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

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

doi:https://doi.org/10.5194/egusphere-2025-3515

Preprints

https://doi.org/10.5194/egusphere-2025-3515

Preprints

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.

Received: 21 Jul 2025 – Discussion started: 11 Aug 2025

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

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RC1:
'Comment on egusphere-2025-3515', Anonymous Referee #1, 03 Sep 2025 reply
The manuscript presents the linear depolarization ratio of small ice crystals (< 70 µm) at 178° scattering angle under controlled laboratory conditions. The size and morphology of the ice crystals are well characterized in the laboratory. Therefore, the chamber studies present a valuable piece of information for atmospheric observations of cirrus clouds with lidar. Furthermore, it serves as constraint for optical models in the description of the complex shape of ice crystals. It is worth to analyze the AIDA measurements for this purpose and the topic is well suited for ACP. My concerns mostly regard the modelling approaches used to fit the observations. Therefore, it should be published after major revisions.
Major comments
A major point of critics is that you compare your measurement results to CGOM calculations for columns or even hollow columns, but only 19% (-40°C) and 40% (-50°C) of your observed ice crystals actually have a columnar shape. And of these, only 46% (-40°C) and 40% (-50°) are hollow columns (Tab. 2). It means that you found hollow columns only in 0.19*0.46 = 9% (-40°C) and 0.4*0.4= 16% (-50°C) of your ice crystal samples. Hollow columns represent only a minority of your lab measurements and it is not comprehensible why the CGOM modelling results of hollow columns should explain your data. The morphologically more complex particles which represent the majority of your ice crystals are more difficult to reproduce in optical models but are dominating your signals.

It seems that the IITM simulations were added later to the study. Sect. 2.4 firstly focuses only CGOM and T-Matrix and much later, you mention that IITM was used additionally. Also, Fig 8 and 9 are split between the models. Actually, the T-Matrix and the IITM both are used to describe the smaller particles up to 9 µm and for AR=1, IITM was extended up to around 17 µm which gives an overlap to CGOM. However, this overlap is not used in your discussions. Also, T-Matrix and IITM are not compared. I am wondering, if the increasing depolarization for small particles as seen for IITM is present in the T-Matrix results as well. How does it look like if you start your calculations at GMD = 0 as done for IITM? A more comprehensive use of the 3 models and corresponding discussion is needed.

The T-Matrix method is used for spheroids. However, the shape of ice crystals is far away from a spheroidal shape. You need a strong motivation to use the spheroidal shape for the representation of the small ice crystals. The agreement of the results for the spheroidal shape might be pure coincidence. Because if it agrees already with spheroidal simulations (L447/448), then you don’t need to ask for more complex optical models.

Do you have to consider possible orientation/alignment effects of the ice crystals? Maybe not, because you’re observing them from the side in a turbulent environment. However, in lidar, specular reflection caused by horizontally oriented ice crystals lead to much lower depolarization ratios in vertically pointing lidars. To avoid the effect of specular reflections in cirrus clouds, most lidars are tilted to an off-zenith angle of 3-5°.

Minor comments:
An outline at the end of the introduction is common to guide the reader through the manuscript. Furthermore, one or two introductory sentences at the beginning of each section would be helpful. It would help to keep the track of your study.

Eq 1+4: Why do you name it delta_parallel and not just delta?

A sketch of the setup and the involved instruments would be nice. I agree that it is already presented in previous publications, but a visualization (even very schematic) would help to better follow the methods section.

Fig 1 Does the scattering pattern depend on particle orientation? And how do you deal with randomly oriented regular particles?

L119-149: These paragraphs should be carefully checked again, because there are some small careless mistakes, e.g., twice a reference to Fig. 1c, a different uncertainty of sigma in Fig 1e and the text (L135), in bracket references not correctly formatted, and some rather slang formulations.

In Sect. 2.3, I don’t like the formulations like “minute 25” in a written text. A suggestion would be to write “at t=25 min”. Furthermore, Fig 2 should be introduced once before presenting the procedure. Is it a typical experiment to give an example? Or were all experiments done like this?

L186: What is mu? Overall, this note is a bit tricky to understand.

L215: What exactly do you mean with spherical equivalent diameter? Which quantity is equivalent to a sphere (volume, surface, …)?

And why you are using the maximum dimension of a spheroid (L215) or the column length (L228) for comparison to the spherical equivalent diameter?

L234: Please provide the refractive index.

It would be recommended to better link Fig 4 with Tab 2 as some images belong to cases presented in the table.

The caption of Fig 4 is quite long. You may consider to add some information, e.g., the supersaturation, already to the images.

L275 and Fig 5: You split your results by campaign. It is important for your analysis. However, for the reader it is a bit confusing as I don’t see the differences between RICE 01 – 03. An overall number for each regime might be sufficient. It is up to you how to best harmonize the description.

L353/354: Please add a reference to the appendix as well.

L392: Actually, it is not in line with your chamber studies, because in the colder temperature regime (CIRRUS-ML, -58 – -63°C), you observe a constant depolarization ratio and then a increase towards smaller particle sizes.

The difference in the scattering angle and the possible implications should also be mentioned in the conclusion.

L460: A better reference for EarthCARE would be Donovan et al., AMT 2024 as they describe the lidar measurements and the retrieval of ice crystal properties.

