The manuscript presents unique in situ observations of a convective overshooting event into the lower stratosphere during a mid-latitude cold-air outbreak (TPEx, June 2024, Sweden). The combined microphysical cloud spectrometer and trace gas measurements document the transport of ice particles and tropospheric water vapor up to ~1.5 km above a low tropopause (~9 km), together with pronounced ozone structure and signatures of gravity-wave-driven mixing. These data fill an important gap in the current literature, which has emphasized (sub)tropical overshooting, and they provide valuable constraints on short-lived stratospheric hydration following overshoot events. I find the study interesting, the measurements novel, and—after addressing the points below—suitable for publication. Brief remarks follow.

37: defined by potential temperatures (Θ) greater than 380 K, a threshold chosen because the 380 K is the lowest isentropic surface lying entirely above the tropopause globally, throughout all seasons, thereby marking the lower boundary of the permanently stratospheric air mass.

110: Typo; “The…”

112: “…  arising from …”

114: Either explain what the modification was, and with respect to what former setup, or simply state that there is an offset.

175: “…masses for the season…”

199: Before what?

230: The explanation suggesting that the constant water vapor mixing ratio and RH ≈ 120% result from sublimation of small ice particles may need reconsideration. At such levels of supersaturation, even small ice particles are generally expected to grow rather than sublimate, despite the influence of the Kelvin effect. It might be helpful to clarify under which specific conditions sublimation would still be expected at RH > 100%, or to explore alternative explanations for the observed features. Rather than sublimation, the apparent loss of small ice particles under RH≈ 120% could be attributed to preferential growth of larger particles due to a Wegener–Bergeron–Findeisen like process, or to instrumental limitations in detecting the smallest size classes. It may be helpful for the authors to clarify whether such factors have been considered as alternative explanations. While the classical Wegener–Bergeron–Findeisen process involves vapor transfer from liquid to ice, a similar size-selective growth mechanism may occur among ice particles of different sizes in a supersaturated environment. In such conditions, larger crystals grow faster due to reduced surface curvature effects, while smaller particles may grow more slowly or become depleted through diffusional competition. Clarifying this distinction might help improve the interpretation of the observed changes in the ice PSD.

236: The explanation invoking diabatic cooling due to ice sublimation is physically sound, but it would be helpful if the authors could quantify the observed temperature decrease in the overshooting filament. Given the latent heat involved, even modest sublimation can cause cooling on the order of 1–3 K depending on the local ice water content. Including this information would help support the proposed interpretation.

247-251: The observed correlation between trace gas fluctuations and potential temperature is interesting, especially in a context where wave breaking is invoked. Given that wave-induced irreversible mixing tends to reduce such correlations, it would be helpful if the authors could clarify here whether the observed structure reflects an early stage of breaking with incomplete mixing, or a coherent transport process preceding the mixing. This also in view of the discussion that follows which strengthen the interpretation of mixing.

352: The microphysical simulations suggest that ice crystals sublimate within 3 minutes after full entrainment in the LS. However, given the observational evidence of ice particles persisting under subsaturated conditions, it would be helpful if the authors could clarify whether such short sublimation times are consistent with the size range of the observed particles and the inferred degree of subsaturation. Additionally, are the authors confident that the observed particles must have been injected so shortly before detection? Further discussion on the timing and plausibility of such recent injection would be of interest.

This manuscript provides a solid case study of an overshooting convection event observed within a low tropopause (~9 km) environment, which has not been comprehensively sampled. The authors make use of an impressively broad toolset (airborne in situ observations, ERA5 reanalysis, trajectory modeling, etc.) to evaluate the event and convincingly demonstrate upper troposphere lower stratosphere (UTLS) composition change driven by overshooting convection. The study is mostly well-constructed and executed, with some clarifications needed in a few places. My assessment is that minor revisions are required for it to be accepted. 

General Comments

1. There is some inconsistent use of defined acronyms throughout. In several cases, text alternates between using full text or previously defined acronyms. I recommend acronyms be used uniformly after definition. 

2. There are several instances where parenthetical citations are provided where in-text citations should be used instead. Please carefully review citations and correct as needed. Some examples can be found at lines 91 & 222. 

