Review of “Air mass history linked to the development of Arctic mixed-phase clouds” by Murray-Watson and Gryspeerdt

This paper uses trajectory analysis combined with various satellite retrievals to examine the evolution of Arctic mixed-phase clouds, differentiating between marine cold-air outbreak (MCAO) and non-MCAO clouds. The mixed-phase clouds under these two conditions (essentially, convective versus non-convective) display different evolutions; namely, the mixed-phase cloud fraction and the cloud top temperature (CTT) regimes under which they exist. The authors link these differences to the time that the airmasses linger over snow- and ice-covered surfaces, as these areas are devoid of biological ice nucleating particles, which are thought to play an important role at warmer CTTs. As a result of their findings, the authors conclude that the prevalence of mixed-phase clouds at relatively warm CTTs cannot be fully explained by the satellite data used in their study. Thus, they discuss three potential causes: secondary ice production, presence of active ice nucleating particles at warmer temperatures, and retrieval biases. They also highlight the need for additional in-situ measurements of Arctic aerosols and mixed-phase clouds to improve fundamental understanding and inform climate models, the latter of which lack proper representation of these components in the Arctic.

I find this is a nice study overall. The manuscript is well written, and the explanations throughout the paper are informative. I think the authors provide a good discussion of the limitations of their study, and I appreciate its systematic nature; the logic was easy to follow. This study should be a useful contribution for the literature covering Arctic mixed-phase clouds. I have just one major comment and otherwise some minor comments that should be addressed before the manuscript is suitable for publication.

Major comment:

1. In my opinion, Section 2 (Methods) does not provide sufficient detail. I understand that this study uses methods from two other papers that are cited, but I think some basic information is not reported. For example, which years are considered in the analysis? Which region do you use to generate the forward trajectories? For the MODIS retrievals described in Section 2.3, you filter out quite a bit of data; what fraction of the data do you end up considering? What does the so-called heterogeneity index tell you and why filter using it? Moreover, I see that you later cite Khanal and Wang (2018), so you’re aware of the study, but I wonder how confident you are that MODIS is properly classifying “single-layer, liquid clouds” in this rather challenging environment. It would be good to say something about that.

Specific comments:

1. L133: Please define “TSI”.
   
   
   
   
   
   
2. L137-139: Presumably warm air intrusion clouds make up an important fraction of these “other” clouds when the flow is poleward?
   
   
   
   
   
   
3. L157: “The temperature data are obtained from ERA5 and processed as the wind data”; what does this mean?
   
   
   
   
   
   
4. L198: I see an increase in liquid cloud fraction over time in the “Towards” clouds, not a shift toward mixed or ice phases. Am I missing something?
   
   
   
   
   
   
5. Figure 1: Is the x-axis showing time since tracking the airmass? Also, what does the solid line and shading represent?
   
   
   
   
   
   
6. Figures 2, 9, and 10: I think it should be possible to determine which bins are statistically significant and which are not. Please add hatching or something similar to indicate this.
   
   
   
   
   
   
7. Section 4: You discuss secondary ice production and link it to previous studies that have examined the role of strong updrafts. Considering that MCAOs are largely surface-driven, the M value provides a good proxy for surface turbulent heat fluxes. I wonder if you could filter your data by M value (e.g., bin positive values by increments of 3 or 4) to address your hypothesis.
   
   
   
   
   
   
8. L364: COMBLES --> COMBLE
   
   
   
   
   
   
9. L364-366: Please also note that COMBLE was measuring CAO conditions ~100s-1000 km downstream of the ice edge.
   
   
   
   
   
   
10. L445: We know that secondary ice production can be at play; I suggest changing the phrasing to something like, “if the mechanism is ubiquitous”.

Murray-Watson and Gryspeerdt use pseudo-Lagrangian framework to investigate the air-mass history during the development of mixed-phase MCAO clouds. While doing so, they elucidate on possible explanation behind the observed development by predominantly tying it to the role played by the biological INPs. The study seems to have a clear focus. It is also clearly presented and argued. The analysis and the use of satellite data are done appropriately. In general, I think it is a very good contribution and it should eventually be accepted.

There are a couple of aspects that I wish the authors would hopefully elaborate on.

• The meteorological context seems to be lacking. I believe providing the meteorological context (showing and contrasting temperature (not just the cloud top temp), humidity, dynamics) would be very useful for the potential readers. This would also help to indirectly evaluate if the role of changing INPs makes physically sense. I understand that this can be a manuscript on its own. So, I am certainly not expecting a detailed analysis, but I think at least a brief overview is warranted to fully grasp what is going on as the air masses are transported large distances off the sea ice edges.
• Another thing that got me thinking is the use of AOD as a proxy of INPs here. Given the persistence of clouds, high solar zenith angles and challenging meteorological and surface conditions, it is to be honest difficult to believe AOD changes that are in the order of 0.01 based on CAMS reanalysis. It would be better if the authors show a bit more information on aerosol variability. For example, are the changes in AOD really consistent with the hypothesis along the trajectories?
• Furthermore, it is to be noted that the atmospheric circulation and the aerosol vertical structure in itself (at the cloud base, free troposphere) could be different under non-MCAO and MCAO scenarios, which could complicate the use of AOD.