Preprints
https://doi.org/10.5194/egusphere-2025-3515
https://doi.org/10.5194/egusphere-2025-3515
11 Aug 2025
 | 11 Aug 2025
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Cloud Chamber Studies on the Linear Depolarisation Ratio of Small Cirrus Ice Crystals

Adrian Hamel, Martin Schnaiter, Masa Saito, Robert Wagner, and Emma Järvinen

Abstract. Space-borne lidar, in combination with other remote sensing instrumentation, has been used to infer vertical profiles of ice cloud properties from A-train satellites, and more recently, also from the newly launched EarthCARE mission. However, accurately retrieving ice crystal microphysical properties from lidar signals requires a thorough understanding of their relationship to backscattering characteristics. Cloud chambers can be used to study the link under a controlled environment. This study investigates the link between the linear depolarisation ratio in the near-backscattering direction (178°) and the ice microphysical properties for 47 cloud experiments at cirrus temperatures between -75 °C and -39 °C. Predominantly small (diameter < 70 µm) columnar and irregularly shaped ice crystals were grown under distinct conditions of supersaturation with respect to ice. A statistical and visual analysis of size, shape and morphological complexity reveals that more than 40 % of the columnar particles exhibit hollowness on the basal facets. Ice crystals larger than 10 µm show depolarisation ratios below 0.3, which is lower than typical values observed in mid-latitude cirrus but in agreement with polar cirrus observations. Two temperature-dependent depolarization ratio - size modes were found and successfully reproduced with ray tracing simulations of hollow columns incorporating surface roughness, hollowness and internal scattering. These results are important for the interpretation of the linear depolarisation ratio of small ice crystals in active remote sensing and evaluating the performance of state-of-the-art optical particle models, especially for small size parameters below 100.

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Adrian Hamel, Martin Schnaiter, Masa Saito, Robert Wagner, and Emma Järvinen

Status: open (until 22 Sep 2025)

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Adrian Hamel, Martin Schnaiter, Masa Saito, Robert Wagner, and Emma Järvinen
Adrian Hamel, Martin Schnaiter, Masa Saito, Robert Wagner, and Emma Järvinen

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Short summary
The depolarisation ratio of ice clouds is commonly measured by satellites and ground-based instruments to learn about ice particle shapes. In our cloud chamber experiments, we found that for small ice crystals, the depolarisation ratio is more strongly influenced by particle size than by nano-scale structure. The measured trends could be reproduced with numerical simulations. This result helps improve the interpretation of remote sensing data and the accuracy of light scattering models.
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