the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 04 Jul 2025
Development and validation of a ground-based Asymmetric Spatial Heterodyne Spectroscopy (ASHS) system for sounding neutral wind in the mesopause region
Abstract. Winds in the mesopause region are key to understanding atmospheric dynamics. This study presents a novel ground-based Asymmetric Spatial Heterodyne Spectroscopy (ASHS) system designed to measure these winds by observing the nighttime green line airglow of atomic oxygen. The system’s configuration, thermal behavior, and calibration procedures are detailed. The as-built configuration meets the performance expectations established during the design phase, enabling wind measurements with an accuracy better than 2 m/s. Field observations from Mohe Station (53.5° N, 122.3° E) during geomagnetically quiet conditions demonstrate good agreement with co-located LiDAR data, validating the ASHS system's capability to derive mesopause winds from interferograms.
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RC1: 'Comment on egusphere-2025-2486', Anonymous Referee #1, 24 Jul 2025
This manuscript presents a novel and comprehensive study on developing and validating a ground-based ASHS system tailored for measuring neutral winds in the mesopause region. The authors have provided detailed descriptions of the system's configuration, thermal performance, and calibration methods, which facilitate the replication and further development of the ASHS technology. The laboratory and field validation results presented in the manuscript are highly encouraging, showing good agreement with co-located LiDAR data, which underscores the potential of the ASHS system as a valuable tool for atmospheric dynamics research. The manuscript is overall well-written, and I have several minor comments about the paper in its current form.
- How much does the ASHS instrument drift during the Doppler velocity calibration? In the field configuration you have described, a krypton lamp is employed to monitor instrument drift. However, the krypton lamp was used during laboratory calibration to simulate airglow, and no synchronous light source was available to monitor instrument drift. How much does this affect the calibration results?
- In Fig.11, some of the observed data have very large error bars. This reason must be discussed, as the effective observation sampling rate is also a crucial indicator of an instrument.
- How is the data from LiDAR weighted and averaged?
- The manuscript only compares data from different systems for a few days. Perhaps there should be more comparative data since 2024?
Citation: https://doi.org/10.5194/egusphere-2025-2486-RC1 -
AC1: 'Reply on RC1', Guangyi Zhu, 29 Aug 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2486/egusphere-2025-2486-AC1-supplement.pdf
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RC2: 'Comment on egusphere-2025-2486', Anonymous Referee #2, 27 Jul 2025
This manuscript describes an Asymmetric Spatial Heterodyne Spectrometer (ASHS) used to measure Doppler wind from green line airglow emissions in the mesosphere. The authors present the design of the instrument, and provide a characterization of the thermal drift behaviour of the instrument. In laboratory Doppler measurements were presented demonstrating the capability of the instrument. Finally, atmospheric green line Doppler winds were observed and compared to co-located lidar wind measurements, with the ASHS winds being slightly lower magnitudes than the lidar.
This manuscript presents the first ground-based measurements of mesospheric green line airglow Doppler winds using an ASHS, which is a slight advancement with respect to previous ASHS style instruments, as there have been satellite instruments that measure Doppler winds from the upper mesosphere through to the thermosphere, and ground-based measurements of red line Doppler winds. In addition, many different Fabry-Perot and Michelson interferometers have been used to measure Doppler winds in the mesosphere. Overall, this manuscript could be acceptable for publication after the authors have addressed some major to minor comments. I will begin by describing the my major comments, and then follow with the minor comments.
Major comments:
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There is a lack of sufficient references, notably in sections 2 and 3.2. Please include at least some references in these sections.
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The intensity is written as a function of the etendue, equation 7, with a max amplitude described for a particular solid angle. Liu et al. (2018) is referenced for this equation, but this equation does not appear in that paper. Please verify and find the correct citation. In addition, it is my understanding that the limitation of large incident angles is that the fringe phase starts to change too rapidly with respect to typical pixel sizes, which causes the reduction in fringe contrast. Field-widening the instrument reduces the phase change, allowing for larger incident angles to be included.
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Line 225: Does having the interferometer attached to a large aluminum plate affect the thermal stability of the system? Although a thermally insulated box would help to reduce temperature changes, the interferometer being attached to the highly thermally conductive aluminum plate would cause the interferometer to be more sensitive to ambient temperature changes.
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Please include uncertainties in Figures 7, 9, 11 (lidar), and 12. When possible, uncertainties should be included when presenting experimental data.
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For the wind wheel experiment, why were thermal drift calibration measurements not included? If it were too difficult to implement a simultaneous calibration line, a thermal drift estimate could be made by alternating 'wind' measurements and thermal drift measurements (e.g. 1000 rpm, 2000 rpm, 1000 , 3000, 1000 etc.). Given the high sensitivity of the interferometer to thermal drift, this must be properly accounted for.
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Line 306: It is stated that the thermal drift is estimated using continuous sampling and piece-wise fitting. How do you determine what is attributable to thermal drift and what to Doppler shift? Continuous sampling of what? Temperature? What type of piece-wise fitting? Linear, cubic spline? This process needs to be described in more detail.
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Line 333: The nightly average provided a zero-wind phase, was the variability of the vertical wind taken into account? Given that the horizontal wind measurements are made at 45°, there would be an equal contribution of the vertical wind. For many of the measurements, the vertical wind would be negligible relative to the horizontal winds, but this is not always the case. Do you expect the zenith variability is associated with vertical wind?
