Review of “Relative importance and interactions of primary and secondary ice production in the Arctic mixed-phase clouds” by Xi and Liu in ACPD, 2021.

In this work, the authors contrasted several parameterizations of primary ice production (PIP), combined with a new set of parameterizations of secondary ice production (SIP) in the NCAR CESM2/CAM6 model. The model simulations are compared with observations from the DOE M-PACE campaign. The scientific questions include: What are the impacts of SIP parameterizations on the simulation results? What are the effects of SIP on PIP? How does the PIP process influence SIP? As the authors mentioned, the interactions of SIP and PIP have not been carefully examined before, and the mechanisms of how they affect each other are still unclear.

Overall, this is a well-written manuscript. It is very easy to follow the simulation experimental design since the logics are very clear and straightforward. The reviewer recommends that the paper be accepted after a minor revision on the following points.

Main comments:

1. About the comparison of ice crystal number concentration (ICNC) between observations and simulations, the observations are restricted to > 100 micron, while the simulations use the entire size range from zero to infinity. Since ICNC is dominated by smaller ice particles, the simulations may overestimate ICNC when a wider size range is used. The reviewer suggests a revision on the simulation dataset to delete ice particles < 100 micron. In addition, a scaling factor of 1/4 is applied to the observations due to potential ice shattering effect. But as the author mentioned, previous studies showed that the scaling factor may be around 1 to 1/4.5. Thus, using 1/4 seems to provide a lower end of ICNC from observations. If the authors apply another scaling factor, such as 1/2, how will it change the result? Some discussions on this sensitivity test can be added.

2. Another main comment is about the mechanism used to explain how introduction of SIP leads to weaker PIP. The authors described this mechanism around line 339 - 346, that is, “Since temperature and supersaturation are similar in these nudged simulations, the decreased cloud droplet number concentration with the introduction of SIP leads to weaker PIP in B53_SIP and M92_SIP”. Can the authors clarify which variables in the SCAM simulation are nudged, such as temperature, U and V wind? Is the specific humidity nudged as well? The reviewer tries to understand why ice supersaturation is similar between the two simulations. If there are more ice crystals produced by SIP, these ice crystals could provide more deposition of water vapor to ice phase, and thereby relaxing ice supersaturation back to ice saturation faster. Then it could lead to a suppression of PIP when ice supersaturation frequency and/or magnitude is reduced, since PIP requires a certain magnitude of ice supersaturation to occur. Also, are the ice crystals formed from SIP able to provide seeding for lower levels when they sediment? Can the seeding lead to suppression of PIP?

3. Following the previous comments on Figure S7, some parts of this figure do not make sense to the reviewer. For example, accumulation mode dust decreases at 880 – 1000 hPa, but increases at 880 – 700 hPa in N12_SIP compared with N12. Why does the accumulation mode dust increase at 880 – 700 hPa in N12_SIP, if the mechanism of SIP is to increase wet deposition (line 341)? In addition, the accumulation mode deposition in panel (e) only significantly increases in N12_SIP near the surface around 980 – 1000 hPa. This pressure level does not match with the location of changes seen in panel (d), and the increasing deposition doesn’t explain the increase of accumulation mode dust at 880 – 700 hPa as mentioned above. The change of coarse mode deposition also doesn’t match with the vertical locations of changes seen in coarse mode dust. Can the authors explain this figure a bit more?

Some minor comments on Figure S7, the (d) panel x axis label is out of bound on the page. Also some x axes are suggested to use the same range for an easier comparison. For example, c, d, and e can use the same scale and unit; f and g can use the same scale.

4. In several analyses, the authors use relative altitude to the cloud layer, that is, 0 refers to cloud base and 1 refers to cloud top. There is no discussion about how this relative altitude is derived. Is it derived based on ground-based observations or in-situ observations? Please clarify.

Minor comments:

1. Several simulations, CNT, N12 and D15, as well as CNT_SIP, N12_SIP and D15_SIP, provide similar results to each other. Can the authors provide some explanations why these three PIP parameterizations provide very similar results? Is it because they were derived based on similar observation data?

