude (Ls)
Northern
spring
equinox
Norther
n fall
equinox
Planetocentric solar longitude (Ls)
Northern
spring
equinox
Huygens
Ls 90°
Ls 180° Ls 270°
(Oct
(Nov
2002)
1995)
Today
Ls 0°
(Aug
2009)
Ls 90°
(May
2017)
Ls 180° Ls 270°
Planetocentric solar longitude (Ls)
Ls 0°
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)
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)
3 Titan years:
Jan:
2015 2020 2025
Huygens Today
2005 2010 2015 2020 2025
Huygens Today
2005 2010 2015 2020 2025
Planetocentric solar longitude (Ls)
Huygens Today
2005 2010 2015
3 Titan years:
Jan:
2015 2020
Huygens Today
2000 2005 2010 2015 2020
Huygens Today
2000 2005 2010 2015 2020
Large cloud
outbursts at the
south pole in
summer
Planetocentric solar longitude (Ls)
Huygens Today
2000 2005 2010 2015
3 Titan years:
Jan:
2015 2020
Huygens Today
Huygens Today
2000 2005 2010 2015 2020
Clouds (with
occasional
rain) follow
the ITCZ as
it crosses the
equator in
northern
spring
2000 2005 2010 2015 2020
Note yearto-year
differences
Planetocentric solar longitude (Ls)
Huygens Today
2000 2005 2010 2015
3 Titan years:
Jan:
2015 2020
Huygens Today
2000 2005 2010 2015 2020
Huygens Today
2000 2005 2010 2015 2020
Appear
more
extended
in latitude
than in
the south
Large cloud
outbursts at the
north pole in its
summer
Planetocentric solar longitude (Ls)
Huygens Today
2000 2005 2010 2015
3 Titan years:
Jan:
2015 2020
Huygens Today
2000 2005 2010 2015 2020
Far fewer clouds
as the ITCZ
crosses the
equator again in
southern spring
Huygens Today
2000 2005 2010 2015 2020
Again, note
year-toyear
differences
Planetocentric solar longitude (Ls)
Huygens Today
2000 2005 2010 2015
3 Titan years:
Jan:
2015 2020
Huygens Today
Huygens Today
2000 2005 2010 2015 2020
2000 2005 2010 2015 2020
Some cloud
activity at the
poles before the
‘main events’;
more at north
than south
Planetocentric solar longitude (Ls)
Huygens Today
2000 2005 2010 2015
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
Titan years
So what is the net effect on surface methane?
Red = surface methane
increase > 70°N
Blue = surface methane
increase > 70°N
Note: remaining non-polar
surface methane now
resides in atmosphere
Note: results
shown
previously
came from
here
Green = surface methane
decrease outside polar
regions
Titan years
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
Titan years
What happens if we reverse perihelion (so it now
occurs during northern summer instead)?
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
Titan years
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]
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
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
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.