Overview

This is an interesting and well-written manuscript, which investigates the QBO and large-scale tropical waves in zonal wind data from Aeolus, three reanalyses datasets, and radiosondes. The authors have found a good agreement between Aeolus wind and the reanalyses on zonal averages. The differences are more pronounced near the strong shear zones, which might be related to the differences in vertical resolutions of the datasets, and also before the QBO disruption. Furthermore, the differences are much more noticeable when the reanalysis are locally compared to radiosonde observations. The results also show good agreement between reanalyses and Aeolus zonal wind in capturing the variances related to global-scale stationary and travelling tropical waves. The focus of the manuscript aligns well with the journal's special issue on Aeolus data and their application. I have only a few minor comments that should help to clarify a few points.

General Comments:

• Given the following points that are mentioned and discussed in the paper, I am wondering what value of a difference between zonal winds in Aeolus and reanalysis could be considered a (statistically) significant difference:
  • The precision of Aeolus winds is about 3 to 7 m/s (line 104)
  • The vertical and meridional components of winds are ignored (Eq. 3)
  • The winds are interpolated to fixed geopotential heights (Eq. 4).
• Has any kind of tapering been applied to the 31-day period windows before applying the 2D Fourier transforms? If not, do you see any spectral leakage at higher frequencies?
• Related to the point above, when a 31-day window is being used, and assuming no tapering in time, the lowest resolved frequency should be ~0.03 cycles/day. However, in isolating the Kelvin waves, the frequency of 0.015 cycles/day has been used as the lower band (i.e., a period of ~67 days). I think this results in some contamination of the Kelvin wave signal by the quasi-stationary waves.

Specific Comments:

Line 52: It might be useful to mention that the gravity waves are usually not resolved in climate models and their effects are approximated using different parameterization schemes.

Lines 130 to 150: My understating is that the Aeolus and radiosondes winds are interpolated to 0.25 km vertical resolution, but the reanalysis are interpolated to a 0.5 vertical resolution. If this is the case, then how you subtract them in the next sessions?

Figure 1: I am not sure why the one standard deviation is shown with respect to zero. I think it should be shown with respect to the mean differences (i.e., the red line).

Figure 6: Have you tried using a log-scale for plotting the power? I think it would be useful in extracting the interesting features in higher frequencies.

Section 4.2.4. You might want to mention that the peak variances at ~30 km are due to extratropical Rossby waves propagating equatorward from the winter hemisphere and are not related to the equatorial Rossby waves.

Figure A2: I am a little bit surprised by the panel for MRGW. I expected to see a stronger signal in meridional wind than in the zonal wind for MRG waves. In isolating the MRG signal based on the wavenumber-frequency spectrum, have you considered the fact the MRG waves are symmetric in meridional wind (while they are antisymmetric in zonal wind)?

Line 207: The comma before “Also”, should change to a dot.

 [ General comments ]

The paper analyzes tropical zonal winds observed by Aeolus for the recent 5-year period, particularly the zonal mean and perturbations due to tropical waves. The result is compared to the wind fields in three modern reanalyses and in radiosonde data. There may be no doubt that the global wind observation is invaluable, and the comparison of its first result to other independent datasets is highly motivated. The analysis made here is therefore timely. It covers most of the major tropical wave modes, with interpretations of the result. Some limitations of the measurements (such as the vertical resolutions) are also discussed. Overall, the structure of the paper is easy to read. I would recommend this paper for publication after a minor revision.

My comments are listed below, which are all minor or technical. The most interesting observation from my point of view was to see the shear-related differences between the radiosonde and Aeolus (as well as between the radiosonde and reanalyses). See the comments on L279 and L272.

[ Specific comments ]

L36 : “one of the main ...” : What would be other processes in the tropics ?

L43 : “... MRGWs” : It may be arguable whether the MRGW is a prominent wave mode in zonal winds, as also seen in the result of this paper.

L48 : “in the troposphere” : Isn’t the OLR a quantity at the top of atmosphere, observed from the space ?

L51 : I would suggest including Bushell et al. (2022, doi:10.1002/qj.3765) too.

L61 : Could you please add a reference ?

L52 : Holt et al. should be removed in this line (but kept in L53) as they did not analyze gravity waves but resolved waves (mainly Kelvin waves and MRGWs).

L82 : “models” → “reanalyses”

L154, 159 : “T639”, “T319” → “TL639”, “TL319”

L162 : “~21 km in the tropics”

Figures : Reference mean-wind contours : Why has MERRA-2 been chosen (while the difference of the reanalysis outputs from Aeolus winds is minimum for ERA-5) ?

L236–237 : Would traveling waves remain in monthly mean (which is usually used for the QBO) ?

L250 : Aren’t the intervals at 3 days ? (L188)

L265 : Please provide the corresponding period here again.

L268 : Where is the SAO shear-related signal found in the figure ?

