Comparing TitanWRF and Cassini Results at the End of the Cassini Prime Mission Claire E. Newman, Mark I. Richardson, Anthony D. Toigo and Christopher Lee.

Comparing TitanWRF and Comparing TitanWRF and Cassini Results at the End Cassini Results at the End

of the Cassini Prime of the Cassini Prime MissionMission

Claire E. Newman,Claire E. Newman,

Mark I. Richardson, Anthony D. Toigo and Mark I. Richardson, Anthony D. Toigo and Christopher LeeChristopher Lee

GPS Division, California Institute of TechnologyGPS Division, California Institute of Technology

AGU Fall Meeting 2008AGU Fall Meeting 2008

What is TitanWRF?What is TitanWRF? Global, 3D numerical climate model for Titan based on Global, 3D numerical climate model for Titan based on

NCAR’s WRF (Weather Research and Forecasting) modelNCAR’s WRF (Weather Research and Forecasting) model

Uses Titan gravity, surface pressure, rotation rate etc..Uses Titan gravity, surface pressure, rotation rate etc..

Titan solar forcing (diurnal & seasonal cycle) with radiative Titan solar forcing (diurnal & seasonal cycle) with radiative transfer, boundary layer and surface/sub-surface schemestransfer, boundary layer and surface/sub-surface schemes

Can be run as a limited area or global model, or as a global Can be run as a limited area or global model, or as a global model with high resolution ‘nests’model with high resolution ‘nests’

Can be run with gravitational tides due to SaturnCan be run with gravitational tides due to Saturn

Can be run with a simple methane cloud schemeCan be run with a simple methane cloud scheme

Model description

Early simulations of Titan’s stratosphere Early simulations of Titan’s stratosphere Stratospheric results

Northern winter (Ls~293-323) period observed by Cassini [Achterberg et al. 2008]

Zonal mean T

Zonal mean u

Pre

ssu

re (

mb

)

Latitude (deg N)

Zonal mean T

Zonal mean u

Peak wind < 30m/s

The same time period in the original version of TitanWRF [Richardson et al. 2007]

Stratospheric results

Northern winter (Ls~293-323) period observed by Cassini [Achterberg et al. 2008]

Zonal mean T

Zonal mean u

Recent simulations of Titan’s stratosphere Recent simulations of Titan’s stratosphere

Zonal mean T Zonal

mean u

Same period in the latest version of TitanWRF: no horizontal diffusion

Pre

ssu

re (

mb

)

Latitude (deg N)

Stratospheric results

mean meridional circulation

Angular momentum transport in TitanWRFAngular momentum transport in TitanWRF

total advection

transient eddies

poleward transport

equatorward transport

Mean meridional circulation transports momentum polewardsMean meridional circulation transports momentum polewards

Eddies begin transporting significant momentum equatorwards after Eddies begin transporting significant momentum equatorwards after ~3 Titan years (once the winter zonal wind jet has become strong)~3 Titan years (once the winter zonal wind jet has become strong)

Stratospheric annual mean Stratospheric annual mean northwardnorthward transport of angular momentum transport of angular momentum

Stratospheric results

mean meridional circulation

total advectiontransient eddies

Northern winter solstice Northern spring equinox

poleward transport

equatorward transport

Strongest mean transport poleward; strongest eddy transport equatorward

Weak equatorward eddy transport opposes poleward mean transport

Stratospheric results

Reducing horizontal diffusion was Reducing horizontal diffusion was vitalvital for a realistic stratosphere for a realistic stratosphere

An improved match to observed seasons increases our confidence in An improved match to observed seasons increases our confidence in predictions for predictions for otherother seasons - e.g.: seasons - e.g.:

Strong gradients at high latitudes require better treatment of the polar Strong gradients at high latitudes require better treatment of the polar boundary condition, so we are currently improving this in TitanWRFboundary condition, so we are currently improving this in TitanWRF

Northern fall circulation in TitanWRFNorthern fall circulation in TitanWRF

Zonal mean T

Zonal mean u

Pre

ssur

e (m

b)

Latitude (deg N)

Stratosphere summaryStratosphere summary

Future workFuture work

Surface results

Surface winds and observed dune featuresSurface winds and observed dune featuresMap of inferred dune directions (Lorenz, Radebaugh and the Cassini radar team)

Lat

itud

e (d

eg N

)

-

Longitude (deg W)

Dunes mostly within 30° of equatorDunes mostly within 30° of equator

Surface features suggest that dunes Surface features suggest that dunes formed in westerly formed in westerly (from the west)(from the west) winds winds

Cassini radar image

-60

-3

0

0

30

60

But models / basic atmospheric dynamics predict But models / basic atmospheric dynamics predict easterlieseasterlies here:here:

Surface results

0.5 m/s

Annual mean winds (45S-45N) from TitanWRF with tides included

Longitude (deg E)

Lat

itud

e (d

eg N

)L

atit

ude

(deg

N)

-

-30

0

30

Surface results

NNE

ENE

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0. 45 0.5 0.55 m/s

Plots show annual mean wind magnitude at each gridpoint in the chosen direction

ESE

SSE

Do find some of the strongest winds from 30S-30N pointing NNE or SSE

But from 15S-15N they spend < 5% of their time in these directions

And for 30-15S and 15-30N it’s still only 15-20%

*

*

*

What about instantaneous winds?

