Simulation of a Pulsed Metastable Helium Lidar

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review of: Simulation of a Pulsed Metastable Helium LiDAR by Jiaxin Lan et al.

General comments:
The article does a detailed job of simulating what is expected from future metastable He measurements in China.  There are a lot of general lidar information, technical details and equations presented here, which are not needed or used. It distracts from the central story of the article: simulations of saturation and relative errors.

Missing is any discussion of how the simulated results from the Chinese metastable He lidar will compare to the observations by Kaifler.  The paper would also be improved by showing some preliminary measurements.  The paper would also benefit for using an online spelling and grammar checker as there are some errors.

I suggest that major revisions be conducted.

Specific Comments:
1) Line 33-34: sentence is repeated from the abstract.  Please rephrase.

2) Line 39-40:  Please cite some modern papers for wind and temperature lidar:
Gerding, Michael, et al. "The Doppler wind, temperature, and aerosol RMR lidar system at Kühlungsborn/Germany–Part 1: technical specifications and capabilities." EGUsphere 2023 (2023): 1-29.

Baumgarten, Gerd. "Doppler Rayleigh/Mie/Raman lidar for wind and temperature measurements in the middle atmosphere up to 80 km." Atmospheric Measurement Techniques 3.6 (2010): 1509-1518.

Khaykin, Sergey M., et al. "Doppler lidar at Observatoire de Haute-Provence for wind profiling up to 75 km altitude: performance evaluation and observations." Atmospheric Measurement Techniques 13.3 (2020): 1501-1516.

3) Line 43-45:  Same comment, please add some references for each metal and discuss briefly.  It is more important to have a geophysical background of winds, temperatures, and metals and how they relate to He than to discuss the details of a lidar when measurements are not presented.

4) Figure 1: resolution is a bit low compared to other figures

5) Figure 2: Cite the model in the caption.  The reader should be able to read this by only having the figure caption

6) Line 108: citation format problem "WALDROP L S et al.,"

7a) Section 3:  A lidar system is described in detail, but no measurements are ever shown.  This is a paper about simulations why does the reader need the technical details of a lidar? This section could be deleted without impacting the paper.

7b) Is the lidar finished or still in design?

8)  Equation (7) and equation (8):  They are not needed.  None of your figures plot He density.  There are only plots of photon counts.

9) Equation (8):  N_L is never used

10) Section 4.2:  Was the objective in this section to determine what the optimal laser output is to minimize saturation?  If yes, please consider stating the goal for the reader before the derivations.  Additionally, there are multiple equation formatting errors in this section.  Please use LaTex.

11) Equation (9) is too difficult to read.  This equation needs some derivation, and it needs to be formatted in a way that is easier to read.  Also, an error with N_He'

12) "a severer" --> a more severe

13) Line 307-308:  Make clear the origin of the seasonal variation is from number density.  A person reading Equation (9) quickly might forget this.

14)  What is shown in Figure 8?  What doe dashed and solid lines mean?  You derive them with Equation (9)?  If yes, what are the parameters used in the equation?

15a) Table 1:  Atmospheric transmittance = 0.85?  Please provide some justification of what factors were used for light at 1083 nm.  What are the extinction terms being considered?

15b) If geometric correction = 1 then say in the text that it is not being considered in the study.

16) Line 320-322:  You've not demonstrated any connection between the He and your photon count SNR.  You have done a lot in terms of lidar signal but never show an expected profile of He.

17) Section 4.4:  This section isn't really needed.  Since the dominant source of uncertainty is statistical error.  Figure 11 is just 1/SNR from Figure 10?

18) Line 340:  Do you mean day and night?  Solar cycle is not really relevant during a night of lindar measurements.

19) Lines 367-369:  Delete.  There are no measurements in the paper.  This work was about simulations of saturation and relative errors.

 

Comments on the Quality of English Language

There are a few minor spelling and grammar mistakes.  As well, a few places where the paper doesn't read clearly.  I suggest using a free online tool to check the text (Grammarly, LanguageTool, etc)

Author Response

Please see the attachment. 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Review of " Simulation of a Pulsed Metastable Helium LiDAR " by Jiaxin Lan et al.

