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

What is TitanWRF? Global, 3D numerical climate model for Titan based on NCARs WRF (Weather Research and Forecasting) model Global, 3D numerical climate model for Titan based on NCARs 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 transfer, boundary layer and surface/sub-surface schemes 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 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 gravitational tides due to Saturn Can be run with a simple methane cloud scheme Can be run with a simple methane cloud scheme Model description

Early simulations of Titans stratosphere Stratospheric results Northern winter (Ls~ ) period observed by Cassini [Achterberg et al. 2008] Zonal mean T Zonal mean u Pressure (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~ ) period observed by Cassini [Achterberg et al. 2008] Zonal mean T Zonal mean u Recent simulations of Titans stratosphere Zonal mean T Zonal mean u Same period in the latest version of TitanWRF: no horizontal diffusion Pressure (mb) Latitude (deg N)

Stratospheric results mean meridional circulation Angular momentum transport in TitanWRF total advection transient eddies poleward transport equatorward transport Mean meridional circulation transports momentum polewards 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) Eddies begin transporting significant momentum equatorwards after ~3 Titan years (once the winter zonal wind jet has become strong) Stratospheric annual mean northward transport of angular momentum Stratospheric annual mean northward transport of angular momentum

Stratospheric results mean meridional circulation total advection transient eddies Northern winter solsticeNorthern 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 vital for a realistic stratosphere 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.: 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 Strong gradients at high latitudes require better treatment of the polar boundary condition, so we are currently improving this in TitanWRF Northern fall circulation in TitanWRF Zonal mean T Zonal mean u Pressure (mb) Latitude (deg N) Stratosphere summary Future work

Surface results Surface winds and observed dune features Map of inferred dune directions (Lorenz, Radebaugh and the Cassini radar team) Latitude (deg N) - Longitude (deg W) Dunes mostly within 30° of equator Dunes mostly within 30° of equator Surface features suggest that dunes formed in westerly (from the west) winds Surface features suggest that dunes formed in westerly (from the west) winds Cassini radar image

But models / basic atmospheric dynamics predict easterlies here: But models / basic atmospheric dynamics predict easterlies here: Surface results 0.5 m/s Annual mean winds (45S-45N) from TitanWRF with tides included Longitude (deg E) Latitude (deg N)

Surface results NNE ENE 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 its still only 15-20% * * * What about instantaneous winds?

Surface results Surface temperature variations in TitanWRF Planetocentric longitude (L s ) Latitude (deg N) For Ls ~ , 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 summary Mean low latitude winds in TitanWRF dont match directions inferred Mean low latitude winds in TitanWRF dont match directions inferred Winds with some westerly component occur < 5% of the year for 15S- 15N and <20% for 30-15S and 15-30N, though are relatively strong Winds with some westerly component occur < 5% of the year for 15S- 15N and <20% for 30-15S and 15-30N, though are relatively strong Surface temperatures match Cassini observations fairly well 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 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 Look at effect of including variable topography / surface properties Future work

Surface methane evaporation Surface methane evaporation Condensation and immediate fall-out when methane mixing ratio exceeds specified saturation ratio Condensation and immediate fall-out when methane mixing ratio exceeds specified saturation ratio Precipitation if condensate doesnt re-evap on way down Precipitation if condensate doesnt re-evap on way down In results shown, no latent heat and infinite surface methane In results shown, no latent heat and infinite surface methane Methane cycle Simple methane cloud model The two dominant controlling factors are: 1.Near-surface temperatures (=> ability to hold methane) 2.Upwelling in atmosphere (=> cooling => clouds)

Methane cycle Controls on evaporation Time of year (°L s ) => + Time of year (°L s ) Latitude (deg N) Latitude (deg N) Solar heating of troposphere Near-surface air temperature Near-surface methane needed for saturationActual near-surface methane Amount needed to saturate near-surface air Evaporation

Methane cycle Upwelling in TitanWRFs troposphere Latitude (deg N) Planetocentric longitude (°L s ) 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 Pressure (mbar)

Methane cycle Controls on clouds and precipitation Maximum vertical velocity in troposphere Latitude (deg N) Cloud condensationSurface precipitation Planetocentric longitude (°L s ) =>

Methane cycle Net transfer from South to North Planetocentric longitude (°L s ) Latitude (deg N) Net increase in surface methane since start Evaporation Precipitation More evaporation during S summer More precipitation during N summer Column mass of methane More transport from south to north than north to south Latitude (deg N) Planetocentric longitude (°L s )

Methane cycle Methane 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 Current Current TitanWRF results are not definitive But 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 Will 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 work