2. Some of the analyses and figures are based on ground-based remote sensing observations (such as Figure 1) while the other ones are based on in-situ aircraft observations. It would be beneficial to clarify in Section 3, such as line 182, which type of observations is used in a specific figure or analysis.

3. Please clarify how the variables related to “rate” are defined in the model. For example, is the variable ice production rate describing the amount of ice crystals (in kg) being produced in every unit mass of dry air (in kg) per second in the entire grid box, or only in the in-cloud section of the grid box?

4. Figure 2, is it possible to add sub-panels of observations to compare with the model results?

5. Figure 3, since the INP concentrations in CNT, N12 and D15 are significantly lower than the observations, can the authors apply a scaling factor to INPs in these parameterizations to match with the observations better, and see how the results change? Also, the reviewer wonders why with such low INP concentrations, these parameterizations are able to produce quite a similar amount of ICNC compared with observations?

6. Figure 5, please clarify how the normalization was calculated in this figure. It seems that the PDF is calculated by the number of samples of each bin divided by the total number of samples in each temperature bin (the sum of % in each temperature range equals one), instead of divided by the total number of samples of the entire temperature range. Is that correct? The reviewer wonders how this figure will change, if the latter type of normalization is also provided (i.e., the sum of % in all bins equals one).

7. Figure 9, for the accretion rate of cloud water by snow, does cloud water include both cloud droplets and rain? Some minor revisions on the sub-title of g and h are recommended. For example, h can be “Droplet number” instead of “Cloud number”, and h can be “Accrete water by snow”.

Review of Manuscript # acp-2021-686 in ACPD: “Relative importance and interactions of primary and secondary ice production in the Arctic mixed-phase clouds” by Zhao and Liu.

General comments:

The authors examined five different ice nucleation schemes and secondary ice production (SIP) processes in the simulations of Arctic mixed-phase clouds during the M-PACE campaign using single column mode of CESM2 CAM6 model. They concluded that the simulations using aerosol-aware ice nucleation schemes and including SIP processes resemble the observed single-layer mixed-phase clouds during the M-PACE. In these simulations, SIP plays a key role, and there is a competition between ice nucleation and SIP. Overall, the manuscript is well organized, and the logic is clear. However, there are several concerns that should be clarified before considering the manuscript for publication. The reviewer would recommend major revision for this manuscript in case the authors need more time for revision.

Major comments:

1. Analyses: The analyses in the manuscript are full of qualitative phrases. Some examples are listed in the minor comments. Please conduct quantitative analyses.
2. How did the authors attain the simulated ice crystal number concentration (ICNC) for comparison with observations? Did the authors consider snow particles? Because observations should include all types of ice particles, the authors should include all ice categories for comparison. Meanwhile, in the comparison only the observed ICNC with sizes larger than 100 microns are considered, while the entire size range of simulated ICNC is used. So, the comparison is also unfair. Please use the same size range of all types of ice particles for comparison.
3. Lines 199-203: “M-PACE observed ICNCs were scaled by a factor of 1/4”, have the data collected by the authors been scaled by a factor to remove the shattering effect during the data quality control? Are the conclusions sensitive to this correction factor?
4. The reviewer was surprised as the results shown in Figures 1 and 2. N12 (N12_SIP) seems to be the same as CNT (CNT_SIP), but their INPs are obviously different in Figure 3. Why?
5. It is not clear that how the authors attained the INP number concentrations from observations and simulations especially for B53 scheme. Did the author conduct a fair comparison between them? Did the authors include all types of ice nucleation for comparison? Please provide a more detailed description.
6. “Section 4.3 Interactions between PIP and SIP”: SIP suppressed the PIP. Did the authors consider whether some setups in the microphysics scheme lacking physical meaning result in or enhance this suppression? For example, suppression is due to decreasing difference between total ice nucleation number from parameterization and increasing ice particle number. Please provide a discussion.
7. Some “rate”s in the manuscript are confusing. If the reviewer understood correctly, the production rates in the manuscript are mainly for ice mass based on Figures 8-10. The question is how IIC increases ice mass? The “ice” in the manuscript all means “cloud ice” and does not include “snow”? If yes, following comment #2, different categories of ice are defined artificially in microphysics schemes, and it might not be true in observations. The authors should clarify it. The reviewer would recommend conducting analyses including simulated snow particles.

