Long-term studies of the summer wind in the mesosphere and lower thermosphere at middle and high latitudes

Jaen, Juliana; Renkwitz, Toralf; Liu, Huixin; Jacobi, Christoph; Wing, Robin; Kuchař, Aleš; Tsutsumi, Masaki; Gulbrandsen, Njål; Chau, Jorge L.

doi:https://doi.org/10.5194/egusphere-2023-1465

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https://doi.org/10.5194/egusphere-2023-1465

Preprints

11 Jul 2023

| 11 Jul 2023

Long-term studies of the summer wind in the mesosphere and lower thermosphere at middle and high latitudes

Juliana Jaen, Toralf Renkwitz, Huixin Liu, Christoph Jacobi, Robin Wing, Aleš Kuchař, Masaki Tsutsumi, Njål Gulbrandsen, and Jorge L. Chau

Abstract. Continuous wind measurements using partial reflection radars and specular meteor radars have been carried out for nearly two decades (2004–2022) at middle and high latitudes over Germany (∼54° N) and northern Norway (∼69° N), respectively. They provide crucial data for understanding the long-term behavior of winds in the mesosphere and lower thermosphere. Our investigation mainly focuses on the summer season, characterized by the absence of intense planetary wave activity and relatively stable stratospheric conditions. This work presents the long-term behavior, variability and trends of the maximum velocity of the summer eastward, westward and southward winds. In addition, the geomagnetic influence on the summer zonal and meridional wind is explored at middle and high latitudes. The results show that a westward summer maximum is located around 75 km with velocities of 35–54 m/s, while the eastward wind maximum is observed at ∼97 km with amplitudes of 25–40 m/s. A weaker southward wind peak is found around 86 km ranging from 9–16 m/s. The findings indicate significant trends at middle latitudes in the westward summer maxima with increasing winds over the past decades, while the southward winds show a decreasing trend. On the other hand, only the eastward wind in July has a decreasing trend at high latitudes. Evidence of oscillations around 2–3, 4 and 6 years modulate the maximum velocity of the summer winds. Particularly a periodicity between 10.2–11.3 years found in the westward component is more significant at middle latitudes than at high latitudes, possibly due to solar radiation. Furthermore, stronger geomagnetic activity at high latitudes causes an increase in eastward wind velocity, whereas the opposite effect is observed in zonal jets at middle latitudes. The meridional component appears disturbed during high geomagnetic activity, with a notable decrease in the northward wind strength below approximately 80~km at both latitudes.

Received: 30 Jun 2023 – Discussion started: 11 Jul 2023

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Journal article(s) based on this preprint

01 Dec 2023

Long-term studies of the summer wind in the mesosphere and lower thermosphere at middle and high latitudes over Europe

Juliana Jaen, Toralf Renkwitz, Huixin Liu, Christoph Jacobi, Robin Wing, Aleš Kuchař, Masaki Tsutsumi, Njål Gulbrandsen, and Jorge L. Chau

Atmos. Chem. Phys., 23, 14871–14887, https://doi.org/10.5194/acp-23-14871-2023,https://doi.org/10.5194/acp-23-14871-2023, 2023

Short summary

Juliana Jaen, Toralf Renkwitz, Huixin Liu, Christoph Jacobi, Robin Wing, Aleš Kuchař, Masaki Tsutsumi, Njål Gulbrandsen, and Jorge L. Chau

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1465', Anonymous Referee #1, 08 Aug 2023

