Gravity waves above the Northern Atlantic and Europe during streamer events using ADM-Aeolus

Wüst, Sabine; Küchelbacher, Lisa; Trinkl, Franziska; Bittner, Michael

doi:https://doi.org/10.5194/amt-2024-18

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https://doi.org/10.5194/amt-2024-18

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27 Mar 2024

| 27 Mar 2024

Status: this preprint is currently under review for the journal AMT.

Gravity waves above the Northern Atlantic and Europe during streamer events using ADM-Aeolus

Sabine Wüst, Lisa Küchelbacher, Franziska Trinkl, and Michael Bittner

Abstract. Information about the energy density of gravity waves (GWs) is crucial for improving atmosphere models. So far, most space-based studies report on the potential energy, E_pot, of GWs, as temperature measurements from satellite are more common.

We used ADM-Aeolus (Atmospheric Dynamics Mission) wind data to derive a lower limit of the kinetic energy density, E_kin, of GWs above the Northern Atlantic and Europe. Aeolus on ADM, ESA’s fourth Earth Explorer Mission, was the first Doppler wind lidar in space and measured vertical profiles of the horizontal line-of-sight wind from the ground to the lower stratosphere (20 – 30 km) between 2018 and 2023. With a vertical resolution of 0.25–2 km, Aeolus measurements are in principle well suited for the analysis of GWs. However, the data quality is a challenge for such analyses, as the accuracy of the data is in the range of typical GW amplitudes in the tropo- and stratosphere.

In this study, we derive daily resolved time series of the lower limit of the E_kin, called E_kin,low, before, during and after two so-called streamer events above the Northern Atlantic and Europe. Streamers are large-scale tongue-like structures of meridionally deflected air masses, which are due to enhanced planetary wave activity. They are linked to vertical shear of horizontal wind and a pressure system, two possible GW generation mechanisms. We find that there is a temporal coincidence between the daily averaged E_kin,low and occurrence of the streamer events. The results indicate, that the derivation of GW signals based on Aeolus data is possible. However, we collected about 100 profiles to statistically reduce the uncertainty of the daily averaged E_kin,low. Compared to non-satellite measurements those daily averaged values are at the upper border.

Received: 06 Feb 2024 – Discussion started: 27 Mar 2024

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Sabine Wüst, Lisa Küchelbacher, Franziska Trinkl, and Michael Bittner

Status: final response (author comments only)

RC1:
'Comment on amt-2024-18', Anonymous Referee #3, 10 Apr 2024

In their paper "Gravity waves above the Northern Atlantic and Europe during streamer events using ADM-Aeolus" the authors use ADM Aeolus data to derive the kinetic energy density (Ekin) of gravity waves over the Northern Atlantic and Europe during periods of streamer events. During these events enhanced gravity wave energy densities would be expected. One major problem is the high noise level of Aeolus which complicates the derivation of gravity wave signals. Therefore, the authors average over a larger region to reduce the uncertainty of time series of daily averages for relating them to the streamer occurrences. Indeed, minor enhancements of Ekin are found that may be related to the streamers. Horizontal distributions of Ekin show enhancements that may be related also to other gravity wave sources.
Overall, the paper is an interesting study, fits into the scope of AMT, and the results are of interest for the community of atmospheric dynamics and people who intend to derive gravity wave distributions from Aeolus soundings.

The paper is therefore recommended for publication in AMT after addressing my minor, but important comments.

MAIN COMMENTS are:
(1) The expression "Ekin,low" is somewhat misleading and should be avoided! This expression as is suggests to be a lower estimate of the gravity wave kinetic energy density. However, Aeolus Ekin estimates should be significantly high-biased because of the high Aeolus noise level.

Therefore it should be stated more clearly in the manuscript that Ekin derived from Aeolus should not necessarily be a lower estimate!

Using a better statistics will reduce the "uncertainty" of Ekin, but will not remove the bias introduced by Aeolus noise. Of course, a better statistics will help to obtain more reliable relative variations of Ekin assuming that the noise produces a constant Ekin offset.

(2) Gravity waves found in the region of interest are not necessarily excited in the same region by the streamer events. It has been shown before by, for example Krisch et al. (2017) that gravity waves can travel large distances horizontally until where they are observed. This information should be added in the discussion.

