Preprints
https://doi.org/10.5194/egusphere-2026-3865
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/egusphere-2026-3865
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
07 Aug 2026
| 07 Aug 2026
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).
Investigating ice microphysical processes in the dendritic growth zone by combining Monte-Carlo Lagrangian particle modelling with multi-frequency polarimetric radar observations
Abstract. The dendritic growth zone (DGZ) is associated with distinct polarimetric and multi-frequency radar signatures, yet the governing microphysical processes remain uncertain. We analyse characteristic DGZ observations showing a concurrent increase in dual-wavelength ratio (DWR), enhanced specific differential phase shift (KDP) and the maximum of the spectrally resolved ZDR (sZDRmax), a pronounced reduction in mean Doppler velocity (MDV), and the occurrence of a secondary Doppler spectral mode near −15 °C.
To investigate the governing processes, radar observations are combined with the Lagrangian particle-based Monte Carlo model McSnow, which includes an updated ice habit scheme and a new fragmentation parametrization. Forward radar simulations use a discrete dipole approximation scattering database.
The simulations show that enhanced sZDRmax requires local formation of dendritic or plate-like crystals near −15 °C; sedimentation of pre-existing particles alone cannot reproduce the signal. The observed KDP enhancement is only reproduced when secondary ice production via collisional fragmentation is included, which also strengthens aggregation-related signatures. The reduction in mean Doppler velocity is explained by a habit change and aggregation of sedimenting columnar ice particles. Together, these signatures provide the most diagnostic constraints on DGZ microphysical processes identified so far.
This study demonstrates that multi-frequency polarimetric radar observations combined with Monte Carlo Lagrangian particle simulations can disentangle competing ice microphysical processes in the DGZ. The results identify collisional fragmentation as a key unifying mechanism, with the DGZ radar fingerprint emerging from the interplay of depositional growth, aggregation, and secondary ice production.
How to cite. von Terzi, L., Seifert, A., Siewert, C., Jakub, F., and Kneifel, S.: Investigating ice microphysical processes in the dendritic growth zone by combining Monte-Carlo Lagrangian particle modelling with multi-frequency polarimetric radar observations, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2026-3865, 2026.
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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Status: open (until 18 Sep 2026)
Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor
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- RC1: 'Comment on egusphere-2026-3865', Anonymous Referee #1, 29 Aug 2026 reply
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RC2: 'Comment on egusphere-2026-3865', Anonymous Referee #2, 04 Sep 2026
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This is an interesting and insightful study by von Terzi et al., which works to address questions which remain regarding processes active in the dendritic growth zone. The manuscript is clear, well structured, and enjoyable to read. This study is worthy of publication, subject to changes made in response to some minor comments detailed below.Minor commentsL32: It would be good to specify at what temperatures it is thought that collisional ice fragmentation contributes to the discrepancy between INP and ice particle concentrations. Is it specifically in the DGZ, or at a wider range of temperatures?L45: It is interesting that approximately a quarter of your observations had regions of high KDP within the DGZ; this is broadly consistent with the frequency of multimodal spectra identified in the DGZ from other studies (https://amt.copernicus.org/articles/18/6569/2025/amt-18-6569-2025.pdf, Figure 8, https://egusphere.copernicus.org/preprints/2026/egusphere-2026-2928/, Figure 14).L58-9: "Similar signatures have also been identified in spectrally resolved observations, where the slowdown tends to be most pronounced among slowly falling particles." The way this sentence is worded is confusing. Which slowly falling particles are you referring to - slowly falling secondary ice populations, or particles which fall slowly due to their open geometry, like dendrites? If I understand correctly, you are referring to the latter, but this should be clarified.L60: I think it is worth acknowledging the possibility that particles which sediment into the DGZ from above may themselves slow down. I'm not suggesting that this process replaces those you mention, but I believe it is at the very least a contributing factor.Figure 1: This is a useful figure and nicely demonstrates the features you discuss in the text, but I would like to see the same statistics for the subset of data where KDP<1degree/km in the DGZ. This could be as extra panels, but I think you have room to add extra lines to the existing panels without the plot becoming overly cluttered. I think it would be useful to see a more direct comparison of how the plotted variables change between profiles where KDP>1degree/km and KDP<1degree/km in the DGZ.L213: I would argue that we do not have enough evidence to state that fragmentation activity is "known to be" highest in the DGZ, so would prefer the phrasing here to be softened.L225: I