Review of “Features of mid- and high-latitude low-level clouds and their relation to strong aerosol effects in the Energy Exascale Earth System Model version 2 (E3SMv2)” by Wan et al.

This paper investigates ultra-low (<10 cm^-3) cloud droplet number concentrations (CDNC) occurring in stratiform and convective clouds in E3SMv2, identifying the locations and some of the circumstances behind these events. These events are of particular concern because they appear to influence the global mean effective radiative forcing of anthropogenic aerosols (ERF_aer). Through a series of sensitivity experiments, the authors find that ultra-low CDNC events specifically in lower troposphere gridboxes with high cloud fractions and weak turbulence have a strong influence on ERF_aer sensitivity. However, while enhancing activation and turbulent mixing in the simulations increases CDNC in these regimes, the magnitude of ERF_aer increases simultaneously. Although this finding doesn’t bring ERF_aer closer to expected values, the results indicate that tropospheric stratus clouds in weak turbulence are a useful regime to focus on to help better understand the causal root behind these events.   

Despite the many complex relationships and ideas discussed in this paper, the authors’ precise prose and clean, frequently-referenced figures clearly explain the processes involved with minimal sources of confusion. It’s a nicely put-together analysis that doesn’t overwhelm the reader despite the high volume of figures in the paper and supplementary materials. In particular, their timely references to Figure 1 throughout helped the overall analysis stay grounded in a familiar, understandable framework. Before publication, I recommend a few minor corrections below:

Comments:

1. Line 231: What model levels are nudged? What’s the relaxation time? More information about the nudging would be useful here.
2. Lines 384-387: The nc10_f0.9 experiment can account for ~65% of the reduction in ERF_aer, indicating that the high cloud fraction regime plays a larger role in this change. Is the other ~35% wholly or mostly attributable to the small cloud fraction regime?
3. In Section 5, you introduce Figure 10, which illuminates some of the involved processes for these Arctic summer low CDNC events. In this section and in this figure caption, it is difficult at first to follow which simulation (out of the many described in this paper) is being analyzed. While it is eventually mentioned in the caption for Figure 11, an in-text reminder that these are results from the free-running version of nc00 towards the beginning of this section would be useful to help keep the reader on track.

Typos:

Line 24: Should these units be in Wm^-2 (instead of Wm^-1)?

Line 152: “The decrement in the grid box mean droplet number mixing”- missing word?

This study investigates ultra-low cloud droplet number concentrations in the E3SMv2 global climate model, focusing on their occurrence and impact on the effective radiative forcing of anthropogenic aerosols. The analysis identifies where ultra-low CDNCs occur in stratiform and shallow convective clouds and examines their influence on ERFaer. Process-level insights into cloud droplet formation and removal mechanisms are provided. The study suggests that increasing aerosol activation and enhancing turbulent mixing can raise CDNCs but may also undesirably amplify global mean ERFaer. The findings highlight the importance of further investigating mid- and high-latitude low-level stratus clouds under weak turbulence to address the root causes of ultra-low CDNCs and their strong influence on ERFaer in E3SMv2.

Overall I think it's a very well-written and info-dense manuscript and would support as-is publication, even though the style is a bit unconventional - L374 science questions in the middle of the paper, and L445 L512's question style titles, they did bring intrigue for the reader. Thanks for making it an interesting read.