Summary of The Dynamics Behind Titan's Methane Clouds
by Mitchell, Pierrehumbert, Frierson, and Caballero, which appears
in the Proceedings of the National Academy of Sciences.
Primary arguments:
- Titan's midlatitude cloud band could be due to large-scale ascent by the
mean circulation, instead of surface sources of methane (cryovolcanoes).
- Titan has a "tropical" circulation at -180 Celsuis! The Hadley cell is
global and methane condensation is the dominant heat source.
Discussion:
Titan is a moon of Saturn, that's been in the news a lot lately due to the
Cassini
mission to study Saturn, and the Huygens probe which flew through
Titan's atmosphere and landed on its surface. Titan has a similar size
as Earth, but is much more slowly rotating. Therefore, Titan could be
considered a "tropics-only" planet, due to the weakness of the Coriolis
force at all latitudes.
There is another feature that makes Titan similar to the Earth's tropics:
the presence of a condensible substance that releases latent heat
and influences the dynamics. On Earth, that substance is water
vapor; on Titan it is methane. In order to simulate Titan's
troposphere and explain the observed cloud distribution, we used a simple
moist convection scheme (Frierson 2007a) coupled with various other simple
physical parameterizations.
One of the great things about working with idealized physical
parameterizations, which aren't tuned to Earth's climate, is that
they're easily extendable to situations such as these. We only
needed to change the relevant physical parameters, and the convection
scheme worked the first time!
We ran the simulations with the model that showed that large scale
dynamics can indeed explain both the polar clouds and midlatitude
clouds observed on Titan. This suggests that localized surface
sources such as volcanos, are not necessary to explain the observed
clouds. There are some interesting properties of convection and
ITCZs that one can learn from the simulations that we ran for this
study, that may be interesting to some atmospheric scientists in
particular. I'll highlight one of these results here, that concerns the
above figure.
We compare dry and moist simulations that are run over the same
boundary conditions to evaluate the effect of methane condensation on
the ITCZ movement (the above figure shows latitude-time plots of
convection in dry and moist simulations). In the dry simulations, the
updraft regions is significantly larger than in the moist simulations,
which has a much more tightly confined ITCZ. The reason for
this is the different effective stratifications felt in updrafts and
downdrafts when condensation occurs. Convective updrafts feel a
reduced moist stratification, while downdrafts feel a larger dry
stratification. Conservation of mass and energy then
requires fast and narrow updrafts, and slow and wide downdrafts.
With only dry convection, both upward and downward regions each
feel a similar stratification, so updrafts and downdrafts are more
symmetric.
The idea of a frigid planet being "tropical" in nature has received quite
a bit of attention in the press; see our
press release,
and one example of a
news article.
Full citation:
Mitchell, J. L., Pierrehumbert, R. T., Frierson, D. M. W., and R. Caballero.
The Dynamics Behind Titan's Methane Clouds.
Proceedings of the National Academy of Sciences, 103, 18421-18426,
doi:10.1073/pnas.0605074103, 2006.
The official journal link can be found here.
A PDF download of the full paper can be found here.
This download is courtesy of the National Academy of Sciences, who owns sole
rights to it.
The download is subject to copyright laws and statutes. For more
information, please visit the PNAS website.
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