SPECIFIC COMMENTS:
(1) l.15 and l.21: Later in the manuscript it turns out that Ekin derived from Aeolus winds should not necessarily be a lower limit. Therefore these statements in the abstract should be revised.

(2) l.32: please add the citation Holton, 1982 in addition to Houghton, 2002
Holton, J. R.: The role of gravity wave induced drag and diffusion in the momentum budget of the mesosphere, J. Atmos. Sci., 39, 791-799, 1982.

(3) l.32/33, l.41 onward: Please be more specific! For a gravity wave pseudomomentum flux (Fpx,Fpy) is conserved, but not the wave energy in general. Specifically, in your paper you are considering Ekin, which is not conserved! Even if a wave propagates conservatively, Doppler-shifting by the background wind will change the intrinsic frequency, which then leads to changes in wave kinetic energy.

(4) l.51: momentum -> pseudomomentum

(5) l.84: accuracy -> precision (see comment (8))

(6) l.85: This measurement noise will result in a high-bias of Ekin.

(7) l.127-130: To my knowledge, Metop-A is no longer operational. You should refer to the GOME-2 instruments on Metop-B and Metop-C, instead!

(8) l.223: What do you mean by "accuracy"? Usually, in the field of science and engineering "accuracy" refers to systematic errors/biases, while "precision" refers to random errors. See also:
https://en.wikipedia.org/wiki/Accuracy_and_precision
Alternatively, you could use the notation after ISO 5725 (which is rarely used in our field), but this should then be stated clearly and, accordingly, the expression "bias" should be generally avoided.
Please check the manuscript throughout for the correct use of "accuracy" and "precision"!

(9) L.264/265: This assumption is not valid for polar latitudes. Please refer to Krisch et al., AMT, 2022 for details.
Krisch, I., Hindley, N. P., Reitebuch, O., and Wright, C. J.:

On the derivation of zonal and meridional wind components from Aeolus horizontal line-of-sight wind,

Atmos. Meas. Tech., 15, 3465-3479, https://doi.org/10.5194/amt-15-3465-2022, 2022.

(10) l.265: The expression Ekin,low is misleading as Ekin is calculated from squared wind fluctuations such that random noise will not average out and will produce biases. This should be particularly the case for Aeolus.

(11) l.293 onward: please check for the use of "accuracy" and "precision", see comment (8)

(12) Fig.6: "accuracy" should be "precision", or just "error"??

(13) l.333-336: I think it is encouraging that the positive and negative mean residuals are about symmetric with respect to zero, suggesting that incomplete removal of the background does not introduce strong biases on average. This could be mentioned in the manuscript.

(14) l.401-412: In addition, Aeolus noise will produce a considerable offset of Ekin causing a high-bias.

(15) l.415-430: It is remarkable that in Fig.10 enhanced Ekin is found in the vicinity of Greenland. The southern part of Greenland is a prominent source of gravity waves as can be seen in different gravity wave climatologies (for example, Hoffmann et al., 2013; Ern et al., 2018).
Hoffmann, L., Xue, X., and Alexander, M. J.:

A global view of stratospheric gravity wave hotspots located with Atmospheric Infrared Sounder observations,

J. Geophys. Res. Atmos., 118, 416-434, doi:10.1029/2012JD018658, 2013.
Ern, M., Trinh, Q. T., Preusse, P., Gille, J. C., Mlynczak, M. G., Russell III, J. M., and Riese, M.:

GRACILE: a comprehensive climatology of atmospheric gravity wave parameters based on satellite limb soundings,

Earth Syst. Sci. Data, 10, 857-892, https://doi.org/10.5194/essd-10-857-2018, 2018.

(16) l.415-430: You should mention in the discussion that gravity waves can travel larger distances from their source to the location where they are observed (for example, Krisch et al., 2017). Therefore gravity waves of origin other than the streamer events could be superimposed in the region of interest and partly obscure the streamer gravity wave signal. Possible candidates for gravity waves of different origin could be mountain waves from Greenland and Iceland.
Krisch, I., Preusse, P., Ungermann, J., Doernbrack, A., Eckermann, S. D., Ern, M., Friedl-Vallon, F., Kaufmann, M., Oelhaf, H., Rapp, M., Strube, C., and Riese, M.:

First tomographic observations of gravity waves by the infrared limb imager GLORIA,

Atmos. Chem. Phys., 17, 14937-14953, https://doi.org/10.5194/acp-17-14937-2017, 2017.

