CESM2/CARMA Cloud and CARMA Aerosol Model Descriptions

Zhu, Yunqian; Liu, Cheng-Cheng; Bardeen, Charles; Wang, Lu; Tilmes, Simone; Quaglia, Ilaria; Maloney, Christopher M.; Vitt, Francis; Toon, Owen Brian

doi:10.22541/essoar.177135577.71771059/v1

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https://doi.org/10.22541/essoar.177135577.71771059/v1

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11 Mar 2026

| 11 Mar 2026

Status: this preprint is open for discussion and under review for Geoscientific Model Development (GMD).

CESM2/CARMA Cloud and CARMA Aerosol Model Descriptions

Yunqian Zhu, Cheng-Cheng Liu, Charles Bardeen, Lu Wang, Simone Tilmes, Ilaria Quaglia, Christopher M. Maloney, Francis Vitt, and Owen Brian Toon

Abstract. The Community Aerosol and Radiation Model for Atmospheres (CARMA) is a sectional microphysical model widely used to understand the formation of aerosols and clouds. This paper is an updated description of the CARMA model algorithms since Toon et al 1988, with examples from the newly developed CARMA Cloud model. This paper describes the general algorithms and solvers for nucleation, condensation/evaporation, coagulation, and sedimentation shared among all CARMA aerosols and clouds models; specific microphysical processes needed for water droplet and ice; how the CARMA Cloud is set up and how it interacts with other processes inside the Community Earth System Model.

Received: 02 Mar 2026 – Discussion started: 11 Mar 2026

Yunqian Zhu, Cheng-Cheng Liu, Charles Bardeen, Lu Wang, Simone Tilmes, Ilaria Quaglia, Christopher M. Maloney, Francis Vitt, and Owen Brian Toon

Status: open (until 06 May 2026)

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CEC1:
'Comment on egusphere-2026-1185', Astrid Kerkweg, 01 Apr 2026 reply
Dear authors,
in my role as Executive editor of GMD, I would like to bring to your attention our Editorial version 1.2: https://www.geosci-model-dev.net/12/2215/2019/
This highlights some requirements of papers published in GMD, which is also available on the GMD website in the ‘Manuscript Types’ section: http://www.geoscientific-model-development.net/submission/manuscript_types.html
In particular, please note that for your paper, the following requirement has not been met in the Discussions paper:
"The main paper must give the model name and version number (or other unique identifier) in the title."

Code must be published on a persistent public archive with a unique identifier for the exact model version described in the paper or uploaded to the supplement, unless this is impossible for reasons beyond the control of authors. All papers must include a section, at the end of the paper, entitled "Code availability". Here, either instructions for obtaining the code, or the reasons why the code is not available should be clearly stated. It is preferred for the code to be uploaded as a supplement or to be made available at a data repository with an associated DOI (digital object identifier) for the exact model version described in the paper. Alternatively, for established models, there may be an existing means of accessing the code through a particular system. In this case, there must exist a means of permanently accessing the precise model version described in the paper. In some cases, authors may prefer to put models on their own website, or to act as a point of contact for obtaining the code. Given the impermanence of websites and email addresses, this is not encouraged, and authors should consider improving the availability with a more permanent arrangement. Making code available through personal websites or via email contact to the authors is not sufficient. After the paper is accepted the model archive should be updated to include a link to the GMD paper.

Please provide the version number of CARMA in the title of your revised manuscript.
As GitHub is not a persistent archive, please provide a persistent release for the exact source code versions for CARMA and CESM/CARMA used for the publication in this paper. As explained in https://www.geoscientific-model-development.net/about/manuscript\_types.html the preferred reference to this release is through the use of a DOI which then can be cited in the paper. For projects in GitHub a DOI for a released code version can easily be created using Zenodo, see https://guides.github.com/activities/citable-code/ for details. Finally note, that according to our new Editorial (v1.2) all data and analysis / plotting scripts should be made available.
Additionally, the part of the model output on which the results of your paper are based need to be made pupblicly available.
Yours,
Astrid Kerkweg

