Performance Evaluation of an A Band Differential Absorption LIDAR Model and Inversion for the Ocean Surface Pressure from Low-Earth Orbit

Round 1

Reviewer 1 Report

The concept using differential absorption measurements in the oxygen A band to measure the atmospheric pressure was proposed almost 40 years ago. This manuscript appears to reevaluate the pressure measurements using this concept from space. Although there is a value in the manuscript, the manuscript is not well written based on my reading. Incomplete sentences are used. The flow of the manuscript is not very logical. Equations are duplicated and assigned multiple numbers. All these led to the readability of the manuscript is not good. I suggest the authors to rewrite the manuscript, have some English speakers proofread, and resubmit the manuscript. Some of my comments are provided in below for authors to consider.

1.       The manuscript made a lengthy discussion and derivation regarding a paper by Schwemmer et al and derived a conclusion implying that the situation considered in Schwemmer et al is not realistic. I think it is not necessary to discuss a paper published lone ago back to 1987. If the paper is not appropriate, the authors should cite other papers or derive correct equations to use in their simulation. Numerous new papers have been published over the past decades discussing pressure measurement using the lidar differential absorption measurement technique (see an example by Lin and Liu in below). It seems that Eq. (1-3) (12) is not appropriate, implying that Eq.1 from Schwemmer et al is not correct. If so, the authors should not include them in the manuscript. The inclusion of these equations just cause confusion. Some equations are duplicated, e.g., Eq.(2) following line 102 and Eq.(2) (10) following line 127. It is not necessary at all and does not follow the general format of journals. I suggest the author to rewrite section 1 Introduction and the first part of section 2 lines 89 - 139.

Lin, B., & Liu, Z. (2021). Martian atmospheric CO2 and pressure profiling with differential absorption lidar: System consideration and simulation results. Earth and Space Science, 8, e2020EA001600. https://doi.org/10.1029/2020EA001600

2.       Regarding section 3.2 Differential absorption LIDAR system model, some of the parameters in Table 2 are not realistic or currently unavailable. In Table 2, the laser pulse energy is assumed to be 100 mJ at a pulse repetition rate of 100 Hz. In the text, lines 265-267 state that “To achieve narrow line width of the laser operation, cavity injection seeding is employed via two CW fiber coupled tunable distributed feedback (DFB) lasers, a EDFA (Erbium-Doped Fiber Amplifier)…” There are technical difficulties to build high power fiber amplifiers. To the best of my knowledge, commercial fiber amplifiers can typically generate output energy of sub-millijoules. Very recently, some research module of fiber amplifiers with outputs of only few millijoules just became available. It is impossible to get ~100 mJ output energy from fiber amplifiers. The simulation should be based on the reality. You may consider other type of amplifiers or fiber amplifiers with low pulse energy and high repetition rate, such as 1mJ at 10 kHz. A 10 kHz repetition rate corresponds to an aliasing range of 15 km.

3.       Regarding the uncertainty estimation. It appears that an important error source of random noise in the lidar return from the atmosphere is omitted in the simulation. Atmospheric lidar returns from different altitudes are involved in your OD retrieval from 75 km to the altitude under consideration. Hence, noises in the lidar returns can contribute to the pressure retrieval.

4.       Clarify the units of errors in tables 3-10. Are they relative error? It would be useful to calculate relative error for each error source in Table 11 and sort them by value so that readers can easily know which error sources are dominant and which are ignorable. Is there a better way to visualize Table 11?

Author Response

1.The manuscript made a lengthy discussion and derivation regarding a paper by Schwemmer et al and derived a conclusion implying that the situation considered in Schwemmer et al is not realistic. I think it is not necessary to discuss a paper published lone ago back to 1987. If the paper is not appropriate, the authors should cite other papers or derive correct equations to use in their simulation. Numerous new papers have been published over the past decades discussing pressure measurement using the lidar differential absorption measurement technique (see an example by Lin and Liu in below). It seems that Eq. (1-3) (12) is not appropriate, implying that Eq.1 from Schwemmer et al is not correct. If so, the authors should not include them in the manuscript. The inclusion of these equations just cause confusion. Some equations are duplicated, e.g., Eq.(2) following line 102 and Eq.(2) (10) following line 127. It is not necessary at all and does not follow the general format of journals. I suggest the author to rewrite section 1 Introduction and the first part of section 2 lines 89 - 139.

