Turbulent mixing at the polar jet and transport of cirrus particles into the lowermost extratropical stratosphere

Emig, Nicolas; Miltenberger, Annette; Bozem, Heiko; Krämer, Martina; Afchine, Armin; Kunkel, Daniel; Lachnitt, Hans-Christoph; Li, Yun; Reutter, Philipp; Rolf, Christian; Spelten, Nicole; Tost, Holger; Hoor, Peter

doi:10.5194/egusphere-2026-4614

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https://doi.org/10.5194/egusphere-2026-4614

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15 Sep 2026

| 15 Sep 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Turbulent mixing at the polar jet and transport of cirrus particles into the lowermost extratropical stratosphere

Nicolas Emig, Annette Miltenberger, Heiko Bozem, Martina Krämer, Armin Afchine, Daniel Kunkel, Hans-Christoph Lachnitt, Yun Li, Philipp Reutter, Christian Rolf, Nicole Spelten, Holger Tost, and Peter Hoor

Abstract. The composition of the extratropical transition layer (ExTL), in particular the mixing ratios of ozone and water vapour, has a high impact on the radiative budget of the atmosphere. The ExTL is a transition region above the extratropical tropopause, characterized by smooth vertical gradients between typical tropospheric and stratospheric properties. It is shaped by several transport pathways, including quasi-isentropic mixing at the subtropical jet, diabatic descent from the stratosphere as part of the overturning circulation, and diabatic cross-tropopause exchange. The latter pathway is partially suppressed by the strong static stability above the tropopause, so that diabatic processes are necessary to facilitate mixing between the troposphere and the ExTL.

Here we present in situ measurements taken during the TPEx campaign in summer 2024 over the North Sea, that provide evidence for transport of ice particles through turbulent mixing across the tropopause caused by strong wind shears above the jet stream. The measurements are complemented by Lagrangian analyses of ICON model simulations for atmospheric context and history. High resolution acceleration measurements indicate turbulence while Lagrangian analysis shows suitable conditions for the turbulent mixing one hour prior to the measurements. Trace gas correlations and ice particle measurements confirm cross tropopause mixing through simultaneous occurrence of a stratospheric chemical signature and ice particles in subsaturation. With this combined analysis of measurements and simulations, we demonstrate that turbulent mixing is a potential pathway across the extratropical tropopause and that ice particles mixed into the stratosphere by this process possibly constitute a source of stratospheric water.

Received: 31 Jul 2026 – Discussion started: 15 Sep 2026

Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

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'Comment on egusphere-2026-4614', Anonymous Referee #1, 29 Sep 2026 reply
Review of: “Turbulent mixing at the polar jet and transport of cirrus particles into the lowermost extratropical stratosphere”

By: Nicolas Emig, Annette K. Miltenberger, Heiko Bozem, Martina Krämer, Armin Afchine, Daniel Kunkel, Hans-Christoph Lachnitt, Yun Li, Philipp Reutter, Christian Rolf, Nicole Spelten, Holger Tost, and Peter M. Hoor

Recommendation: Reconsidered after major revisions

General comments:

The manuscript reports a case encountered during the TPEx campaign in 2024, which demonstrated ice particle transported from the upper troposphere to the lowermost stratosphere by turbulence associated with the mid-latitude jet. In addition to trace gas in situ measurements that display a turbulent mixing signature, the study also incorporates model simulation and trajectory analysis to reveal the presence of conditions favourable to turbulence occurrence shortly before, supporting that turbulent mixing was the source of the measured ice particles in the lowermost stratosphere.

While the methodology looks rigorous, the presentation quality, including both the text and figures, should be improved before publication. For instance, the objective or major research questions of the study should be introduced more clearly, allowing readers to recognise the novelty and significance of the study. Also, the description of the figures and the presentation of the evidence can be considerably improved to highlight the major contribution of this study (further details below). Therefore, I recommend a major revision of the manuscript.

Major comments:

To begin with, I think the case study is interesting as in situ measurements reveal concurrent turbulent conditions and the presence of cirrus particles, suggesting turbulent mixing as a pathway to bring moisture into the lowermost stratosphere. The study also demonstrates that ice particles in subsaturated conditions can be considered as a short-lived tracer. The case is also analysed in detail with different measurements and model data presented.

