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 re-evaporates 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

Planetocentric solar longitude (Ls)

Lat

itud

e (d

egre

es n

orth

)Looking at two Titan years of model output:

Planetocentric solar longitude (Ls)

Lat

itud

e (d

egre

es n

orth

)

One Titan year

Planetocentric solar longitude (Ls)

Lat

itud

e (d

egre

es n

orth

)Northern

spring equinox

Northern fall

equinox

Northern spring

equinox

Planetocentric solar longitude (Ls)

Lat

itud

e (d

egre

es n

orth

)Ls 0° (Aug 2009)

Ls 180°Ls 270° (Oct

2002)

Ls 90° (May 2017)

Ls 180° (Nov 1995)

Ls 270° Ls 0°Ls 90°

TodayHuygens

One possible methane cycle with latent heating onColumn mass of methaneSurface temperature (K)

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

Planetocentric solar longitude (Ls) Planetocentric solar longitude (Ls)

Surface evaporation

Peak vertical velocity in troposphere Integrated column cloud mass

Precipitation at surface

TodayHuygens

Jan: 2015 2020 2025 2005 2010 2015 2020 2025 2005 2010 2015 2020 2025 2005 2010 2015

TodayHuygens TodayHuygens3 Titan years:

Planetocentric solar longitude (Ls)

TodayHuygens

Jan: 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015

TodayHuygens TodayHuygens3 Titan years:

Planetocentric solar longitude (Ls)

Large cloud outbursts at the south pole in summer

TodayHuygens

Jan: 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015

TodayHuygens TodayHuygens3 Titan years:

Planetocentric solar longitude (Ls)

Clouds (with occasional rain) follow the ITCZ as it crosses the equator in northern spring

Note year-to-year differences

TodayHuygens

Jan: 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015

TodayHuygens TodayHuygens3 Titan years:

Planetocentric solar longitude (Ls)

Large cloud outbursts at the north pole in its summer

Appear more extended in latitude than in the south

TodayHuygens

Jan: 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015

TodayHuygens TodayHuygens3 Titan years:

Planetocentric solar longitude (Ls)

Far fewer clouds as the ITCZ crosses the equator again in southern spring

Again, note year-to-year differences

TodayHuygens

Jan: 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015 2020 2000 2005 2010 2015

TodayHuygens TodayHuygens3 Titan years:

Planetocentric solar longitude (Ls)

Some cloud activity at the poles before the ‘main events’; more at north than south

So what is the net effect on surface methane?C

hang

e in

sur

face

mas

s

Titan years

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

So what is the net effect on surface methane?C

hang

e in

sur

face

mas

s

Titan years

Red = surface methane increase > 70°N

Blue = surface methane increase > 70°N

Green = surface methane decrease outside polar regions

Note: results shown previously came from here

Note: remaining non-polar surface methane now resides in atmosphere

So what is the net effect on surface methane?C

hang

e in

sur

face

mas

s

Titan years

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

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)?

Cha

nge

in s

urfa

ce m

ass

Titan years

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

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.