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Overview

This is a review of "Water isotope characteristics of the cloud-circulation coupling in the North-Atlantic trades. Part 1: A process-oriented evaluation of COSMO_iso simulations with EUREC4A observations" by Villiger et al.. This paper is part of a multi-part paper of which I have only reviewed Part 1. This paper is an evaluation of COSMO simulations that are to be used to interpret the mechanisms behind trade wind cumulus.

I find the paper and the methods interesting, but there are some things that can be improved, therefore I suggest major revisions.

First of all, I would like to comment that the paper is thoroughly written, it is clear what has been done and what the results and interpretation of the experiments are. Also, I appreciate that the authors have made nice figures with a consistent layout and sizing throughout the paper, showing that the paper has been made with care.

Despite the results being clearly explained, I did not find it easy to extract the true motivation of the paper. From the title I expected the paper to be primarily about validating the model for isotopes studies, but in the end it seems to be more about evaluating COSMO_iso for trade wind cumulus simulations. Maybe the whole study could benefit from reducing this paper largely in size and merge it with the other Part 2 paper, in order to have a very original isotopes paper.

One thing that I found particularly striking, and maybe even worrying is that the authors state that the ultimate goal is to use isotopes and simulations to interpret the cloud system better, but at the same time, they conclude that while precipitation, cloud fraction and liquid water content are sensitive to grid spacing, temperature, humidity, and isotopes are not. If this is the case, how can isotopes then be ultimately be of help in learning about cloud processes? Also, if cloud processes are key, wouldn't it make more sense to do high-resolution large-eddy simulations with isotopes, as these capture the cloud processes much better? I would like to ask the authors to clarify the motivation and to convince the reader better that their goal has chance in succeeding.

Also, the authors present suggestions for improvement in the conclusion that in my view should be conducted as the suggested improvements would largely increase the qualities of the simulations and the work does not look that extensive.

Detailed points

- Line 1: Why do isotopes have this potential? Please state this

- Line 10: Doesn't this statement show that the simulations will not help much in interpreting the isotope measurements?

- Line 22-25: I do not understand how the results stated before lead to the conclusion that COSMO_iso evaluation suggests that the simulations can be used for studying the impact of circulation on clouds, as cloud properties do not converge with resolution.

- Introduction: the introduction is lacking hypotheses how isotopes could help in improving understanding. It would help the reader (I am a reader that knows very little about isotopes) to state how isotope measurements in combination with simulations could show something that the simulations, nor the data analysis of isotopes alone could not.

- Line 204: The cited papers from Seifert & Beheng describe a 2 moment scheme. Please clarify.

- Line 206: How does one explicitly resolve shallow convection at grid spacings larger than the size of a shallow cloud?

- Section 2.2 is very well written and explains the data collection very well.

- Line 410: What is your opinion on the quality of the precipitation distribution? Is it good enough? Should we expect further improvements if we move to LES resolution?

- Section 4.1: One open question connecting to my main comments is how well do we understand isotopic processes? Are there indications that the results presented here also teach us something new about isotopic fractionation itself?

- Conclusions: I sometimes wonder if this separation in Flower, Sugar, Gravel, Fish is bringing so much progress to the field. It is a rather arbitrary classification and one could wonder if these are really the fundamental types. Do the isotope characteristics also vary significantly between the types?

- Line 685: should this sensitivity analysis not be part of this paper? The goal is to let COSMO reproduce the case as good as possible, and there are clear deficiencies.

- Line 695: Same as the previous comment. Please try this, as you need high quality simulations for your purpose.