Study on the Impact of the Doppler Shift for CO2 Lidar Remote Sensing
Round 1
Reviewer 1 Report
Please see attchment
Comments for author File: 
Comments.pdf
Author Response
Point 1: Line 42-44, I agree with the authors about the importance of CO2 in global warming. However, it is definitely appreciated to mention the importance of CH4 and N2O in global warming too which are getting more and more attentions recently, with references such as Khalil (1999, doi: 10.1146/annurev.energy.24.1.645), Zhao et al. (2009, doi: 10.1029/2008JD011671), Montzka et al. (2011, doi: 10.1038/nature10322), Jeong et al. (2012a, b, doi: 10.1029/2011JD016896, doi: 10.1029/2012GL052307), Zhang et al. (2015, doi: 10.1016/j.ecoinf.2014.01.009), Zhang et al. (2018, doi: 10.1002/2017EF000707).
Response 1: Line 49-51, the importance of CH4 and N2O in global warming has been added.
Point 2: Line 49, not only at global scale, but also at regional scale. I would suggest using “at both regional and global scales”.
Response 2: Line 59, ”on the global scales” has been replaced by “at both regional and global scales”.
Point 3: Line 56-58, please rephrase this sentence, including the periods.
Response 3: Line 69-72, the sentence has been rephrased.
Point 4: Line 60-62, To be fair, please also indicate the limitation/shortcoming of active remote sensing observations.
Response 4: Line 79-80, the limitation of the active remote sensing observations has been added.
Point 5: Line 83-84, “because of missing technology readiness”? I would suggest simply removing this.
Response 5: Line 103, “because of missing technology readiness” has been removed.
Point 6: Line 94, “compared with” or “were compared with”?
Response 6: Line 113, “were” has been added.
Point 7: Line 112-113, “Zhu Yadan”? Please provide the reference.
Response 7: Line 134-136, the reference has been added.
Point 8: Line 143, “were” should be “was”
Response 8: Line 172, “were” has been replaced by “was”.
Point 9: Line 145, Why does “weather conditions” can result in datasets acquired over a variety of surfaces? In my understanding, “weather conditions” can only make the observations available or not.
Response 9: Line 174-176, “as well as the weather conditions” has been removed.
Point 10: Figure 2, it might be okay for current organization. Personally, I think people should be familiar with Doppler shift, and the contents here could be shortened.
Response 10: In Section 2.3, the derivation of equation 14 (including Equations 7-13) has been shortened. The Figure 2(a) is the principle of the Doppler shift, so it was also removed. Figure 2(b) represents the Doppler shift during the flight campaign. It will help the readers to understand the Doppler shift in the airborne measurements, so I keep it.
Point 11: Line 257-259, oen value for “DAOD values”?
Response 11: Line 298-299, do you mean open value for “DAOD” values? The range of "DAOD" values is not unlimited. The single DAOD values mostly stay in the range of 0-1.1. The mean DAOD values for airborne measurements are in the range of 0.42-0.48. The mean DAOD values for spaceborne measurements are larger than airborne measurements due to the longer laser path, about 0.4-1.2. Or do you mean the ocean value for “DAOD” values? The sentence has been rephrased. The mean DAOD values in residential areas are slightly larger than that in the ocean by 0.01.
Point 12: Figure 5, an explanation for the relatively stable and low XCO2 over mountainous area is appreciated.
Response 12: Line 336-341, the explanations for the relatively stable and low XCO2 over mountainous area have been added.
Point 13: Line 309-310, there are two “initially” here. Please remove one.
Response 13: Line 362-364, the latter “initially” has been removed.
Point 14: Line 367, “Doppler shifts”
Response 14: All the “Doppler” in the text have been capitalized.
Point 15: Line 407-410, why do not use the sounding from observations, considering the potential large uncertainties from NWP?
