the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Aug 2026
Measurement report: Intercomparison of cloud spectrometers and a holographic probe during the ACTRIS sub-zero in-cloud Campaign at Sonnblick Observatory
Abstract. This study presents results from the Aerosol, Clouds and Trace Gases Research Infrastructure Cloud In Situ (ACTRIS-CIS) European Centre for Cloud Ambient Intercomparison (ECCINT-INT01) campaign, an intercomparison of cloud spectrometers conducted at the Sonnblick Observatory (3,106 m a.s.l.) from 21 November to 3 December 2022. Seven instruments measuring cloud microphysical properties were deployed, including three Fog Monitors (FM-120), three Cloud Droplet Analyzers (CDAs), and an open-path Single Particle Holographic Imaging probe (ICEMET), which served as the reference instrument. The campaign evaluated instrument performance in measuring cloud droplet number concentration, median volume diameter, effective diameter, liquid water content, and cloud droplet size distributions under sub-zero conditions. Significant sampling losses were observed in vertically oriented instruments compared with ICEMET, particularly for cloud droplets with median volume diameters of 10–15 µm. Cloud droplet number concentration and liquid water content were the most affected parameters, whereas median volume diameter and effective diameter remained comparatively robust. Horizontally aligned instruments showed improved agreement with ICEMET, highlighting the importance of wind-aligned sampling geometry. Despite these losses, all spectrometers showed strong consistency in derived sizing parameters, indicating that droplet size structure can be reliably retrieved under different deployment configurations. In contrast, liquid water content showed larger discrepancies, highlighting the need for correction methods accounting for sampling losses of larger droplets. Our results provide recommendations for instrument deployment, demonstrate the influence of inlet design on measurement quality, emphasize the value of multi-instrument intercomparisons, and support efforts to harmonize ground-based cloud microphysical observations within the ACTRIS framework.
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Status: open (until 15 Sep 2026)
- RC1: 'Comment on egusphere-2026-3898', Anonymous Referee #1, 11 Aug 2026 reply
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RC2: 'Comment on egusphere-2026-3898', Darrel Baumgardner, 14 Aug 2026
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My company, Droplet-ENVEA, developed the FM-100/120, one of the instruments used in this study, and so I want to state up front that my review does my reflect a particular bias, since I am equally supportive of the CDA, which is another instrument similar in measurement principles as the FM and its response is also similar to the FM in this comparative study.
As another reviewer has suggested, this journal is not a good venue for this study; however, given that this study is deeply flawed, I regret that it was even published as a discussion paper in this journal because now the conclusions, which are highly questionable, will be propagated and lead some researchers to think that holography is the golden standard, given that in this paper it is being used as such.
The fatal flaws in this study are the following:
- The holographic system in this study, the ICEMET, is used as the standard against which the six other instruments are compared, with no justification whatsoever.
- There is minimal uncertainty analysis of the instruments with none at all for the ICEMET.
- The ICEMET seriously overcounts with respect to all of the FMs and CDAs and yet the paper concludes that the problem is with all six of the single particle light scattering instruments with no admission that perhaps there is a problem with the ICEMET.
- The Nc, MVD and LWC derived from FM and CDAs are compared against the ICEMET but never against each other except in the size distribution figures where the differences do not look nearly as large as the X-Y comparisons. The least squares fits of Nc and LWC between FM and ICEMET and CDA and ICEMT have similar slopes, i.e. it would appear that it is the ICEMET that is seriously over counting rather than the FMs and CDAs undercounting.
- The argument as to why there is a difference between the ICEMET and other instruments is based solely on sampling efficiency with only the FMI orientation as an example while the explanation that the vertical orientation is the culprit for undercounting is not backed up by any serious modeling. Spiegel et al does not show the types of particle losses that are seen in these results.
I cannot offer a recommendation as how the author should proceed with this study, but I would like to see a review by a holography expert like Jacob Fugal or Jan Henneberger to weigh in on this study.
Citation: https://doi.org/10.5194/egusphere-2026-3898-RC2
Data sets
Data set: Intercomparison of cloud spectrometers and a holographic probe during the ACTRIS sub-zero in-cloud Campaign at Sonnblick Observatory K. Doulgeris et al. https://doi.org/10.5281/zenodo.21642323
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The Measurement report submitted focuses on instruments intercomparison and inlet modification/orietation impact analysis on sampling efficiency in a natural environment, in an ACTRIS observatory on top of a mountain. In ACP Aims, the authors are advised to consider whether their article fits better in other journals published by Copernicus Publications, such as Atmospheric Measurement Techniques....". While I understand that the measurement report doesn't meet the research article bar for AMT, it also struggles to meet the measurement report bar for ACP with no sufficient contextual description of the measurements in the atmospheric physics or meso scale context and highlighting the implications of measurements biases.
More specifically, there is lack of information about full PSD spectrum, TWC, and vertical orographic wind. No adequate analysis of flow for different inlet designs (either from simulations or a controlled environment) essential prior to such field intercomparison, including analysis of flow variability (prior to clogging) due to severe ice accretion observed in the pictures and given that the reports focuses on sampling in icing conditions. There is a mention of snowfall but no contextual information about precipitation or particles above 94um.
I suggest the authors to consider either an AMT research article submission with additional results and simulations or tests results in a controlled environment, which could be of substantial value for the community or improving the Measurement Report in ACP with more contextual connection to the atmosphere e.g., presenting how your findings and analysis impact past measurements or past conclusions about microphysical processes from this cite or providing a new correction factor or other meaningful recommendation beyond what the report currently offers to improve future measurements or measurements reported in other similarly positioned cites.