Climatology Low-Latitude Sporadic Sodium Layers over Hainan Based on Long-Term Observations and Their Relationship with Es Layers

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

l. 44: I recommend reformulating "enhancement of atoms".
l. 54: "SSLs are not merely natural fluctuations" should mean that SSLs are of an artificial nature? I recommend reformulation.
l. 61: Please specify "this field" here.
l. 62: Please add a corresponding reference
Table 1: There is enough space for "time resolution" and "spatial resolution". I recommend using them.
Figure 1. The legend duplicates labels in the figure. I recommend removing the legend and increasing the label size. 
I also recommend extending the map in longitude to fill the blank space and give more context on the location.
Is it possible to show the field of view for each instrument at a given altitude (e.g., 100 km)?
Figure 2, caption: What do you mean by "effective duration"? The same question is for line 153.
l. 97: Speaking of seasons, I believe it ought to state what you mean by each of them.
l. 100: Are there limitations in 2014 and 2016 connected with the short summertime measurement period?
l. 103: In 2018 and 2023, the spring hours were longer than any other. Consider reformulation by giving some numerical values (e.g., interannual mean duration of season)
ll. 107-108: The formulation is confusing. Consider the following: Despite the good hour coverage data from 2021 showing insufficient altitude over March-October (see the Appendix), we decided to exclude this data from the investigation.
ll. 112-113: Support "typical structure" by a reference.
ll. 113-115: According to the figure, seasonal variation is about 25%. I believe such a variation to be moderate, not weak.
l. 116: I recommend providing some quantity (e.g., percents) describing the seasonal variation.
Figure 3: Please add a year range to the caption.
l. 141: Please give a reference.
Figure 4: Please add grids to a) and b) similar to the one in c)
Table 2: It is necessary to introduce the criteria for high SSL in the text.
Replace commas with standard ones ",".
I believe it is better to present this data graphically: X - year, Y - month, color - availability or quality of data.
Figure 5: Please add error bars to each point and the year range to the figure's title.
ll. 182-183: I believe the monthly median should be more valid than the mean.
ll. 191-193. Could the authors provide more detailed references within [Mathews, 1998], as I did not find any systematic analysis of seasonal dependencies there. Is the reference correct? The paper has only one author (no "et. al.")
ll. 196-197: With all the respect, the Es and SSL enhancements shown in Figure 5 are not simultaneous: Es shows close high values in May-June, while SSL has a clear peak in June and 30% lower value in May; Es's minimal value is in March, while SSL's is in April. 
Figure 6: What are the quantities in 6d and 6e?
l. 220: As far as I see, both the peaks are at period 21-01 UT.
ll. 233-234: The term "relatively concentrated peak heights" is confusing. Does it mean heights within a thin layer near 95 km?
Figure 7: I recommend adding the SSL height distribution to this figure.
ll. 252-254: What do you mean by "latitudinal characteristic"?
ll. 249-252: I believe such a conclusion should be made with bigger statistics. The referred paper is based on data from two points, one of which is Haikou. Please support the statement with some independent data from a wider latitudinal range.
l. 254: Why is it named "Studies" when the only day of 21 June 2019 was investigated? Either add more cases or rename the subsection. According to the paper title, this section should contain electron concentration data showing Es during the investigated period.
Figure 8: Please describe in the text how the data in 8 b was calculated.
Figure 9: I recommend increasing the fonts. The term FWHM was neither introduced nor used in the text.
l. 281: Typical SSL in these latitudes? Is it possible for the reader to see this consistency? I recommend adding more detail here about typical SSL and supporting it with a reference.
l. 304: To be precise, 4 for February and 5 for October. I recommend specifying it here.
l. 308: The observed enhancements are not synchronous. Please reformulate the statement.
ll. 321-322: The final "other related factors" makes this statement so wide that it makes no sense. 
Figure 10: I recommend increasing the font size and using logscale for the color range.

