Comparing TitanWRF and Cassini Results at the End of the Cassini Prime Mission

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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
• GPS Division, California Institute of Technology
• AGU Fall Meeting 2008

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What is TitanWRF?
Model description

• Global, 3D numerical climate model for Titan
  based on NCARs WRF (Weather Research and
  Forecasting) model
• Uses Titan gravity, surface pressure, rotation
  rate etc..
• Titan solar forcing (diurnal seasonal cycle)
  with radiative transfer, boundary layer and
  surface/sub-surface schemes
• Can be run as a limited area or global model, or
  as a global model with high resolution nests
• Can be run with gravitational tides due to Saturn
• Can be run with a simple methane cloud scheme

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Early simulations of Titans stratosphere
Stratospheric results
Northern winter (Ls293-323) period observed by
Cassini Achterberg et al. 2008
Zonal mean u
Zonal mean T
Pressure (mb)
The same time period in the original version of
TitanWRF Richardson et al. 2007
Zonal mean T
Peak wind lt 30m/s
Zonal mean u
Latitude (deg N)
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Recent simulations of Titans stratosphere
Stratospheric results
Northern winter (Ls293-323) period observed by
Cassini Achterberg et al. 2008
Zonal mean u
Zonal mean T
Pressure (mb)
Same period in the latest version of TitanWRF no
horizontal diffusion
Zonal mean T
Zonal mean u
Latitude (deg N)
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Angular momentum transport in TitanWRF
Stratospheric results

• Stratospheric annual mean northward transport of
  angular momentum

mean meridional circulation
transient eddies
total advection
poleward transport
equatorward transport

• Mean meridional circulation transports momentum
  polewards
• Eddies begin transporting significant momentum
  equatorwards after 3 Titan years (once the
  winter zonal wind jet has become strong)

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Stratospheric results
Northern winter solstice
Northern spring equinox
Strongest mean transport poleward strongest eddy
transport equatorward
Weak equatorward eddy transport opposes poleward
mean transport
mean meridional circulation
poleward transport
transient eddies
total advection
equatorward transport
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Northern fall circulation in TitanWRF
Stratospheric results
Stratosphere summary

• Reducing horizontal diffusion was vital for a
  realistic stratosphere
• An improved match to observed seasons increases
  our confidence in predictions for other seasons -
  e.g.
• Strong gradients at high latitudes require better
  treatment of the polar boundary condition, so we
  are currently improving this in TitanWRF

Pressure (mb)
Zonal mean u
Zonal mean T
Latitude (deg N)
Future work
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Surface winds and observed dune features
Surface results
Map of inferred dune directions (Lorenz,
Radebaugh and the Cassini radar team)
Latitude (deg N)
-60 -30 0 30
60
-
Longitude (deg W)
Cassini radar image

• Dunes mostly within 30 of equator
• Surface features suggest that dunes formed in
  westerly (from the west) winds

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Surface results

• But models / basic atmospheric dynamics predict
  easterlies here

Annual mean winds (45S-45N) from TitanWRF with
tides included
0.5 m/s
Latitude (deg N)
Longitude (deg E)
-30 0 30
Latitude (deg N)
-
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Surface results

What about instantaneous winds?
Plots show annual mean wind magnitude at each
gridpoint in the chosen direction
NNE
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

• Do find some of the strongest winds from 30S-30N
  pointing NNE or SSE
• But from 15S-15N they spend lt 5 of their time
  in these directions
• And for 30-15S and 15-30N its still only 15-20

ENE
ESE

SSE

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Surface temperature variations in TitanWRF
Surface results
For Ls 316-357, Cassini found Jennings et al.
2008
Drop from equator to north pole 3K
Drop from equator to south pole 2K
Peak at 10S of 93.7K
TitanWRF
Drop from equator to north pole 4K
Latitude (deg N)
Peak at 20S of 92.3K
Drop from equator to south pole 1.5K
Planetocentric longitude (Ls)
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Surface summary
Surface results

• Mean low latitude winds in TitanWRF dont match
  directions inferred
• Winds with some westerly component occur lt 5 of
  the year for 15S-15N and lt20 for 30-15S and
  15-30N, though are relatively strong
• Surface temperatures match Cassini observations
  fairly well
• Look at correlations between predicted winds that
  are close to the observed wind direction and the
  near-surface environment
• Look at effect of including variable topography /
  surface properties

Future work
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Simple methane cloud model
Methane cycle

• Surface methane evaporation
• Condensation and immediate fall-out when methane
  mixing ratio exceeds specified saturation ratio
• Precipitation if condensate doesnt re-evap on
  way down
• In results shown, no latent heat and infinite
  surface methane

• The two dominant controlling factors are
• Near-surface temperatures (gt ability to hold
  methane)
• Upwelling in atmosphere (gt cooling gt clouds)

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Controls on evaporation
Methane cycle
gt
Solar heating of troposphere
Near-surface air temperature
Latitude (deg N)
-60 -30 0 30 60
gt
Near-surface methane needed for saturation
Actual near-surface methane

Latitude (deg N)
-60 -30 0 30 60
gt
Evaporation
gt
Amount needed to saturate near-surface air
Latitude (deg N)
-60 -30 0 30 60
330 0 30 60 90 120
150 180 210 240 270 300

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

Time of year (Ls)
Time of year (Ls)
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Upwelling in TitanWRFs troposphere
Methane cycle
Northern summer solstice (1 pole-to-pole cell)
Plot the upwelling region by plotting the maximum
vertical velocity (in the troposphere) through
one Titan year
Equinox (2 symmetric cells)
Single, persistent pole-to-pole Hadley cells
around the solstices
Pressure (mbar)
-60 -30 0 30 60
Southern summer solstice (1 pole-to-pole cell)
Latitude (deg N)
Double Hadley cell upwelling region moves rapidly
330 0 30 60 90
120 150 180 210 240 270
300 330
Planetocentric longitude (Ls)
Latitude
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Controls on clouds and precipitation
Methane cycle
Maximum vertical velocity in troposphere
gt
Cloud condensation
Surface precipitation
gt
-60 -30 0 30 60
Latitude (deg N)
330 0 30 60 90
120 150 180 210 240 270
300
330 0 30 60 90
120 150 180 210 240 270
300
Planetocentric longitude (Ls)
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Net transfer from South to North
Methane cycle
Evaporation
Column mass of methane
Latitude (deg N)
More transport from south to north than north to
south
-60 -30 0 30 60
More evaporation during S summer
Net increase in surface methane since start
Precipitation
More precipitation during N summer
Latitude (deg N)
-60 -30 0 30 60
330 0 30 60 90 120
150 180 210 240 270 300
330
330 0 30 60 90 120
150 180 210 240 270 300
Planetocentric longitude (Ls)
Planetocentric longitude (Ls)
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Methane cycle summary analogy with Mars
Methane cycle
gt More water vapor / methane gas transported
into northern hemisphere during/after southern
summer than vice versa
Mars
Titan
Both
Warmer southern summer (since perihelion occurs
here) gt Atmosphere can hold more water vapor /
methane gas
Cooler northern summer gt Surface build-up of
water ice / methane liquid
S pole
N pole
Future work

• Current TitanWRF results are not definitive
• But 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 scheme