(17) l.442: This is not correct! Aeolus is not a limb viewer! It views downward with an angle of just 35deg to the nadir.

(18) l.443/444: "GWs with phase fronts oriented meridionally will cancel out entirely or at least to a large part."

This is not correct because Aeolus observes in "near-nadir" geometry!

(19) l.456: However, gravity waves of phase speed opposite to the background wind would not be filtered out. These gravity waves could undergo considerable amplitude growth and overcompensate the filtering effect.

(20) l.458: "the generation of secondary GWs"

This statement is very speculative. Of course, this process can also happen at relatively low altitudes, however, usually it becomes important at altitudes well above 20km not covered by Aeolus.

(21) l.495: The relatively high values of Ekin are understandable because the high noise level of Aeolus will cause an offset.

(22) l.513: There are more satellite instruments that currently observe temperature altitude profiles in the lower stratosphere and are suited for gravity wave analysis, for example, AIRS and MLS, see Hoffmann and Alexander (2009) and Ern et al. (2022).
Hoffmann, L., and Alexander, M. J.:

Retrieval of stratospheric temperatures from Atmospheric Infrared Sounder radiance measurements for gravity wave studies,

J. Geophys. Res., 114, D07105, doi:10.1029/2008JD011241, 2009.
Ern, M., Hoffmann, L., Rhode, S., and Preusse, P.:

The mesoscale gravity wave response to the 2022 Tonga volcanic eruption: AIRS and MLS satellite observations and source backtracing,

Geophys. Res. Lett., 49, e2022GL098626, https://doi.org/10.1029/2022GL098626, 2022.

TECHNICAL COMMENTS:
(1) l.23 due to -> caused by
(2) l.25: between the daily averaged -> between enhanced daily averaged
(3) l.33 are can -> can
(4) l.59: by -> be
(5) l.71: doppler -> Doppler
(6) l.81: data has -> data have
(7) l.112: air of low -> air of low total column ozone (TO3) ???
(8) l.121: The identification -> Our identification
(9) l.134: comparable large -> comparably large
(10) l.139:

longitudes, so it has a strong meridional structure

->

latitudes, so it has a strong zonal structure
(11) caption of Fig.1: starting data -> starting date
(12) l.147: November 2020 -> September 2020
(13) l.180: horizontal position -> horizontal orientation
(14) Table 1: dependant -> dependent
(15) l.397: ofKramer -> of Kramer
(16) l.438 waves 2 -> wave 2

Citation: https://doi.org/10.5194/amt-2024-18-RC1
- AC2: 'Reply on RC1', Sabine Wüst, 29 May 2024
  
  Please find our answers to the comments in the attachment
  
  Citation: https://doi.org/10.5194/amt-2024-18-AC2
RC2:
'Comment on amt-2024-18', Anonymous Referee #1, 22 Apr 2024

Throughout this review, the format "PXX.LXX Comment" is used to refer to the page number "PXX" and line number "LXX" in the originally submitted manuscript corresponding to each comment.
SHORT SUMMARY

This is a review of the paper titled "Gravity waves above the Northern Atlantic and Europe during streamer events using ADM-Aeolus", submitted to Atmospheric Measurement Techniques. This paper investigates the potential for Aeolus measurements to provide gravity wave (GW) signals during so-called "streamer events" above the Northern Atlantic and Europe, and in particular, to derive a lower limit for the GW kinetic energy density (E_kin_low) during such events. Two example cases are analysed, and a temporal correlation is found between the daily averaged E_kin_low and the occurence of each streamer event, with enhanced values observed for each. The authors also consider the spatial distribution of the kinetic energy density signals from Aeolus during one of these events, however no significant pattern can be found. Cubic splines are used to approximate the atmospheric background and retrieve wind residuals, and the analysis is split into a tropospheric and a stratospheric part to exclude the tropopause wind maximum.
This manuscript is interesting and has the potential to fulfil the scope of AMT, the results are presented in a balanced manner and some of the scientific quality is good. However, a major revision is likely to be necessary in order to address the following three issues.
(i) The study itself is rather focused and could be improved by both a broader development of the GW analysis technique and a wider analysis of the streamer events mentioned and/or of other similar events.
(ii) There are some outstanding questions regarding the validity of the analysis technique for measuring GWs, with the inherent uncertainties in the Aeolus data. This may just require some clarification.
(iii) Although there is a good clarity and concision to the overall writing of the manuscript, the figures and presentation of results will require further improvement to a higher quality.
Therefore, publication can be recommended only after the following issues and suggestions are resolved or considered.
GENERAL COMMENTS