3. Figure labeling needs some improvement. Pressure axes in Figs. 3b, 4b, & 4g list no values, but they should. Not all colors utilized in Figs. 5b-d are defined - namely, what is the difference between light and dark blue? Also, it would be helpful in many instances if illustrations were added to tie into the discussion more. I found it hard to follow interpretations/arguments in some places. 

4. Were vertical velocities measured aboard the aircraft? If so, those would be a valuable addition to the analysis, particularly to support the arguments with respect to the role of gravity wave breaking. If not, it should be acknowledged somewhere that such measurements do not exist.

5. There is a third enhancement in H₂O (and attendant changes in other gases) at higher altitudes near 13:15 UTC which is not addressed in the manuscript. It should be acknowledged and an explanation of its possible (or known) source provided as it is clearly evident in Figure 3. 

Specific Comments

There are a few places where the H₂O acronym has an italicized "O". This should be fixed.

The "Homeyer & Bowman (2014)" reference is incorrect. It is missing many authors (should be Homeyer et al. 2014). See http://dx.doi.org/10.1002/2014JD021485. 

Line 41: there is another limitation of the Ueyama et al. 2023 & Dauhut and Hohenegger (2022) studies not mentioned here - they primarily capture convection in the tropics, missing much (or nearly all) of the midlatitude overshooting. 

Line 48: unnecessary line break; merge with former paragraph.

Lines 71-73: this sentence is unnecessary. 

Line 107: revise "Additionally" to "In addition" 

Line 110: "he" should be "The"

Line 112: "arose" should be "resulting", and "temperature" should be "temperatures"

Line 128: "NASA'S" should be "NASA's"

Lines 133-134: both instances of "in XX hPa" should be "at XX hPa"

Line 141-142: This sentence can be more simply stated as "In this study, hourly ERA5 data are used at a longitude-latitude resolution of 0.25º."

Line 144: please specify the method of interpolation. Linear in space and time??

Lines 206-208: this sentence seems like too much of a detour and not very relevant to the focus of the analysis (certainly not beyond all that has already been said). 

Line 221: there are many more studies that can be cited here, especially some of the early modeling studies of Pao Wang and others. 

Lines 265-267: the role of downward transport is also discussed and shown in a more appropriate analog of midlatitude overshooting simulations - https://doi.org/10.1029/2022JD036713. 

Line 357: another worthwhile citation to bolster this argument would be the overshoot trajectory analyses of https://doi.org/10.1029/2021JD034808. 

This paper provides a detailed description of in situ data acquired in the upper troposphere and lower stratosphere (UT/LS) in the vicinity of an overshooting convective event at high latitudes in Boreal spring during the TropoPause composition gradients and mixing Experiment (TPEx)  campaign. The in situ data, measured by instrumentation aboard the Learjet research aircraft, include trace gases (e.g., Water Vapor, Ozone, and Nitrous Oxide) and a full spectrum of aerosol and ice size distributions. The data show clear evidence for overshooting convection transporting water, predominantly as ice, into the LS. In contrast, there is little evidence for the convective transport of Ozone and Nitrous Oxide from the troposphere to the stratosphere. Instead, these passive trace species show that older stratospheric air from higher altitudes has been brought downward into the LS following the collapse of the overshoot. The authors also show water vapor data from the MLS instrument on NASA's Aura satellite, output from the ERA5 reanalysis, and model results from a high-resolution convective simulation (ICON) to provide more context for the in situ data. Finally, forward trajectory calculations suggest that the stratospheric lifetime of the convectively influenced air is relatively short-lived, a few days for air initialized at 320 K, up to a few weeks for air initialized above 345 K. 

This is a straightforward study that provides a solid foundation and motivation for further work examining the impact of extratropical overshooting convection over Northern Europe on lower stratospheric water. A couple of follow-on research directions are briefly mentioned in the summary and conclusions. The paper constitutes an important contribution to the field, especially as it addresses a topic that has received very little attention to date. I recommend publication after some minor revisions. Please see the uploaded "marked-up" .pdf for more detailed suggestions, questions, and comments.