Surface results

Surface temperature variations in TitanWRFSurface temperature variations in TitanWRF

Planetocentric longitude (Ls)

Lat

itu

de

(deg

N)

For Ls ~ 316-357, Cassini found [Jennings et al. 2008]:

TitanWRF

Drop from equator to north

pole = ~ 4K

Drop from equator to south pole = ~ 1.5K

Peak at ~ 20S of ~92.3K

Drop from equator to north pole = ~ 3K

Peak at ~ 10S of ~ 93.7K

Drop from equator to south pole = ~2K

Surface results

Surface summarySurface summary Mean low latitude winds in TitanWRF don’t match directions inferredMean low latitude winds in TitanWRF don’t match directions inferred

Winds with Winds with somesome westerly component occur < 5% of the year for 15S- westerly component occur < 5% of the year for 15S-15N and <20% for 30-15S and 15-30N, though are relatively strong15N and <20% for 30-15S and 15-30N, though are relatively strong

Surface temperatures match Cassini observations fairly wellSurface temperatures match Cassini observations fairly well

Look at correlations between predicted winds that are close to the Look at correlations between predicted winds that are close to the observed wind direction and the near-surface environmentobserved wind direction and the near-surface environment

Look at effect of including variable topography / surface propertiesLook at effect of including variable topography / surface properties

Future workFuture work

Surface methane evaporationSurface methane evaporation

Condensation and immediate fall-out when methane mixing Condensation and immediate fall-out when methane mixing ratio exceeds specified saturation ratioratio exceeds specified saturation ratio

Precipitation if condensate doesn’t re-evap on way downPrecipitation if condensate doesn’t re-evap on way down

In results shown, no latent heat and infinite surface methaneIn results shown, no latent heat and infinite surface methane

Methane cycle

Simple methane cloud modelSimple methane cloud model

The two dominant controlling factors are:

1. Near-surface temperatures (=> ability to hold methane)

2. Upwelling in atmosphere (=> cooling => clouds)

Methane cycleControls on evaporationControls on evaporation

Time of year (°Ls) 330 0 30 60 90 120 150 180 210 240 270 300

=>

=>

+

=>

=>

Time of year (°Ls)

Lat

itud

e (d

eg N

) -

60

-30

0

30

6

0 -

60

-30

0

30

6

0 -

60

-30

0

30

6

0

Lat

itud

e (d

eg N

)L

atit

ude

(deg

N)

Solar heating of troposphere Near-surface air temperature

Near-surface methane needed for saturation Actual near-surface methane

Amount needed to saturate near-surface air Evaporation

330 0 30 60 90 120 150 180 210 240 270 300

Methane cycle

Upwelling in TitanWRF’s troposphereUpwelling in TitanWRF’s troposphere

Lat

itud

e (d

eg N

) -

60

-

30

0

30

60

Planetocentric longitude (°Ls) 330 0 30 60 90 120 150 180 210 240 270 300 330

Double Hadley cell; upwelling region

moves rapidly

Single, persistent pole-to-pole Hadley cells around the solstices

Equinox (2 ~symmetric cells)

Northern summer solstice (1 pole-to-pole cell)

Southern summer solstice (1 pole-to-pole cell)

Plot the upwelling region by plotting the

maximum vertical velocity (in the troposphere) through one Titan year:

Latitude

Pre

ssur

e (m

bar)

Methane cycleControls on clouds and precipitationControls on clouds and precipitation

Maximum vertical velocity in troposphere

Lat

itud

e (d

eg N

)

Cloud condensation Surface precipitation

-60

-30

0

30

60

Planetocentric longitude (°Ls) 330 0 30 60 90 120 150 180 210 240 270 300 330 0 30 60 90 120 150 180 210 240 270 300

=>=>

Methane cycle

Net transfer from South to NorthNet transfer from South to North

330 0 30 60 90 120 150 180 210 240 270 300 330

Planetocentric longitude (°Ls)

-60

-

30

0

30

60

Lat

itud

e (d

eg N

)

Net increase in surface methane since start

Evaporation

Precipitation

More evaporation

during S summer

More precipitation during N summer

Column mass of methane

330 0 30 60 90 120 150 180 210 240 270 300

More transport from south to north than

north to south

-60

-

30

0

30

60

Lat

itud

e (d

eg N

)

Planetocentric longitude (°Ls)

Methane cycle

Methane cycle summary: analogy with MarsMethane cycle summary: analogy with Mars

S pole

Mars

Warmer southern summer (since perihelion occurs here)

=>Atmosphere can hold more water vapor / methane gas

Titan

Both

=> More water vapor / methane gas transported into northern hemisphere

during/after southern summer than vice versa

CurrentCurrent TitanWRF results are not definitive

ButBut we expect TitanWRF to show preferential accumulation of methane at northern high latitudes once we allow regions to dry out

Will also have latent heat effects and a better tracer advection schemeWill also have latent heat effects and a better tracer advection scheme

N pole

Cooler northern summer =>

Surface build-up of water ice / methane liquid

Future workFuture work