Mean age from observations in the lowermost stratosphere: an improved method and interhemispheric differences

Wagenhäuser, Thomas; Jesswein, Markus; Keber, Timo; Schuck, Tanja; Engel, Andreas

doi:https://doi.org/10.5194/egusphere-2022-1197

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https://doi.org/10.5194/egusphere-2022-1197

Preprints

11 Nov 2022

| 11 Nov 2022

Mean age from observations in the lowermost stratosphere: an improved method and interhemispheric differences

Thomas Wagenhäuser, Markus Jesswein, Timo Keber, Tanja Schuck, and Andreas Engel

Abstract. Age of stratospheric air is a concept commonly used to evaluate transport timescales in atmospheric models. The mean age can be derived from observations of a single long-lived trace gas species with a known tropospheric trend. Commonly, deriving mean age is based on the assumption that all air enters the stratosphere through the tropical (TR) tropopause. However, in the lowermost stratosphere (LMS) close to the extra-tropical (exTR) tropopause cross tropopause transport needs to be taken into account. We introduce the new exTR-TR method, which considers exTR input into the stratosphere in addition to TR input. We apply the exTR-TR method to in situ SF₆ measurements from three aircraft campaigns (PGS, WISE and SouthTRAC) and compare results to those from the conventional TR-only method. Using the TR-only method, negative mean age values are derived in the LMS close to the tropopause during the WISE campaign in northern hemispheric (NH) fall 2017. Using the new exTR-TR method instead, the number and extent of negative mean age values is reduced. With our new exTR-TR method we are thus able to derive more realistic values of typical transport times in the LMS from in situ SF₆ measurements. Absolute differences between both methods range from 0.3 to 0.4 years among the three campaigns. Interhemispheric differences in mean age are found when comparing seasonally overlapping campaign phases from the PGS and the SouthTRAC campaigns. On average, within the lowest 65 K potential temperature above the tropopause the NH LMS is 0.5 years ± 0.3 years older around March 2016 than the southern hemispheric (SH) LMS around September 2019. The derived differences between results from the exTR-TR method and the TR-only method, as well as interhemispheric differences are higher than the sensitivities of the exTR-TR method to parameter uncertainties, which are estimated to be below 0.22 years for all three campaigns.

Received: 03 Nov 2022 – Discussion started: 11 Nov 2022

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03 Apr 2023

Mean age from observations in the lowermost stratosphere: an improved method and interhemispheric differences

Thomas Wagenhäuser, Markus Jesswein, Timo Keber, Tanja Schuck, and Andreas Engel

Atmos. Chem. Phys., 23, 3887–3903, https://doi.org/10.5194/acp-23-3887-2023,https://doi.org/10.5194/acp-23-3887-2023, 2023

Short summary

Thomas Wagenhäuser et al.

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-1197', Anonymous Referee #1, 05 Dec 2022

This paper makes a significant improvement to the calculation of mean age in the lowermost stratosphere from in situ trace gas measurements by quantifying the effect of extratropical entry into this region. This study builds on previous work that has shown the lowermost stratosphere to be composed of a mixture of air masses from different source regions with a large seasonal dependence. This mixture can have an important impact on mean age estimates, different in each hemisphere, and the interhemispheric contrast in mean ages that are revealed with this method are quite interesting. The techniques and methods used are well described and the uncertainties are clearly shown. The paper is laid out well and easy to read. Really excellent work! I have no significant suggestions for improvement, and only a very few minor comments listed below. I suggest publication in the present form.

Specific comments:

Line 192: add ‘of’ after ‘instead’

Table 2 could go in the supplement.

Line 246: ‘datasets were processed in three steps.’

Line 250: I don’t see this age correction formula in the Leedham Elvidge et al. paper. How was this derived?

Lines 313, 385, 390, 420: ‘extend’ should be ‘extent’

Citation: https://doi.org/10.5194/egusphere-2022-1197-RC1
- AC1: 'Reply on RC1', Thomas Wagenhäuser, 17 Feb 2023
  
  Thank you for your kind words and constructive comments. Attached please find our detailed answer.
  