Response 15: Line 485-488, the atmospheric state (vertical profiles of temperature, moisture, and pressure) can be measured by multiple platforms, such as weather balloons, aircraft and satellite observations. Figure 1 shows the distribution of the upper-air observation stations based on weather balloons. Figure 2 shows the distribution of the aircraft observations. These measurements can provide accurate concentrations. Nonetheless, there remain significant gaps at global scales due to the limited spatial coverage and uneven distribution. The ERA5 dataset is derived from the numerical weather prediction (NWP) model. It provides a synthesized estimate of the climate state generated from a combination of a numerical model and as many observations as possible of the Earth system, such as satellite radiometers, ground, ship, airborne weather stations, moored buoys, radiosondes, and ground-based radars. It has the advantage of wide coverage compared to ground observations. In addition, the quality of the ERA5 data has been evaluated by many researchers, such as Bryukhanov (2022, doi: 10.20944/preprints202205.0086.v1) and Hersbach et al. (2020, doi: doi.org/10.1002/qj.3803). Figures 3-4 show the vertical profiles of RMSE, bias, and correlation of the the air temperature and relative humidity based on ERA5 relative to the radiosonde observations (RAOB) data for 2016-2020. The temperature from the ERA5 reanalysis consists well with the RAOB data, with small RMSE values of 0.8-2.8K. The correlation between ERA5 and RAOB is greater than 75% at 400-1000hPa, which is within the acceptable range for simulations. So, it can provide reasonable data for the data simulations. The sentence (“And quality control was performed to screen out NWP data with large uncertainties.” ) has been removed due to improper expression. A more appropriate expression would be that the ERA5 data for the period of 1950-1978 is worse than that from 1950 onwards due to the limited accumulated data for this period.
Point 16: Line 464, unit should be given.
Response 16: Line 554, the unit of the bias (ppm) has been added.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The authors present the effect that the Doppler shift has on XCO2 derivations from IPDA LIDAR measurements. The data from the March 14th, 2019 airborne experiment was examined with and without Doppler shift considerations and the results were found to be in better agreement with in situ measurements when accounting for the Doppler shift due to aircraft instability. Similarly the effect of the Doppler shift was considered, via simulation, for the ACDL experiment on the AEMS satellite. Changes due to the Doppler effect were seen most evidently over the Qinghai-Tibet Plateau due to the higher surface elevation, the effect being most prevalent at higher altitudes (lower pressures), and the fact that the experiment performs a line-of-sight integration.
The paper is mostly well written and only requires some editing. Specific line changes, some suggested only and some required, are enumerated below.
Lines 68, 74, 78: When used as an adjective I believe units get hyphens like, "1.57-um laser"
Line 76 2006 - 2009. Hyphen rather than twiddle?
Lines 111, 115, 119, 130 etc Doppler effect needs capitalization. Be consistent throughout
Line 122 influence of what?
Line 134 is not a sentence.
Line 136 Do you mean "A distributed-feedback (DFB) laser is used as a reference and is modulated..."
Line 138 Are there multiple reference lasers?
Line 143 "was" not "were"
Line 166 LIDAR continue to capitalize?
Line 170 Sentence beginning with "The echo signals" is grammatically confusing. See line 235 for a clearer explanation.
Line 184 Is P power? Perhaps intensity of the pulse signal?
Line 191 XCO2 is written differently than it is in the introduction.
The derivation of equation 14 (including Eqs 7-13, Figure 2) may not be necessary for this paper. Depends on the intended audience.
Line 320 "solid and dashed vertical lines"
Line 325 Needs better connection to the prior sentence if that is what you mean. "Owing ... online, this positive shift will..."
Line 329 Not technically a complete sentence. connect to prior. "...frequency shifts, while the derivative online is 16 times larger. Thus, a small..."
Figure 9 caption needs a description of plot a and b. Line 350 about 9b needs to say difference between original and Doppler shifted results.
Figure 10 caption could use more too.
Line 371 Low stability of the aircraft? What does that mean?
Line 375 Might mention the old bias value again here so reader can compare how much better the new value is. "...reduced to 0.16ppm (from 1.XXppm)..."
Line 410 Sentence should not start with AND
Line 411 "priors" ?
Figure 11 caption "of a) XCO2 and b) DAOD simulations ... "
Line 432 "due to less human activities on the Plateau"
Line 433 No AND
Line 488 Need to say difference between a1 and a2 in the text. with and without Doppler shift
Line 454 You say the absolute difference is 1.25. in what units? 1.25 isn't even on your scale for a3.
Figure 12 I see now. a3 needs a % indication in caption and on the figure itself. It is not an absolute difference you've plotted but a % difference according to the text and color scale. b3 might be absolute.
Figure 12 I'm confused again. You say a1 and a2 are simulations(?) with and without Doppler, but in the text you say b1 and b2 are Truth and simulation with Doppler. It's unclear what's being plotted.
Line 455 proper English but slightly confusing. consider: "... Plateau, and this difference corresponds to..."