My major concern is the presentation quality of the manuscript. While the measurements, model simulation, and analysis performed seems to me rigid, the current manuscript cannot clearly show the scientific significance of the study and has some essential information for the study being unclear. For instance, the introduction is rich in information, but the motivation or the objective of the study is not highlighted there. Some details of the method, or information about the flight are scattered in the text or missing, making readers difficult to understand the figures or results (will be listed in “Specific comments” below). I therefore recommend a major revision to improve these aspects.

I would also like to ask for more clarification on the proxy variable used to approximate TKE. While the proxy variable is introduced in Sect. 2.2, more details should be included to show and justify the choices. For example, the variability is only mentioned as the rolling mean standard deviation, without stating the averaging period. Also, it is mentioned that it “yields surprisingly sound results for short, potentially turbulent sections of the flight” in lines 165-166, but no justification is shown. I think a bit more details on this TKE-proxy, even with some mathematical derivation on how it correlates with 1D-TKE, would be very helpful (can be in the Appendix).

Specific comments:

In the introduction, there is a very long paragraph (lines 21-76). I suggest the authors separate it into paragraphs for readers to follow.

More importantly, this paragraph does not introduce clearly the research need and/or the objective of the study. Lines 21-56 describe the knowledge on stratosphere-troposphere exchange (STE), and lines 57-76 introduce the distribution of ice particles or cirrus near the tropopause. While the relevant background is introduced, it is unclear what is missing in previous studies (such that the current study can contribute) or what motivates the current study. I therefore recommend the authors restructuring the introduction section. A well-structured introduction can highlight the contribution of the study and allow readers to recognise the importance of the study. For example, a short paragraph describing the missing pieces in previous studies would help readers to understand the novelty of the case presented in the manuscripts. Also, an explicit list of research questions may be considered to highlight the objective of the study.

In Sect. 2, the technical details and the description of the derived quantities are a bit overwhelming. I recommend the authors using tables to associate the measurement device and the quantities they measured, so that readers can better understand. (Or even which instruments are on Learjet, which are on TPC-TOSS)

Line 133: What is “cloudindex”? Do you mean particles with diameters > 3 micrometres are counted as cloud particles?

Lines 159-166: As mentioned in the major comment, more clarification and justification on the proxy variable would be helpful.

In Sect. 3, I find Fig. 2 fairly complicated, and the variables shown are not necessarily relevant for subsequent analysis. For instance, the equivalent potential temperature at 850 hPa is plotted but it is only used to demonstrate the low baroclinicity (lines 198-199). The surface pressure and geopotential height at 500 hPa are also slightly redundant. In contrast, the upper-level wind speed (indication of the upper-level jet) is not shown, and readers can only refer to Fig. 3 for the vertical wind shear (which is only an indirect indicator for the jet). I therefore suggest reducing the number of variables shown in Fig. 2 and only retaining those that are necessary for this case study.

More importantly, the flight track is not shown in Fig. 2 but already discussed in line 193 (also line 201). Readers can only refer to Fig. 3 for the flight track, but it shows a different domain. I suggest also including the flight track in Fig. 2.

Line 191: I think only using the northern edge of the 500 hPa geopotential height ridge as the indicator for the jet stream is not appropriate, especially when the jet core is shown later to be at 10.5 km altitude. I think directly showing wind speed at, e.g. 250 hPa, would be much more useful.

Lines 192-194: Is the statement “air masses originating as far south as Gibraltar…” confirmed through backward trajectory calculation?

Line 201: The “near-jet cirrus cloud” is difficult to locate, especially without any direct reference for the jet (wind speed) and the measurement area (flight path).

Fig. 3 caption: Could you include the vertical wind shear contour values?

Fig. 2, 3, and 4: While the flight F10 went from 07:00 to 10:00 UTC, can you explain why Fig. 2 is at 07 UTC, Fig. 3 at 09 UTC and Fig. 4 at 08 UTC? I believe the conclusion would not change drastically, but a consistent time can help the comparison as suggested in lines 205-207.

Fig. 4 and 7, A1, A2: Is there a reason for the two different kinds of vertical cross sections (or curtains)? I think a consistent choice would be helpful for readers to compare across figures.

Line 207: The stratospheric chemical signature is not stated before. I suggest delaying the discussion to Sect. 4 or including more explanation here.

