Superrotation Mechanisms and Seasonal Changes on Titan in the Ashima Research Titan

General Circulation Models

Claire Newman, Yuan Lian and Mark Richardson

Work funded through the OPR program, and simulations performed on the NASA Ames High End Computing cluster

Goals of this work

• To reproduce key aspects of Titan’s circulation in a 3-dimensional general circulation model (GCM)

• To form a robust understanding of the dynamical mechanisms responsible

• To produce robust predictions of seasonal changes in Titan’s circulation

In previous work, we simulated strong, realistic stratospheric superrotation and seasonal change similar to that observed --

Zonal winds in northern summer Zonal winds in southern summer

Pres

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(mba

r)Comparing the TitanWRF GCM with observations

Zonal winds from CIRS in 2005 (Ls~293-323°)

Zonal winds from CIRS in 2011 (Ls~20-26°) [from Achterberg et al.]

-60 -30 0 30 60Latitude (deg N)

-90 -60 -30 0 30 60 90Latitude (deg N)

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-90 -60 -30 0 30 60 90Latitude (deg N)

TitanWRF predictions for same times

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CIRS zonal winds for Ls~293-323° TitanWRF zonal winds for same period

Zonal wind peaks at a lower altitude than observed (likely due to the ‘low’ model top and/or the lack of active haze advection)

Discrepancies between TitanWRF and observations

Also, no lower stratosphere zonal wind minimum as seen by Huygens and also Cassini (Flasar, 2012)

Stratospheric superrotation in TitanWRFWe find superrotation is produced by episodic ‘transfer events’

Planetocentric solar longitude (in ° Ls)

d(an

gula

r mom

entu

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s2

Southern dM/dt(Pole to 22.5°S)

Northern dM/dt(Pole to 22.5°N)

Equatorial dM/dt(22.5°S to 22.5°N)

Rate of change of angular momentum in 3 regions over a Titan year

Stratospheric superrotation in TitanWRF

Planetocentric solar longitude (in ° Ls)

Angular momentum ‘transfer events’ between northern/southern hemisphere and equatorial region, in northern/southern late fall-spring

Unstable region develops on low-latitude flank of

~winter zonal jet

Waves carry westward angular momentum -> jet

=> accelerate low latitudes

Waves break depositing westward angular

momentum => decelerate high latitudes

Momentum transport during a ‘transfer event’

In TitanWRF we found too much atmospheric mixing disrupts these delicate wave processes, leading to weak stratospheric circulations

Question 1: Does another GCM using identical radiative forcing produce a similar circulation?- This is actually a well-known problem in Titan modeling

Question 2: Do we see episodic ‘transfer events’ in a different Titan GCM?- How robust is our proposed superrotation mechanism?

Question 3: How delicate are the wave interactions involved in driving Titan’s equatorial superrotation?- Was the need to minimize mixing limited to TitanWRF?

Questions we wanted to answer

We examined two GCMs and four ‘set-up’s:

1. TitanWRF [Newman et al., Icarus, 2011]• Lat-lon grid, finite-difference solver

2. TitanWRF with a ‘rotated pole’• Numerical pole and ‘polar’ filtering now at the equator

Filtering to avoid instabilities where grid spacing is small

Grid rotated through 90°

We examined two GCMs and four ‘set-up’s:

3. Titan MITgcm [Mars version described in Lian et al., Icarus, 2012]• Lat-lon grid, finite-volume solver

4. Titan MITgcm using ‘cubed-sphere’ grid• No singularities at poles• ‘Special points’ at cube corners

Which produced realistic superrotation?

1. TitanWRF with a standard lat-lon grid

2. Titan MITgcm with a standard lat-lon grid

Which had problems?2. TitanWRF with rotated pole 4. MITgcm with cubed-sphere grid

What do the problem set-ups have in common?

In both cases, we’re ‘messing with’ the low- to mid-latitudes where the waves are produced that are

crucial to driving superrotation

TitanWRF with rotated pole Titan MITgcm with cubed-sphere grid

Has filtered regions at both numerical poles

Has 6 special ‘corner’ points

Superrotation index for the MITgcm lat-lon gridSuperrotation index = mass-weighted angular momentum of layer

that of same layer at rest wrt the solid surface

Superrotation index for the MIT cubed-sphere grid

Far weaker superrotation is achieved with the cubed-sphere grid

Question 3: How delicate are the wave interactions involved in driving Titan’s equatorial superrotation?- Was the need to minimize mixing limited to TitanWRF?

Answer from this work: The dynamics of the low- to mid-latitudes should be treated very carefully to avoid disrupting vital wave-mean flow interactions- This does not seem to be limited to TitanWRF

Questions we wanted to answer

Planetocentric solar longitude (in ° Ls)

dM/d

t in

kg m

2/s2

TitanWRF

Another year of TitanWRF

dM/d

t in

kg m

2/s2

Planetocentric solar longitude (in ° Ls)

dM/d

t in

kg m

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TitanWRF

Titan MITgcm

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kg m

2/s2

Question 2: Do we see episodic ‘transfer events’ in a different Titan GCM?- How robust is our proposed superrotation mechanism?

Answer from this work: Momentum transport in the Titan MITgcm is remarkably close to that in TitanWRF despite big differences in dynamical core / numerics - The mechanism appears to be quite robust

Questions we wanted to answer

The circulation in TitanWRF and the MITgcmComparing winds at Ls = 270°: TitanWRF has larger peak wind

speeds, but Titan MITgcm simulates a strong zonal wind minimum

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TitanWRF Titan MITgcm

The circulation in TitanWRF and the MITgcmComparing temperatures at Ls = 270°: largely look very similar, but

slight variations can have a big impact on dynamics

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TitanWRF Titan MITgcm

Question 1: Does another GCM using identical radiative forcing produce a similar circulation?- This is actually a well-known problem in Titan modeling

Answer: Many similarities, but differences in detail: e.g. superrotation strength; sharper vertical gradients- Much more to investigate here!

Questions we wanted to answer

Conclusions• At Ashima we now have two superrotating Titan GCMs with

similarly realistic circulations: TitanWRF and Titan MITgcm

• Our proposed mechanism for the production of equatorial stratospheric superrotation in TitanWRF – via ‘episodic transfer events’ – is supported by Titan MITgcm results

• As found before, GCM set-ups that disrupt the low- to mid-latitudes (e.g. too much diffusion, filtering, etc.) disrupt the delicate wave momentum transports responsible

• Titan MITgcm also captures far more of the observed zonal wind minimum above the tropopause than TitanWRF– Could be due to improved accuracy of temperature advection– However, tropospheric wind speeds in Titan MITgcm are too small

Tropospheric methane ice cloud predicted by TitanWRF

More future work: study the CH4 cycle in the MITgcm!