Line 458 need a comma. "... other regions, about 0.09%"
Line 462 "In this region the mean difference"
Figure 13b looks VERY similar to 13a. Needs a better caption. Am I to understand from 13d that there's a 14 unit difference at above 200 hPa between 13a and 13b? I find that hard to believe unless 13a and 13b are x10^-25, which is not indicated. Also, label says "molecu"
Line 487 No AND. Longitudinal? What longitude? or what temperature was used?
Line 497 Consider something like: "The greater deviations from higher altitudes (lower pressure) will be diluted during line-of-sight integration with signal from the lower altitudes, which exhibit smaller deviations."
Line 524 "on spaceborne ACDL measurements"
Line 535 "lower surface elevation"
Author Response
Point 1: Lines 68, 74, 78: When used as an adjective I believe units get hyphens like, "1.57-um laser"
Response 1: Line 85, 92, and 97, the hyphens have been added.
Point 2: Line 76, 2006 - 2009. Hyphen rather than twiddle?
Response 2: Line 94, the twiddle has been replaced by hyphen.
Point 3: Lines 111, 115, 119, 130 etc Doppler effect needs capitalization. Be consistent throughout
Response 3: All the “Doppler” in the text have been capitalized.
Point 4: Line 122 influence of what?
Response 4: Line 146, the influence of the Doppler shift, “of the Doppler shift” has been added.
Point 5: Line 134 is not a sentence.
Response 5: Line 160-161,the sentence has been rephrased.
Point 6: Line 136 Do you mean "A distributed-feedback (DFB) laser is used as a reference and is modulated..."
Response 6: Yes. Line 163-166, the sentence has been rephrased.
Point 7: Line 138 Are there multiple reference lasers?
Response 7: Line 166, only one reference laser. “lasers” has been replaced by “laser”.
Point 8: Line 143 "was" not "were"
Response 8: Line 172, “were” has been replaced by “was”.
Point 9: Line 166, LIDAR continue to capitalize?
Response 9: Line 197, “LIDAR” has been capitalized .
Point 10: Line 170, Sentence beginning with "The echo signals" is grammatically confusing. See line 235 for a clearer explanation.
Response 10: Line 203-204, the sentence has been rephrased.
Point 11: Line 184, Is P power? Perhaps intensity of the pulse signal?
Response 11: Line 218, I think power is more reasonable. The IPDA lidar equation can be expressed as following (Ehret et al., 2008 (doi: 10.1007/s00340-007-2892-3)):
where is the echo power, is the wavelength, is the receiving optical efficiency, is the overlap factor, is the area of the telescope, is the height of the surface above sea level, is the altitude of the aircraft platform, is the emission energy of the laser, is the effective pulse width of the echo pulse, is the target reflectivity, is the two- way integral optical depth caused by CO2, and is the atmospheric transmission efficiency. But P can also be converted to intensity by dividing by dividing by A.
Point 12: Line 191, XCO2 is written differently than it is in the introduction.
Response 12: Line 225, ”XCO2” has been replaced by “XCO2”
Point 13: The derivation of equation 14 (including Eqs 7-13, Figure 2) may not be necessary for this paper. Depends on the intended audience.
Response 13: In section 2.3, the derivation of equation 14 (including Equations 7-13) has been shortened. The Figure 2(a) is the principle of the Doppler shift, so it was removed. Figure 2(b) represents the Doppler shift during the flight campaign. It will help the readers to understand the Doppler shift in the airborne measurements. So I was wondering if you would mind keeping it?
Point 14: Line 320, "solid and dashed vertical lines"
Response 14: Line 374, ”solid and dash lines” has been replaced by “solid and dashed vertical lines”
Point 15: Line 325, Needs better connection to the prior sentence if that is what you mean. "Owing ... online, this positive shift will..."
Response 15: Line 381-382, the sentence has been rephrased.
Point 16:. Line 329 , Not technically a complete sentence. connect to prior. "...frequency shifts, while the derivative online is 16 times larger. Thus, a small..."
Response 16: Line 386-387, the sentence has been rephrased.
Point 17: Figure 9 caption needs a description of plot a and b. Line 350 about 9b needs to say difference between original and Doppler shifted results.
Response 17: Line 421-428, the description of plot a and b has been added. Line 407-409, “between original and Doppler shifted results in IWF” has been added to the sentence.
Point 18: Figure 10 caption could use more too.