Sect. 3: In general, I find the information about flight F10 important for the interpretation of the results are scattered in various later sections. I think a more precise and detailed description of the flight in Sect. 3 would be helpful. For instance, would it be possible to have a time series of the altitude during the flight (similar to that shown in Fig. 5 (top), but for altitude) together with a text description? It helps informing the readers on the flight track and provide the context for the “FL340” and “FL350” marked in the vertical cross section shown in Fig. 4.

Line 234: Probably some readers would appreciate an explanation that 2-PVU isosurface is used to indicate the dynamical tropopause.

Fig. 7: Which time is shown in Fig. 7? May I ask why is this northeast-southwest curtain chosen (instead of the north-south one shown in Fig. 4)?

Line 269: I would suggest stating the time period for the leg at FL350. It helps reader to better understand Fig. 8-10.

Lines 282-284: I suggest being more specific in the description. “Regions before and after potential local turbulence” are described to have small differences in ozone mixing ratios between the two platforms, but the period 08:46-08:48 seems to show a significant difference. Maybe “before 08:45 and after 08:49” fits better for this description?

Fig. 9: I think the description of how the adjusted power spectra are constructed in panel d is missing.

Lines 322-323: The sentence “On the other hand…already partly evaporated in subsaturated conditions.” has an unnatural transition to the following discussion. Please consider rephrasing or restructuring this.

Fig. 10: The description of Fig. 10 is currently insufficient for readers to understand the method. Could you explain how the curtains are obtained? It seems not a pure spatio-temporal interpolation from ICON data, but with backward trajectories. How exactly are the trajectories used to construct this curtain plot (e.g. how to get a vertical profile from trajectories launching at the flight path on a single flight level)? Also, the ICON data is hourly, how are the vertical profile obtained at each time step to match the measured time series? More explanation is needed.

Lines 327-330: Probably related to the confusion in understanding Fig. 10, the first sentence looks redundant (enhancement of TKE are co-located to enhanced TKE at the time of measurements). Could you clarify the message here?

Lines 330-332: What does it mean “following the backward trajectories” in Fig. 10c,d?

Lines 333-335: Please consider rephrasing or adding punctuations to this long sentence to improve readability.

I would like to clarify the interpretation of the 1-hour shift in ICON: Fig. 9 showed turbulent periods at 08:45 and 08:49 UTC at FL350 with concurrent ice particles measured, so turbulence appears to be active at the measurement time. In Fig. 10, the ICON model shows that the TKE peaks match better with the data 1 hour earlier. Is this 1-hour shift interpreted as a model bias (i.e. turbulence forecast earlier than in reality) or an indication for the advection of a previously turbulently mixed air parcel to the measurement (as stated in lines 337-339)?

In my view, the current presentation in Sect. 5 does not highlight the additional value of the trajectory analysis performed with the ICON model. The major evidence of turbulence occurrence appears to be the vertical acceleration variations recorded by TPC-TOSS (which also indicates turbulent conditions during measurement?). Could you further explain how the ICON TKE or cloud ice water content shown in Fig. 10 brings new evidence of turbulence as the underlying process leading to the observed cirrus particles in the stratosphere? I suggest the authors clearly stating the aim of using backward trajectories, and what questions are being addressed.

A general question: In Fig. 7, there seems to be STT indicated at FL320 by the presence of low N2O, stratospheric air on the tropospheric side of the tropopause. Is this also an interesting leg to investigate? For instance, does RHi drop when N2O mixing ratio drop along that section of the flight? It probably lies outside the scope of the current study, and the question is purely out of curiosity.

Technical corrections:

Please check the consistency in introducing acronyms. Sometimes the full name is first stated with the acronyms in bracket, but sometimes the opposite (e.g. line 21 for ExTL and line 37 for TST).

The dates are also stated in different conventions (20.06.2024; 20th June 2024, etc.)

Both “cirrus particles” and “ice particles” are used in the manuscript. Do they carry the same meaning? To me, they seem to be used interchangeably, and I find it sometimes confusing when “ice particles” is used in figure legends/labels, but “cirrus particles” in the corresponding text description.

Line 61: “Theta>360K” is probably a typo from the Greek letter “θ”.

“FISH” is referred to at line 129, but only introduced at line 135. Consider adding “(see later)” or other methods.

