htrdr-planets & Titan PCM: coupling Monte Carlo radiative transfer and Planetary Climate Models

Titan’s atmosphere has been extensively studied and modeled over the last twenty years. Global Climate Models (GCMs) successfully reproduce the main features of Titan’s atmosphere, including the detached haze layer and the polar clouds (Lebonnois et al., 2012; Lora et al., 2015; de Batz de Trenquelléon et al., 2025a,b, 2026). However, those models rely on simple plane-parallel radiative transfer algorithms which fail to capture the effects of heterogeneity and sphericity, important for the extended atmosphere of Titan. To address this limitation, we are coupling htrdr-planets (https://www.meso-star.com/projects/htrdr/htrdr.html He et al., 2026), a 3D backward Monte Carlo radiative transfer model, able to account for sphericity and heterogeneity, with the Titan LMDZ Planetary Climate Model (PCM) (de Batz de Trenquelléon et al., 2025a,b, 2026). htrdr-planets incorporates recent developments in computational science (Galtier et al., 2013; Villefranque et al., 2019), and is able to calculate the radiative budget within each PCM cell with minimal approximations and relatively low computational cost.

Figure 1: Heating rates at northern summer solstice at 300 km altitude. Upper panels: 2-stream plane-parallel results. Middle panels: htrdr-planets results. Bottom panels: differences htrdr-planets minus plane-parallel. The plots on the left are zonal averages.

 

Comparisons between the original 2-stream plane-parallel model used in the Titan PCM and htrdr-planets indicates that significant differences are expected at high altitudes in the terminator region (Fig. 1). This behavior is explained by the high incident angles of the stellar radiation near the terminator, which results in larger extinction in a plane-parallel model compared to a spherical scenario. Fig. 1 also demonstrates that, on a zonal average, the main differences are concentrated at the limits of the polar night and polar day, since those regions spend a significant amount of time close to the terminator. In addition, light can directly reach regions beyond the terminator, as a result of sphericity, where plane-parallel calculations cannot be conducted. As a consequence, the spherical model produces additional heating in the regions beyond the terminator in the spherical model relatively to the plane-parallel.

To explore how the observed heterogeneity and sphericity effects impact the thermal structure and dynamic of Titan’s atmosphere, we couple htrdr-planets to the Titan PCM. This is realized by running htrdr-planets every 11 Titan days, with optical properties extracted from the Titan PCM. The calculated heating rates are then used in the Titan PCM with an interpolation in solar longitude and local time. For the sake of computation time, we focus our efforts on the solar  heating and keep the original 2-stream model to calculate the thermal cooling.

 

Figure 2: Wind maps around the northern fall equinox. The color scale presents the zonal winds, white lines represent the streamfunction, where dashed lines indicate counter-clockwise circulation and solid lines indicate clockwise circulation. Upper panels: reference PCM calculation. Bottom panels: PCM simualtion coupled to htrdr-planets. Panels on the left are at L_S = 135° , middle panels are at L_S = 180° and right panels are at L_S = 225° .

 

Preliminary results of this coupling demonstrate that the changes brought by the modified heating rates on the dynamics of Titan’s atmosphere are important. We observe changes in the stability of the Hadley cell as well as modifications in the timing of appearance of this structure (Fig. 2). In addition, smaller yet stable cells appear in the upper stratosphere and are expected to affect the transport of various species, as well as haze particles. Such changes have major ramifications for the thermal structure of the atmosphere.

In this presentation, we detail those initial comparisons between htrdr-planets and a 2-stream plane-parallel model and we explore the preliminary results of the coupling between htrdr-planets and the Titan PCM.

 

This work has been funded by the French National Research Agency (ANR), project RaD3-net, grant number ANR-21-CE49-0020.

 

References
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de Batz de Trenquelléon, B., et al. (2025b). The New Titan Planetary Climate Model. I. Seasonal Variations of the Thermal Structure and Circulation in the Stratosphere. The Planetary Science Journal, 6, 78.
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