# 3D Monte Carlo Radiative Transfer for Parameter Retrieval in Planetary Atmospheres

Zili He, Sandrine Vinatier, Bruno Bézard, Anthony Arfaux, Vincent Eymet, Vincent Forest, Pascal Rannou, Sébastien Rodriguez, Emmanuel Marcq, Richard Fournier, et al.

► To cite this version:

Zili He, Sandrine Vinatier, Bruno Bézard, Anthony Arfaux, Vincent Eymet, et al., 3D Monte Carlo Radiative Transfer for Parameter Retrieval in Planetary Atmospheres. EPSC-DPS Joint Meeting 2025, Sep 2025, Helsinki, Finland. pp.EPSC-DPS2025-681, 10.5194/epsc-dps2025-681. hal-05436229

HAL Id: hal-05436229

https://hal.science/hal-05436229v1

Submitted on 31 Dec 2025

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3D Monte Carlo Radiative Transfer for Parameter Retrieval in
Planetary Atmospheres

zili he<fnref n="1" />, **Sandrine Vinatier**<fnref n="1" />, Bruno Bézard<fnref n="1" />, Anthony Arfaux<fnref n="1" />, Vincent Eymet<fnref n="2" />, Vincent Forest<fnref n="2" />,
Pascal Rannou<fnref n="3" />, Sébastien Rodriguez<fnref n="4" />, Emmanuel Marcq<fnref n="5" />, Richard Fournier<fnref n="6" />, Stéphane Blanco<fnref n="6" />,
Nada Mourtaday<fnref n="7" />, Yaniss Nyffenegger-Péré<fnref n="8" />, Sébastien Lebonnois<fnref n="9" />, and Anni Määttänen<fnref n="10" />

Retrieving planetary atmospheric parameters from observational data is particularly challenging
under large observation angles, in thick and highly scattering media (such as Titan and Venus), and
in the presence of horizontal heterogeneities, like clouds and hazes. Traditional radiative transfer
models, often based on plane-parallel or pseudo-spherical approximations, typically assume
horizontally homogeneous layers, which limits their applicability in such scenarios.

To overcome these limitations, we have developed a novel 3D radiative transfer solver, htrdr-
planets, based on the Monte Carlo method that solves models considering spherical and
heterogeneous atmospheres[1]. This solver leverages recent advances from the computer graphics
and statistical physics communities to ensure computational efficiency.

htrdr-planets supports arbitrary ground geometry, represented as triangular meshes with user-
defined surface materials, and atmospheric properties defined on unstructured tetrahedral meshes.
Gas absorption is modeled using the k-distribution method, and multiple aerosol and cloud
populations with their own radiative properties can be described on separate spatial grids.

Critically, we address the need for gradients (i.e., sensitivities) in parameter retrieval. Since
conventional finite-difference methods are inefficient or infeasible with Monte Carlo, we differentiate
the Monte Carlo estimator itself [2]. By reusing the same radiative paths, we construct a Monte
Carlo estimator that computes both the radiance and its gradient with respect to atmospheric and
surface parameters at negligible additional time cost.

We apply this method to Titan and Venus, producing spatially resolved maps of sensitivity with
respect to scattering, absorption, and surface-reflection properties. This framework enables
retrievals in geometrically complex cases that defy traditional models, including Titan's polar cloud
structure and haze distribution, using Cassini and JWST datasets. This work is supported by the
Agence National de la Recherche (ANR) through the RaD3-net project (ANR-21-CE49-0020-01).

<footnote n="1">¹LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, CY Cergy Paris Université, CNRS, 5 place Jules Janssen, Meudon, 92190, France</footnote>

<footnote n="2">²Méso-Star, France, Toulouse, France</footnote>

<footnote n="3">³GSMA, UMR CNRS 6089, Université de Reims Champagne-Ardenne, Reims, France</footnote>

<footnote n="4">⁴Université Paris Cité, Institut de Physique du Globe de Paris, CNRS, Paris, France</footnote>

<footnote n="5">⁵LATMOS/IPSL, UVSQ Université Paris-Saclay, Sorbonne Université, CNRS, Guyancourt, France</footnote>

<footnote n="6">⁶Laboratoire Plasma et Conversion d'Energie, UMR-CNRS 5213, Université Paul Sabatier, Toulouse, France</footnote>

<footnote n="7">⁷Laboratoire d'Optique Atmosphérique, CNRS, UMR 8518, Université de Lille, Lille, France</footnote>

<footnote n="8">⁸Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain</footnote>

<footnote n="9">⁹Laboratoire de Météorologie Dynamique (LMD/IPSL), Sorbonne Université, ENS, PSL Research University, Ecole Polytechnique,
IP Paris, CNRS, Paris, France</footnote>

<footnote n="10">¹⁰LATMOS/IPSL, Sorbonne Université, UVSQ Université Paris-Saclay, CNRS, Paris, France</footnote>
[1] htrdr-planets, https://www.meso-star.com/projects/htrdr/htrdr.html

[2] He, Zili, et al. "Simultaneous Estimation of Radiance and its Sensitivities to Radiative Properties in a Spherical-Heterogeneous Atmospheric Radiative Transfer Model by Monte Carlo: Application to Titan." (Submitted to Journal of Quantitative Spectroscopy and Radiative Transfer.)