A reversed Monte Carlo radiative transfer model for Titan PCM

Titan and particularly its thick atmosphere, unique among solar system objects, has been a center of interest for many decades. Titan’s atmosphere has been thoroughly studied, notably with the use of Global Climate Models (GCM) (Lebonnois et al. 2012; Lora et al. 2015; de Batz de Trenquelléon et al. 2025a;
de Batz de Trenquelléon et al. 2025b). All of them currently consider a plane-parallel atmosphere for the radiative transfer calculation (Lora et al. 2015; de Batz de Trenquelléon et al. 2025a). However, this assumption has limitations in the case of Titan.

First, its thick atmosphere makes sphericity effects prominent. For instance, the Titan PCM (Planetary Climate Model, developed mainly at IPSL) simulates up to 500 km altitude, while Titan radius is 2575 km, therefore the atmosphere extension is not negligible, representing 20% of Titan’s radius. In such a case,
sphericity effects are important, especially at high altitudes, and may result in variations of the radiative budget with retroactions on the circulation and cloud formation. Another effect from sphericity is the absence of polar night above ∼300 km altitude, matching the altitude of the polar cloud observed by West
et al. 2016. We also note that the multiple scattering in a 3D atmosphere can allow for the propagation of light into the polar night at lower altitudes. Both effects can affect the thermal properties of the polar regions with implications for the formation of the polar clouds.

Additionally, the horizontal heterogeneity of the atmosphere can have a role in the radiative transfer. Indeed, the radiative budget can be affected by the optical properties of neighboring columns, which is not accounted for in GCMs, where the radiative transfer calculations are performed independently for each
column. For instance, the shadowing produced by large polar clouds or sharp haze variations in the optical properties affecting the horizontal propagation of light, can reduce or increase the radiative budget in the neighboring columns.

Therefore accounting for those effects necessitates a more sophisticated approach. We have developed a new 3D radiative transfer model, with spherical geometry and heterogeneous layers: htrdr-planets (https: //www.meso- star.com/projects/htrdr/htrdr.html; He et al. submitted), to be implemented in Titan
PCM and study its atmosphere. This model is based on a reversed Monte Carlo algorithm incorporating recent developments in computing science (Villefranque et al. 2019), able to calculate radiative budget within each GCM cell with very little approximations.

We present here comparisons between simulated radiative budget computed with the new model and the two stream plane-parallel model used in the Titan PCM and we explore the expected effects on the cloud microphysics and atmospheric circulation.

 

Acknowledgements:
This work is supported by the Agence National de la Recherche (ANR) through the RaD³-net project (ANR-21-CE49-0020-01).

 

References:

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