This study explores ozone variability in the upper troposphere and lower stratosphere using plenty of coordinates. The results show obvious differences in either ozone concentration its variability while choosing different coordinates. Such results give important hints in detecting changes in ozone on different time scales and in different regions, and support to the OCTAV-UTLS activity. The manuscript is well organized and written in English. I would recommend an acceptance after some minor revisions.

Comments:

1. Abstract: It would be better to summarize the main findings in this manuscript.
2. L60-70: Here, the authors describe the ‘geophysical variability’ and its importance. However, it is not clear for how to distinguish the ‘geophysical variability’ and the true variability. In the analysis of this study, the authors evaluate different coordinates by comparing the relative standard deviation of ozone presented by different coordinates, but did not explain why reduced relative standard deviation is better. I think the relative standard deviation includes both ‘geophysical variability’ and the true variability, what is the scientific meaning of a reduced relative standard deviation?
3. Section 2.2.2: In this section, the authors want to examine the effects of different coordinate systems on the representation of geophysical variability. However, the descriptions to each figure are very simple. I would suggest the authors to describe in more details to help the audience to understand the ‘geophysical variability’.
4. L274: it is evident that
5. L398: the use of tropopause or subtropical jet ‘vertical coordinates’, should be horizontal coordinates? This sentence is confusing, please rewrite it.

Review of (egusphere-2024-144) by Millan et al.

In this paper, the authors present the analysis of several relative coordinate systems to define the transition layer between the troposphere and the stratosphere, dealing with the regimes in the upper troposphere, the tropopause itself, and the lowermost stratosphere. They use ozone concentrations as a key fingerprint.

First of all, I have co-authored some works with some of the authors of this manuscript. However, we have not collaborated over the last few years; therefore, I have not perceived a conflict of interest, and I think I can provide an objective review of this paper. 

Also, I recommend citing several of my works here. I am not trying to impose their citations on the authors. I suggest them because I think they cover gaps in this case and will help create a more balanced, complete, and informative manuscript for the readers. I let the authors and the editor judge on it.

My main comment is that the manuscript would benefit from explaining why this topic is relevant and explaining the potential applications of these coordinates. 

As a general comment, given that equivalent latitude is the best-performing coordinate system here and the resolution is 5 degrees (table 3), I think it is important to note that finner resolutions could improve the result using it. Also, the authors use the known piecewise-constant method to compute the equivalent latitude, which results could be improved up to an additional 5% using a Region of Interest technique (Añel et al. 2013). This could be noted in the Discussion or Summary.

Añel JA, Allen DR, Sáenz G, Gimeno L, de la Torre L (2013) Equivalent Latitude Computation Using Regions of Interest (ROI). PLoS ONE 8(9): e72970. https://doi.org/10.1371/journal.pone.0072970

(https://doi.org/10.1371/journal.pone.0072970). 

Line 24: multiple tropopause and tropopause fold conditions play an essential role, introducing uncertainty on whether the region is under tropospheric or stratospheric conditions. The authors should mention some relevant literature here: Randel et al. 2007, Añel et al. 2008, Wang and Polvani, Añel et al. 2013. 

Randel, W. J., D. J. Seidel, and L. L. Pan (2007), Observational characteristics of double tropopauses, J. Geophys. Res., 112, D07309, doi:10.1029/2006JD007904.

Añel et al. (2008) Climatological features of global multiple tropopause events, J. Geophys. Res., 113, D00B08, doi:10.1029/2007JD009697.

Wang, S., and L. M. Polvani (2011), Double tropopause formation in idealized baroclinic life cycles: The key role of an initial tropopause inversion layer, J. Geophys. Res., 116, D05108, doi:10.1029/2010JD015118.

Añel, J.A., de la Torre, L. and Gimeno, L., 2012. On the origin of the air between multiple tropopauses at midlatitudes. The Scientific World Journal,2012.  https://doi.org/10.1100/2012/191028

Line 25: it would be valuable to add information on the uncertainty by satellite measurements because of vertical resolution

Line 29: I would mention the cold-point tropopause (Gettelman et al., Pan et al. 2018) as it is relevant for water vapour and has been used many times instead of other definitions. Also, I suggest making it more explicit that Potential Vorticity (PV) can be used to distinguish between stratospheric air masses and tropospheric ones and track them, as they have very different values. They could add some examples, e.g. Chen et al. (2013).

