Seasonal variation of Titans atmospheric structuresimulated by a general circulation model

The seasonal variation of Titans atmospheric structure with emphasis on thestratosphere is simulated by a three-dimensional general circulation model. The model includesthe transport of haze particles by the circulation. The likely pattern of meridional circulation isreconstructed by a comparison of simulated and observed haze and temperature distribution. TheGCM produces a weak zonal circulation with a small latitudinal temperature gradient, in conflictwith observation. The direct reason is found to be the excessive meridional circulation. Underuniformly distributed opacity sources, the model predicts a pair of symmetric Hadley cells nearthe equinox and a single global cell with the rising branch in the summer hemisphere belowabout z = 230 km and a thermally indirect cell above the direct cell near the solstice. Theinterhemispheric circulation transports haze particles from the summer to the winter hemisphere,causing a maximum haze opacity contrast near the solstice and a smaller contrast near theequinox, contrary to observation. On the other hand, if the GCM is run under modified coolingrate in order to account for the enhancement in nitriles and some hydrocarbons in the northernhemisphere near the vernal equinox, the meridional cell at the equinox becomes a single cell withrising motions in the autumn hemisphere. A more realistic haze opacity distribution can bereproduced at the equinox. However, a pure transport effect (without particle growth bymicrophysics, etc.) would not be able to cause the observed discontinuity of the global hazeopacity distribution at any location. The stratospheric temperature asymmetry can be explainedby a combination of asymmetric radiative heating rates and adiabatic heating due to verticalmotion within the thermally indirect cell. A seasonal variation of haze particle number density isunlikely to be responsible for this asymmetry. It is likely that a thermally indirect cell covers theupper portion of the main haze layer. An artificial damping of the meridional circulation enablesthe formation of high-latitude jets in the upper stratosphere and weaker equatorial superrotation.The latitudinal temperature distribution in the stratosphere is better reproduced.