Minor comments:

1. Lines 107-108: Please describe how the graupel mass and number are diagnosed in the scheme briefly.
2. Lines 209-225: Please quantify the analyses, e.g., percentage of enhancement, reduction, “largest”, “smallest”, “modest”, “closest”, “significantly decreases/increases”, …
3. Lines 233-234: “appears an inversely linear relationship”, “this relationship is not as clear”, do they have statistical significance?
4. Lines 234-238: Please quantify the analyses, e.g., “reduces dramatically”, “much higher”, …
5. Lines 253-264: Why is SIP not active in B53_SIP and M92_SIP? Is there a maximum threshold of ICNCs in the microphysics scheme?
6. Line 269: Please quantify “slightly higher”
7. Lines 281 and 283: Please quantify “overestimate”, “predominantly”
8. Lines 287-288: How about the TWC in these simulations?
9. Lines 294-308: It is confusing whether the authors talked about ice number or mass in Figure 7. If the authors talked about ice mass in Figure 7, how do IIC contribute to ice mass?
10. Line 328: Please quantify “substantially weakened”
11. Lines 342-343: Based on Eq. (5), M92 seems dependent on supersaturation not temperature and cloud droplet number concentration.
12. Lines 362-367: Please quantify the analysis.
13. Figure 1: Please provide uncertainties of these observations.
14. Figure 4: How did the authors determine the cloud top and cloud base for observations and simulations?
15. Figure 5: x-axis in (h), “CTL” -> “CNT”? What are the bin sizes for x and y variables?
16. Figure 7: “total ice production rate”, is the “production rate” for mass or number?
17. Figure 9: “(h) accretion rate of cloud water by snow”, how about the accretion of rainwater by snow?
18. Figure 10: (c), (e), (F), IIC influences the mass mixing ratio of ice particles?

Review for “Relative importance and interactions of primary and secondary ice production in the Arctic mixed-phase clouds” by Zhao & Liu

This manuscript compares the impacts of primary ice production (PIP) and secondary ice production (SIP) as well as their interactions on the simulation of multiple Arctic mixed-phase cloud microphysical and macrophysical properties observed during the M-PACE field campaign.  The authors design a set of 10 simulations, 5 of which differ only in their treatment of ice nucleation schemes and the other 5 which utilize the same 5 aforementioned ice nucleation schemes but with representations of SIP via the ice-ice collisional breakup (IC) and rain droplet fragmentation (FR) mechanisms in addition to the Hallett-Mossop process which is represented in all 10 simulations.  The authors find that 3 of the ice nucleation schemes that are aerosol-aware (CNT, N12 and D15) exhibit similar behaviour to each other in terms of their simulated ice crystals number concentration vertical profiles, supercooled liquid fraction (SLF), IWP, LWP and relative contributions from primary and SIP rates to the total ice production rate.  They also find that these variables are also similar to each other for the other two ice nucleation schemes (B53 and M92).  One of the main is that PIP and SIP actively influence each other.  The authors also conclude that the aerosol-aware ice nucleation schemes with the IC and FR mechanisms represented best represent the single-layer mixed-phase clouds observed during M-PACE. 

This is an interesting and valuable study at the forefront of effort to improve cold cloud microphysics in global climate models and their impact on cloud properties.  There are however, a number of ways that the manuscript can be improved, particularly pertaining to the writing including the description of the model used and the experimental design, description of the observations and grammar.  Overall, I recommend major revisions that are provided below.

