Research at our Institute: Atmosphere
1. Global Atmospheric Circulation
The Cologne group has been developing since the 1990s a three-dimensional global circulation of the atmosphere (GCM), which is specifically devoted to Titan. This model cannot reproduce the observed stratospheric superrotation, in accordance with several GCMs of other groups. The reason for this ‘failure’ is unknown. The structure of the mean meridional circulation (Hadley circulation) predicted by this model is largely consistent with predictions by other models as well as with observations.
The model predicts a substantial seasonal transport of stratospheric aerosols from one hemisphere to the other and vice versa by the Hadley circulation. This causes a seasonal variation in the optical depth of the atmosphere, which can be telescopically monitored. Comparisons with simulations could contribute to a better understanding of the mechanism that lead to the variation in the brightness distribution of Titan.
The angular momentum of the atmospheric superrotation is believed to have been extracted in the past from the surface/interior. Possible mechanisms to transfer angular momentum between the surface and atmosphere are the surface friction caused by the surface winds and atmospheric pressure differences between the west and east face of mountains.
Publications:
Tokano, T., F. M. Neubauer, M. Laube, C. P. McKay. Seasonal variation of Titan’s atmospheric structure simulated by a general circulation model. Planet. Space Sci., 47, 493-520, 1999.
Tokano, T. Mountain torque and its influence on the atmospheric angular momentum on Titan. Icarus, 220, 863-876, 2012.
Tokano, T. Wind-induced equatorial bulge in Venus and Titan general circulation models: Implication for the simulation of superrotation. Geophys. Res. Lett., 40, 4538-4543, 2013.
2. Methane Hydrological Cycle
The same GCM was used to investigate the global methane hydrological cycle. Methane is subject to transport from the winter to summer hemisphere near the surface, converges near the summer pole, where it ascents and encounters condensation. Therefore, the most precipitation is predicted to occur near the summer pole, in agreement with the cloud observations. The precipitation amount predicted by the model is relatively small despite the large abundance of methane.
The original model version predicted extremely strong precipitation right at the equator. This anomaly was caused by unrealistic assumptions of the surface temperature and disappeared altogether in the revised model version.
Polar hydrocarbon seas discovered by Cassini change the methane hydrological cycle particularly at high latitudes. Large seas cause sea breeze or land breeze, depending on the composition of the seas. Should they contain a high methane concentration, they could even cause tropical cyclones (typhoons, hurricanes) in summer. However, there are so far no observations of tropical cyclones on Titan.
Publications:
Tokano, T., F. M. Neubauer, M. Laube, C. P. McKay. Simulation of Titan's atmospheric methane cycle by a general circulation and the effect of supersaturation of methane on the atmospheric circulation. Icarus, 153, 130-147, 2001.
Tokano, T. Impact of seas/lakes on polar meteorology of Titan: simulation by a coupled GCM-sea model. Icarus, 204, 619-686, 2009.
Tokano, T. Precipitation climatology on Titan. Science, 311, 1393-1394, 2011.
Tokano, T. Arrows in Titan's sky. Nature Geosci., 4, 582-583, 2011.
Tokano, T. Are tropical cyclones possible over Titan's polar seas? Icarus, 223, 766-774, 2013.
3. Atmospheric Tides
Strong tidal forces act on the entire atmosphere of Titan due to the elliptical orbit of Titan and the vicinity to the giant planet Saturn. Considering the relatively weak thermal forcing of the atmosphere, gravitational tides on Titan are not only relevant for the oceanography of the seas/lakes but also for meteorology. In the equations of motion of the GCM the tidal force, which does not appear in terrestrial GCMs, has been implemented. The tidal force causes a diurnal periodical oscillation of the surface pressure with amplitudes of ~1 hPa, which is much larger than any short-term pressure variation on Titan. Periodically reversing winds with amplitudes of 1-2 m/s are induced, which are superposed on the background wind. The tides generate planetary waves of wavenumber 2, which migrate eastward. The gravitational tides are not significant in the stratosphere, where other forces are stronger.
Contrary to previous belief, GCM predictions contain signals of thermal tides in the stratosphere. Thermal tides cause planetary waves of wavenumber 1, which migrate westward with the apparent speed of the Sun. Another interesting effect of thermal tides is the tilt of the atmospheric angular momentum vector by a few degrees and a westward precession of this vector. This tilt was first recognized in the temperature data of Cassini and temporal variation in the hemispheric brightness distribution of Titan.
Publications:
Tokano, T., F. M. Neubauer. Tidal winds on Titan caused by Saturn. Icarus, 158, 499-515, 2002.
Tokano, T., R. D. Lorenz. GCM simulation of balloon trajectories on Titan. Planet. Space Sci., 54, 685-694, 2006.
Tokano, T. Westward rotation of the atmospheric angular momentum vector of Titan by thermal tides. Planet. Space Sci., 58, 814-829, 2010.
4. Lightning
In Titan’s troposphere convective clouds can occasionally develop, which morphologically resemble terrestrial thunderclouds. They consist of frozen methane in the upper part and of a liquid methane-nitrogen mixture in the lower part. Due to the sudden growth such clouds could attach free electrons outside the cloud and be transitionally electrically charged. The electric field in the cloud might get strong enough to initiate lightning discharges. Free electrons in the lower atmosphere are produced by galactic cosmic rays, which were detected by Huygens.
However, Huygens landed on Titan on a cloud-free day, so no thunderstorms were encountered.
Publications:
Tokano, T., G. J. Molina-Cuberos, H. Lammer, W. Stumptner, Modelling of thunderclouds and lightning generation on Titan, Planet. Space Sci., 49, 539-560, 2001.
Lammer, H., T. Tokano, G. Fischer, W. Stumptner, G. J. Molina-Cuberos, K. Schwingenschuh, H. O. Rucker. Lightning activity on Titan: can Cassini detect it? Planet. Space Sci., 49, 561-574, 2001.
Fischer, G., T. Tokano, W. Macher, H. Lammer, H. O. Rucker, Energy dissipation of possible Titan lightning strokes, Planet. Space Sci., 52, 447-458, 2004.