Thank you for your valuation

Respect the opinions of experts, delete comments the on the relationship equation between atmospheric pressure and optical depth in the article of Schwemmer et al, 1987, NASA. References [4] Schwemmer, G. K.,  Dombrowski, M.,  Korb, C. L. Milrod, J., Walden, H. and Kagann, R. H.: A LIDAR system for measuring atmospheric pressure and temperature profiles, Rev. Sci. Instrum.58(12), 2226~2237, doi:10.1063/1.1139327, 1987.

Add new references

References [15]Lin, B., and Liu, Z. (2021). Martian atmospheric CO₂ and pressure profiling with differential absorption lidar: System consideration and simulation results. Earth and Space Science, 8, e2020EA001600. https://doi.org/10.1029/2020EA001600

2. Regarding section 3.2 Differential absorption LIDAR system model, some of the parameters in Table 2 are not realistic or currently unavailable. In Table 2, the laser pulse energy is assumed to be 100mJ at a pulse repetition rate of 100 Hz. In the text, lines 265-267 state that “To achieve narrow line width of the laser operation, cavity injection seeding is employed via two CW fiber coupled tunable distributed feedback (DFB) lasers, a EDFA (Erbium-Doped Fiber Amplifier)…” There are technical difficulties to build high power fiber amplifiers. To the best of my knowledge, commercial fiber amplifiers can typically generate output energy of sub-millijoules. Very recently, some research module of fiber amplifiers with outputs of only few millijoules just became available. It is impossible to get ~100 mJ output energy from fiber amplifiers. The simulation should be based on the reality. You may consider other type of amplifiers or fiber amplifiers with low pulse energy and high repetition rate, such as 1mJ at 10 kHz. A 10 kHz repetition rate corresponds to an aliasing range of 15 km.

Answer: Due to the poor English expression of the manuscript, the reviewers were misunderstood. In fact, the output energy of 100mJ is not from the high-power fiber amplifier, but from the all-solid-state alexandrite laser and its optical amplifier, referring to the research results of Coney, A. T. and Damzen, M. J. The fiber amplifier is only a part of the seed injection locking module of alexandrite laser.

References [20]Coney, A. T. and Damzen, M. J.: High-energy diode-pumped alexandrite amplifier development with applications in spacecraft-based LIDAR, Journal of the Optical Society of America B38(1), 209-219, doi:10.1364/JOSAB.409921, 2021.

The reflectivity of the tropical and subtropical sea surface is significantly lower than that of the Mars surface dust (MOLA) or the ice at the two poles of the earth (ICESat-2 mission). If the sea surface is irradiated with low-energy laser pulses, the signal-to-noise ratio of the receiver will be very low. Photon counters are required, and more echo accumulation and averaging are required. In fact, increasing the pulse repetition frequency will not improve the horizontal distance resolution.

3. Regarding the uncertainty estimation. It appears that an important error source of random noise in the lidar return from the atmosphere is omitted in the simulation. Atmospheric lidar returns from different altitudes are involved in your OD retrieval from 75 km to the altitude under consideration. Hence, noises in the lidar returns can contribute to the pressure retrieval.

Answer: Due to the poor readability of the English expression of the manuscript, it brings misunderstanding to experts. In the manuscript, "3.3.1 Random error of differential optical depth caused by noise" discusses the random noise of lidar returns. The fundamental method to improve it is 1) simultaneous transmission and reception of online/offline dual pulse; 2) Multi-pairs pulse echo accumulation average; 3) Ultra-narrow band solar filter (Fabry-Perot etalon 25pm bandpass).

4.larify the units of errors in tables 3-10. Are they relative error? It would be useful to calculate relative error for each error source in Table 11 and sort them by value so that readers can easily know which error sources are dominant and which are ignorable. Is there a better way to visualize Table 11?

