Dear Editor,

The paper by Hwang et al. titled “Propagating characteristics of mesospheric gravity waves observed by an OI 557.7 nm airglow all sky camera at Mt. Bohyun (36.2N, 128.9E)” provides a comprehensive summary of the 557.7 nm greenline all sky camera observation of gravity waves.    The authors found that majority of the gravity waves are vertically free propagating waves.   The horizonal propagating directions are consistent with seasonal variations of the neutral winds from the reanalaysis data base and locally installed meteor radar observation.   It is nice to have combined wind and all sky image observations, which allows the derive of the intrinsic frequency of the gravity wave.   The paper is well written and should be considered for publication.   I have some minor comments about the paper. 

1. The authors used “observed wave parameters “ referring to the neutral wind Doppler shifted values.   I think it is better to refer them as “apparent”.
2. L182 “… Korean peninsula can propagate any directions …’.   Change to ‘… in any directions …’
3. L252 “The small percentage of evanescent waves may imply that the majority of the observed waves is not locally originated from, at least the altitude range of 90 – 100 km.” Probably should just say that majority of the waves is free propagating wave. 
4. L286 “The evanescent waves may be generated in situ at the airglow layer, probably as secondary waves, not propagated from the lower atmosphere. The evanescent waves were very rare (less than 2%) in our analysis of the BHO images. “ It will be good to show some examples of all sky images of the evanescent waves.      As to the sources of the wave, do you have any references?  Do you see any sign of the instability in the mesospheric region?
5. Did you do any binning of the CCD images?

Manuscript title: Propagating characteristics of mesospheric gravity waves observed by an OI 557.7 nm airglow all-sky camera at Mt. Bohyun (36.2°N, 128.9°E)

Manuscript #: angeo-2021-65

The authors have analyzed short-period gravity wave in the MLT region using the all-sky imager (557.7 nm filter) and collocated meteor radar over Mt. Bohyun observatory (36.2°N, 128.9°E) for the period of 2017-2019. The intrinsic parameters (phase velocity, wavelength, and periodicities) and propagation direction have been estimated seasonally. The season-wise preferential direction has been attributed to wind filtering in the lower atmosphere. They analyzed the background condition of GWs propagation for each event by calculating the m² profile using the temperature from NRLMSISE-00 and wind from meteor data. The background analysis suggests that most of the waves are observed to be freely propagating.

There is a significant concern regarding the m² calculation based on NRLMSISE-00 data, which is utilized to find the ducting, evanescent and free propagation conditions. The paper has some serious issues which need to be addressed properly. This paper can be published after a major revision. The comments are included below:

Major comments:

1. The authors used NRLMSISE-00 temperature data, which is not useful for background analyses of individual cases. The author did not mention the role of thermal ducting due to the presence of mesospheric temperature inversion which plays a vital role in the ducting of GWs. Therefore, the statistical observations of free propagating, ducting, or evanescent waves are biased.

2. Model temperature cannot record any of these events. The authors should use SABER temperature data for this purpose.

3. The seasonal results of propagation direction are similar to previously reported from the mid-latitude location. However, a serious question arises from the low number of data set for statistical analysis. On average, the observation day is only 13% of the total days of 3 years which is very small for statistical analyses. Can the authors justify this?

4. In previous studies, the neutral instability has been attributed to the in-situ generations of ripples structure (<10-15 km). Figures 5a & 5d show the wavelength data in the range of 10 km. How are they sure that those small-scale waves are not ripples? In this regard, the authors are suggested to calculate the Richardson number to analyze dynamic instabilities.

5. Line 170-171: “…. the spring/summer the westward wind is dominant in the middle

171 atmosphere, whereas in the fall/winter the eastward wind is dominant …”. The observation is during nighttime. Is the seasonal wind pattern mentioned above during nighttime? Authors are suggested to provide mean wind map or profiles of MERRA-2 over Mt. Bohyun observatory season-wise to support the conclusions in lines 321-324.

6. Why did the authors use interquartile range (IQR) instead of mean and standard deviation?

7.Is it possible to detect the nature of GWs (ducted, evanescent, or free propagating waves) based on airglow imager alone?

Minor comments:

 Line 17: Please mention the bandwidth of the 557.7 nm filter.

Line 217: The authors did not include the first and second derivative terms in the GWs dispersion relation (Eqn 2), which is also essential in the case of Doppler duct analyses.

Line 225: Please correct the scale height (Hs) and recalculate those profiles.

Line 252: What is the reason behind the observation of evanescent waves during spring only?

Figure 6: The X-axis limits of all subplots in the left (wind speed) should be same and also applicable to right subplots (m²). It will be easy to compare.