Line 178: JGU not introduced (or is this needed here?)

Line 211: repeated “(bottom), bottom panel)”

Fig. 6 caption: labels (a,b,c) are plotted, but referred to as “Left”, “Middle”, and “Right”. Also, the title of panel b seems misleading, as green dots do not contain ice particles. Would it be better to keep only “FL350”? Description “Middle: flight level FL350 with ice particles on FL350…” seems repetitive.

Flight level and the short form “FL” are used inconsistently in the manuscript.

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Citation: https://doi.org/10.5194/egusphere-2026-4614-RC1
RC2: 'Comment on egusphere-2026-4614', Anonymous Referee #2, 10 Oct 2026 reply

The manuscript egusphere-2026-4614 entitled “Turbulent mixing at the polar jet and transport of cirrus particles into the lowermost extratropical stratosphere” explores turbulent mixing due to strong wind shear near the jet stream as a potential transport pathway of ice particles to the stratosphere across the extratropical tropopause. In situ measurements from the TPEx campaign and Lagrangian analyses of ICON model simulations are used to explore this transport. Specifically, ice particle measurements, trace gas correlations, vertical acceleration measurements, and model derived turbulent kinetic energy (TKE) are used to explore potential mixing. The authors find evidence that ice particles were transported into subsaturated stratospheric air at the northern edge of the anticyclonic portion of the polar jet stream, owing to turbulent mixing associated with strong wind shear above the jet core. I found the presence of ice particles above and below the tropopause coinciding with elevated TKE on this flight to be an interesting case study and appreciate the variety of measurements presented. My main comments are bulleted below, and I recommend a decision of Minor Revisions.

Main Comments:

The details about the campaign and analyzed flight currently feel scattered throughout the manuscript, which makes it more challenging to discern the conditions of the flight that made these measurements and insight towards this transport pathway possible. Including a dedicated sub-section towards an overview of the field campaign and specific flight discussed in this manuscript towards the of the beginning of the methods or results would help make the analysis presented more cohesive. There is currently campaign/flight information at the beginning of Section 2.1, Section 3, and Section 4 that would benefit from being combined. There are a few comments on figures in the specific comments section below that support this theme as well.

To provide the reader with a clearer idea of the flight and conditions encountered throughout, I would recommend the authors include a flight curtain figure, either as a panel within one of the main figures or as a supplemental figure. Figure 4 demonstrates the altitude of level legs and spatially where the particles are encountered along the legs. However, when examining particles and the turbulence proxy in timeseries figures (ex: Figs. 5 and 8), it would be advantageous to know the altitude the aircraft was at and whether it was completing a level leg or increasing in altitude with time.

L77: The motivation towards this study should be emphasized more in the Introduction prior to introducing the objective of this study. Additional emphasis on limitations of previous work would convey the value of this study. One example could be the timescale aspect (discussed in L366-376), where previous work incorporated longer lived tracers.

L159-166: Additional details regarding the TKE proxy variable from vertical accelerations would be beneficial. How are portions of the level legs versus increasing in altitude considered during this flight? Would it be possible to quantify or give examples of the long versus short timescales mentioned in this paragraph? For the rolling variability, was a specific time window used?

L168-169: How is this flight section supposedly laminar? How long is this section of the flight compared to the rest of the flight? Additional details would be advantageous.
Specific Comments
Abstract
L10: Please spell out campaign name for TPEx.

L13: Please spell out full name for ICON before using abbreviation.
Introduction
L21-76: This is a long paragraph which can make it more challenging to focus, consider breaking it up into multiple paragraphs to help with pacing for the reader.

L37: Switch “TST” and “Troposphere-to-Stratosphere Transport” so that TST is in the parenthesis.

L41: Introduce abbreviation of “STE” for stratosphere-troposphere exchange here instead of in line 54.

Methods
L110: FISH is introduced here but mentioned in greater detail below in line 135 – consider spelling out FISH here too or adding something along the lines of ‘(discussed in greater detail below)’ so the reader knows to expect additional details about the instrument.

L174: Spell out ICON model.

L177-178: Would this information be better suited in the data availability statement?

Results
L199: It currently states equivalent potential temperature is color coded with opacity, but in the figure it appears to be blue to yellow and reflectance is opacity?

