11 Nov 2022
11 Nov 2022
Status: this preprint is open for discussion.

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 Thomas Wagenhäuser et al.
  • Institute for Atmospheric and Environmental Sciences, Goethe-University of Frankfurt, Frankfurt, Germany

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.

Thomas Wagenhäuser et al.

Status: open (until 29 Dec 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2022-1197', Anonymous Referee #1, 05 Dec 2022 reply

Thomas Wagenhäuser et al.

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.

Model code and software

exTR-TR-method Python code Wagenhäuser, T. and Engel, A.

origin fraction parameterization Python code Wagenhäuser, T.

sf6-timeshifts-from-rigby2010 Python code Wagenhäuser, T.

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.