  Citation: https://doi.org/10.5194/egusphere-2022-1197-AC1
RC2:
'Comment on egusphere-2022-1197', Anonymous Referee #2, 24 Dec 2022

This manuscript reports on a new method to calculate the mean age in the lowermost stratosphere that explicitly takes into account the contributions of air masses that cross the tropopause into the stratosphere via the extratropics (not tropics). Physically, this is well motivated as studies have long shown evidence of extratropical stratosphere-troposphere exchange occurring through isentropic exchange over subtropical and middle latitudes. Despite this, standard methods to calculate the mean age still assume a tropical entry point. To this end, the authors propose a so-called “exTR-TR” method which they apply to aircraft data to obtain new observational constraints on the mean age in the lowermost stratosphere. The method does not contribute much physical understanding, given that it consists mainly of developing an empirical parameterization of origin fractions computed in a previous study and then using these, in combination with measurements, to obtain new observational age estimates. In that respect, this manuscript should be really be considered an extension/follow-up to that work. Nonetheless, the study is overall very clear, well motivated and generally well-written. I have a few minor comments, however, that should be addressed before I can recommend publication.

Minor Comment 1, Line 86: It is not clear to me why G(x,t’) in equation (2) does not depend on the source region x_i. That is, G should also be conditionally dependent on x_i, such that G(x,t’) should be rewritten as G(x,t’|x_i). The authors here have assumed that the transport operators propagating tracer concentrations for all regions i are the same, but I can envision several cases where this would not be true. For example, air propagating into the stratosphere at high latitudes will have no clear path into the stratosphere, as opposed to air straddling the midlatitude tropopause, where isentropic surfaces provide a clear pathway for stratosphere-troposphere exchange. The authors need to provide their rationale here.

Minor Comment 2, Section 2.2.2: I am curious about the calculation of t_xi. The procedure outlined in steps (i)-(iii) essentially sounds like a description of how to calculate the SF6-age, which previous studies have used to calculate the tropospheric mean age (albeit using an SF6 surface boundary condition that only averages stations over northern midlatitudes). The details of the regions considered may be slightly different, but the procedure is basically the same. So why not reference this literature? In particular, the authors should review these studies:

Waugh, Darryn W., A. M. Crotwell, E. J. Dlugokencky, G. S. Dutton, J. W. Elkins, B. D. Hall, E. J. Hintsa et al. "Tropospheric SF6: Age of air from the Northern Hemisphere midlatitude surface." Journal of Geophysical Research: Atmospheres 118, no. 19 (2013): 11-429.

Orbe, Clara, Darryn W. Waugh, Stephen Montzka, Edward J. Dlugokencky, Susan Strahan, Stephen D. Steenrod, Sarah Strode et al. "Tropospheric AgeâofâAir: Influence of SF6 Emissions on Recent Surface Trends and Model Biases." Journal of Geophysical Research: Atmospheres 126, no. 19 (2021): e2021JD035451.

Technical Comments:

Line 83: The concept of “origin fraction” referred to here certainly precedes the Hauck et al. (2020) study and the authors should properly reference the literature. For example, see these studies:

Orbe, Clara, Mark Holzer, Lorenzo M. Polvani, and Darryn Waugh. "Airâmass origin as a diagnostic of tropospheric transport." Journal of Geophysical Research: Atmospheres 118, no. 3 (2013): 1459-1470.

Orbe, Clara, Darryn W. Waugh, and Paul A. Newman. "Airâmass origin in the tropical lower stratosphere: The influence of Asian boundary layer air." Geophysical Research Letters 42, no. 10 (2015): 4240-4248.

Citation: https://doi.org/10.5194/egusphere-2022-1197-RC2
- AC2: 'Reply on RC2', Thomas Wagenhäuser, 17 Feb 2023
  
  Thank you for your kind words and constructive comments. Attached please find our detailed answers.
  
  Citation: https://doi.org/10.5194/egusphere-2022-1197-AC2
RC3:
'Comment on egusphere-2022-1197', Anonymous Referee #3, 26 Dec 2022

Wagenhäuser et al. derive stratospheric mean age of air in the lowermost stratosphere from observational data, using a novel method that considers the addition of extra-tropical input into the stratosphere. This is an important contribution to this field of study as previous work assumes that the tropical tropopause is the only entry point of air into the stratosphere when deriving mean age of air. Overall, the paper is clearly written and easy to read. The approach and techniques are very well described and the results and uncertainties clearly presented.

I have no suggestions for improvement and recommend publication in the present form, with minor comments listed below:

Line 25. … isentrope, and approximates

Line 39. … made contact with

Line 48. making fewer assumptions compared to deriving age spectra.

Line 51. … measurements, an infinite lifetime is commonly assumed.

