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

Preprints

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

Preprints

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

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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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