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Citation: https://doi.org/10.5194/egusphere-2026-1185-CEC1
- AC1: 'Reply on CEC1', Yunqian Zhu, 01 Apr 2026 reply
  
  Dear Editor, thank you for the comments.
  1. the CARMA version is version 4, which will be add to the title during the revision.
  2. We created a new permanent doi to archive the model code:
  
  NCAR CARMA Group, & Owen Brian Toon Research group. (2026). Community Aerosol and Radiation Model for Atmospheres (CARMA) standalone 4.11 code. University of Colorado Boulder. https://doi.org/10.25810/SNMH-NW73
  3. And the data for the output are achived at: Zhu, Y. (2026). CARMA Cloud model standalone test results [Data set]. Zenodo. https://doi.org/10.5281/zenodo.19373048
  
  Reply
  
  Citation: https://doi.org/10.5194/egusphere-2026-1185-AC1
RC1: 'Comment on egusphere-2026-1185', Parker Case, 06 Apr 2026 reply

This paper is an impressive description of a highly complex and detailed microphysical model. The paper represents a triumph in moving forward the CESM model. For the most part the paper is well written, technically correct, and ready for publication. Below I outline a few general comments as well as smaller "in-line" comments. The primary issue with the manuscript as is is that the naming conventions for the various models is inconsistent and unclear. Once the below comments are rectified, this manuscript should be published.
General comments
What is the name of the overall model you are presenting? Does CARMA Cloud Model refer to the CESM2 configuration or just the configuration of CARMA?
Make sure it is clear, for each of the given experiments (sedimentation, coagulation, etc.) what the model is you’re using. Is it the box-model? Is it the 1-d column model?
Throughout the paper, CARMA Cloud is, I think, being contrasted with other CESM2 modules. I’ve made line-specific comments below about this but in general, make it clear when you are contrasting a something to your novel CARMA Cloud model.
Some processes (contact nucleation, for example) are mentioned but either not implemented in the CARMA Cloud codebase or not turned on. An explanation as to why these design choices were made would clarify the paper significantly.

1 Introduction:
Line 60-76: The second sentence of this paragraph seems to be restating something that is clear from the first sentence. I suggest removing it.
There is some amount of confusion about the naming of the different configurations you are considering here as well. At the beginning of the paragraph, you refer to “CESM2 CARMA Cloud” but later say “this paper links CARMA Cloud with the MAM4 model aerosol model”. Are these referring to the same thing? Make sure it is clear when you are talking about a full model configuration (like a configuration of CESM2) versus the individual components (like CARMA or MAM4).
Line 90-91: For readability, this sentence should read, “CARMA has also been developed and applied to study planetary atmospheres”
Line 96-98: The final sentence of this paragraph means nothing to someone who doesn’t use CESM2—I don’t know what the namelist is. Could it be removed?

2 General Model Structure
Lines 122-134: Processes, like “When ice particles fall, they redistribute dust and sulfate,” should be in the body of the paper, not the figure description. Make sure that all of the colors and symbols in the figure are described.
Lines 136-137: Model naming conventions: here it seems that “CARMA Cloud model” is actually referring to a configuration of CESM2 that includes the cloud-enabled version of CARMA.
Lines 169-170: Does the extra size information from CARMA improve the representation of the optical properties for ice/graupel or are the same assumptions made as MG?
Line 184: Should be RRTMG, not RRTGM
Lines 190-233: This is an excellent overview of the CARMA module and routines!
Lines 247-263: This description of the number concentration element/numerical problem should be its own paragraph, not attached to the discussion of grids above.