Line 308. Our findings indicate that… (no comma needed after ‘indicate’)

Lines 313/385/390/420. Change 'extend' to 'extent'

Citation: https://doi.org/10.5194/egusphere-2022-1197-RC3
- AC3: 'Reply on RC3', Thomas Wagenhäuser, 17 Feb 2023
  
  Thank you for your kind words and comments. Attached please find our point-by-point reply.
  
  Citation: https://doi.org/10.5194/egusphere-2022-1197-AC3

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2022-1197', Anonymous Referee #1, 05 Dec 2022

This paper makes a significant improvement to the calculation of mean age in the lowermost stratosphere from in situ trace gas measurements by quantifying the effect of extratropical entry into this region. This study builds on previous work that has shown the lowermost stratosphere to be composed of a mixture of air masses from different source regions with a large seasonal dependence. This mixture can have an important impact on mean age estimates, different in each hemisphere, and the interhemispheric contrast in mean ages that are revealed with this method are quite interesting. The techniques and methods used are well described and the uncertainties are clearly shown. The paper is laid out well and easy to read. Really excellent work! I have no significant suggestions for improvement, and only a very few minor comments listed below. I suggest publication in the present form.

Specific comments:

Line 192: add ‘of’ after ‘instead’

Table 2 could go in the supplement.

Line 246: ‘datasets were processed in three steps.’

Line 250: I don’t see this age correction formula in the Leedham Elvidge et al. paper. How was this derived?

Lines 313, 385, 390, 420: ‘extend’ should be ‘extent’

Citation: https://doi.org/10.5194/egusphere-2022-1197-RC1
- AC1: 'Reply on RC1', Thomas Wagenhäuser, 17 Feb 2023
  
  Thank you for your kind words and constructive comments. Attached please find our detailed answer.
  
  Citation: https://doi.org/10.5194/egusphere-2022-1197-AC1
RC2:
'Comment on egusphere-2022-1197', Anonymous Referee #2, 24 Dec 2022

This manuscript reports on a new method to calculate the mean age in the lowermost stratosphere that explicitly takes into account the contributions of air masses that cross the tropopause into the stratosphere via the extratropics (not tropics). Physically, this is well motivated as studies have long shown evidence of extratropical stratosphere-troposphere exchange occurring through isentropic exchange over subtropical and middle latitudes. Despite this, standard methods to calculate the mean age still assume a tropical entry point. To this end, the authors propose a so-called “exTR-TR” method which they apply to aircraft data to obtain new observational constraints on the mean age in the lowermost stratosphere. The method does not contribute much physical understanding, given that it consists mainly of developing an empirical parameterization of origin fractions computed in a previous study and then using these, in combination with measurements, to obtain new observational age estimates. In that respect, this manuscript should be really be considered an extension/follow-up to that work. Nonetheless, the study is overall very clear, well motivated and generally well-written. I have a few minor comments, however, that should be addressed before I can recommend publication.

Minor Comment 1, Line 86: It is not clear to me why G(x,t’) in equation (2) does not depend on the source region x_i. That is, G should also be conditionally dependent on x_i, such that G(x,t’) should be rewritten as G(x,t’|x_i). The authors here have assumed that the transport operators propagating tracer concentrations for all regions i are the same, but I can envision several cases where this would not be true. For example, air propagating into the stratosphere at high latitudes will have no clear path into the stratosphere, as opposed to air straddling the midlatitude tropopause, where isentropic surfaces provide a clear pathway for stratosphere-troposphere exchange. The authors need to provide their rationale here.

Minor Comment 2, Section 2.2.2: I am curious about the calculation of t_xi. The procedure outlined in steps (i)-(iii) essentially sounds like a description of how to calculate the SF6-age, which previous studies have used to calculate the tropospheric mean age (albeit using an SF6 surface boundary condition that only averages stations over northern midlatitudes). The details of the regions considered may be slightly different, but the procedure is basically the same. So why not reference this literature? In particular, the authors should review these studies:

Waugh, Darryn W., A. M. Crotwell, E. J. Dlugokencky, G. S. Dutton, J. W. Elkins, B. D. Hall, E. J. Hintsa et al. "Tropospheric SF6: Age of air from the Northern Hemisphere midlatitude surface." Journal of Geophysical Research: Atmospheres 118, no. 19 (2013): 11-429.

