Three-dimensional modeling of the tropospheric methane cycle on Titan

The tropospheric methane cycle on Titan including advection, diffusion, con densation, precipitation, evaporation, and surface source is simulated by a three-dimensional general circulation model. The model takes into account the different nucleation of methane in Titan's troposphere, and assumes con densation whenever the saturation ratio (relative humidity) exceeds a criti cal value of either 100 or 150%. The simulation with the latter critical sa turation is shown to be more compatible with the Voyager data. The meridion al circulation accounts for a seasonal methane transport from the winter to summer hemisphere in the mid-troposphere, causing a higher supersaturation in late summer and early autumn. Condensation preferentially occurs in thi s season at 15 km altitude at low latitudes. The occasional character may b e explained by the high critical saturation ratio, which cannot be often ex ceeded. Falling rain evaporates during the descent, so no rain arrives at t he surface. The hydrological cycle of methane on Titan takes place mostly w ithin the atmosphere. The latitudinal methane distribution near the surface cannot be reproduced by transport, evaporation, condensation, and combinat ions of them, indicating some undetermined surface processes affecting the methane distribution. The zonal and meridional circulation is sensitive to the methane distribution. (C) 2001 Academic Press.