Response 18: Line 449-456, the description of plot a and b has been added.
Point 19: Line 371, Low stability of the aircraft? What does that mean?
Response 19: Line 440-441, the sentence has been rephrased.
Point 20: Line 375, Might mention the old bias value again here so reader can compare how much better the new value is. "...reduced to 0.16ppm (from 1.XXppm)..."
Response 20: Line 444-446 ”from 1.312ppm” has been added.
Point 21: Line 410, Sentence should not start with AND
Response 21: Line 488-489, the sentence has been removed due to improper expression. A more appropriate expression would be that the ERA5 data for the period of 1950-1978 is worse than that from 1950 onwards due to the limited accumulated data for this period.
Point 22: Line 411, "priors" ?
Response 22: Line 489-491 the sentence has been rephrased.
Point 23: Figure 11 caption "of a) XCO2 and b) DAOD simulations ... "
Response 23: Line 529-532, the description of plot a and b has been added.
Point 24: Line 432, "due to less human activities on the Plateau"
Response 24: Line 513-514, ”the ” has been removed.
Point 25: Line 433, No AND
Response 25: Line 514, ”And” has been removed.
Point 26: Line 488, Need to say difference between a1 and a2 in the text. with and without Doppler shift
Response 26: Line 585 ”between a1 and a2” has been added. Line 503, 548, 558 and 632, “with and without Doppler shift” has been added.
Point 27: Line 454, You say the absolute difference is 1.25. in what units? 1.25 isn't even on your scale for a3.
Response 27: Line 540-542, Figure 12(a-3) shows the relative difference of IWF between the (a-1) and (a-2). -1.25 is the absolute difference, and this difference corresponds to a relative difference of -0.07%.
Point 28: Figure 12 I see now. a3 needs a % indication in caption and on the figure itself. It is not an absolute difference you've plotted but a % difference according to the text and color scale. b3 might be absolute.
Response 28: Figure 12 and Line 566, ”%” has been added to Figure12(a-3) and caption.
Point 29: Figure 12 I'm confused again. You say a1 and a2 are simulations(?) with and without Doppler, but in the text you say b1 and b2 are Truth and simulation with Doppler. It's unclear what's being plotted.
Response 29: Figure 12(b-1) and Figure 12(b-2) show the XCO2 results retrieved from the ACDL simulations with and without Doppler shift. Initially, Figure 12(b-1) was referred to as the true values because the XCO2 retrievals with Doppler shifts are almost identical to the CO2 columns obtained from the TCCON dataset, shown in Figure11(a), and the TCCON dataset is considered as true values. To avoid misinterpretation, Line 35, 502, 547, 555 and 632, and the text was reorganized.
Point 30: Line 455, proper English but slightly confusing. consider: "... Plateau, and this difference corresponds to..."
Response 30: Line 540-542, the sentence has been rephrased.
Point 31: Line 458, need a comma. "... other regions, about 0.09%"
Response 31: Line 545，comma has been added.
Point 32: Line 462, "In this region the mean difference"
Response 32: Line 559 ”In this region” has been added.
Point 33: Figure 13b looks VERY similar to 13a. Needs a better caption. Am I to understand from 13d that there's a 14 unit difference at above 200 hPa between 13a and 13b? I find that hard to believe unless 13a and 13b are x10^-25, which is not indicated. Also, label says "molecu"
Response 33: Line 600-604, the caption of Figure 13 has been redescribed. The unit of Figure 13(a) and 13(b) is 10-23cm2/molecu, which was accidentally missed. The unit has been added to the new Figure 13.
Point 34: Line 487, No AND. Longitudinal? What longitude? or what temperature was used?
Response 34: Line 583-584, ”And” has been removed. Figure 13(c) represents a longitudinal section of the deviations between (a) and (b) at 296K. Line 584, the temperature “296K” has been added to the text.
Point 35: Line 497, Consider something like: "The greater deviations from higher altitudes (lower pressure) will be diluted during line-of-sight integration with signal from the lower altitudes, which exhibit smaller deviations."
Response 35: Line 593-595, the sentence has been changed.
Point 36: Line 524, "on spaceborne ACDL measurements"
Response 36: Line 629, ”spaceborne” has been added.
Point 37: Line 535, "lower surface elevation"
Response 37: Line 643, ”lower altitude” has been replaced by “lower surface elevation”
Author Response File: Author Response.docx