Major revisions:

• The title is wordy and unclear. Perhaps revise to something like “primary and secondary ice production: interactions and their relative importance”?
• An interesting conclusion of this manuscript is the interaction between SIP and PIP which compete with one another. The suppression of SIP via PIP is intuitive, however, the suppression of PIP via SIP is less intuitive since one would initially expect that more ice crystals nucleated via PIP would allow more SIP.  The explanation for the latter phenomenon provided in the manuscript relates to the lack of precipitation particles in B53 and M92 due to the enhanced glaciation of mixed-phase clouds.  A description of the graupel scheme (which seems to be diagnostic based on line 364) the authors implemented would help the readers more clearly understand the mechanism instead of referring to Zhao et al. 2021.  The mechanism of SIP and PIP suppression could also be summarized in the Abstract.  Also, the discussion on lines 73-78 in the Introduction can also be elaborated on in this aspect when describing the work of Phillips et al. 2017b.
• An 80% contribution of SIP to total ice formation seems very large. Are these any observations to gauge how realistic this value is?  Similarly, on lines 297-301, are there any observations to gauge how realistic these contributions are?  Otherwise, this should be declared in the main text.
• In addition to the graupel implementation mentioned above, the description of the ice nucleation schemes could also be described in more detail. All ice nucleation schemes appear to be implemented as immersion freezing schemes --- please confirm.  How are deposition, condensation, and contact freezing represented?  To be consistent with the other naming conventions used in the manuscript, I would also recommend changing “CNT” scheme to reflect the reference that was used (was it Wang et al. 2014 or Hoose et al. 2010)?  The description of this scheme also does not include the equation and the units of all equations that are provided are missing.  For N12, is the dry diameter of dust particles predicted by MAM4?  For the D15 scheme, please include more information on the instruments that were used for the measurements and the location where the observations were taken from.  To be clear, are marine organic aerosols and sea salt not included as INPs in any of the parameterizations?  Please include in the description.
• Lines 96-97: It would be better to clarify that this is the case for the default CAM6 model with MG2 microphysics.
• More on the model description: line 165: What were the aerosols initialized with in SCAM and what are the aerosol types that are represented? Line 168: what aerosol-cloud interactions are represented?  g. Twomey, Albrecht, glaciation indirect effect, etc.?  Lines 171-172: can the cloud-borne aerosols released as interstitial aerosols be reactivated?  Were the simulations not free-running or nudged to MPACE meteorology?
• Line 194: please cite the original source of the observations. The ground-based observations are not directly comparable with the model and should be stated.
• Line 200: Dividing by a factor of 4 seems very approximate to account for shattering effects. I would suggest using a dataset that has been revised according to the interarrival times for more accurate comparisons (Korolev et al. 2015)
• Why don’t B53, B53_SIP, D15 and D15_SIP not appear in Figs. 1 and 2? Please include.  Please also include the observations in Fig. 2.
• Fig 5: I find the “enhancement ratio” confusing because the relative enhancement in Figures b-j are compared relative to Figure a, but they all use the same colour bar. Wouldn’t it make more sense to use a separate colour scheme for b-j?
• Please include error bars in the observations and all simulations.

Minor revisions:

• Line 12: “of” needed after “importance”
• Line 32-34: another source of ice particles in mixed-phase cloud could be from ice crystals that fell from overlying cirrus clouds.
• Lines 42-43: Ice crystal fall speed is a cloud microphysical process that is also quite important for mixed-phase cloud properties such as SLF according to the CAM5 model shown by Tan & Storelvmo 2016.
• Line 70: “Albeit these studies, how…” is grammatically incorrect.
• Line 188: “rather than” I think should be “in addition to” since Hallett-Mossop is included in all simulations?
• Line 248: suggest replacing “in accompany with” with “accompanied by” and again on line 409.
• Line 370: add “rate” after “nucleation”
• Lines 423-426: Not necessary to discuss here since there is no associated figure and discussion and not central to the manuscript?

References:

Tan, Ivy, and Trude Storelvmo. "Sensitivity study on the influence of cloud microphysical parameters on mixed-phase cloud thermodynamic phase partitioning in CAM5." Journal of the Atmospheric Sciences 73.2 (2016): 709-728.

Korolev, A., and P. R. Field. "Assessment of the performance of the inter-arrival time algorithm to identify ice shattering artifacts in cloud particle probe measurements." Atmospheric Measurement Techniques 8.2 (2015): 761-777.