would like to see an explanation of why fragments produced by fragmentation are assumed to be plate-like. Intuitively, the fragments produced would resemble dendritic branches, like those seen in Grzegorcyzk et al. (2023, Figure 10a).L302: Again, I would soften the statement that fragmentation is "known to be" active in the DGZ. Whilst I agree that it is likely, my understanding is that our current knowledge of fragmentation is mostly based on lab studies, which, whilst critical to our understanding of certain ice microphysical processes, cannot perfectly replicate real atmospheric conditions.L333: "Since the observed Ze values lie mostly above the 75th percentile of the statistical climatology in Figure 1, slight underestimations in this temperature range were deemed acceptable." Underestimations of what? Ice nucleating particles? Ze? Please clarify.Figure 5: In Figure 5IIa, the legend is covering the blue lines and making the detail quite hard to see, particularly where the "monomer" Ze line begins to deviate from the "aggregate" Ze. Personally, I would be happy for these lines to be defined in the figure caption if moving the legend is impractical.Figure 6: Figure 6 needs to be discussed in much greater detail in the text; as far as I can see, it is mentioned only once. At a minimum, I would like to see an explanation of the variables plotted and how the panels should be interpreted, as these are non-intuitive to me.L459: Simulation 3 does produce the greatest MDV slowdown, but I would like to see a brief discussion on how realistic the shape of this profile is. The slowdown you see here appears to be very localised between temperatures of -15degC and -12degC, whereas the shape of this profile in previous literature has this reduction in MDV occurring over a broader temperature range.Typos/formatting pointsL42: KDP exceeding 1degree/km does not need to be in brackets.
L184: A space is missing after "sedimentation".
L214: I believe it should be collisional kinetic energy.
L216: There is no capital needed on graupel.
L279: "launch time for"
L305: IMP is not defined.
When discussing the slowing down of particles/MDV in the DGZ, be consistent with either "slowdown" or "slow-down"; you currently have a mixture of both.Citation: https://doi.org/10.5194/egusphere-2026-3865-RC2
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Latest update: 17 Sep 2026
Ludwig-Maximilians University Munich
Axel Seifert
Deutscher Wetterdienst
Christoph Siewert
Deutscher Wetterdienst
Fabian Jakub
Deutscher Wetterdienst
Stefan Kneifel
Ludwig-Maximilians University Munich
Short summary
A distinct temperature layer in clouds promotes growth of dendritic ice crystals, producing characteristic radar signatures whose origin has remained unclear. We combined radar observations with microphysical simulations to identify the dominant microphysical processes. Collisions breaking particles into fragments prove essential to reproducing the signatures, alongside crystal growth and aggregation, clarifying the link between radar signals and ice processes for weather models.
A distinct temperature layer in clouds promotes growth of dendritic ice crystals, producing...
This is an interesting study combining multi-frequency polarimetric radar observations with the McSnow Lagrangian particle model to investigate microphysical processes in the dendritic growth zone. I like the use of radar to constrain the modelling.
My main concern is that the conclusions about collisional fragmentation are stronger than can be supported by the modelling framework as currently described. The new fragmentation treatment requires substantially more implementation detail, including how physical ice–ice collisions and fragment production are represented in the super-particle framework, the values and sensitivities of key parameters, and how fragment size, habit, collision energy and particle type/rime state are treated. Is it just the fragmentation of the initialised ice with the same ice. Is there any graupel? I think not as you say there was no riming; however, frontal clouds often contain embedded convection.
Therefore I am also concerned that the 1-D shaft configuration excludes other potentially important secondary-ice pathways by construction. The treatment and initialisation of cloud liquid water, drizzle and rain are not clear - I don't think there is any initialised, and the absence of embedded convection/vertical transport makes it difficult to generate or maintain supercooled liquid drops. Freezing-drop fragmentation and fragmentation associated with liquid–ice collisions could therefore be important alternatives that are not tested. The observations should be used, where possible, to establish whether supercooled liquid/drizzle/rain and rimed particles were present or can reasonably be excluded.
There are uncertainties in the primary ice nucleation profile and in the representation of ice habits at colder temperatures. These assumptions may affect the comparison between particles sedimenting from above and particles generated locally within the DGZ.
Overall, I consider the manuscript suitable for public discussion, but I expect that major revision will be needed. In particular, I think the conclusions should presently be framed as showing that an additional local source of small asymmetric ice particles is required to reproduce the observed radar signatures, with collisional fragmentation being one plausible mechanism, rather than as uniquely identifying ice–ice fragmentation.