The Titan PCM : a fully coupled climate model to study thermal structures, haze, clouds and their seasonal variations

We have developed a new version of the IPSL Titan GCM, now called the Titan Planetary Climate Model (Titan PCM), including a new microphysical model for haze and clouds. Observations of Titan have long revealed the presence of seasonal cycles on Titan (haze, clouds, organic compounds), the ins and outs of which are still poorly understood. In particular, the lack of information on the different flows that govern these cycles prevents us from understanding all the phenomena taking place in Titan’s atmosphere. The need to develop a complete climate model, including microphysics, therefore becomes essential.

The latest improvements in the Titan PCM radiative transfer, now based on a flexible correlated-k method and up-to-date gases spectroscopic data, lead to a better modelling of the temperature profiles in the middle atmosphere. The photochemical solver extends computation of the composition above the top of the PCM (roughly 500 km) up to 1300 km. Radiative transfer is coupled with a new microphysics model in moments. This model includes phenomena such as the nucleation and condensation of clouds, and precipitation that shape the satellite’s landscape.

We are now able to model the processes involved in the formation of tropospheric (CH4) and polar (C2H2, C2H6 and HCN) clouds on Titan. Cloud formation induces new seasonal cycles, particularly at the tropopause where clouds empty the lower layers of the atmosphere of aerosols, featuring two boundary, the main haze layer and a layer of condensed organic compounds. Higher up, in the lower stratosphere, the haze follows a new cycle constrained solely by the circulation, leading to a better modelling of the temperature profiles in the low stratosphere and the troposphere.

From the results of coupled simulations, we can discuss multiple questions raised by observations. Special interest is bear on the overall control of the thermal structure, and impact of the coupling on equinoctial circulation reversal. We also discuss the radiative destabilization of the lower polar winter stratosphere, observed by Cassini radio-occultations.