3 CARMA Continuity Equation Algorithms and Solvers
Lines 309-313: Is the cldfrc scaling used only when converting CARMA-calculated values to CESM2? Or do you scale the values inside CARMA?
Lines 324-325: Wording, I suggest, “Coagulation is expensive to compute and slow, therefore it is often time split.”
Line 324-335: Are you using coalescence and coagulation interchangeably here? It might be good to be consistent or to define the difference explicitly.
Line 414: These sedimentation tests are done in the standalone, 1-d CARMA? It might be good to explicitly mention this.
Line 510: “Avoid” should be “Avoids”
Lines 519-520: This sentence is confusingly worded. I suggest “To conserve mass between the vapor and particle phases, particles are not allowed to grow out of the largest mass bin”
Lines 539-542: “In Nature” should be “In nature”. Additionally “lead to broadening the size distribution or make it multimodal” could be simplified for clarity to “broaden the size distribution or make it multimodal”.
Lines 549-558: How often does the sub stepping happen? i.e. for a given time-step, how much of the global grid is sub stepping?
Lines 564-566: The parenthetical on this sentence is hard to understand—what does it mean to “setup the convergence criteria to do that”?
Lines 571-585: Are there physical inconsistencies in the way that CLUBB treats the bulk variables that CARMA passes it during this process? In other words, does CLUBB make its own size assumptions for any processes that are different than the size information inside of CARMA?
Lines 608-609: Is Ak=1 and g1 = 0 okay because of the size? Could you explain these choices?
Lines 633-636: Some additional information is needed in this caption about what each frame of the figure means, especially (j) and (k).
Lines 687-689: How were these parameters chosen?
Line 715: This shows the same experiment as Figure 11, just with different parameters, correct? This sentence makes it sound like it is entirely separate.
Lines 738-739: This sentence does not make sense, “The model uses 100 vertical levels resolutions in a Cartesian coordinate system.” Perhaps remove “resolutions”?

4 Physical Process Equations
Lines 780-782: “at every time” should be “at every tilmestep”.
Lines 782-785: This sentence is unclear. Are these the options in CESM/CARMA? The third option (where everything except coagulation is updated) is complex and maybe warrants its own sentence.
Lines 789-791: There’s certainly a more recent version of Seinfeld and Pandis than 1998—it seems preferable to cite a more recent/updated version.
Figure 16: Why not include the Beard (1976) values on the right panel?
Lines 853-854: “Oddly, this term is not discussed in textbooks when coalescence is considered despite being dominant in some size ranges”. This is not clear from Figure 18. “cbr” does not seem to be dominant in any of the size ranges shown.
Lines 862-864: “The collision efficiencies of liquid cloud, ice, and raindrops based on the table in (Hall, 1980) can be used in CARMA Cloud since they are widely used in other models”. This is a poor reason. Maybe “they have been widely validated in other models”? Similar for the “Ecoalescence” value a few sentences later.
Lines 890-893: This comment, I assume, is about the non-CARMA cloud models in CESM2. This should be made explicit.
Lines 942-947: The Gettleman and Morrison model is not used if CARMA Cloud is being used, correct? Is this meant to contrast with CARMA Cloud?
Lines 989-991: How common is this process? How big of a problem is it that CARMA Cloud does not return the sulfate in melting ice particles to liquid droplets?
Line 1042: “That model uses” should be “Those models use,” referring to both Bardeen’s model and M&G.
Lines 1053-1057: “We’ve comment out these nucleation processes”. The verb tenses don’t make sense. I think it should be “We’ve commented out”.

5 Conclusions
Line 1088: “Cloud sized particle” should be “cloud sized particles”.
Line 1099: “MG” has not been defined up to this point, it has previously been referred to as Gettelman and Morris or Morris and Gettelman.

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Citation: https://doi.org/10.5194/egusphere-2026-1185-RC1

Yunqian Zhu, Cheng-Cheng Liu, Charles Bardeen, Lu Wang, Simone Tilmes, Ilaria Quaglia, Christopher M. Maloney, Francis Vitt, and Owen Brian Toon

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Short summary

The Community Aerosol and Radiation Model for Atmospheres (CARMA) is a sectional microphysical model widely used to study the formation of aerosols and clouds. This paper presents an update to the CARMA model algorithms since Toon et al. 1988, with a focus on the newly developed CARMA Cloud model.


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