Docstoc

Modeling Titan_s methane cycle with the TitanWRF GCM

Document Sample
Modeling Titan_s methane cycle with the TitanWRF GCM Powered By Docstoc
					        Titan’s methane cycle in the
    TitanWRF general circulation model

                 Claire E. Newman
  Yuan Lian, Mark I. Richardson and Christopher Lee
                 Ashima Research

                      Anthony D. Toigo
                            APL

Work was supported by NASA’s OPR program and the NASA
Astrobiology Institute, and all simulations were conducted on
NASA’s High End Computing facility at NASA Ames.
                       Overview
• The TitanWRF General Circulation Model (GCM)

• Using a GCM as a global, integrated retrieval tool

• Example: stratospheric superrotation in TitanWRF

• TitanWRF’s methane cloud scheme and an example of one
  possible methane cycle produced

• North-south asymmetry of polar methane in TitanWRF

• Cloud movies

• Conclusions and further work
             The TitanWRF GCM
• 3D atmospheric model from surface to ~400km

• Includes thermal and gravitational tides, seasonally
  and diurnally-varying solar forcing, and full radiative
  transfer

• Simulates observed magnitude of stratospheric
  superrotation [Newman et al., 2011]

• Includes a simple methane cloud scheme with latent
  heat effects and finite surface methane
        A GCM as a global retrieval tool
• A GCM is the encapsulation of what we think we know and a
  collection of other hypotheses to be tested

• If a GCM doesn’t match observations it’s either missing or
  incorrectly representing (e.g., incorrect parameters; inadequate
  complexity) a physical process that’s actually present

• The more disparate the observations the better: it’s highly
  unlikely that a GCM will be able to match them all if a physical
  process is missing or inadequately represented

• ‘Tuning’ a GCM = retrieving quantities with a real physical
  meaning (e.g. thermal inertia of surface; total methane mass)
     Example: stratospheric superrotation
• TitanWRF produces realistic amounts of stratospheric
  superrotation (see movie next and at end)

• We find that low latitudes receive ‘kicks’ of eastward angular
  momentum from the strong winter jet, during infrequent wave
  -driven ‘transfer events’ [Newman et al., 2011]

• We (and others) have found that to produce stratospheric
  superrotation we must limit the amount of horizontal
  dissipation / diffusion imposed in the model

• Note this has a real physical meaning: horizontal diffusion
  is used to represent sub-grid scale mixing, but too much
  appears to ‘mix away’ the smaller perturbations that develop
  into the large-scale waves responsible for superrotation
     Next slide: zonal mean zonal wind movie



• Zonal mean zonal winds predicted by TitanWRF, from the
  surface to ~400km over a period of ~3 Titan years
              Methane cloud scheme
• Methane is advected as a tracer in the atmosphere,
  and tracked at the surface (surface methane = initial
  surface methane + precipitation – evaporation)

• Surface evaporation occurs if lowest atmospheric layer
  is sub-saturated, provided surface methane is present

• Condensation occurs when the atmosphere is saturated

• Condensate falls to surface as precipitation, unless re-
  evaporates in sub-saturated layers en route
   Moist convection & latent heat effects

• Latent heat is released / used when atmospheric
  methane condenses / re-evaporates

• δT due to moist processes is limited to a maximum rate
  => condensation and evaporation are limited also

• Vertical diffusion scheme mixes methane mmr and
  temperature following phase changes


• Evaporation of surface methane also cools the surface
            Looking at two Titan years of model output:




egrees north)




                     Planetocentric solar longitude (Ls)
                                 One Titan year




egrees north)




                Planetocentric solar longitude (Ls)
                        Northern      Northern        Northern
                         spring         fall           spring
                        equinox       equinox         equinox




egrees north)




                Planetocentric solar longitude (Ls)
                               Huygens   Today

          Ls 90°   Ls 180° Ls 270°   Ls 0°       Ls 90°   Ls 180° Ls 270°   Ls 0°
                    (Nov     (Oct    (Aug        (May
                    1995)   2002)    2009)       2017)




egrees north)




                          Planetocentric solar longitude (Ls)
One possible methane cycle with latent heating on
         Surface temperature (K)                 Column mass of methane




    Near-surface methane abundance          Peak vertical velocity in troposphere




      Planetocentric solar longitude (Ls)      Planetocentric solar longitude (Ls)
One possible methane cycle with latent heating on
   Peak vertical velocity in troposphere     Integrated column cloud mass




           Precipitation at surface                Surface evaporation




       Planetocentric solar longitude (Ls)   Planetocentric solar longitude (Ls)
3 Titan years:          Huygens Today                 Huygens Today                Huygens Today



Jan:   2015 2020 2025    2005 2010 2015 2020 2025       2005 2010 2015 2020 2025    2005 2010 2015




                                 Planetocentric solar longitude (Ls)
3 Titan years:         Huygens Today                 Huygens Today             Huygens Today



Jan:   2015 2020   2000 2005 2010 2015 2020     2000 2005 2010 2015 2020   2000 2005 2010 2015




