The New Titan Planetary Climate Model. I. Seasonal Variations of the Thermal Structure and Circulation in the Stratosphere

Abstract

Observations of Titan through Cassini’s mission allowed Saturn’s moon’s stratospheric thermal structure and composition to be mapped over half a Titan year. Seasonal variations revealed various unexplained phenomena, such as mechanisms within the polar vortex, thermal structure evolution at high latitudes, and the impact of the enrichment in trace compounds during winter polar nights. We have developed the Titan Planetary Climate Model (Titan PCM)—an improved version of the IPSL Titan Global Climate Model (GCM)—including upgraded radiative transfer, now based on a flexible correlated-k method, updated spectroscopic data on gases, and integration of a new microphysics model for haze and clouds. Our photochemical solver extends computation of the composition above the top of the model up to 1300 km. The radiative transfer is now coupled with microphysics, consisting of the first full radiative coupling of microphysics within a Titan GCM. The model is presented along with its dynamics, thermal structure, and seasonal variations. Despite biases above 10 Pa due to upper boundary limitations, the Titan PCM leads to better modeling of the temperature profiles in the middle atmosphere. Consequently, it is now possible to address some scientific issues about thermal structure in polar regions. Investigations into fall and winter polar phenomena, particularly focusing on thermal structure control and equinoctial circulation reversal, were conducted. This study is linked to the radiative destabilization of the lower polar stratosphere, observed at the end of winter by Cassini radio-occultations. The results confirm a dynamic interplay between haze and gas distributions, which influence the thermal structure.