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Line 335: What is the weighted average used for the lidar measurements? Are they associated with the lidar uncertainties or nominal airglow layer height? Please provide a complete description of the lidar measurements and weightings.
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Line 337: ‘the standard deviation between the fitted phase and the retrieved phase.’ This statement is not clear. Please describe the fitted phase and the retrieved phase. Is the fitted phase a thermal drift correction?
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Line 368: It is stated that differences between the lidar and ASHS winds can be attributed to inaccuracies in the weighted average calculation of lidar detection results. What inaccuracies? Do you mean increased uncertainty? This could be verified by including uncertainties in the figures. If you intend to say biases in the lidar, what biases? What are you basing this on? It is a bit unsatisfactory to simply say that when the results differ, the ASHS is correct and the lidar is biased.
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Figure 12: Please explain the discontinuities in the meridional winds. Are there gaps in the time series? If so, it may be easier to follow if the gaps are left in the figure, as this would not imply that there were such discontinuities. In addition, it would be useful to include the lidar and ASHS uncertainties either as error bars or shaded regions. It is important to include uncertainties in measurements.
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Line 388: ‘In the linear fitting equation for zonal wind, the slope is observed to be less than 1, which may be attributed to potential system biases between the two systems or errors introduced during the weighted averaging of wind measurement results from various altitudes recorded by LiDAR.’ I think a full description of the weighted lidar average would be useful here. Please attempt to explain why the lidar winds are, on average, larger than the ASHS. What systematic biases between the systems? Given that the slope is less than 1, this is not likely a simple bias, but related to a systematic underestimation of the wind by the ASHS. Could this be attributable to a difference in atmospheric scattering of the different wavelengths?
Minor Comments
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Remove ‘region’ from the title.
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Line 30: Change ‘<’ to better than.
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Line 45: change ‘high etendue device’ to ‘high etendue’.
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Line 60: change ‘observation’ to ‘observations’.
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Figure 2: What are the angles of these prisms.
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Please include the proper punctuation after equations. Equations should be treated as part of the paragraph, and punctuated in such a manner.
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Line 146: change ‘is to ‘are’.
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Line 150: ‘decreases’ should be ‘increases’.
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Line 170: Please define ω.
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Line 179: Is ω the same incident angle as η?
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Line 184: ‘... the parameters, which minimizes the phase difference...’ should be ‘... the parameters that minimize the phase difference...’
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Line 191: Change ‘minimize’ to ‘minimizes’.
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Line 210: Change ‘...allowing observations in different directions and enabling zenith direction viewing to obtain zero wind measurements.’ to ‘...allowing observations in the four cardinal directions and zenith to obtain zero wind measurements.’
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Line 216: place parentheses around ND.
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Line 217: Change ‘on’ to ‘by’.
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Line 218: change ‘lens’ to ‘lenses’.
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Line 229: change ‘impacting’ to ‘being impacted’.
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Line 230: Change ‘environment’ to ‘temperatures’.
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Line 231: Perhaps ‘housing’ would be better than ‘thermostat’
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Line 237: remove ‘stability’
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Line 276: It is stated that ‘the temperature sensitivity is sufficiently small that it does not impact the wind data quality’, however, this is misleading as a temperature change of 0.1° would correspond to approximately 30 m/s for the green line emission. It is rather that due to this thermal sensitivity, calibration measurements must be synchronously made.
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Line 283: ‘system’ should be ‘systematic’.
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Line 291: It is stated that ‘reflection of light significantly decreases when the disk is not rotating.’ This statement is unclear. Why would the reflection of light significantly decrease when the disk is not rotating? The retro-reflective material should still work if the disk is motionless, however, there could be biases introduced if there are imperfections in the retro-reflective material that would be averaged out with a rotating wheel.
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Line 332: How are outliers identified? Is this done using standard deviation?
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Line 354: The nightly average provided a zero-wind phase, was the variability of the vertical wind taken into account? Given that the horizontal wind measurements are made at 45°, there would be an equal contribution of the vertical wind. For many of the measurements, the vertical wind would be negligible relative to the horizontal winds, but this is not always the case. Do you expect the zenith variability is associated with vertical wind?
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Figure11: These wind patterns appear to have a period of approximately 12 hours. Is this attributable to a semi-diurnal tide?
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Line 354: It is stated that the zenith wind is close to zero indicating strong instrument stability. Were calibration lines not used for these measurements? Or is this the variability after thermal drift correction? If the latter, this is not really indicative of instrument stability.
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Line 358: remove ‘were utilized’
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Line 364: It is stated that black dots in Figure 12 are lidar and red are ASHS, but this is opposite of what is shown in the figure legend. Which is correct?
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Line 398: it is stated that the uncertainty of less than 2 m/s is due to the thermal sensitivity. Although this could be partially attributable to the thermal drift, there would also be contributions related to SNR, line visibility, phase precision.
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Line 400: For the lidar comparisons, please be more specific and quantitative.
Citation: https://doi.org/10.5194/egusphere-2025-2486-RC2 -
AC2: 'Reply on RC2', Guangyi Zhu, 29 Aug 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2486/egusphere-2025-2486-AC2-supplement.pdf
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