Novel Possibilities of In-Situ Cloud Observation with Drone-Mountable Holographic Imager

Leskinen, Ari; Radlwimmer, Antonia; Isopahkala, Uula; Calderón, Silvia; Brus, David; Doulgeris, Konstantinos; Kaikkonen, Ville; Molkoselkä, Eero; Moiseev, Dmitri; Hirschy, Marie Lou; Mäkynen, Anssi; Romakkaniemi, Sami; Komppula, Mika

doi:10.5194/egusphere-2026-2088

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https://doi.org/10.5194/egusphere-2026-2088

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30 Jun 2026

| 30 Jun 2026

Status: this preprint is open for discussion and under review for Atmospheric Measurement Techniques (AMT).

Novel Possibilities of In-Situ Cloud Observation with Drone-Mountable Holographic Imager

Ari Leskinen, Antonia Radlwimmer, Uula Isopahkala, Silvia Calderón, David Brus, Konstantinos Doulgeris, Ville Kaikkonen, Eero Molkoselkä, Dmitri Moiseev, Marie Lou Hirschy, Anssi Mäkynen, Sami Romakkaniemi, and Mika Komppula

Abstract. We present a lightweight holographic imaging instrument, the UAV-CPS (Unmanned Aerial Vehicle Cloud Particle Sensor), designed for high-resolution in-cloud particle measurements at a rate of 10 holograms per second. When mounted on a UAV, the instrument samples a well-defined volume of ~3 cm³ within a single hologram, providing sub-meter spatial resolution within targeted cloud regions such as stratiform cloud tops and cumuliform cloud edges. Compared to conventional platforms, such as research aircraft or tethered balloons, the UAV-based approach offers a cost-effective and versatile alternative, capable of accessing confined or remote areas. This enables improved validation of remote sensing products and provides observational constraints for cloud process rates in numerical cloud models. During a 16-day campaign comprising 14 flights at Pallas, Finland, UAV-CPS measurements were compared against reference instruments installed at the hilltop Sammaltunturi station. On average, UAV-CPS showed lower droplet concentrations in the 5–13 µm diameter range, while larger particles were captured with high accuracy. As with other holographic sensors, detecting and sizing the smallest droplets in a large measurement volume per hologram remains challenging. However, these limitations are quantified and can be mitigated through calibration, complementary measurements, and ongoing instrument refinement, such as optimizing hologram reconstruction procedures and employing a shorter-wavelength light source. In addition to introducing UAV-CPS, we present a case study that highlights the potential of such instrumentation for high-resolution cloud observations.

Received: 13 Apr 2026 – Discussion started: 30 Jun 2026

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RC1:
'Comment on egusphere-2026-2088', Anonymous Referee #2, 20 Jul 2026 reply
The manuscript presents a promising lightweight holographic imager for UAV-based cloud measurements and addresses a topic well suited to Atmospheric Measurement Techniques. The field deployment and combination with aerosol, ground-based, and remote-sensing observations are valuable. However, key aspects of the instrument characterization, particularly the effective sample volume, small-particle detection efficiency, and number-concentration uncertainty, require clearer treatment. In addition, several atmospheric interpretations are stronger than can be supported by the spatially and temporally separated observations. I therefore recommend major revisions before the manuscript can be considered for publication.
Major comments:

Instrument description and hologram processing

Sect. 2.3.1 requires more technical detail, even if the general design is described by Kaikkonen et al. (2024). Please specify:

whether the optical beam is diverging or collimated;

theoretical and practical resolution limits;

camera pixel size;

dimensions of the reconstructed and accepted measurement volume;

relevant reconstruction, focusing and particle-detection criteria.

For a diverging-beam system, optical resolution and detection efficiency vary with distance from the detector. The effective sample volume may therefore depend on particle size. Please clarify whether this depth- and size-dependence was considered when calculating particle size distributions and number concentrations, and provide an uncertainty estimate.

Please also describe how droplets, ice particles and pollen were classified. Was classification automated, based on morphological thresholds or manually verified?

Atmospheric interpretation in Sect. 3.3

Several interpretations in Sect. 3.3 are stronger than supported by the observations. The measurements cannot uniquely distinguish between cloud-top activation, entrainment, evaporation, aerosol nucleation, boundary-layer decoupling and terrain-induced effects.

For example, the statement that “activation is taking place in both layers” is not demonstrated. The small-droplet mode could also result from evaporation, mixing, spatial heterogeneity or temporal differences between the UAV flights. Likewise, elevated 7-15 nm concentrations are consistent with recent particle formation but do not demonstrate local nucleation at cloud top.

The proposed cloud-top activation mechanism also requires further explanation. Entrainment normally introduces drier free-tropospheric air and initially promotes evaporation. How would this process generate sufficient supersaturation for additional activation?

Please distinguish clearly between direct observations, model-supported interpretations and unconfirmed hypotheses, and use more cautious wording throughout Sect. 3.3.

Number-concentration uncertainty is assumed rather than demonstrated

The statement that holographic instruments provide reliable number concentrations because their sampling volume is well defined should be qualified. A known geometric volume does not necessarily represent the effective detection volume, particularly when detection efficiency, optical resolution, image contrast, or reconstruction performance vary with particle size and position within the volume.

The bead measurements characterize sizing bias and repeatability, but they do not appear to validate particle-detection efficiency or absolute number concentration. The authors should therefore:

determine the detection efficiency as a function of particle diameter and reconstructed depth, if the calibration data allow;

derive a size-dependent effective sample volume and use it when calculating number concentrations;

quantify the resulting uncertainty in number concentration and propagate it to CDNC and LWC;

clearly distinguish sizing uncertainty, sample-volume uncertainty, and detection-efficiency uncertainty.

