Reviewed by Minghui Diao

Review for ACPD manuscript “Microphysical and thermodynamic phase analyses of Arctic low-level clouds measured above the sea ice and the open ocean in spring and summer” by Moser et al.

This manuscript uses airborne in-situ observations during two Arctic flight campaigns – AFLUX and MOSAIC-ACA – to investigate the cloud microphysical properties during spring and summer in Arctic. Two main comparisons are conducted, between two seasons spring and summer, and between two types of surface conditions – open ocean and sea ice. Several main differences are identified, including summer time having more spatially extensive cloud coverage and more liquid droplets than spring, and spring time having more ice particles (either in mixed phase or pure ice phase) and more patchy cloud coverage than summer. Also open ocean water is found to correlate with clouds with larger Deff, smaller N, and larger cloud water content (CWC) compared with clouds over sea ice. This is possibly due to thermodynamic condition of air above open ocean water (i.e., warmer and more humid air) and aerosol concentrations (i.e., sea ice may provide higher aerosol number concentrations than open ocean).

The manuscript is overall very well written. The analysis is easy to follow, and the literature review is thorough. Below are some comments from the reviewer. I suggest minor revisions before being accepted.

1. The main comment from the reviewer is about how Figures 4 and 5 and Table 3 are not separated into different cloud phases or different types of cloud hydrometeors. The reviewer recommends adding figure sub-panels similar to the original Figure 4 and 5, but separately plot the distributions of the 3 types of cloud thermodynamic phases (ice, mixed and liquid) as identified by the group number 1, 2 and 3 in Figure 6, respectively. This way the readers can see if the vertical distributions of these three cloud phases change between summer and spring, and between over ocean and sea ice. In addition, for table 3, more rows can be added to compare N, Deff, CWC, d_cloud_mean that are calculated for each of the 3 cloud phases separately.

2. This is a related question to comment 1, can the authors separate each instrument measurement into liquid or ice hydrometeor? In Figure 6, the Deff and N values seem to use the combined measurements of multiple probes. It would be helpful to trace back how these ice, mixed and liquid phase 1-Hz measurements are contributed by individual probes. For example, the authors can add sub-panels to this Figure 6, using CAS, CDP, CIP and PIP, individually, and then calculate just the Deff and N for that probe alone (in a limited size range), and show how their own Deff vs N would look, color code where the 1-Hz samples of ice, mixed and liquid exist in that probe’s measurement space. This way one can understand how individual probes contribute to the groups 1, 2 and 3 of cloud phases in the combined Deff vs N plot.

Another suggestion is, if the authors can separate the ice and liquid hydrometeors within one second, then the authors can calculate the N, Deff, CWC just for liquid or just for ice hydrometeors. Note that this type of calculation wouldn’t have the mixed phase because it is based on the type of hydrometeors, not the type of cloud segment. The reviewer would like to point to some previous methods of defining ice and liquid from CDP, 2DC and 2DS probes in the following papers. These probes have some similar size range to the ones used in this study.

Yang, C.A., M. Diao, A. Gettelman, K. Zhang, J. Sun, W. Wu, G. McFarquhar, Ice and Supercooled Liquid Water Distributions over the Southern Ocean based on In Situ Observations and Climate Model Simulations, Journal of Geophysical Research: Atmosphere, 126, e2021JD036045. https://doi.org/10.1029/2021JD036045, 2021.

D’Alessandro, J., M. Diao, C. Wu, X. Liu, B. Stephens, and J.B. Jensen, “Cloud phase and relative humidity distributions over the Southern Ocean in austral summer based on in-situ observations and CAM5 simulations”, J. Climate, doi:10.1175/JCLI-D-18-0232.1, 2019.

3. Because the Deff and N currently used in Section 3.1 seems to be a combined value of both ice and liquid, the reviewer suggests the description of higher or lower Deff not be directly referring to liquid droplets. Line 280, the author said “also during summer a decrease of Deff of the liquid droplets is observed”. This is more likely because there are more liquid droplets in summer which tend to be smaller than ice, not because these liquid droplets in summer are smaller than the liquid droplets in spring.

4. Comparisons between summer and spring and between ocean and sea ice need some more statistical significance tests. For instance, the analysis in Figure 4, the whiskers represent 97.5 and 2.5th percentile, and Tabel 3 has 75 and 25th But it would be helpful to provide a t-test to check if the two groups of data (summer vs spring, or ocean vs sea ice) are significantly different statistically.

5. Minor typos. Line 376, We observe lager (should be larger) ice…

6. Line 394, typo Figures have been design (should be designed).

7. In data availability, there is currently no description about where the AMSR2 satellite data for sea ice coverage during the AFLUX and MOSAIC-ACA campaigns are stored. It would be helpful for follow-up studies if the authors can provide other data used in this analysis, such as satellite and HYSPLIT back trajectory data.