Parameterization adaption needed to unlock the benefits of increased resolution for the ITCZ in ICON

Kroll, Clarissa A.; Jnglin Wills, Robert C.; Kornblueh, Luis; Niemeier, Ulrike; Schneidereit, Andrea

doi:10.5194/acp-25-16915-2025

Articles | Volume 25, issue 22

https://doi.org/10.5194/acp-25-16915-2025

© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-25-16915-2025

© Author(s) 2025. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 25, issue 22

Research article

|

26 Nov 2025

Research article |

| 26 Nov 2025

Parameterization adaption needed to unlock the benefits of increased resolution for the ITCZ in ICON

Clarissa A. Kroll, Robert C. Jnglin Wills, Luis Kornblueh, Ulrike Niemeier, and Andrea Schneidereit

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Final revised paper (published on 26 Nov 2025)
Preprint (discussion started on 25 Mar 2025)

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-1212', Anonymous Referee #1, 10 Apr 2025

Review in file

Citation: https://doi.org/10.5194/egusphere-2025-1212-RC1
- AC1: 'Reply on RC1', Clarissa Kroll, 10 Jul 2025
  
  We would like to thank Reviewer 1 for their detailed and constructive comments on our manuscript.
  
  In the attached document, we provide point-by-point responses addressing each of their comments.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1212-AC1
RC2:
'Comment on egusphere-2025-1212', Anonymous Referee #2, 04 May 2025

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1212/egusphere-2025-1212-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2025-1212-RC2
- AC2: 'Reply on RC2', Clarissa Kroll, 11 Jul 2025
  
  We thank Reviewer 2 for their helpful feedback on our manuscript.
  
  In the attached document we provide detailed responses on their comments.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1212-AC2

Peer review completion

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

AR by Clarissa Kroll on behalf of the Authors (08 Aug 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (11 Aug 2025) by Michael Byrne

RR by Anonymous Referee #1 (22 Aug 2025)

RR by Anonymous Referee #3 (06 Oct 2025)

Suggestions for revision or reasons for rejection

This study investigated the double-ITCZ bias in the AMIP simulations of ICON at different horizontal resolutions and explored the impacts of the minimum gustiness parameter. They showed that the double-ITCZ bias still exists with 5km horizontal resolution. They found that while increasing Umin over ocean reduces the tropical precipitation bias to some extent, it has some adverse impacts on the large-scale circulation and evaporation and does not fix the biases in humidity and its source.

While the manuscript presents a lot of results which are worthwhile to be published, the manuscript is not clearly structured (partly because there are too many results) and the writing is sometimes too casual for publication standard. Also, I feel the manuscript lacks a clear focus. I would recommend a major revision so the authors can improve their presentation and sort out their focus.

There are a lot of experiments with different values in the perturbing parameters. While I understand the workflow, I think it will be much clearer if the authors can focus on just one or two experiments. For example, they may focus on PTB-5_1 and remove the results of PTB_5. It makes sense to me to keep Umin =1m/s over land.

Figures: for unit in the colorbar, one may use “[ ]” instead of “/” which often means “divided by”.

Figure 3,4: Are these biases in humidity and temperature shared by CMIP models?

Figure 5: Since the different treatment for land and ocean, why not show the RMSE for land and ocean separately?

Figure 5: better to move the box of RMSE and mean error out of the figure.

For 2d spatial map, is it necessary to show the global map? Maybe focus on the tropical region since the paper is about tropical precipitation bias.

Figure 11: Is it necessary to rotate it?

Line 204: in the origin of

Line 509: Here, it is more related to the double-ITCZ bias in AMIP (instead of CMIP), which exists without the bias in SST.

Line 512: It is better to avoid using two “focus on”.

Given the results presented here, what are the implications for the double-ITCZ bias in AMIP-style simulations (in the CMIP project).

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ED: Publish subject to minor revisions (review by editor) (06 Oct 2025) by Michael Byrne

AR by Clarissa Kroll on behalf of the Authors (17 Oct 2025) Author's response Author's tracked changes Manuscript

ED: Publish as is (20 Oct 2025) by Michael Byrne

AR by Clarissa Kroll on behalf of the Authors (21 Oct 2025) Manuscript

Short summary

The double Inter-Tropical Convergence Zone is a prominent precipitation bias in climate models. We demonstrate its persistence from a resolution of 160 km up to 5 km. Its root cause lies in biased moisture transport from the subtropics to the inner tropics reducing convection and weakening tropical circulation. Increasing the surface wind speed addresses the bias, but deteriorates the global circulation. This highlights the importance of resolution hierarchies and parametrization development.