          Large cloud
          outbursts at the
          south pole in
          summer




                                Planetocentric solar longitude (Ls)
3 Titan years:             Huygens Today                 Huygens Today             Huygens Today



Jan:   2015 2020    2000 2005 2010 2015 2020        2000 2005 2010 2015 2020   2000 2005 2010 2015




          Clouds (with
          occasional
          rain) follow
          the ITCZ as                        Note year-
          it crosses the                     to-year
          equator in                         differences
          northern
          spring


                                    Planetocentric solar longitude (Ls)
3 Titan years:          Huygens Today                 Huygens Today             Huygens Today



Jan:   2015 2020    2000 2005 2010 2015 2020     2000 2005 2010 2015 2020   2000 2005 2010 2015




                                                   Appear
                                                   more
                                                   extended
                                                   in latitude
                                                   than in
                                                   the south
          Large cloud
          outbursts at the
          north pole in its
          summer




                                 Planetocentric solar longitude (Ls)
3 Titan years:         Huygens Today                 Huygens Today                 Huygens Today



Jan:   2015 2020   2000 2005 2010 2015 2020     2000 2005 2010 2015 2020       2000 2005 2010 2015




          Far fewer clouds
          as the ITCZ
                                                                 Again, note
          crosses the
                                                                 year-to-
          equator again in
                                                                 year
          southern spring
                                                                 differences



                                Planetocentric solar longitude (Ls)
3 Titan years:         Huygens Today                 Huygens Today             Huygens Today



Jan:   2015 2020   2000 2005 2010 2015 2020     2000 2005 2010 2015 2020   2000 2005 2010 2015




                                       Some cloud
                                       activity at the
                                       poles before the
                                       ‘main events’;
                                       more at north
                                       than south




                                Planetocentric solar longitude (Ls)
    So what is the net effect on surface methane?
          Red = surface methane
          increase > 70°N

          Blue = surface methane
          increase > 70°N

            Green = surface methane
                                                    Net gain in
            decrease outside polar
in surface mass                                     northern polar
            regions
                                                    surface methane




                                      Titan years
    So what is the net effect on surface methane?
          Red = surface methane
          increase > 70°N                      Note: remaining non-polar
                                               surface methane now
          Blue = surface methane               resides in atmosphere
          increase > 70°N
                                                              Note: results
            Green = surface methane                           shown
            decrease outside polar
in surface mass                                               previously
            regions                                           came from
                                                              here




                                      Titan years
    So what is the net effect on surface methane?
          Red = surface methane
          increase > 70°N

          Blue = surface methane
          increase > 70°N

            Green = surface methane                 Net gain in
            decrease outside polar
in surface mass                                     NORTHERN
            regions                                 polar surface
                                                    methane




                                      Titan years
What happens if we reverse perihelion (so it now
  occurs during northern summer instead)?
     What happens if we reverse perihelion (so it now
       occurs during northern summer instead)?
          Red = surface methane
          increase > 70°N

          Blue = surface methane
          increase > 70°N

            Green = surface methane                 Net gain in
            decrease outside polar
in surface mass                                     SOUTHERN
            regions                                 polar surface
                                                    methane




                                      Titan years
                            Why?
• There is increased methane transport into high latitudes by the
  tropospheric circulation in spring/summer

• Rainout to surface over this period (increasing surface
  methane) is balanced by increased evaporation (decreasing
  surface methane); timings vary annually even in steady state

• Summer not containing perihelion (currently northern) is
  longer and cooler => more precipitation and less evaporation
  => gains more surface methane

• Both similarities and differences to Schneider et al. [2012]
        Next slide: methane cloud map movie
• Integrated column mass of ice ‘cloud’ in troposphere

• Actually, integrated column mass of methane ice that
  condenses out in all tropospheric layers and falls to lower
  layers – does not subtract that which re-evaporates before
  reaching the surface


 Following slide: zonal mean methane cloud movie
• Zonal mean of methane ‘clouds’

• Actually, zonal mean condensation (in yellow / bright green)
  and evaporation (blue / purple) in units of mass mixing ratio
                      Conclusions
• TitanWRF has a simple methane cycle scheme with latent heat
  effects and a finite methane inventory (i.e., surface can dry)

• The tropical surface dries out and high latitude surface
  moistens

• For present day (warmer southern summer) we predict more
  surface methane in northern high latitudes at steady state

• With timing of perihelion reversed (warmer northern
  summer) we predict more surface methane in southern high
  latitudes
                  Ongoing and future work
• Now performing detailed comparisons between methane cycle
  observations and GCM predictions using steady state results

• How can we improve the realism of our steady state results?

• Vary physical parameters:
   –   Surface thermal inertia (uniform or global map)
   –   Maximum δT per second allowed due to latent heat
   –   Total methane inventory
   –   Etc.
• Add / modify representations of processes:
   –   More complex clouds (e.g. entrainment effects; microphysics)
   –   Sub-surface diffusion of methane
   –   Treat solar insolation properly (discussed in Lora’s talk yesterday)
   –   Etc.

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:1
posted:7/15/2014
language:Unknown
pages:33