References to Sato and Okamoto are missing. In several publications, they discussed the modelling of the depolarization ratio of cirrus clouds. Furthermore, you can search for more lidar observations of the depolarization ratio of cirrus clouds.

The plots of Fig B2 and B4 are swapped. It really created a lot of confusion for me until I’ve got that the figures are not the ones explained in the caption.

Technical Corrections
L 436: Skip the word “about”. Or is the number of experiments not sure?

Fig B3: According to the figure, C is shown for hollow columns not solid ones. The caption is probably wrong.

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Citation: https://doi.org/10.5194/egusphere-2025-3515-RC1
RC2: 'Comment on egusphere-2025-3515', Darrel Baumgardner, 12 Sep 2025 reply

This is a very nicely written discussion of what I consider to be a very important subject matter, i.e. interpretation of polarization ratios derived from both passive and active remote sensors. The authors have conducted very thorough research and have approached the problem from multiple directions in order to achieve some semblance of closure.
What I think is missing, or perhaps has been overlooked, is a thorough uncertainty analysis, not only of the measurements but of the models, as well. There are also some open questions that I have about the SIMONE measurement system that are related to my request for the analysis of uncertainties.
Before evaluating the differences in the models and measurements, I think that it is imperative to discuss uncertainties. I am skeptical of the depolarization uncertainty that is stated as 1.4%, based solely on what is stated as calibration cycles. I didn't read the Schnaiter (2012) paper but my opinion is that referring the reader to this paper is insufficient due to the importance of this derived ratio to the main focus of the paper. Although it is not clearly explained in this paper, I believe that the SIMONE does not measure single particle scattering but detects scattering from an ensemble of particles. That being said, what impact do the following have on the derived polarization ratios: 1) number concentration, 2) size distribution, 3) crystal orientation, 4) spatial distributions within the sample volume (if not random, then how will this impact the measurement?).
My second concern is that there does not appear to be any mention of the difference between 178 degrees and 180 degrees backscattering on the polarization ratios. From my own modeling of polarized backscattering from single particles, there can be quite a difference in polarization ratios when looking at 178 degrees versus 180 degrees. Was a sensitivity analysis done in order to evaluate the differences between lidar at 180 and SIMONE and models at 178 for ensembles?
Once a detailed uncertainty analysis is include in this submission, I will be ready to accept it for publication.

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Citation: https://doi.org/10.5194/egusphere-2025-3515-RC2
CC1: 'Comment on egusphere-2025-3515', Zbigniew Ulanowski, 18 Sep 2025 reply

This is a potentially very valuable contribution to cloud property retrieval using polarisation lidar. It is supported by extensive measurements using a whole array of instruments to not only measure depolarisation but also to characterise chamber-generated ice particle clouds.
However, I concur with Darrel Baumgardner's concern (https://doi.org/10.5194/egusphere-2025-3515-RC2) about the accuracy of the measurements and possible bias due to the difference between backscattering at the exact 180 and 178 degree angles. For example, differences may arise due to the phenomenon of coherent backscattering, which tends to create a sharp peak at 180 degrees. This also raises the possibility that the methods used are unable to accurately represent depolarisation at or near to the backscattering direction. In particular, it has been shown that the geometric optics method used here produces erroneous results in this context.
I also have serious misgivings about the methods used to size the ice particles. The authors write that they use a simple expression for size: D = a · I^b, where I is the scattered flux, and a and b≈0.5 are constants, citing Vochezer et al. (2016) who in turn cite the paper by Cotton et al. (2010) containing this expression. Then the authors state that the unknown "factor a is calibrated with spherical droplets". Either some details have been omitted from the description of this method, or it is inaccurate. The original paper by Cotton et al. (2010) goes on to point out that "the scattered flux from the sphere is larger by a factor of 3.0 [compared to ice particles]". Hence the omission of this correction would lead to undersizing of ice crystals by a factor of ~1.7. While the magnitude of this correction will vary with the detection geometry (range of scattering angles that are detected), in general it cannot be neglected. Has an attempt been made to extablish the magnitude of this correction factor for the specific geometry of the PPD-2K instrument that is reported to have been used here? If not, the sizing data may be invalid.
Finally, the last statement in the abstract gives the impression that the authors somehow aim to eat their cake and eat it too: can the study simulateously contribute to "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"? I would claim that it should be either, or: either the measurements are used to verify the models, or to provide interpretation of depolarisation as a remote sensing tool. The only exception to this dichotomy would be if evidence of complete closure between the ice properties, depolarisation, and model results was provided, but this is not seriously attempted. So the aims should be clarified, and appropriate emphasis adopted.
References:

Cotton, R., Osborne, S., Ulanowski, Z., Hirst, E., Kaye, P. H., and Greenaway, R. S.: The ability of the Small Ice Detector (SID-2) to characterize cloud particle and aerosol morphologies obtained during flights of the FAAM BAe-146 research aircraft, J. Atmos. Oceanic Technol., 27, 290–303, https://doi.org/10.1175/2009JTECHA1282.1, 2010.

Vochezer, P., Järvinen, E., Wagner, R., Kupiszewski, P., Leisner, T., and Schnaiter, M.: In situ characterization of mixed phase clouds using the Small Ice Detector and the Particle Phase Discriminator, Atmos. Meas. Tech., 9, 159–177, https://doi.org/10.5194/amt-9-159-2016, 2016.

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Citation: https://doi.org/10.5194/egusphere-2025-3515-CC1

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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