Answer: The last row in Table 11 is the relative error of pressure (%), the penultimate row is the absolute error of pressure (Pa), and the absolute error of optical depth of other categories.

Reviewer 2 Report

The article may be published. However, there are a some of small remarks.

1. The authors supposed that the vertical temperature profile is known with an error of less than 1K, and the uncertainty of profile for water vapor mixture ratio is less than 20%. The choice of these values is not explained. In particular, for satellite sounding, the typical error is 2K.

2. It would be useful to take into account other factors that may lead to additional sources of error. For example, take into account the effect of Doppler-broadened Rayleigh backscattering. As a result of volcanic activity, an aerosol is formed. Volcanic aerosol is characterized by the formation of stratus clouds. This can lead to additional sources of error when using DIAL measurements. (e.g., Ansmann A. Errors in ground-based water–vapor DIAL measurements due to Doppler-broadened Rayleigh backscattering // Appl. Opt. 1985. V. 24. No 21. P. 3476-3480).

Author Response

Comments and Suggestions for Authors

The article may be published. However, there are a some of small remarks.

1. The authors supposed that the vertical temperature profile is known with an error of less than 1K, and the uncertainty of profile for water vapor mixture ratio is less than 20%. The choice of these values is not explained. In particular, for satellite sounding, the typical error is 2K.

Thank you for your comments,

Answer: The evaluation of vertical temperature profile error and uncertainty of water vapor mixing ratio in the paper is based on the active detection of atmospheric temperature, humidity and pressure parameters by future satellites;

1）European Space Program WALES（Water Vapor Lidar Experiment in Space）

Lifetime [year]                                                         2-3

Data reliability [%]                                                      95

Timeliness [hour]                                                       < 3

Accuracy (bias) [%]                                                     < 5

Random error (1σ) [%]                                                   20

Dynamic Range [g kg^-1]                                             0.01 – 15

Horizontal Integration [km]                              25    100    150    200

Horizontal Domain Global Vertical Resolution [km]          1.0    1.0    1.0    1.5

Altitude Range [km]                                 0-2   2-5   5-10*  10*-16*

2）Paolo Di Girolamo, Andreas Behrendt, and Volker Wulfmeyer，Spaceborne profiling of atmospheric temperature and particle extinction with pure rotational Raman lidar and of relative humidity in combination with differential absorption lidar: performance simulations，APPLIED OPTICS  Vol. 45, No. 11，10 April 2006，2474-2494

Fig.1 Nighttime measurement precision DT in cloud-free conditionsfor different values of the average power of the lidar transmitter(45, 100, 200, 400 W). S, Stokes; AS, anti-Stokes.

Fig.2daytime measurement precision DT in cloud-free conditions for different values of the average power of the lidar transmitter (45, 100, 200, 400 W). S, Stokes; AS, anti-Stokes.

It can be seen from Figure 1 and Figure 2 that the surface-lower stratosphere (0-15km), measurement precision ∆T can reach 1K

3）AIRS - the Atmospheric Infrared Sounder，was launched on the Aqua research satellite, a major component of NASA's Earth Observing System, in May 2002.In addition to temperature profiles - with a vertical resolution of 1 km in the troposphere and an accuracy of 1 K, and water vapor profiles - with a vertical resolution of 2 km in the troposphere and an accuracy of 15%.

2. It would be useful to take into account other factors that may lead to additional sources of error. For example, take into account the effect of Doppler-broadened Rayleigh backscattering. As a result of volcanic activity, an aerosol is formed. Volcanic aerosol is characterized by the formation of stratus clouds. This can lead to additional sources of error when using DIAL measurements. (e.g., Ansmann A. Errors in ground-based water–vapor DIAL measurements due to Doppler-broadened Rayleigh backscattering // Appl. Opt. 1985. V. 24. No 21. P. 3476-3480).