In this work, the authors analyze radar wind datasets from two sites at different latitudes, infer summer wind maxima in wind components and derive trends. They find a robust strengthening of the mesospheric westward wind at mid latitudes. The influence of daily varying geomagnetic activity on the winds is studied and excluded as source for significant trends. Gravity waves are suggested as cause of the trends.
The paper is well structured and the figures are clear. In few places there are descriptions that can be confusing, mainly when the authors speak of "maximum velocity amplitudes" when actually absolute wind values are meant, which is likely a language problem. I notice that title is a bit unspecific, being almost identical to the special issue title.
The authors list a number of similar observational studies, sometimes based on the same datasets, that sometimes come to contradicting results regarding the trends. This is attributed to different years or altitude ranges. If trends depend so sensitively on the selection of years or altitude ranges, this should be investigated in more detail to be of scientific value. For example, if altitude of wind maxima change with time, this could have been documented.
Another possible source of variability, longitude, is not mentioned at all. The authors generally refer to "middle and high latitudes" and "zonal mean winds" and therefore make the impression that their results are valid for all longitudes. This however cannot be inferred from the datasets the authors used, and some discussion of this topic should be included. Whether the obtained results are only valid locally or in the zonal mean is relevant.
No references to model studies were made, and models were mentioned only briefly in one sentence at the end of the manuscript. The work is mainly a report of measurements without detailed exploration of the findings regarding the underlying mechanisms. For example, periodograms with periods of 2-4 and 11 years are presented but discussion with references to QBO and ENSO remain very vague. The same applies to gravity waves being the suggested cause of the trends, but no proposals or attempts were made at how this could be tested or verified. If this is out of scope, but similar studies are available, that sometimes agree, sometimes contradict, the value of presenting measurements only is limited.
I noticed a mismatch in the presented data between Fig. 1d that shows meridional wind data above 70 km only (where the wind is southward) and Fig. 4c and 4d where, I think, this same data was partitioned regarding Ap and that shows meridional wind data also below 70 km altitude, and the wind is suddenly northward.
I also wonder why a larger Ap threshold of 20 is used for mid latitudes than for high latitudes. The authors refer to different geomagnetic latitudes of the radar sites, but I don't see why this is an argument. I would appreciate more substantial explanations and discussions, e.g. of the mechanisms of how wind maxima are a "proxy for MLT dynamics", or how geomagnetic activity affects wind maxima. In Fig. 4b, is the enhancement in 2020 related to any major solar event? Do other studies exist as it seems to be a major effect?
Regarding the spectral analysis I wonder why a generalized Lomb-Scargle analysis is preferred over a Fourier transform. Isn't the data evenly spaced? The authors do not mention measurement uncertainties. I guess they are smaller than the variability. In Fig. 3, significance levels could have been added to the plots.
I come back to the author's nomenclature of "the maximum velocity amplitude" as their proxy for MLT dynamics which confused me. I expected an "amplitude" to relate to an oscillation, for example a tide or a gravity wave. The meaning of "maximum" was unclear, it could have related to a period of time, or altitude, or a peak-to-peak amplitude.. ? In l. 79 the authors write "the maximum velocity amplitude of the horizontal winds, independent of altitude, variability, and trends..", and I wondered how this value could be independent of altitude, variability and trends. Then, in l. 111, "the maximum amplitude of the velocity per month" seemed to indicate some deviation from a monthly mean. And indeed in l. 115 a reference to "monthly median values" was made, but Fig. 2 shows maximum values of wind components, and not amplitudes (that is differences of wind values) of any kind. I suggest to improve the language in these descriptions or add more details, and not use the term "amplitude".
A similar language problem might apply to "zonal mean wind" (l. 196 and others), which implies a global zonal mean, when in fact the authors probably meant "mean zonal wind". Also, the terms "a mid latitude" or "a high latitude" might be more honest than generally speaking of "mid latitudes" or "high latitudes", as only data from sites at one specific longitude was used.
In general, grammar could be improved. Examples are "Below the mesosphere is located the stratosphere..", "linear functions were adjusted to…" instead of "fitted to", "periodograms were extracted…" instead of "calculated", "The zonal component is built with the combination of two datasets ". Some sentences lack subjects, e.g. in l. 91, l. 138, l. 308 or in Fig. 6 caption. Specific comments and questions are listed by line number:
line 4: "mainly focuses on the summer season" what part of the study does not focus on the summer season?
line 6: is there no northward wind? Northward wind is mentioned in l. 16
line 23: 1980s
line 28: regarding the broad subject of greenhouse gas monitoring, the authors only cite the works of two authors from the author's institute
line 91: FMCW is not defined. In l. 103, abbreviations are defined that are not used a second time
line 104: what is the horizontal dimension of the observational volume?
line 111: do the "different ranges in the … data used for the climatologies" refer to the range of the color bar in the plot?
line 130: what is meant by the "complete" 19-year time series?
line 134: what was the result of this study? How did the MLT respond to the change in the index?
line 140: what is meant by "the complete time series from the selected Ap index"? Isn't it rather data from all days with Ap index values above or below the respective threshold?
line 154: "at the mesopause"
line 230: the study contradicting the author's results, was it done at a different longitude?
line 262: what is a "missing solar cycle"? Do you mean conditions of solar minimum?
line 295: please add year range
line 300: "three studied months" this could me misinterpreted in two ways. First, the dataset is larger than three months, and second, the decline does not occur over the course of three months.

