New type of evidence for secondary ice formation at around −15&thinsp;°C in mixed-phase clouds

Mignani, Claudia; Creamean, Jessie M.; Zimmermann, Lukas; Alewell, Christine; Conen, Franz

doi:https://doi.org/10.5194/acp-19-877-2019

Articles | Volume 19, issue 2

https://doi.org/10.5194/acp-19-877-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-19-877-2019

© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 19, issue 2

Research article

|

23 Jan 2019

Research article |

| 23 Jan 2019

New type of evidence for secondary ice formation at around −15 °C in mixed-phase clouds

Claudia Mignani, Jessie M. Creamean, Lukas Zimmermann, Christine Alewell, and Franz Conen

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Final revised paper (published on 23 Jan 2019)
Supplement to the final revised paper
Preprint (discussion started on 22 Aug 2018)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Review of Direct evidence for secondary ice formation at around -15C in mixed-phase clouds', Anonymous Referee #1, 28 Aug 2018
- SC1: 'Clarification regarding the main issue of concern in RC#1', Claudia Mignani, 05 Sep 2018
  - RC2: 'Sampling discussion', Anonymous Referee #1, 10 Sep 2018
    - AC1: 'Author response to Referee #1', Claudia Mignani, 28 Sep 2018
RC3: 'Reviews for Mignani et al. 2018', Anonymous Referee #2, 11 Sep 2018
- AC2: 'Author response to Referee #2', Claudia Mignani, 28 Sep 2018
  - RC4: 'Reviews Mignani et al 2018', Anonymous Referee #2, 03 Oct 2018
    - AC3: 'Author response to Referee #2 RC4', Claudia Mignani, 11 Oct 2018

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Claudia Mignani on behalf of the Authors (15 Nov 2018) Author's response Manuscript

ED: Referee Nomination & Report Request started (16 Nov 2018) by Markus Petters

RR by Sylvia Sullivan (27 Nov 2018)

RR by Anonymous Referee #3 (29 Nov 2018)

Suggestions for revision or reasons for rejection

I support publication. See below for a more complete explanation. An overview of how I reviewed the paper is also warranted. I read the paper after reviews 1 and 2 (and the corresponding replies from the authors) had already been posted. I read the paper first, so that I could form an independent opinion, then read the other reviews and replies. Finally, I read the revised version of the manuscript (and associated correspondence from the authors) before I wrote out my own review.

The authors have addressed a particularly pernicious issue in cloud physics -- secondary ice formation in a regime where riming-splintering is not occurring. As the authors have noted, field observations indicate that there's more ice than can be explained with the measurements of ice nuclei. There are hypotheses as to what processes produce this ice, but data is scarce.

This study is rare, in that the authors attempt to derive a multiplication factor from field data. While there are uncertainties associated with their technique, it's quite intriguing and worthy of publication. Using the ice crystals as a measure of the temperature is the key step in this procedure, and I think that it is justified. The (pristine) habit is a good indicator of temperature and saturation ratio. Using planar, branched crystals in mixed phase cloud conditions restricts the temperature range to $\approx$ -15 C. By collecting individual crystals, melting them, then re-freezing them, the authors establish whether there is an ice nucleating particle within the crystal capable of nucleating ice at -15 or higher. The absence of such an entity is strong evidence that the crystal was formed through a multiplication process. If the crystal is the result of a secondary process, and it collects other interstitial aerosol particles or aerosol particles embedded in supercooled droplets (\textit{i.e} through riming) before being sampled, those particles will most likely not be effective ice nucleating particles for $T \geq -15$.

Those facts, taken together, are strong indicators of primary vs. secondary formation processes. The habit indicates the temperature of formation, and the re-freezing temperature indicates the presence or absence of a particle capable of catalyzing freezing at -15, which in turn is an indication of primary vs. secondary formation.

Are there assumptions and uncertainties inherent in this method? Yes. The other reviewers have pointed out several; the changes that the authors have made in the revised manuscript have improved it substantially. \vskip .2cm

The key issue is whether the assumptions and uncertainties that are inherent in this technique are so great as to preclude publication. I do not think they are. \vskip .2cm

This is just musing, and a not-quite-rhetorical question on my part... Does this data imply that the secondary process here produces ice fragments that are quite small? If the process produced large fragments, wouldn't those show up as irregularities in the collected crystals. (In other words, the habits wouldn't be so pristine..?) To be clear, I'm not asking that the authors settle this question, just posing it as something to consider.

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ED: Reconsider after major revisions (29 Nov 2018) by Markus Petters

AR by Claudia Mignani on behalf of the Authors (10 Jan 2019) Author's response Manuscript

ED: Publish as is (11 Jan 2019) by Markus Petters

AR by Claudia Mignani on behalf of the Authors (14 Jan 2019) Manuscript

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Short summary

A snow crystal can be generated from an ice nucleating particle or from an ice splinter. In this study we made use of the fact that snow crystals with a particular shape (dendrites) grow within a narrow temperature range (−12 to −17 °C) and can be analysed individually for the presence of an ice nucleating particle. Our direct approach revealed that only one in eight crystals contained such a particle and was of primary origin. The other crystals must have grown from ice splinters.