L269 : “drop” : what does this mean ?

L272–274 : Could the uncertainty range of altitude estimates, derived from the temperature and pressure profiles, be calculated/provided ? Would it be as large as ~1 km at z = 20–25 km ?

L279–280 : The shear-related differences found between reanalysis and radiosonde winds (e.g., positive differences along the dashed contour in the stratosphere in 2021–2022, followed by negative differences along the solid contour in 2022; seen more clearly in ERA-5 and JRA-55 than in MERRA-2) are observed at all four stations, thus it is arguable whether the cancellation effect due to zonal averaging could hide these signals. Moreover, these shear-related differences from radiosonde winds are also found in Aeolus data in 2021–2022 along the dashed and solid contours with generally the same signs: positive and negative, respectively (although a bit noisy). The fact that the shear-related differences (from the radiosonde) are rather consistently found in Aeolus and reanalyses can therefore lead to the expectation that differences between Aeolus and reanalysis winds (as in Fig. 1) regarding the shear zone would be less pronounced than the differences from the radiosonde.

L327 : “1 cycle/day” : How is this determined ?

L337–339 : That the phase shift is mentioned here leads readers to deduce the whole procedures to obtain the two spectra (while there could exist different ways) which have been omitted in the text. If the authors would mention the phase shift in the text, an explanation for the procedures before/after the shifting should also be supplemented. Or it may also be okay if the phase shift --- a specific way to obtain the two spectra --- is not mentioned at all, while still referring to the reference for the details.

L363–364 : Is there any reason that the different frequency ranges are used between reanalyses and Aeolus ? Would it be more consistent if the frequencies higher than 1 cycle/day are not used in reanalyses too ?

L417 : What would be a possible reason that the reanalyses could not capture this ? (ENSO-related phenomena must be in large scales in space and time.)

L464–465 : Regarding the underrepresentation of MRGWs in JRA-55, Kim et al. (2019, doi:10.5194/acp-19-10027-2019) could also be added here (in which the meridional winds are used for MRGWs).

Fig. 10 : It is interesting to observe the MRGW peak above the easterly, around January 2022 (z ~ 30 km). Would there be no statement to mention about this ?

L471 : “are similar (about 10–15 m² s^–2)”

L484 : “n=0 inertia gravity waves” (with no parenthesis)

L546 : The La Nina condition may be a possible factor for the less performance seen in 2021–2022. Could you please revise the sentence to be a bit less conclusive ? (The 5-year record is too short to generalize this finding.)

Appendix A1 : “Rossby-wave-like”, “gravity-wave-like” : I would suggest using a less ambiguous term.

L582 : Could you please explain this a bit more ? (I could not understand it.)

L591 : The stability change around the tropopause also leads to the decrease in the vertical wavelength which then could result in the change in the zonal wind amplitudes. Would it be justified to consider the constant wind amplitude here ?

[ Technical comments ]

L32 : “sensitive” : (to what ?) not clear what this means

L40 : “These” → “The”

L68 : “observation” : repetitive

L84 : “tropical” : repetitive

L88 : “mean zonal wind” (to distinguish from what is investigated in Sect. 4)

L115 : “and neglecting” → “neglecting” ?

L161 : “pressures below” → “pressures less than”

Fig. 1 caption : (1) The period “2018-2022” is applied not only for the mean but also for the standard deviation. Please rephrase the sentence. (2) Currently what the standard deviation is of is not specified (which must be of the difference). Please rephrase it. (3) Black solid and dashed lines : What are the corresponding values ? (0 and –20 m/s ?)

Fig. 1 : Could the late months in 2022 in ERA-5 now be filled ?

L207 : comma → period

L288–289 : Please re-write “This … stations” to, e.g., “The magnitude of this pattern differs among the stations.”

L293 : “differences” → “errors” ? (such advection or wave forcing cannot be derived from sonde data, and thus the comparison/difference cannot be made.)

L296 : “shear, deficiencies” (comma)

Fig. 6 caption : (1) Is it actually the “squared spectral amplitudes”, not the spectral variances ? (2) Would it be clearer if n = 1 (sym.) and n = 2 (antisym.) were indicated for Rossby waves, to distinguish from n > 2 modes ? (Or, could the term “equatorial” Rossby waves already imply those ?)

L358 : “In addition” → “Here” ; “... avoided in the calculation of the background”

L373–374 : I could not understand this sentence.

L428–433 : Too many “Again” (4 out of 6 sentences in the paragraph begin with this.)

L496 : “initiatives” : plural or singular ?

L580 : “z” in the equation may also need to be defined (log-pressure altitude).

L585 : “gravity-wave-like Kelvin” → “Kelvin”

Please remove commas ahead of ‘and’ in, e.g., L20, 29, 345.

Please remove commas ahead of ‘or’ in, e.g., L41, 45, 312.