 

General Comment:

The prime objective of this manuscript is to systematically describe a carefully designed powerful metastable helium resonance fluorescent LiDAR by the authors. A numerical simulation is implemented to evaluate the performance of their He-LiDAR system. Key results from the simulation and comprehensive analysis indicate that there would be a relative error of less than 10% above 350 km in winter and less than 20% in 250-570 km in summer for metastable helium density measurements. In addition, the discussion of saturation effect is helpful for the design of laser power and the correction of simulation results.

Overall, most of the analysis in this manuscript is well done and the conclusions well-reasoned. However, the present manuscript has some shortcomings and that is why the manuscript can be considered for publication after minor revisions from my point of view. I would like to give my comments below that need to be addressed. 

Major Comments:

From the content of the manuscript, the author has a high level of professionalism in instrument design and development. However, an important part of the simulation process was not analyzed, which is the calculation of atmospheric radiation transfer. This is an important factor affecting the intensity of the LiDAR echo signal. Therefore, I hope the author can add some content on the calculation of radiation transfer and a discussion on its impact in the revised manuscript.

Specific Comments:

There are some small details that I hope the author can take seriously.

1. Line 102: In Fig.1, 'wavelength' should be 'Wavelength'. Should units be enclosed in '()' or separated by '/'?

2. Line 168: In Fig.4, 'frequency' should be 'Frequency'. 

3. Line 315: In Table.1, 'power' 'repetition' 'dark counts' should be 'Power' 'Repetition' 'Dark counts'.

Comments on the Quality of English Language

Minor editing of English language required

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

This manuscript presents the simulation of pulsed metastable helium lidar. The structure is well organized and I think this article will be helpful to other research if the content can be polished. In general, there is no significant mistake in this manuscript. However, the content is more like to evaluate the performance of the proposed helium lidar project. Because the technology regarding metastable helium lidar has been developed for a decade, I expect the content should be more generic and symbolic, e.g. the error analysis. My specific comments are listed below.

 

Major:

1. The performance evaluation should consider all possible situations. For example, the noise level in the daytime and the nighttime. I expect the dead-time effect in the photon counting will play an important role.

2. The sensitivity of N_He on each factor should be present. In this manuscript, the simulation is done for a pre-defined helium lidar system. I expect the simulation should tell us what is the minimum hardware requirement.

3. The error analysis should be based on error propagation. I don't see how the relative error is derived.

4. The first part of this manuscript should be a generalized simulation. And the evaluation of the defined system can be put in the second part.

5. Table 1 shows the range resolution is 50 km. But in Fig. 7-11, it seems the range resolution is much higher?

 

Minor.

1. LiDAR should be LIDAR or lidar. LiDAR is more commonly used for mapping lidar.

2. In Eq. (4), does M represent metastable helium?

3. LN90-LN101, many references should be cited in this paragraph. For example, how does the backscatter cross-section in Fig. 1 be calculated?

4. LN155, PID loop should be shown in Fig. 3. 

5. Sec. 4.4. Make a table to list the considered error factor. And discuss the related error for each error factor.

Comments on the Quality of English Language

Professional English editing will help to improve the quality.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

Sufficient changes have been made to the manuscript.  Thank you.

Author Response

Dear reviewer,

 

On behalf of all the contributing author, I would like to thank you again for your helpful suggestions, which have helped us to improve this manuscript quite much.

 

Sincerely,

Jiaxin Lan

Reviewer 3 Report

Comments and Suggestions for Authors

The revised manuscript is much improved and clear. Thought there were some mistakes, I think this manuscript can be published after corrected and polished. The major mistake is regarding the calculation of saturation.

Major:

1.        Sec. 4.2: The saturation should consider the laser power attenuated during propagation. It can be easily understood that the saturation is related to the total photons injected into the helium layer so the power level of the laser attenuated by air molecules or aerosols/clouds before the laser gets into the helium layer is needed to be considered. The photon counts of the backscatter at the bottom of the helium layer should also needed to be included to evaluate the saturation would happen or not.

 

Minor:

1.        L136. The reference number of Waldrop et al.

2.        L277. Suggest using E(z)≈1 instead of E(z)=1.

3.        L 344. LiDAR  Lidar

4.        L367. The “i” in “Where the subscript i” should be italic.

5.        L383-384. Please consider rearranging this one-sentence paragraph.

 

 

Comments on the Quality of English Language

no comment

Author Response

Please see the attachment.

Author Response File: Author Response.docx