the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Mar 2025
Towards an improved understanding of the impact of clouds and precipitation on the representation of aerosols over the Boreal Forest in GCMs
Abstract. General circulation models (GCMs) face uncertainties in estimating Earth's radiative budget due to aerosol-cloud interactions (ACI). Accurate aerosol number size distributions are crucial for improving ACI representation in GCMs, requiring precise modelling of aerosol source and sink processes throughout their lifetime. This study employs a Lagrangian trajectory framework to analyse how clouds and precipitation influence aerosol lifecycles during transport in the boreal forest. A comparison of two GCMs, the United Kingdom Earth System Model (UKESM1) and ECHAM6.3-HAM2.3-MOZ1.0 with the SALSA2.0 aerosol module (ECHAM-SALSA), is conducted. An evaluation against in-situ observations and reanalysis-based trajectories is performed. Results show that overall aerosol-precipitation trends are similar between GCMs and observations. However, seasonal differences emerge: in summer, UKESM1 exhibits more efficient aerosol removal via precipitation than ECHAM-SALSA and observations, whereas in winter, the opposite is observed. These were found to coincide with differences in key variables controlling aerosol activation, such as sub-grid scale updraughts and number size distributions. For example, in winter the removal of the total aerosol mass in ECHAM-SALSA was stronger compared to UKESM1, coinciding with higher activated fractions during airmass transport, which, on the other hand, were likely due to the larger sub-grid scale updraughts in ECHAM-SALSA. For both GCMs, investigation of aqueous-phase chemical processing along the trajectories showed clear increase of SO4 mass for cloud-processed air masses when compared to clear sky conditions, in-line with the observations. As expected, based on the model parametrizations, these increases in SO4 were mostly distributed to the accumulation mode aerosols.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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.- Preprint
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Status: closed
- RC1: 'Comment on egusphere-2025-721', Anonymous Referee #1, 10 Apr 2025
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RC2: 'Comment on egusphere-2025-721', Anonymous Referee #2, 30 May 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-721/egusphere-2025-721-RC2-supplement.pdf
- RC3: 'Comment on egusphere-2025-721', Anonymous Referee #3, 14 Jul 2025
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AC1: 'Response to reviewers', Sini Talvinen, 22 Aug 2025
We thank all three referees for their thorough and constructive reviews, which have significantly improved the quality and clarity of our manuscript. The comments provided have helped us to refine the storyline, improve the structure, and better emphasize the core findings. A recurring point raised by all reviewers concerned the length of the manuscript. We acknowledge that the original submission was extensive, as we aimed to ensure robustness and transparency by including substantial technical detail and supporting information. In response, we have substantially revised the text to improve conciseness and flow without omitting essential material. The main text has been reduced from approximately 15,500 to 13,900 words (from Introduction to Conclusions/Outlook), with several sections streamlined for clarity. Additionally, Appendix B and the comparison between all AeroCom models—originally presented early in Sect. 4—have been moved to the Supplementary Material (reorganized section S4), allowing the main paper to focus more tightly on the key results and interpretations. Longer sections have been carefully divided into subsections to enhance readability; for example, dedicated subsections have been introduced for the airmass transport comparison (Sect. 3.1) and the comparison between surface and vertically resolved precipitation (Sect. 4.1). These structural improvements significantly enhance the manuscript’s clarity and navigability.
Our detailed responses for each referee are presented in the attached supplement.
Status: closed
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RC1: 'Comment on egusphere-2025-721', Anonymous Referee #1, 10 Apr 2025
The authors compare output from two general circulation models that were nudged to reanalysis conditions against aerosol measurements taken at a boreal forest ground site. The analysis includes both Eulerian and Lagrangian perspectives. The latter is used to infer if aerosol sinks and sources are plausible in both models. The manuscript is of great relevance and should be published after major revisions.
Major concerns
I think the manuscript (not considering appendix and supporting material) is too long and should be more concise in style. A good example is the “Conclusion and Outlook” section, which spans over 2 pages. While there are key conclusions in this section, it is hard to find them when mingled with discussion material. Instead, the authors could split this section into a conclusion sections (as also suggested by the ACP guidelines) that is very concise and an outlook (or else discussion) section. In the same manner, the authors should attempt to shorten sections 3-5, by perhaps moving discussion-style material to a dedicated discussion section.
Throughout the Lagrangian analysis the authors mostly show median values. I’m wondering how large the interquartile range is and whether the main conclusions are affected when considering this range.
Minor concerns
l. 57 Please define “sectional approaches”.
ll. 428ff (and Fig. 3). Why is rainfall rate increasing closer to the site?
ll. 805ff. Is there a chance that “in-cloud” and “clear-sky” groups correspond to very different regions (with unique local aerosol sources)?
Typos/style
ll. 923-924 Please check this sentence.
l. 943 “extremeties”
Citation: https://doi.org/10.5194/egusphere-2025-721-RC1 -
RC2: 'Comment on egusphere-2025-721', Anonymous Referee #2, 30 May 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-721/egusphere-2025-721-RC2-supplement.pdf
- RC3: 'Comment on egusphere-2025-721', Anonymous Referee #3, 14 Jul 2025
-
AC1: 'Response to reviewers', Sini Talvinen, 22 Aug 2025
We thank all three referees for their thorough and constructive reviews, which have significantly improved the quality and clarity of our manuscript. The comments provided have helped us to refine the storyline, improve the structure, and better emphasize the core findings. A recurring point raised by all reviewers concerned the length of the manuscript. We acknowledge that the original submission was extensive, as we aimed to ensure robustness and transparency by including substantial technical detail and supporting information. In response, we have substantially revised the text to improve conciseness and flow without omitting essential material. The main text has been reduced from approximately 15,500 to 13,900 words (from Introduction to Conclusions/Outlook), with several sections streamlined for clarity. Additionally, Appendix B and the comparison between all AeroCom models—originally presented early in Sect. 4—have been moved to the Supplementary Material (reorganized section S4), allowing the main paper to focus more tightly on the key results and interpretations. Longer sections have been carefully divided into subsections to enhance readability; for example, dedicated subsections have been introduced for the airmass transport comparison (Sect. 3.1) and the comparison between surface and vertically resolved precipitation (Sect. 4.1). These structural improvements significantly enhance the manuscript’s clarity and navigability.
Our detailed responses for each referee are presented in the attached supplement.
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The authors compare output from two general circulation models that were nudged to reanalysis conditions against aerosol measurements taken at a boreal forest ground site. The analysis includes both Eulerian and Lagrangian perspectives. The latter is used to infer if aerosol sinks and sources are plausible in both models. The manuscript is of great relevance and should be published after major revisions.
Major concerns
I think the manuscript (not considering appendix and supporting material) is too long and should be more concise in style. A good example is the “Conclusion and Outlook” section, which spans over 2 pages. While there are key conclusions in this section, it is hard to find them when mingled with discussion material. Instead, the authors could split this section into a conclusion sections (as also suggested by the ACP guidelines) that is very concise and an outlook (or else discussion) section. In the same manner, the authors should attempt to shorten sections 3-5, by perhaps moving discussion-style material to a dedicated discussion section.
Throughout the Lagrangian analysis the authors mostly show median values. I’m wondering how large the interquartile range is and whether the main conclusions are affected when considering this range.
Minor concerns
l. 57 Please define “sectional approaches”.
ll. 428ff (and Fig. 3). Why is rainfall rate increasing closer to the site?
ll. 805ff. Is there a chance that “in-cloud” and “clear-sky” groups correspond to very different regions (with unique local aerosol sources)?
Typos/style
ll. 923-924 Please check this sentence.
l. 943 “extremeties”