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L. Guez, C. P. McKay (NASA ARC), P. Bruston, F. Raulin (LISA)
We have added a model of methane condensation into a radiative equilibrium model of the temperature profile of Titan's atmosphere. We have assumed that particles settling from the upper atmosphere serve as condensation nuclei for methane, and that nucleation of methane occurs readily on every such particle. We have used a tropospheric eddy diffusion of 0.2 m2.s-1, as a nominal value. In that case, the flux of condensation nuclei is high enough and the eddy diffusion coefficient is low enough that supersaturation is negligible. Methane condensation then has a significant effect on the temperature profile. The effect is twofold: through opacity and through latent heat. Substituting cloud opacity to haze opacity in the condensation region leads to an increase of about 1.5 K for the surface temperature. The absorption of latent heat associated to evaporation a few kilometers above the surface brings back the surface temperature about 2 K down. Besides, the release of latent heat accompanying condensation increases the temperature in the middle troposphere, by as much as 5 K at 20 km. All those values vary in the same direction as the eddy diffusion coefficient. The temperature increase in the middle troposphere improves the fit to the profile derived from Voyager radio-occultation data. However, the modeled surface temperature is too low. Limited methane nucleation and condensation, leading to methane supersaturation, may increase the surface temperature while still releasing significant latent heat in the middle troposphere. Alternatively, methane supersaturation could be due to a high eddy diffusion coefficient. Modeling the impact of latent heat release will allow to discriminate between those two possibilities.