1) The term "ADM" (abbreviation of Atmospheric Dynamics Mission) was omitted from the satellite's name by ESA several years ago. Please update all instances of "ADM-Aeolus" to "Aeolus" to conform with convention, noting this previous AMT review in particular, general comment #2: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1924/egusphere-2023-1924-RC1-supplement.pdf
2) P01.L23 With respect to issues (i) and (ii), it is not completely obvious how sharp regions of vertical wind shear, which may define the slanted edges of jet streaks and other non-GW tropospheric wind phenomena, are prevented from contaminating the signals appearing in the GW analysis of Aeolus data. Are these non-GW features captured correctly in the background using the cubic spline method? This is particularly an issue in the troposphere, since the stratosphere is generally stable and stratified, such that vertical perturbations in horizontal wind can be more easily attributed to GWs here. Attached to this review is an along-track Aeolus "quick-look" profile time-series for 2020-02-10 in the region of interest, showing an intense jet streak associated with one of the streamer events in question (Source: https://aeolus.services/). Might there be an increase in non-GW wind perturbations, which inherently occurs during these dynamic streamer events, which could contribute to the results and/or explain some of the temporal coincidence that is found? If this is accounted for, some clarification of this within the text would be helpful. Optional Suggestion: Perhaps an example of the raw Aeolus data from the time period being analysed could be shown at some point to help orient the reader?
3) P23.L450 - P23.L460 The question of whether or not the detrending method is affected by the tropopause wind maximum as an artefact, contaminating the GW signal, is quite an important one. Do the authors think this peak in residual winds is a consequence of this artefact, or is it more attributable to GW filtering, as this paragraph suggests? Will Aeolus have the capability to measure an increase in GW amplitudes which is caused by secondary GWs? An expansion of this discussion and perhaps an exploration of further GW statistics would be useful, particularly to answer these and similar questions.
4) P26.L484 The question of the manuscript seems rather focussed to me, and could be expanded and generalised a little so that this work is more applicable as an atmospheric measurement technique for future gravity wave analyses using Aeolus. Whether this is formed from a slightly broader GW analysis, looking deeper than the kinetic energy density, or whether additional events are required to test the conclusions that have been drawn; both may help to answer some of the current questions that remain about this study.
5) Are the authors aware of work such as Wiegand and Knippertz, 2013 (https://doi.org/10.1002/qj.2112), Wernli and Springer, 2007 (https://doi.org/10.1175/JAS3912.1) and Madonna et al., 2014 (https://doi.org/10.1175/JAS-D-14-0119.1)? These may be useful to read and as a references.
SPECIFIC COMMENTS

P01.L16 Replace "Aeolus on ADM" with "Aeolus". ALADIN is the instrument onboard, as mentioned later in the article.

P01.L17 (also P03.L77) The phrase "from the ground to the lower stratosphere (20 - 30 km)" is a little ambiguous. The ground is of course at 0 km, and the stratosphere also begins lower than 20 km, so this should be rephrased slightly.

P01.L25 Do the results indicate this, or is this an implicit assumption? As the authors mention later, Banyard et al. 2021 has shown that this is possible, although only for the single case of a strong GW.

P02.L31 It's slightly confusing to say that GWs dominate atmospheric dynamics over other wave phenomena, even if this is qualified by specifying that this is "especially above 75 km height". Perhaps this part could be rephrased? This study does not go higher than 24 km.

P03.L84 The phrase "Challenging is the accuracy of Aeolus" reads a little strangely.

P05.L107 Can streamer events be linked to strong cyclones?

P05.L112 low latitude or low potential vorticity?

P08.L172 Could you please clarify how you corrected the hlos wind for the satellite observation geometry?

P08.L176 - P08.L185 Could this paragraph be written a little more succinctly? It's easy to get lost here at the moment.

P12.L251 For clarification, is the use of varying starting points applied in order to combat both the insufficient approximation of extrema in the background and the artificial oscillations generated?