Citation: https://doi.org/10.5194/egusphere-2023-1465-RC1
- AC1: 'Reply on RC1', Juliana Jaen, 11 Sep 2023
  
  Thank you for the comments. The detailed answer is attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1465-AC1
RC2:
'Comment on egusphere-2023-1465', Anonymous Referee #1, 21 Aug 2023

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1465/egusphere-2023-1465-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2023-1465-RC2
- AC2: 'Reply on RC2', Juliana Jaen, 11 Sep 2023
  
  Thank you for the comments. The detailed answer is attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1465-AC2
RC3: 'Comment on egusphere-2023-1465', Anonymous Referee #2, 14 Sep 2023

see comment from 8 Aug, I just resubmit because some website error occurred previously

Citation: https://doi.org/10.5194/egusphere-2023-1465-RC3

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2023-1465', Anonymous Referee #1, 08 Aug 2023

In this work, the authors analyze radar wind datasets from two sites at different latitudes, infer summer wind maxima in wind components and derive trends. They find a robust strengthening of the mesospheric westward wind at mid latitudes. The influence of daily varying geomagnetic activity on the winds is studied and excluded as source for significant trends. Gravity waves are suggested as cause of the trends.
The paper is well structured and the figures are clear. In few places there are descriptions that can be confusing, mainly when the authors speak of "maximum velocity amplitudes" when actually absolute wind values are meant, which is likely a language problem. I notice that title is a bit unspecific, being almost identical to the special issue title.
The authors list a number of similar observational studies, sometimes based on the same datasets, that sometimes come to contradicting results regarding the trends. This is attributed to different years or altitude ranges. If trends depend so sensitively on the selection of years or altitude ranges, this should be investigated in more detail to be of scientific value. For example, if altitude of wind maxima change with time, this could have been documented.
Another possible source of variability, longitude, is not mentioned at all. The authors generally refer to "middle and high latitudes" and "zonal mean winds" and therefore make the impression that their results are valid for all longitudes. This however cannot be inferred from the datasets the authors used, and some discussion of this topic should be included. Whether the obtained results are only valid locally or in the zonal mean is relevant.
No references to model studies were made, and models were mentioned only briefly in one sentence at the end of the manuscript. The work is mainly a report of measurements without detailed exploration of the findings regarding the underlying mechanisms. For example, periodograms with periods of 2-4 and 11 years are presented but discussion with references to QBO and ENSO remain very vague. The same applies to gravity waves being the suggested cause of the trends, but no proposals or attempts were made at how this could be tested or verified. If this is out of scope, but similar studies are available, that sometimes agree, sometimes contradict, the value of presenting measurements only is limited.
I noticed a mismatch in the presented data between Fig. 1d that shows meridional wind data above 70 km only (where the wind is southward) and Fig. 4c and 4d where, I think, this same data was partitioned regarding Ap and that shows meridional wind data also below 70 km altitude, and the wind is suddenly northward.
I also wonder why a larger Ap threshold of 20 is used for mid latitudes than for high latitudes. The authors refer to different geomagnetic latitudes of the radar sites, but I don't see why this is an argument. I would appreciate more substantial explanations and discussions, e.g. of the mechanisms of how wind maxima are a "proxy for MLT dynamics", or how geomagnetic activity affects wind maxima. In Fig. 4b, is the enhancement in 2020 related to any major solar event? Do other studies exist as it seems to be a major effect?
Regarding the spectral analysis I wonder why a generalized Lomb-Scargle analysis is preferred over a Fourier transform. Isn't the data evenly spaced? The authors do not mention measurement uncertainties. I guess they are smaller than the variability. In Fig. 3, significance levels could have been added to the plots.
I come back to the author's nomenclature of "the maximum velocity amplitude" as their proxy for MLT dynamics which confused me. I expected an "amplitude" to relate to an oscillation, for example a tide or a gravity wave. The meaning of "maximum" was unclear, it could have related to a period of time, or altitude, or a peak-to-peak amplitude.. ? In l. 79 the authors write "the maximum velocity amplitude of the horizontal winds, independent of altitude, variability, and trends..", and I wondered how this value could be independent of altitude, variability and trends. Then, in l. 111, "the maximum amplitude of the velocity per month" seemed to indicate some deviation from a monthly mean. And indeed in l. 115 a reference to "monthly median values" was made, but Fig. 2 shows maximum values of wind components, and not amplitudes (that is differences of wind values) of any kind. I suggest to improve the language in these descriptions or add more details, and not use the term "amplitude".
A similar language problem might apply to "zonal mean wind" (l. 196 and others), which implies a global zonal mean, when in fact the authors probably meant "mean zonal wind". Also, the terms "a mid latitude" or "a high latitude" might be more honest than generally speaking of "mid latitudes" or "high latitudes", as only data from sites at one specific longitude was used.
In general, grammar could be improved. Examples are "Below the mesosphere is located the stratosphere..", "linear functions were adjusted to…" instead of "fitted to", "periodograms were extracted…" instead of "calculated", "The zonal component is built with the combination of two datasets ". Some sentences lack subjects, e.g. in l. 91, l. 138, l. 308 or in Fig. 6 caption. Specific comments and questions are listed by line number:
line 4: "mainly focuses on the summer season" what part of the study does not focus on the summer season?
line 6: is there no northward wind? Northward wind is mentioned in l. 16
line 23: 1980s
line 28: regarding the broad subject of greenhouse gas monitoring, the authors only cite the works of two authors from the author's institute
line 91: FMCW is not defined. In l. 103, abbreviations are defined that are not used a second time
line 104: what is the horizontal dimension of the observational volume?
line 111: do the "different ranges in the … data used for the climatologies" refer to the range of the color bar in the plot?
line 130: what is meant by the "complete" 19-year time series?
line 134: what was the result of this study? How did the MLT respond to the change in the index?
line 140: what is meant by "the complete time series from the selected Ap index"? Isn't it rather data from all days with Ap index values above or below the respective threshold?
line 154: "at the mesopause"
line 230: the study contradicting the author's results, was it done at a different longitude?
line 262: what is a "missing solar cycle"? Do you mean conditions of solar minimum?
line 295: please add year range
line 300: "three studied months" this could me misinterpreted in two ways. First, the dataset is larger than three months, and second, the decline does not occur over the course of three months.

