Seasonal variation of Titan's atmospheric structure simulated by a generalcirculation model

The seasonal variation of Titan's atmospheric structure with emphasis on th e stratosphere is simulated by a three-dimensional general circulation mode l. The model includes the transport of haze particles by the circulation. T he likely pattern of meridional circulation is reconstructed by a compariso n of simulated and observed haze and temperature distribution. The GCM prod uces a weak zonal circulation with a small latitudinal temperature gradient , in conflict with observation. The direct reason is found to be the excess ive meridional circulation. Under uniformly distributed opacity sources, th e model predicts a pair of symmetric Hadley cells near the equinox and a si ngle global cell with the rising branch in the summer hemisphere below abou t z = 230 km and a thermally indirect cell above the direct cell near the s olstice. The interhemispheric circulation transports haze particles from th e summer to the winter hemisphere, causing a maximum haze opacity contrast near the solstice and a smaller contrast near the equinox, contrary to obse rvation. On the other hand, if the GCM is run under modified cooling rate i n order to account for the enhancement in nitriles and some hydrocarbons in the northern hemisphere near the vernal equinox, the meridional cell at th e equinox becomes a single cell with rising motions in the autumn hemispher e. A more realistic haze opacity distribution can be reproduced at the equi nox. However, a pure transport effect (without particle growth by microphys ics, etc.) would not be able to cause the observed discontinuity of the glo bal haze opacity distribution at any location. The stratospheric temperatur e asymmetry can be explained by a combination of asymmetric radiative heati ng rates and adiabatic heating due to vertical motion within the thermally indirect cell. A seasonal variation of haze particle number density is unli kely to be responsible for this asymmetry. It is likely that a thermally in direct cell covers the upper portion of the main haze layer. An artificial damping of the meridional circulation enables the formation of high-latitud e jets in the upper stratosphere and weaker equatorial superrotation. The l atitudinal temperature distribution in the stratosphere is better reproduce d. (C) 1999 Elsevier Science Ltd. All rights reserved.