P13.L268 This sentence is a little unclear, could it be rephrased slightly?

P14.L291 By "CIRA" are you referring to the COSPAR International Reference Atmosphere? This needs defining.

P21.L378 Is it possible to suggest reasons for the GWs captured in the analysis of these two streamer events? E.g. Is it unstable shears, geostropic adjustment, the result of interaction between other GWs? It would be good to have some additional discussion on this topic here.

P22.L442 Aeolus measurements are "off-nadir", even though ALADIN looks to the side of Aeolus, it is not a limb viewer.

P23.L447 It might be useful to explain here a little more about why it is the lower limit of the kinetic energy density that you are measuring.

P26.L503 - P27.L517 This is a good and useful discussion which highlights some of the potential future benefits of a succeeding mission to Aeolus and raises important suggestions for optimising its suitability for GW analysis.
FIGURE COMMENTS

Figure 1 The latitude and longitude labels need to be larger and not obscured by the colour bar. Is there a higher resolution version of the image shown?

Figure 2 Please add latitude and longitude labels.

Figure 7 It could be made a little clearer within the plot itself that 7b shows the negative and 7c the positive residuals.

Figure 8 Since 8c is just the green plot from 8b repeated, is there a way to simplify the reading of the entire figure somehow? Having labels on the figure to quickly identify which subplots correspond to the tropospheric and stratospheric parts may be helpful.

Figure 9 Can these figures be superimposed on one another, or is it better to separate the tropospheric and stratospheric components for all figures? It feels like this could be presented in a slightly clearer way, as you have to look quite carefully to understand what each subplot shows at the moment.

Figure 10 The filled contour plotting here is a little confusing, would a different colour scale be better to use? Perhaps a different plotting technique and layout would make it easier to see the changes from week to week? Please put the dates as well as the week numbers.

Figure 11 It is not immediately clear that these figures show the heights of the profile's first and second wind maxima. Is it possible to show this as some sort of density plot with height on the y axis and two variables on the x axis (Var 1 being the height of the first maxima, Var 2 being the heights of the first and second maxima)? Histograms may be fine if they are slightly altered, but they are a bit unclear at present.

All Figures: In general, most of the figures need some improvement in quality to make them clearer and easier to understand. An additional figure to give more context for Aeolus measurements may be helpful, and a couple more may be required for any further analysis that is conducted.
TYPOGRAPHICAL ERRORS

PXX.LXX New (Old)

P02.L33 which can (which are can)

P02.L35 GWs (GW)

P02.L37 GWs (GW)

P02.L38 improving (improve)

P03.L59 However, parts of the GW spectrum, equation (3) can be simplified (However, for parts of the GW spectrum equation (3) can by simplified)

P03.L63 frequently (frequent)

P03.L71 Doppler (doppler)

P03.L75 Aeolus (Aeolus on ADM)

P03.L81 These data have (These data has)

P05.L135 clearly (definitely)

P06.L139 reaches latitudes of 70 degrees N (reaches regions latitude of 70 degrees N)

P08.L174 netCDF (ncdf)

Check Kruger

P12.L243 adaptation (adaption)

P20.L365 Figure 7 (Figure 6)

P26.L484 The question which we addressed in this manuscript was whether (The question, we addressed in this manuscript was, whether)

Citation: https://doi.org/10.5194/amt-2024-18-RC2
- AC1: 'Reply on RC2', Sabine Wüst, 29 May 2024
  
  Please find our answers to the comments in the attachment
  
  Citation: https://doi.org/10.5194/amt-2024-18-AC1
- AC3: 'Reply on RC2', Sabine Wüst, 29 May 2024
  
  Please find our answers to the comments in the attached pdf
  
  Citation: https://doi.org/10.5194/amt-2024-18-AC3

Sabine Wüst, Lisa Küchelbacher, Franziska Trinkl, and Michael Bittner

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Short summary

Information about the energy of gravity waves (GWs) is crucial for improving atmosphere models. Most space-based studies report on the potential energy. We use ADM-Aeolus wind data to derive a lower limit of the kinetic energy density. However, the data quality is a challenge for such analyses, as the accuracy of the data is in the range of typical GW amplitudes. We find a temporal coincidence between enhanced or breaking planetary waves and enhanced gravity wave kinetic energy density.


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