The authors report on the observation of two cases of ice fogs formed at Dome C, Antarctica. Both ice fogs formed at very cold temperatures which are typical for cirrus clouds in the upper troposphere. In particular, these fogs formed in-situ, most likely due to the homogeneous nucleation of ice crystals, i.e. the freezing of pre-existing aerosol solution particles. This aspect makes this study particularly interesting since in contrast to aircraft based observations of natural (i.e. outside of the laboratory) cirrus clouds the authors are able to show timeseries of key parameters at a stationary location, hence within the forming cloud itself.

The manuscript is very well written and fits well into the scope of ACP. After adressing my comments and questions I have listed below, I recommend the acceptance of the manuscript.

Comments:

(1) I find it a pity that no data is presented that can shed some light on the nucleated ice crystals within the two fogs, such as their shape, mass, number density. If there is such data available, I strongly recommend to include that. 

(2) Line 21-22: You state that the ice crystal properties "such as their size and their number concentration" are different for an ice fog or diamond dust. I suggest adding a sentence to clarify these differences.

(3) Line 30-32: To me it seems that the formation process you describe here is the freezing of supercooled liquid droplets which are already as large as cloud droplets. In other words it is the freezing of a pre-existing liquid cloud. I suggest to refer to this process as a liquid-origin cloud, since the term "homogeneous freezing" is usually understood as the freezing of much smaller solution aerosol particles (you describe this process in line 41-44).

(4) Section 2.2: At very cold temperatures close to 200K, a new formulation of the saturation vapor pressure over liquid water was recently presented by Nachbar et al. This formulation differs from the formulation given in Murphy and Koop (2005), in particular at cold temperatures. What happens to your RHl-values if you use this new formulation? Although Nachbar et al state that their parameterization is only valid for temperatures above 200K, it seems that such a comparison is applicable for observed fog in the case 1. Of course, such a comparison might also affect the results in Appendix A.

Reference:

Nachbar, M., Duft, D., and Leisner, T.: The vapor pressure of liquid and solid water phases at conditions relevant to the atmosphere, J. Chem. Phys., 151, 064504, https://doi.org/10.1063/1.5100364, 2019.

(5) Section 2.5: Does the Global Data Assimilation System employ a rotated grid to avoid a pole-singularity in the Antarctica area? If not, does this singularity affects data that is used to compute the backward trajectories?

(6) Figure 4 and 8: I suggest to indicate the two time periods which you describe in the following subsections with "Initiation" and "Growth+Decay" within the figures, e.g. by adding two vertical arrows below the panels. In addition, I suggest to indicate the time which corresponds to each of the vertical lines shown in the panels (e.g. by adding the times at the top of the first panel). What is the meaning of the solid black horizontal line?

(7) Line 151: Note that Baumgartner et al (2022) describe that the homogeneous freezing of the solution particles already starts at values of RHi below the threshold given in Koop et al (2000). The rate of ice crystal nucleation increases as the values of RHi approach that threshold, but the threshold is not to be understood as a switch. In essence, as long as RHi comes close to the critical value (e.g. the threshold), the homogeneous nucleation starts and there might have been some homogeneous nucleation also at 3m height during your observation.

Minor and technial comments:

(1) Line 27: It should read "pre-conditioned"

(2) Line 59: "...data at Dome C, a site particularly..."

(3) Line 104: "droplets"

(4) Line 114: "to track the trajectories of the air masses probed above Dome C."

(5) Line 120: It should read "0600 LT" ?

(6) Line 149: It should read "0800 LT, 8 March (Fig. 4)." and "2230, 7 March, at the"

(7) Line 209-210: What is the maximum value of RHi?

(8) Line 213: "measurements"

(9) Line 247: "precipitation"

(10) Line 256: Delete "this"

(11) Appendix A: I found it quite hard to understand what exactly is shown in figure A1. Please state this more explicitly. It would also be helpful to add a sentence on how one should "read" these plots.

(12) Line 281: It should read "RHl and RHi"

(13) Equations A1, A2 and line 290: Please substitute the asterisk by a centered dot to indicate multiplication.

(14) Equation A2: The numbers of the regression coefficients should appear as an index.

This paper reviews two fog cases that illustrate some unique fog formation means that have likely not been observed or well studied to date.  This research is worthy of publication.  There are a few key elements that of which some are critical to denote:

Review:

Line 35 - Supercooled liquid has been observed to 240 K at South Pole Station during the SPARCLE experiment (see https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2021JD035182)…While this was at a higher altitude above the ground, it is possible to have cold temperatures and still have liquid… Hence, this should be considered in what is written here to denote this. Please revise the temperature values at which liquid water can and does exist.

Line 70 - Averaging data over 30 minutes is a long time. This reviewer is not in favor of this practice as you are smoothing out the data before analysis….which this is less of an issue with slower changing parameters like pressure (not used in this study) but it has a larger impact on faster moving variables such as temperature and wind.  As a note, 30 minute averaging is at least 3 times beyond the WMO recommendations which recommend averaging over small time frames (1 minute for temperature, 2 minute or 10 minute for wind) see WMO Publication #8).  While we can debate the merits of this, I wonder the impact it would have in interpreting the 25 August case between 6 and 15 LT when the RHi goes above the Koop et al (2000) value.  How might the data observations look in this time period without the 30 minute averaging, but instead 10 minute averaging?  This non-standard method for handling the data impacts future comparisons likely to be made by others and other observational datasets that do not do this.  This contributes to the heterogeneous observing network Antarctic suffers from, and it is not getting any better with divergent observing schemes that are in place

Line 80 - As I read through the cases, I wonder if it would help the reader to know more about the Murphy and Koop methodology, as seeing an RH value of over 100% seems unexpected (but it is fine, correct?). Also, the RHi vs. RHl seem to be the same curves with an offset (?) Using RH overall is a terrible measure of actual moisture anyways…and RHi clearly shows that you are saturated or supersaturated with respect to ice.

Figure 2 - Is this for the March 8th case?  Some indication of dates/times in the caption would be helpful.

Figure 3 - So this case, you have wind speeds clearly over the threshold for blowing snow, yet it is not reported nor happening?  (Also see lines 125 through 130…)

Line 120 - Reference Figure 4 here with the RHi value referenced…

Line 150-155 Is it fluxing downward and the atmosphere is not decoupled at all above?? *** Unlikely there is decoupling?***

Line 205 - Is the 20 meters from human observation?

Line 217 - This stray sentence should be combined with the paragraph above.

Line 243 - This stray sentence should be combined with the paragraph above.

Figure 10 is really helpful - just too small.  Is there anyway it can be published to be larger to see the red text??

Figure B2 is too small to see - hopefully this can be improved in publication

Minor langage/English:

Line 5 - Remove “To our knowledge” is not really needed… Just say “This is the first time…”

Line 40 - Remove “hitherto” as it is not needed

Line  256 - Correct the line “To our knowledge, this our study presents…” to simply say “This study presents…”

Line 262 - Remove “ we raised in the Introduction” as it is not needed

Line 285 - Add “thermometer” at the end of this bullet point.