Titan's Methane Cycle and the Surface Energy Budget

The atmosphere of Titan holds and transports a large reservoir of methane. Previous general circulation models (GCMs) show a divergence of methane from the equatorial regions resulting from seasonal changes in the circulation of the atmosphere. However, previous GCMs used to study the methane cycle have employed simplified radiative transfer that does not accurately capture the distribution of surface insolation. Titan's seasonal convective clouds, which occur despite the atmosphere's huge thermal inertia, result from exchanges of heat and moisture with the surface; thus, the surface energy balance is important for the methane transport. The surface energy budget in equilibrium is a balance between net surface radiation and turbulent surface fluxes of latent and sensible energy. Our GCM simulations, including nongray radiation transfer that uses optical properties derived from Cassini/Huygens data, show that maximum surface insolation, and therefore destabilizing surface energy fluxes, oscillate seasonally between midlatitudes. The GCM produces rain at latitudes and seasons where clouds have been observed, and at the same time produces a buildup of polar surface liquids. Furthermore, we find no buildup of midlatitude surface methane, in contrast to previous work. The upwelling branch of the mean meridional circulation tracks with the maximum sensible heat flux from the surface, and when evaporation is significant at low latitudes, this translates into an abrupt transition of the upwelling across the equator during equinoxes. On the other hand, the latitudinal migration of precipitation is less abrupt than in the case where evaporation is negligible near the equator (in which case precipitation can also be more temporally sporadic). Additionally, the latitudinal extent of the upwelling branch of the mean meridional circulation and the asymmetry in the latitudinal distribution of surface liquids appear to be diagnostic of the amount of methane available to the atmosphere from the surface reservoir.