L205: Please clarify Fig. 4 for “in the figure” for extra clarity when comparing to figures 2 and 3.

L206-207: Detail is given about what the turquoise marks represent, and it is implied that the blue marks are ice particles without the stratosphere signature, however stating this explicitly would be beneficial.

L209: What interval were altitudes of the stacked level legs in the flight?

L227-228: Would it be possible to provide a brief (1-2 sentence) description of the method used to construct the stratospheric marker in addition to the citations given?

L256: Clarify that 2 PVU is used as the dynamical tropopause here. Do the authors have confidence in the 2.0 PVU ICON model contour as the most appropriate tropopause definition?

L272: Change to “as can be seen”

L289-2291: Please consider stating examples of the smaller and larger time scales mentioned here.

L309: Consider “than the atmospheric conditions. The pronounced” for flow.

L330-336: Consider rephrasing this sentence, the takeaways based on the mention of the trajectories and ozone was not particularly clear.

Conclusions
L358: Is “this study” referencing the current manuscript or Emig et al. (2025)?

Figures
Figure 2: Consider adding the flight path overlaid here or some sort of annotation indicating where the flight occurs in the domain.

Figure 2: Increase the text size for the latitude and longitude numerical labels as they are currently much smaller than the remaining text which makes them challenging to read.

Figure 2: The yellow contours can be challenging to distinguish from the white contours and additional shading within the figure. Is there a different color that could provide more contrast? I recognize there are multiple colors already being implemented in this figure so the current color choice may be the best contrast available.

Figure 3: Include what the horizontal wind contour values are and how they align with the color scale.

Figure 3: Would it be possible to zoom in the domain more? It is currently challenging to distinguish which portion of the flight domain aligns with the different horizontal wind contours.

Figure 4: Please include endpoint labels here (Ex: 1, 2, 3 or a, b, c) to relate to figure 2 and increase cohesion of the vertical and horizontal components of this flight. Corresponding endpoint labels on the flight track in Figure 3 would be beneficial.

Figure 5: Label panels a and b instead of top and bottom to be consistent with other multi-panel figures.

Figure 6: The figures panels are labelled a, b, and c so those should be used in the caption as well instead of left, middle, and right.

Figure 7: Please clarify what interval and corresponding color the horizontal wind speed contours are.

Figure 8: Label panels a, b, and c and reference as such.

Figures A1 and 2: Please clarify what interval and corresponding color the horizontal wind speed contours are.

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Citation: https://doi.org/10.5194/egusphere-2026-4614-RC2
RC3: 'Comment on egusphere-2026-4614', Anonymous Referee #3, 26 Oct 2026 reply