If these quantities cannot be derived from the available data, the manuscript should explicitly state that absolute number-concentration accuracy has not been validated, especially for particles near the lower detection limit, and the corresponding conclusions should be qualified.

Minor comments / extensions:
Comparison with an adiabatic cloud profile (Lines 380–383): A comparison with an idealized adiabatic cloud profile would strengthen the interpretation of the observed increase in droplet size with altitude. In particular, the measured vertical evolution of LWC and a characteristic droplet diameter could be compared with the expected adiabatic increase above cloud base. This would help assess whether the observed cloud developed approximately adiabatically or whether processes such as entrainment, evaporation, drizzle formation modified its vertical structure.
Lines 19–21: Also report the volumetric sampling rate in cm³ s⁻¹.

Line 24: State the typical UAV endurance and operational range.

Lines 29–30: Give the full reported particle-size range.

Lines 51–53: Clarify “other concurrent processes” and provide examples.

Lines 60–61: Explain how increased aerosol concentration may increase LWC.

Line 114: Specify which processes are being investigated.

Line 254: Clarify “total count of ambient air sample”.

Line 411: Explain the origin of the reduced sensitivity to small particles, preferably in the instrument section.

Sect. 3.4: Move the uncertainty methodology to the Methods section so that instrument limitations are established before interpreting the observations.

Lines 517–526: Move the discussion of depth-dependent resolution and small-particle detection to the instrument or methods section.

Table 1: Explain how cloud-top altitude was determined.

Technical corrections
Line 1: “In-Situ” -> “in situ”.

Line 44: Replace the colon after “Bony et al., 2015” with a semicolon.

Line 66: Use “cloud droplet number concentration”.

Line 72: “Feingold et al.; 2022” -> “Feingold et al., 2022”.

Line 87: “Henneberg” -> “Henneberger”.

Line 106: Add a citation for the statement on UAV instrument applications.

Line 152: “Raspberry pi” -> “Raspberry Pi”.

Line 215: “non-tracable” and “NIST-tracable” -> “non-traceable” and “NIST-traceable”.

Line 223: “analysis if the data” -> “analysis of the data”.

Line 263: “Le at al.” _> “Le et al.”

Line 332: “cloud laying below” -> “cloud lying below”.

Line 347: “reanalyzed hourly data” -> “hourly reanalysis data”.

Line 410: A work described as “to be submitted” should not be used as an accessible reference.

Lines 423–424: Add a citation supporting the comparison with typical marine stratocumulus inversions.

Line 436: “Keil and Wendish” -> “Keil and Wendisch”; check the DOI.

Line 518: “between approximately from 5 µm to 10 µm” -> “from approximately 5 to 10 µm”.

Line 526: “partly due the” -> “partly due to the”.

Line 577: “have also potential” -> “also have potential”.

Lines 579–581: Rewrite to clarify that UAV-CPS profiles are compared with Cloudnet retrievals and Sammaltunturi measurements.

Line 604: Add the missing unit: “5–10 µm”.

Figures
Throughout the manuscript please refer to the specific sub-figures if not the full figure is meant, e.g. Fig. 9 a) etc
Figure 5: Increase the resolution and enlarge labels and legends.

Figure 6: Replace the 3-D bars with a two-dimensional colour-shaded size–height plot. Use logarithmic scaling where appropriate.

Figure 8: Specify whether the ordinate represents , , or concentration per bin.

Figure 9: Add horizontal lines marking cloud base and cloud top. Panel d) please plot x-axis in log format

Reply
Citation: https://doi.org/10.5194/egusphere-2026-2088-RC1
CC1: 'Comment on egusphere-2026-2088', Matthew Berg, 20 Jul 2026 reply

This article presents the results of the field deployment of a holographic imaging sensor for cloud droplets and compares the data with other instruments. This is important work showing much promise for cloud physics studies. Yet the authors can improve the explanation of the holographic sensor. I realize that the instrument is described in the 2024 IEEE paper, but that paper appears not to be open access and it is difficult otherwise to understand how the holograph instrument works. Perhaps the authors could add a diagram of the optics and maybe a block diagram of the electronics and processing to help others who cannot access the earlier paper. Also, the authors need to me more conscience about placing their work in the context of the many drone holographic instruments that precede them. For example, I was surprised not to see any mention of the drone instrument by Kemppinen et al. [O. Kemppinen et al., "Imaging atmospheric aerosol particles from a UAV with digital holography" Sci. Rep. 10, 16085 (2020). https://doi.org/10.1038/s41598-020-72411-x]. The calibration work is good, but I should mention that if the optical design were changed to involve a collimated beam rather than a diverging beam, this effect should disappear -- just a suggestion. Also, I spotted a POPS in the drone. But, I thought POPS does not respond to particles much larger than about 3um diameter. What is this for in relation to the holography then?

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Citation: https://doi.org/10.5194/egusphere-2026-2088-CC1

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Short summary

We present a lightweight holographic imaging instrument, designed for high-resolution in-cloud particle measurements at a rate of 10 holograms per second, which leads to sub-meter spatial resolution on-board UAV. The approach offers a cost-effective and versatile alternative, capable of accessing confined or remote areas, enabling improved validation of remote sensing products and numerical cloud models. We present the potential of such instrumentation for high-resolution cloud observations.


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