Citation: https://doi.org/10.5194/egusphere-2023-1465-RC1
- AC1: 'Reply on RC1', Juliana Jaen, 11 Sep 2023
  
  Thank you for the comments. The detailed answer is attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1465-AC1
RC2:
'Comment on egusphere-2023-1465', Anonymous Referee #1, 21 Aug 2023

The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1465/egusphere-2023-1465-RC2-supplement.pdf

Citation: https://doi.org/10.5194/egusphere-2023-1465-RC2
- AC2: 'Reply on RC2', Juliana Jaen, 11 Sep 2023
  
  Thank you for the comments. The detailed answer is attached.
  
  Citation: https://doi.org/10.5194/egusphere-2023-1465-AC2
RC3: 'Comment on egusphere-2023-1465', Anonymous Referee #2, 14 Sep 2023

see comment from 8 Aug, I just resubmit because some website error occurred previously

Citation: https://doi.org/10.5194/egusphere-2023-1465-RC3

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Juliana Jaen on behalf of the Authors (05 Oct 2023) Author's response Author's tracked changes Manuscript

ED: Publish as is (18 Oct 2023) by John Plane

AR by Juliana Jaen on behalf of the Authors (19 Oct 2023) Manuscript

Journal article(s) based on this preprint

01 Dec 2023

Long-term studies of the summer wind in the mesosphere and lower thermosphere at middle and high latitudes over Europe

Juliana Jaen, Toralf Renkwitz, Huixin Liu, Christoph Jacobi, Robin Wing, Aleš Kuchař, Masaki Tsutsumi, Njål Gulbrandsen, and Jorge L. Chau

Atmos. Chem. Phys., 23, 14871–14887, https://doi.org/10.5194/acp-23-14871-2023,https://doi.org/10.5194/acp-23-14871-2023, 2023

Short summary

Juliana Jaen, Toralf Renkwitz, Huixin Liu, Christoph Jacobi, Robin Wing, Aleš Kuchař, Masaki Tsutsumi, Njål Gulbrandsen, and Jorge L. Chau

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Investigation of winds is important to understand atmospheric dynamics. In the summer mesosphere and lower thermosphere, there are three main wind flows: the mesospheric westward, the mesopause southward (equatorward), and the lower thermospheric eastward wind. Combining almost two decades of measurements from different radars, we study the trend, their inter-annual oscillations, and the effects of the geomagnetic activity over these wind maxima.


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