comparing titanwrf and cassini results at the end of the cassini prime mission claire e. newman,...
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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