Rapid saturation of cloud water adjustments to shipping emissions

Manshausen, Peter; Watson-Parris, Duncan; Christensen, Matthew W.; Jalkanen, Jukka-Pekka; Stier, Philip

doi:https://doi.org/10.5194/acp-23-12545-2023

Articles | Volume 23, issue 19

https://doi.org/10.5194/acp-23-12545-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-23-12545-2023

© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 23, issue 19

ACP Letters

| Highlight paper

|

09 Oct 2023

ACP Letters | Highlight paper |

| 09 Oct 2023

Rapid saturation of cloud water adjustments to shipping emissions

Peter Manshausen, Duncan Watson-Parris, Matthew W. Christensen, Jukka-Pekka Jalkanen, and Philip Stier

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Final revised paper (published on 09 Oct 2023)
Preprint (discussion started on 16 May 2023)

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Status: closed

RC1: 'Comment on egusphere-2023-813', Anonymous Referee #1, 29 May 2023

Please find my review attached below.

Citation: https://doi.org/10.5194/egusphere-2023-813-RC1
RC2:
'Comment on egusphere-2023-813', Anonymous Referee #2, 16 Jun 2023
The study presented in this manuscript uses satellite together with ship position and emission data to identify so-called invisible ship tracks, and exploit changes in emission regulations to evaluate droplet number and liquid water path changes in ship tracks compared to out-of-track data as a function of SOx emission and time since emission. While it builds on a previous study published in Nature by the same authors, it is not just an extension of the dataset, but focusses on an effect not covered in the previous work: While the Nd perturbation scales with emission, the LWP changes do not. This hints at a non-linear threshold behaviour which is highly relevant for the evaluation of aerosol indirect effects from observations and their representation in climate models.

The paper is well-written with clear graphics. The fact that the entire methods section is in the appendix appears unusual, but is in line with the requirements for an ACP Letter. Given the clear storyline , the conciseness of the manuscript, and the relevance of the results, I support publication as a letter subject to minor revisions.

Detailed comments:

3/Line 163: the region of study is specified as “(-50°S,50°N)” and “(-90°W, 20°E)”, but in Fig. 3, the southernmost row of this domain is partly white. Is this because data is missing here? Or are the values all 0%? Please indicate.

How are the out-of-track regions defined? I was not able to find details on this. Line 145 mentions 30 km distance, but this is not further explained in the methods section. Line 213 mentions control regions, which I assume is the same as out-of-track regions, but it would be good to use consistent terminology.

Only one year of post-IMO regulation change data is available (2021). Please discuss in how far this may introduce an uncertainty when pre- and post-IMO conditions are compared.

Line 175 mentions “best-matched tracks”. What does this mean and how is it tested?

Technical comments:
Sometimes the position of the parentheses for references is not correct, e.g. in line 39, where it should read: “As in Manshausen et al. (2022), …”

Line 118: add “their” before Fig. 6 to make clear that this is referring to Gryspeerdt et al.’s figure.

Line 171: positons -> positions
Citation: https://doi.org/10.5194/egusphere-2023-813-RC2
AC1: 'Comment on egusphere-2023-813', Peter Manshausen, 19 Jul 2023

Thank you to the reviewers for their comments. Please find our response in the attached pdf.

Citation: https://doi.org/10.5194/egusphere-2023-813-AC1

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Peter Manshausen on behalf of the Authors (19 Jul 2023) Author's response Manuscript

EF by Lorena Grabowski (20 Jul 2023) Author's tracked changes

ED: Referee Nomination & Report Request started (28 Jul 2023) by Martina Krämer

RR by Anonymous Referee #1 (29 Jul 2023)

RR by Anonymous Referee #2 (31 Jul 2023)

ED: Publish subject to technical corrections (15 Aug 2023) by Martina Krämer

Dear Peter Manshausen and Co-authors,

I am pleased to let you know that your manuscript is accepted by both referees with excellent ratings. Ref#1 suggests some technical corrections (see below) that I ask you to consider in the final version of the article.

Congratulations and best wishes, Martina Krämer

Senior Editor
Atmospheric Chemistry and Physics
https://www.atmospheric-chemistry-and-physics.net
An interactive open-access journal of the European Geosciences Union

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The authors have addressed my comments carefully, and I fully support the manuscript’s publication
in Atmospheric Chemistry and Physics. Below, I state a few suggestions that the authors may want to
consider.
Suggestions (all line numbers refer to the tracked changes document)
A recent paper by Diamond (2023) also addresses the impact of shipping fuel regulations on clouds. I
suggest referencing this publication.
Ll. 7 – 9: Instead of writing about “increases”, I suggest writing about “anomalies”. I associate a
“constant increase” with a constant dLWP/dNd or dLWP/dNa, which are not independent of emissions,
as they would result in an increase of LWP proportional to Nd or Na.
L. 21: Replace “they” with “these“, or remove “studies”.
L. 43: Define “IR” or replace with “infrared”.
L. 54: Change to “The mixing of the track with the surrounding air, [...]”.
Ll. 118 – 119, Fig. 6a: Any ideas why the Nd anomaly increases with free-tropospheric dryness? Could
this be due to an enhanced suppression of precipitation by the evaporation of LWP via the entrainment
of drier air?
L. 123: “controlling”, not “controllong”.
References
Diamond, M. S. (2023). Detection of large-scale cloud microphysical changes within a major shipping
corridor after implementation of the International Maritime Organization 2020 fuel sulfur
regulations. Atmospheric Chemistry and Physics, 23(14), 8259-8269.

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ED: Publish as is (18 Aug 2023) by Timothy Garrett (Executive editor)

AR by Peter Manshausen on behalf of the Authors (01 Sep 2023) Manuscript

Executive editor

Ship tracks are enhanced regions of cloud brightness that trail behind ships that are known to be created by their effluent. They are widely used as observational test beds to deepen understanding of how pollution might affect cloud microphysical processes and climate at larger regional and global scales. This study uses satellite observations of cloud properties and records of ship position and their effluent to assess perturbations from ship emissions on cloud droplet number and liquid water path. The study found that, as expected, the droplet number perturbation in shiptracks scales with ship emission rates of aerosol particles. Surprisingly, however, the liquid water path in drizzling clouds increased by an amount that was nearly fixed. The observation points to novel non-linear threshold behaviours of relevance to representations of aerosol indirect effects in climate models.

Rapid saturation of cloud water adjustments to shipping emissions

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