see attached file

Review of egusphere-2025-1847

The study presented in this manuscript helps to bridge a critical gap in our understanding of contrail climate effects. It is essential that we examine how contrails evolve in the presence of background cloudiness. The authors have provided a well thought-out and thorough study in this regard. However, their arguments are muddied by their presentation of their results, which is somewhat confusing to follow. As such it is unclear how their results demonstrate that “mitigation strategies based solely on atmospheric thresholds are insufficient to reliably predict contrail evolution or their climate impact”. Additionally, there are a few cases of awkward phrasing or text that is too conversational that detracts from the point the authors are making (a few examples are highlighted below). These factors combined with the fact that the authors do not provide a discussion of their results to provide context on how they compare to other studies, implications for potential contrail mitigation, or how their model assumptions could influence their results, I suggest a major revision to this manuscript. Below I have listed each section with my main comments.

Introduction

• Overall this section is well written and clear. It provides a good background overview.
• Line 54-55: “Merged contrails produce less total extinction compared to individual contrails due to competition for available humidity”. Could the authors make this clearer that it’s the competition of the ice crystals within the contrails?
• Paragraph beginning on Line 58: I suggest the authors also cite Krämer et al. 2016 and 2020 as they provide a great overview cirrus properties that would help this section.

Methodology

• Line 141-142: I wouldn’t use terms like “accurately capture” in terms of modeling. Instead simply “represent” or “simulate”.
• Can the authors explain why they did not choose to use the default cloud cover scheme and instead go for the binary (0 or 1) scheme? Are there implications on their results that they could discuss further in the manuscript?
• Line 154: “pre-existing” ice usually refers to ice that was advected into the grid box that formed by some other process (e.g., mixed phase or ice in a convective cloud). I would cut this terminology for clarity.
• I would combine Sections 2.3 and 2.4 to avoid repetition. The authors can explain their contrail formation criteria in a single section. As an example, in section 2.3 while describing the SAC, it is typical to include the aircraft and engine types for readers who may have more knowledge on how these may influence contrail formation thresholds. I found the information in Section 2.4, so to reduce this repetition of information I suggest combining these sections.
• Line 203-204: “This selection aligns with reported values for the west coast of the United States”. Can the authors provide references for this information?
• Finally, I would add a section on simulation “setup”, i.e., what cases were tested and why? Instead of including a table in the Appendix, can the authors layout clearly what control cases were run (and again, why they were chosen). This will help readers immensely when interpreting the results. It is unclear from reading the results what weather conditions are present in each case.
• Line 235: “While contrail formation does not require atmospheric saturation”. This statement is incorrect. Contrail formation requires at least saturation with respect to liquid water to form cloud droplets that then freeze. Overall in Section 2.5, this appears to be missing in the discussion. For example, on Line 230 the authors state “flying aircraft within saturated regions alone does not necessarily lead to formation of contrail”, which is correct if they are referring to ice saturation. Again, contrail formation needs at least liquid saturation.
• Figure 3: Please label all colorbars for clarity

Results

• Line 269: Can the authors consider using a term other than “water-vapor thirsty”?
• Line 316: Please indicate the the Ni anomaly is relative to each control case
• Line 393: Can the authors provide some quantification of how small the ice crystals remain. Another figure is not necessary here.
• Line 427: Please revise the text in the brackets to something like, “this vertical extension is clearer in the IWC anomaly plots below”.
• Figure 9 is unnecessary, at least in the main text as the authors only use it to illustrate that Case 4 is unstable. The authors could consider simply quantifying it in absolute terms or relative (i.e., _% smaller). If this figure is kept, please move the legend to outside the axes.
• Figure 5: please include “Case X” as titles at the top of each column, also please make the axis labels larger for legibility and use the same format for longitude and latitude that was used in Figure 1.
• Figure 6: It would be useful to include the horizontal coordinate on the x-axis, in I assume meters
• Figures 7, 10, and 11: Please include the y-axis label on each row and the units. Similarly for the x-axis label
• Figure 8: by using the same latitude and longitude formatting as in Figure 1, the authors wouldn’t need to include the labels on the axes here. Additionally, a more colorblind friendly colormap is preferred.
• Figures 13 and 14: a single legend would make this figure easier to understand, with subplot titles indicating each case. Similarly, I don’t recall whether the numbers above the black lines are ever explained.