Review of «Turbulent mixing at the polar jet and transport of cirrus particles into the lowermost extratropical stratosphere»
by N. Emig, A. Miltenberger, H. Bozem, M. Krämer, A. Afchine, D. Kunkel, H.-C. Lachnitt, Y. Li, p. Reutter, C. Rolf, N. Spelten, H. Tost and P. M. Hoor
This manuscript presents evidence for turbulence induced mixing at the tropopause in the vicinity of the jet stream over the North Sea, based on in-situ measurements of ice particles, trace gases and acceleration measurements. The findings are complemented by an ICON simulation including a Lagrangian analysis of the history of the involved air masses. Ice crystals are found in ice subsaturated regions with stratospheric properties, when characterised by trace gas concentrations. The authors therefore conclude that turbulent mixing of ice particles across the tropopause in the proximity of a jet and the associated regions with high wind shear, is a possible source of stratospheric water vapour.
The topic of this manuscript is very relevant and well within the scope of this journal. The combination of in-situ measurements and ICON simulation is a strength of this manuscript. However, the manuscript is not yet of sufficient quality. In particular, the text and figures should be improved to enhance readability and strengthen the overall message of the manuscript. I therefore recommend major revisions before publication.
General remarks:
As briefly mentioned above, the manuscript includes many interesting and important results. However, the text is sometimes not well structured, such that it is very difficult to follow. Furthermore, the selection of figures and/or the variables that are shown can be improved in my point of view to better support your interpretation and findings. I would therefore highly recommend restructuring certain parts of the manuscript.
1) Introduction
The introduction contains a lot of important information. However, it lacks a clear storyline which at the end of the introduction leads the reader to the open research questions. Even though many important aspects are discussed, the order in which they are explained seems not very well thought and connected to each other. Furthermore, it would be very helpful for the reader if the introduction would very clearly formulate the open research questions and objectives of this manuscript, which could then even be taken up again in the summary and conclusion section. Therefore, I would highly recommend revising the Introduction, with a special focus on coherency and the storyline, and the formulation of clear questions that will be addressed.
2) Methods
In the Methods section, many important information is given. However, for a reader who is not familiar with all the measurement devices, this can be overwhelming. I would tailor the information better towards what exactly is needed for the interpretation of your results later on. E.g. I would start the Chapter with an introductory paragraph, explaining the readers which variables have been measured, and how they will be used in the following analysis. Only then I would go into more detail. A table listing all instruments and variables that have been measured, including the uncertainty would also be very helpful as it would provide overview over the many information.
3) Chapter 3 – TPEx F10
I think that the plots to describe the overall synoptic situation and to place the flight path into context should be improved. Here are some suggestions which you might consider.
Figure 2: PV (or the 2 pvu iso-line) either on an isentrope (315, 320 K) or pressure surface (300 hPa, 250 hPa), which would indicate the position of the tropopause. Isotachs, showing the position of the jet (and indirectly the spacing of the isotachs would give information about wind shear), sea level pressure and the Meteosat satellite image. Maybe you could then remove figure 3.
4) Figures 4, 7 and A1, A2 and chapter 4
It is not clear to me how you have chosen which plot goes to the appendix and which one to the main text. I think that the 2 pvu line (like in Fig. 7 and A1) gives important context and helps to understand if the flight path is in the troposphere or stratosphere (even if it is from an ICON simulation). So, I would very fundamentally think about which variables are most important to show to support your text and revise these figures. Instead of shifting one to the appendix, you could also do a Figure 4a, b (and c) and show all relevant variables. This would avoid that the reader needs to jump back and forth between the figures.
In general, chapter 4 is difficult to read, and I partly got lost because there is no clear storyline and paragraphs are not well connected to each other. I would like to suggest to re-organise the chapter by thinking about what is the exact message of this chapter and which figures and variables are needed to support your findings.
5) Chapter 5, calculation of trajectories
I think that the calculation of trajectories is very valuable in this context. However, I do not fully understand what you did and what is shown in Fig. 10. What does “Curtain based on position of FL350 backward trajectories” mean? Do the plots show the curtain along the averaged backward trajectories that have been started along the flight path of FL350? Or is it a curtain along one single trajectory? Or a curtain along the flight path? Please clarify.
Minor comments:
L 110: FISH has not been introduced. Maybe shift paragraph that explains the FISH measurements (L. 136 – 143) to ~line 110.
L178: JGU has not been introduced. But I think it is also not important here.
Figure 3, caption: Contours of vertical shear do not have units.
L210: ice particles are found in the southern part of the flight track. However, on the lower flight altitudes, there are also ice particles measured in the northern part.
L216, Fig.5: You say that only wo single particle events are detected. I have difficulties to see that in figure 5. Can you explain in more detail?
L219-223: This paragraph is not well connected to the previous one. You might want to take this into account when you restructure chapter 4.
L246: …. simultaneous measurements of cirrus particles….what exactly do you mean here?
L250: …..is short compared to the life time…..
L293 – 296: these sentences are repetitive. I would simplify.
L297: You say that the amplitude of the acceleration variation is more pronounced for the TPC-TOSS. I don’t think that this is clear from figure 8, bottom. Please clarify what you mean here or where this can be seen.
L322-336: This paragraph is difficult to understand. Can you rewrite it? See also comment related to the calculation of trajectories and Figure 10 above.
L346: …directly in in situ …. --> in in-situ

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Citation: https://doi.org/10.5194/egusphere-2026-4614-RC3

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Short summary

In this study we show mixing of ice particles across the extratropical tropopause via turbulence in the jet shear zone based on in situ measurements from the TPEx campaign over the North Sea, combined with Lagrangian model analysis. Trace gas correlations and subsaturated ice particles constrain the timing and mechanism, while backward trajectories indicate suitable turbulent conditions beforehand. This process possibly impacts the water vapour budget of the extratropical lowermost stratosphere.


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