Friday, 16 May 2014

Ammonia Snow and Circulation of the Great Red Spot

Back in 2009, while I was still a postdoc at JPL, I worked with Glenn Orton on a paper combining thermal imaging from Cassini and large ground-based observatories to study the structure and composition of Jupiter's Great Red Spot.  We presented the highest resolution infrared images of the GRS ever obtained (from the VISIR instrument on the ESO Very Large Telescope), comparing the temperature contrasts to visible-light imaging from Hubble to understand the 3D properties of the solar system's best-known storm system.  We confirmed the cold core of the anticyclone (first suggested in Voyager thermal imaging by Hanel, Flasar et al.), but discovered a warm heart to the GRS - a central core that was warmer than the surroundings and coincident with a possible reversal of the typically anti-clockwise winds.  This subvortex was considered to be a region of subsidence in an otherwise upwelling anticyclone.  The GRS was surrounded by a peripheral lane of decreased aerosol opacity, suggesting a cloud-free ring of atmospheric subsidence and a barrier, keeping the red GRS aerosols separated from the white aerosols surrounding it.  Everything within the centre of the GRS is trapped by strong gradients of potential vorticity (like a polar vortex), and so it's old and continually zapped by UV rays, which might contribute to the deep red-orange colours.
Thermal imaging of the Great Red Spot and Oval BA
compared to Hubble imaging in 2006 (Fletcher et al., 2010)

Motivated by this, and other studies of the structure of the GRS, Csaba Palotai and Tim Dowling worked to create a model of the vortex using a realistic system of ammonia clouds (leaving the deeper water clouds aside for the time being) and the highest vertical resolution used to date.  They used the EPIC atmospheric model (Dowling et al., 2006) with an active hydrological cycle for ammonia, including the production of ammonia snow.  Their model cloud distributions resemble the images we regularly see from spacecraft and ground-based telescopes, and reproduced a warm core feature at some altitudes, but revealed a good deal more about what's actually going on.  Instead of the clouds being moved into place by upwelling within the core of the vortex, they found that the ammonia clouds condensed as soon as the cloud microphysics packages were switched on.  That means that the clouds form over the GRS as a natural consequence of the cold temperatures in the top half of the vortex.  And the vortex is cold so that it can fit underneath the rigid tropopause, which resists bulging upwards over the high pressure anticyclone.  So although some secondary vertical motions will no doubt exist because of the thermal anomalies, they are not required to explain the cold temperatures and cloudy conditions prevailing at the top of the Great Red Spot.

It's worth considering this in detail, because my own naive impression had always been that the anticyclones were cold because of upwelling and adiabatic expansion.  What we're saying now is that the cold temperatures are a natural consequence of the stratification of the atmosphere above the vortex, and the resulting thermal wind shears that serve to damp the circulating vortex to slower and slower speeds as we move towards the tropopause.  These wind-related features are primary circulation features.  We're not saying that secondary circulations like upwelling and downwelling don't exist (indeed they must to explain some of the properties of the GRS in our 2010 observational paper), but that the cold core and cloudiness don't need vertical motions to explain them.

The vortex is introduced in the model as a high pressure core, which is the vertical centre of the vortex.  Interestingly, whereever this centre was introduced, it seemed to move to around the 500 mbar level.  At higher altitudes, the model vortex was a cold core, as we see in our data.  But deeper down, the simulations suggest the GRS should be warmer than the surroundings, which is incompatable with the observations.  Now, thermal remote sensing only sounds down to around the cloud-tops at 700-800 mbar, but nevertheless, there's never been any hints that the GRS should be warmer at those depths.  Something doesn't add up.  Maybe the vortex centre in the model should be deeper, or maybe turning on the water to an active hydrological cycle would also help improve things.
A possible schematic of secondary circulation pattern within
a jovian anticyclone by Marcus et al., (2013)

In any case, I learned a lot from collaborating with Palotai and Dowling on their recent paper, including their warning not to mix up the primary circulation features (related to winds, high pressure cores, wind shears and stratification) and secondary circulation features (those caused by upwelling and downwelling):  "...unfortunately, this fallacy that clouds and cold cores require upwelling is very deeply ingrained in the planetary-science community... Gill (1982) shows that thermal windshear temperature anomalies are primary circulation features, not secondary ones...."

They propose that the primary circulation creates anomalies that certainly contribute to the existence of the secondary circulation, but they should not be significantly affected by it if the secondary circulation is weak.  Dowling used the example of the anvil head of a thunderstorm cloud, which is not caused by upwelling and sidewelling, but rather by the jump in stratification at the tropopause.  "Put simply, a tropospheric anticyclone’s top centre is cool so that it can fit under the tropopause, which resists bulging up to make room. Likewise, a tropospheric cyclone’s top centre is warm because the tropopause resists being pulled down."  The secondary circulation is certainly important, driving contrasts in passive tracers like phosphine and para-hydrogen, and the peripheral ring of cloud-free conditions (see de Pater et al., 2010 and Marcus 2013, for example).  The eyewalls of the polar cyclones on Saturn may be examples of strong secondary circulations.  In any case, the combination of models with active clouds, condensation and snow, with the high-resolution multi-wavelength imaging available from observers, will hopefully provide new insights into the vertical structures of these enormous anticyclones.

Further Reading:

  • L. N. Fletcher, Orton, G. S. Yanamandra-Fisher, P., Irwin, P. G. J. Baines, K. H. Edkins, E., Line, M. R., Mousis, O., Parrish, P. D., Vanzi, L., Fuse, T., Fujoyoshi, T., 2010, Thermal Structure and Composition of Jupiter’s Great Red Spot from High-Resolution Thermal Imaging, Icarus 208, p 306-328 (
  • Palotai, C., Dowling, T.E., Fletcher, L.N., (2014), 3D Modelling of Interactions Between Jupiter's Ammonia Clouds and Large Anticyclones, Icarus, Volume 232, April 2014, Pages 141–156 (
  • de Pater, I., Wong, M.H., Marcus, P., Luszcz-Cook, S., Ádámkovics, M., Conrad, A., Asay-Davis, X., Go, C., 2010. Persistent rings in and around Jupiter’s anticyclones – Observations and theory. Icarus 210, 742–762.
  • Marcus, P.S., Asay-Davis, X., Wong, M.H., de Pater, I., 2013. Jupiters Red Oval BA: Dynamics, color, and relationship to jovian climate change. J. Heat Trans. 135,1–9.

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