Answer: The effective way to reduce the influence of Rayleigh backscatter Doppler broadening is not to select the peak wavelength of the spectral line as the wavelength value of offline/online, but select offline/online on the slope of the spectral line. however

1) If offline/online is selected on the slope of the spectral line, the jitter of the center frequency of the transmitted laser should be constrained at the order of 1MHz, and the jitter of the center frequency of the laser should be constrained at the order of 10MHz, which is much easier than that of 1MHz.

2) When offline/online is selected in the trough area between two spectral line, the influence of Doppler broadening of Rayleigh backscatter is smaller than that at the peak.

The error caused by the uncertainty of aerosol has been partially discussed by the following Formula, and will be further discussed in the future.

Comments and Suggestions for Authors

The article may be published. However, there are a some of small remarks.

1. The authors supposed that the vertical temperature profile is known with an error of less than 1K, and the uncertainty of profile for water vapor mixture ratio is less than 20%. The choice of these values is not explained. In particular, for satellite sounding, the typical error is 2K.

Thank you for your comments,

Answer: The evaluation of vertical temperature profile error and uncertainty of water vapor mixing ratio in the paper is based on the active detection of atmospheric temperature, humidity and pressure parameters by future satellites;

1）European Space Program WALES（Water Vapor Lidar Experiment in Space）

Lifetime [year]                                                         2-3

Data reliability [%]                                                      95

Timeliness [hour]                                                       < 3

Accuracy (bias) [%]                                                     < 5

Random error (1σ) [%]                                                   20

Dynamic Range [g kg^-1]                                             0.01 – 15

Horizontal Integration [km]                              25    100    150    200

Horizontal Domain Global Vertical Resolution [km]          1.0    1.0    1.0    1.5

Altitude Range [km]                                 0-2   2-5   5-10*  10*-16*

2）Paolo Di Girolamo, Andreas Behrendt, and Volker Wulfmeyer，Spaceborne profiling of atmospheric temperature and particle extinction with pure rotational Raman lidar and of relative humidity in combination with differential absorption lidar: performance simulations，APPLIED OPTICS  Vol. 45, No. 11，10 April 2006，2474-2494

Fig.1 Nighttime measurement precision DT in cloud-free conditionsfor different values of the average power of the lidar transmitter(45, 100, 200, 400 W). S, Stokes; AS, anti-Stokes.

Fig.2daytime measurement precision DT in cloud-free conditions for different values of the average power of the lidar transmitter (45, 100, 200, 400 W). S, Stokes; AS, anti-Stokes.

It can be seen from Figure 1 and Figure 2 that the surface-lower stratosphere (0-15km), measurement precision ∆T can reach 1K

3）AIRS - the Atmospheric Infrared Sounder，was launched on the Aqua research satellite, a major component of NASA's Earth Observing System, in May 2002.In addition to temperature profiles - with a vertical resolution of 1 km in the troposphere and an accuracy of 1 K, and water vapor profiles - with a vertical resolution of 2 km in the troposphere and an accuracy of 15%.

2. It would be useful to take into account other factors that may lead to additional sources of error. For example, take into account the effect of Doppler-broadened Rayleigh backscattering. As a result of volcanic activity, an aerosol is formed. Volcanic aerosol is characterized by the formation of stratus clouds. This can lead to additional sources of error when using DIAL measurements. (e.g., Ansmann A. Errors in ground-based water–vapor DIAL measurements due to Doppler-broadened Rayleigh backscattering // Appl. Opt. 1985. V. 24. No 21. P. 3476-3480).

Answer: The effective way to reduce the influence of Rayleigh backscatter Doppler broadening is not to select the peak wavelength of the spectral line as the wavelength value of offline/online, but select offline/online on the slope of the spectral line. however

1) If offline/online is selected on the slope of the spectral line, the jitter of the center frequency of the transmitted laser should be constrained at the order of 1MHz, and the jitter of the center frequency of the laser should be constrained at the order of 10MHz, which is much easier than that of 1MHz.

2) When offline/online is selected in the trough area between two spectral line, the influence of Doppler broadening of Rayleigh backscatter is smaller than that at the peak.

The error caused by the uncertainty of aerosol has been partially discussed by the following Formula, and will be further discussed in the future.