the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Jul 2025
CrystalTrace: A Monte Carlo Raytracing Algorithm for Radiative Transfer in Cirrus Clouds with Oriented Ice Crystals
Abstract. We present CrystalTrace, a Monte Carlo raytracing algorithm designed for radiative transfer computations involving ice crystals with both random and preferred orientations in the visible spectral range. The algorithm can operate as a standalone tool to compute relative intensities or scattering phase functions for individual hexagonal prisms with different sizes and aspect ratios. The current version of CrystalTrace does not support diffraction and ice absorption. Integrated into the three-dimensional radiative transfer solver MYSTIC, which is part of the libRadtran library of radiative transfer, CrystalTrace enables multiple-scattering simulations of absolute radiances. It seamlessly extends MYSTIC's macroscopic raytracing down to the microscopic scale of individual ice crystals. By computing single-scattering properties online, CrystalTrace significantly reduces memory requirements and removes the need for precomputed look-up tables, which are also subject to limited angular resolution. As an extension of MYSTIC, CrystalTrace supports efficient simulations of complex, realistic atmospheric scenes, including ice and water clouds, aerosols, molecules, and surface properties. CrystalTrace, integrated with MYSTIC, fills a critical gap by providing a computationally efficient forward radiative transfer simulator for ice clouds containing oriented crystals, thereby enabling retrievals of oriented ice crystal properties from ground-based, airborne, and satellite imaging observations.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Measurement Techniques.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.-
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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Journal article(s) based on this preprint
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2025-1369', Anonymous Referee #3, 01 Aug 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1369/egusphere-2025-1369-RC1-supplement.pdfCitation: https://doi.org/
10.5194/egusphere-2025-1369-RC1 - AC2: 'Reply on RC1', Linda Forster, 27 Nov 2025
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RC2: 'Comment on egusphere-2025-1369', Anonymous Referee #5, 04 Aug 2025
The manuscript introduces CrystalTrace, a Monte Carlo raytracing algorithm designed for simulating radiative transfer in cirrus clouds containing ice crystals with both random and preferred orientations in the visible spectral range. It runs as a standalone tool to compute scattering phase functions and relative intensities for hexagonal prisms of varying sizes and aspect ratios, while being integrated into the MYSTIC solver within the libRadtran library to enable multiple-scattering simulations of absolute radiances in complex atmospheric scenes. The algorithm uses geometric optics, accounts for crystal orientations via Euler angles and Gaussian distributions for tilt, and validates its single-scattering properties against established methods like IGOM, showing good agreement except in forward scattering regions where diffraction is not included. CrystalTrace extends MYSTIC's capabilities by computing single-scattering properties online, reducing memory needs and eliminating precomputed look-up tables, and demonstrates its utility through simulations of atmospheric optical phenomena such as sundogs, tangent arcs, and Parry arcs. Overall, the tool addresses a gap in efficient forward modeling for oriented ice crystals, facilitating improved retrievals of cirrus properties from ground-based, airborne, and satellite observations.
Overall, this is an excellent study: well-organized, clearly written, and easy to follow. The development of CrystalTrace represents a valuable advancement in modeling radiative transfer for oriented ice crystals, addressing a key limitation in traditional approaches that assume random orientations. As an integrated component of the widely used libRadtran package, it holds significant potential for applications in remote sensing, and atmospheric optics, particularly for simulating optical phenomena like halos and improving cirrus cloud retrievals. I recommend that the manuscript be accepted for publication after the authors address the following minor clarifications and revisions.
- Currently, CrystalTrace does not account for diffraction, which is a known limitation of pure geometric optics (GO) methods for forward scattering regions. Can the authors provide more details on how diffraction effects are handled (or approximated) in the radiative transfer simulations described in Sections 2.3 and 2.4? For instance, did you apply forward peak truncation to the phase functions in the GO and IGOM simulations to mitigate delta-function transmission issues?
- In Figure 7, there is a small peak around the 170° scattering angle in both the GO and CrystalTrace phase functions, which appears to be absent in the YANG phase function. Could the authors explain the potential cause of this difference?