Author Response

Dear Editor, we are grateful for your support. Our detailed responses to the reviewers are provided in the attached file. Thank you for your consideration.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors conducted a very detailed study of SSLs by processing and analysing 13 years of data and setting up and considering various constrains in order to specify SSLs and Es layers scientifically correctly.  These 13 years of data covered 736 effective observational days and a total of 5779 hours of observations recorded over the Hainan station.  For their climatology study, the authors specified and analysed 249 SSL events and conducted an event study for the day of 21 June 2019.  The results obtained are in good agreements with previous studies covering shorter time periods and add new findings.  The manuscript is well written; the figures and tables are of high quality.  The authors even included an Appendix highlighting the 2021 instrumental anomalies severely affecting the Na density data.  According to my best understanding, there are no errors in the manuscript.  Therefore, I recommend the publication of the manuscript in its present form.   

More details:

1. What is the main question addressed by the research?

The main question addressed by the research: What is the relationship between SSLs and Es layers?  It is well understood that sporadic sodium layers (SSLs) are transient phenomena formed by the rapid enhancement of neutral sodium atoms within a localized altitude range in the mesopause region (approximately 90–100 km).  Usually, it is characterized by narrow-layer structures and significant density increases over short timescales.  Meanwhile, Es layers are mainly formed by the convergence of metallic ions produced by meteoric ablation under the action of wind shear. However, as both statistical studies and event studies show, the relationship between SSLs and Es is not simple and is still not clear.

2. Do you consider the topic original or relevant in the field? Does it address a specific gap in the field?

Yes, the topic is both relevant and original, and the research addresses the existing problem related to the complex relationship between SSLs and Es layers.  Various studies conducted coordinated observations of SSLs and Es layers at middle and low latitudes. These studies show that the underlying atmospheric dynamic processes at low latitudes differ significantly from the mid-latitude processes.  However, long-term statistical studies and typical case analyses of SSLs at low latitudes remain relatively limited creating a gap in the current literature. In order to fill this existing gap, the authors conducted complex investigations by using observations covering the calendar years from 2012 to 2024 for analysing the climatology of SSLs for investigating the relationship between SSLs and Es layers.

3. What does it add to the subject area compared with other published material?

The study adds to the subject area by conducting a detailed investigation, based on sodium lidar observations obtained at the Haikou station (20°N, 110.2°E) of the Chinese Meridian Project, covering the time period of 2012–2024 and systematically investigating both (i) the climatology of sporadic sodium layers (SSLs) at low latitude and (ii) their relationship with ionospheric sporadic E (Es) layers at low latitudes. The results obtained reveal significant interannual variability, the SSLs’ seasonal preference appearing in late winter and autumn and the Es layers’ important role in the formation of SSLs. While the underlying physical and chemical processes also play important modulating roles.

4. What specific improvements should the authors consider regarding the methodology? What further controls should be considered?

The current investigation of SSLs over Hainan revealed significant interannual variability and weak seasonal preference along with the underlying influence of direct meteoric injections and ionospheric and other factors.  However, these ionospheric and other factors are still unspecified. Therefore, future studies will focus on these factors. By combining multi-source observations and numerical simulations, future investigations will quantify the relative contributions of tidal winds, metallic ion transport and chemical processes to SSL formation

5. Are the conclusions consistent with the evidence and arguments presented and do they address the main question posed?

Yes, the conclusions are consistent with the experimental observational results and with the main questions asked.  The main conclusions are related to the main specific results obtained and listed in fine points.

6. Are the references appropriate?

Yes.

7. Please include any additional comments on the tables and figures.

The manuscript is well written; the figures and tables are of high quality.  The authors even included an Appendix highlighting the 2021 instrumental anomalies severely affecting the Na density data.  According to my best understanding, there are no errors in the manuscript.  Therefore, I recommend the publication of the manuscript in its present form. 

Author Response

Response to Reviewer:

We sincerely thank the reviewer for the very positive and thorough evaluation of our manuscript. We are delighted that the reviewer considers our study detailed, well written, with high-quality figures and tables, and free of errors, and recommends publication in its present form.

We greatly appreciate the reviewer’s summary of our main contributions and the recognition that our work fills an existing gap in the climatology of low‑latitude SSLs and their relationship with Es layers. We also note the reviewer’s constructive suggestion that future studies should combine multi‑source observations and numerical simulations to quantify the relative contributions of tidal winds, metallic ion transport, and chemical processes to SSL formation. We fully agree with this perspective and will definitely pursue these directions in our future investigations.

Once again, we thank the reviewer for the valuable time and effort dedicated to our manuscript.