Orbe, Clara, Darryn W. Waugh, Stephen Montzka, Edward J. Dlugokencky, Susan Strahan, Stephen D. Steenrod, Sarah Strode et al. "Tropospheric AgeâofâAir: Influence of SF6 Emissions on Recent Surface Trends and Model Biases." Journal of Geophysical Research: Atmospheres 126, no. 19 (2021): e2021JD035451.

Technical Comments:

Line 83: The concept of “origin fraction” referred to here certainly precedes the Hauck et al. (2020) study and the authors should properly reference the literature. For example, see these studies:

Orbe, Clara, Mark Holzer, Lorenzo M. Polvani, and Darryn Waugh. "Airâmass origin as a diagnostic of tropospheric transport." Journal of Geophysical Research: Atmospheres 118, no. 3 (2013): 1459-1470.

Orbe, Clara, Darryn W. Waugh, and Paul A. Newman. "Airâmass origin in the tropical lower stratosphere: The influence of Asian boundary layer air." Geophysical Research Letters 42, no. 10 (2015): 4240-4248.

Citation: https://doi.org/10.5194/egusphere-2022-1197-RC2
- AC2: 'Reply on RC2', Thomas Wagenhäuser, 17 Feb 2023
  
  Thank you for your kind words and constructive comments. Attached please find our detailed answers.
  
  Citation: https://doi.org/10.5194/egusphere-2022-1197-AC2
RC3:
'Comment on egusphere-2022-1197', Anonymous Referee #3, 26 Dec 2022

Wagenhäuser et al. derive stratospheric mean age of air in the lowermost stratosphere from observational data, using a novel method that considers the addition of extra-tropical input into the stratosphere. This is an important contribution to this field of study as previous work assumes that the tropical tropopause is the only entry point of air into the stratosphere when deriving mean age of air. Overall, the paper is clearly written and easy to read. The approach and techniques are very well described and the results and uncertainties clearly presented.

I have no suggestions for improvement and recommend publication in the present form, with minor comments listed below:

Line 25. … isentrope, and approximates

Line 39. … made contact with

Line 48. making fewer assumptions compared to deriving age spectra.

Line 51. … measurements, an infinite lifetime is commonly assumed.

Line 308. Our findings indicate that… (no comma needed after ‘indicate’)

Lines 313/385/390/420. Change 'extend' to 'extent'

Citation: https://doi.org/10.5194/egusphere-2022-1197-RC3
- AC3: 'Reply on RC3', Thomas Wagenhäuser, 17 Feb 2023
  
  Thank you for your kind words and comments. Attached please find our point-by-point reply.
  
  Citation: https://doi.org/10.5194/egusphere-2022-1197-AC3

Peer review completion

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

AR by Thomas Wagenhäuser on behalf of the Authors (17 Feb 2023) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (01 Mar 2023) by Rolf Müller

AR by Thomas Wagenhäuser on behalf of the Authors (10 Mar 2023) Manuscript

Journal article(s) based on this preprint

03 Apr 2023

Mean age from observations in the lowermost stratosphere: an improved method and interhemispheric differences

Thomas Wagenhäuser, Markus Jesswein, Timo Keber, Tanja Schuck, and Andreas Engel

Atmos. Chem. Phys., 23, 3887–3903, https://doi.org/10.5194/acp-23-3887-2023,https://doi.org/10.5194/acp-23-3887-2023, 2023

Short summary

Thomas Wagenhäuser et al.

Supplement

https://doi.org/10.5194/egusphere-2022-1197-supplement

Data sets

SF6 and CFC-12 measurements and mean age along HALO flight tracks during PGS, WISE and SouthTRAC Wagenhäuser, T., Jesswein, M., Keber, T., Schuck, T., Engel, A. and Grooß, J.-U. https://doi.org/10.5281/zenodo.7275822

Model code and software

exTR-TR-method Python code Wagenhäuser, T. and Engel, A. https://doi.org/10.5281/zenodo.7267203

origin fraction parameterization Python code Wagenhäuser, T. https://doi.org/10.5281/zenodo.7267114

sf6-timeshifts-from-rigby2010 Python code Wagenhäuser, T. https://doi.org/10.5281/zenodo.7267089

Thomas Wagenhäuser et al.

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Short summary

A common assumption to derive mean age from trace gas observations is that all air enters the stratosphere through the tropical tropopause. Using SF₆ as an age tracer, this leads to negative mean age values close to the northern hemispheric extra tropical tropopause. Our improved method also considers extra tropical input into the stratosphere. More realistic values are derived using this method. Interhemispheric differences in mean age are found when comparing data from two aircraft campaigns.


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