- How straightforward is it for users to implement new crystal shapes in CrystalTrace? For example, rare halos like the 9° halo have been linked to irregular ice crystal shapes such as droxtals (Zhang et al., 2004, Appl. Opt., 43, 2490-2501). Would adding such a non-hexagonal or complex geometry require significant code modifications, or is the framework flexible enough for users to extend it with minimal effort? Providing a brief example or guidance in the manuscript could enhance its accessibility.
- Can the authors clarify in which version of libRadtran CrystalTrace will be (or has been) incorporated? As of the latest public release (version 2.0.6, December 2024), there is no mention of it on the official website, so specifying the target version or branch (e.g., a development or upcoming release) would help potential users.
Citation: https://doi.org/10.5194/egusphere-2025-1369-RC2 - AC1: 'Reply on RC2', Linda Forster, 27 Nov 2025
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2025-1369', Anonymous Referee #3, 01 Aug 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1369/egusphere-2025-1369-RC1-supplement.pdf
- AC2: 'Reply on RC1', Linda Forster, 27 Nov 2025
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RC2: 'Comment on egusphere-2025-1369', Anonymous Referee #5, 04 Aug 2025
The manuscript introduces CrystalTrace, a Monte Carlo raytracing algorithm designed for simulating radiative transfer in cirrus clouds containing ice crystals with both random and preferred orientations in the visible spectral range. It runs as a standalone tool to compute scattering phase functions and relative intensities for hexagonal prisms of varying sizes and aspect ratios, while being integrated into the MYSTIC solver within the libRadtran library to enable multiple-scattering simulations of absolute radiances in complex atmospheric scenes. The algorithm uses geometric optics, accounts for crystal orientations via Euler angles and Gaussian distributions for tilt, and validates its single-scattering properties against established methods like IGOM, showing good agreement except in forward scattering regions where diffraction is not included. CrystalTrace extends MYSTIC's capabilities by computing single-scattering properties online, reducing memory needs and eliminating precomputed look-up tables, and demonstrates its utility through simulations of atmospheric optical phenomena such as sundogs, tangent arcs, and Parry arcs. Overall, the tool addresses a gap in efficient forward modeling for oriented ice crystals, facilitating improved retrievals of cirrus properties from ground-based, airborne, and satellite observations.
Overall, this is an excellent study: well-organized, clearly written, and easy to follow. The development of CrystalTrace represents a valuable advancement in modeling radiative transfer for oriented ice crystals, addressing a key limitation in traditional approaches that assume random orientations. As an integrated component of the widely used libRadtran package, it holds significant potential for applications in remote sensing, and atmospheric optics, particularly for simulating optical phenomena like halos and improving cirrus cloud retrievals. I recommend that the manuscript be accepted for publication after the authors address the following minor clarifications and revisions.
- Currently, CrystalTrace does not account for diffraction, which is a known limitation of pure geometric optics (GO) methods for forward scattering regions. Can the authors provide more details on how diffraction effects are handled (or approximated) in the radiative transfer simulations described in Sections 2.3 and 2.4? For instance, did you apply forward peak truncation to the phase functions in the GO and IGOM simulations to mitigate delta-function transmission issues?
- In Figure 7, there is a small peak around the 170° scattering angle in both the GO and CrystalTrace phase functions, which appears to be absent in the YANG phase function. Could the authors explain the potential cause of this difference?
- How straightforward is it for users to implement new crystal shapes in CrystalTrace? For example, rare halos like the 9° halo have been linked to irregular ice crystal shapes such as droxtals (Zhang et al., 2004, Appl. Opt., 43, 2490-2501). Would adding such a non-hexagonal or complex geometry require significant code modifications, or is the framework flexible enough for users to extend it with minimal effort? Providing a brief example or guidance in the manuscript could enhance its accessibility.
- Can the authors clarify in which version of libRadtran CrystalTrace will be (or has been) incorporated? As of the latest public release (version 2.0.6, December 2024), there is no mention of it on the official website, so specifying the target version or branch (e.g., a development or upcoming release) would help potential users.
Citation: https://doi.org/10.5194/egusphere-2025-1369-RC2 - AC1: 'Reply on RC2', Linda Forster, 27 Nov 2025
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Linda Forster
Anna Weber
Bernhard Mayer
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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