A GETTELMAN, P.M. de F FORSTER, A Climatology of the Tropical Tropopause Layer, Journal of the Meteorological Society of Japan. Ser. II, 2002, Volume 80, Issue 4B, Pages 911-924

Pan, L. L., Honomichl, S. B., Bui, T. V., Thornberry, T., Rollins, A., Hintsa, E., & Jensen, E. J. (2018). Lapse rate or cold point: The tropical tropopause identified by in situ trace gas measurements. Geophysical Research Letters, 45, 10,756–10,763. https://doi.org/10.1029/2018GL079573

Chen X, Añel JA, Su Z, de la Torre L, Kelder H, van Peet J, et al. (2013) The Deep Atmospheric Boundary Layer and Its Significance to the Stratosphere and Troposphere Exchange over the Tibetan Plateau. PLoS ONE 8(2): e56909. https://doi.org/10.1371/journal.pone.0056909

Line 60: I think it is important to mention that the coordinates and definitions are also relevant and depend on the different phenomena to study (this probably can be addressed in the paragraph I mentioned before on the applications of this work); in many cases, even more critical than "regional" features.

Fig. 1: The dots and squares to locate the ozosonde and lidar sites are too big to be informative. It would be good to have them in a smaller size.

Line 90: I understand the reasons for it, but it would be good to add a line with the reasons to use Aura-MLS and ACE-FTS: lengthening the time series, measuring principle, etc. For example, I would move the current lines 108-110 here. Also, I have found it quite surprising that Toohey et al. (2013) and Hegglin and Tegtmeier (2017) are not cited in this subsection, as they directly discuss the bias in ozone measurements by the instruments used here, and several of the authors of this manuscript (and myself) are co-authors of both these works.

Characterizing sampling biases in the trace gas climatologies of the SPARC Data Initiative, J. Geophys. Res. Atmos., 118, 11,847–11,862, doi:10.1002/jgrd.50874.

Hegglin and Tegtmeier (2017) https://doi.org/10.3929/ethz-a-010863911.

Line 158: the year for "Smit and Thompson" is missing.

Line 161: It could seem evident that the 50 hPa region is outside the UTLS regime. Therefore, the results should be fine with the mentioned problem with the ozonesondes. However, it would be good to be clear with numbers about the reasons, mentioning that it is because the cases when the tropopause extends up to 50 hPa and above (double and triple tropopause cases reflecting a transition layer yet) are below the 5% for most of the planet and below 20% in only a few regions (which however coincide for example with the ozonesonde for Boulder). This information can be found in Añel et al. (2008)

Line 231: VMR has not been defined before

Line 266. Rather than the studies cited, I would cite the primary studies dealing with the exposition of the multiple tropopause phenomenon (Randel et al. 2007 and Añel et al. 2008). Also, I understand that mentioning intrusions here is a generalization, which is not entirely accurate. MTs in this region are not necessarily associated only with instrusions understood in the sense of vertical movement but also with the latitudinal mix and overlapping of the tropical tropopause over the extratropical one and undergo latitudinal advection. This is mentioned later in line 274, but it should be added here and clarified to avoid misinterpretations.

Line 274: regarding the horizontal mixing, again cite Wang and Polvani (2011) and Añel et et al. (2012).

From Fig. 6, it is clear that the 4.5 PVU value catches better than the 2.0 PVU value in the stratospheric character and does much better in extratropical regions. This is not new at all. Later in the text (in the Discussion), the authors mention that it matches previous findings by Kunz et al. (2011a); however, already a prior work by Hoinka (1998) made clear that values above 3.5 PVU are a better representation of the extratropical tropopause. The result again makes a point against the extended use of the 2 PVU value to define the tropopause, which is clearly an overestimation. This point could be included in the Discussion.

Hoinka, K. P., 1998: Statistics of the Global Tropopause Pressure. Mon. Wea. Rev., 126, 3303–3325, https://doi.org/10.1175/1520-0493(1998)126<3303:SOTGTP>2.0.CO;2.

Lines 282-283: please do not use parenthesis this way. https://eos.org/opinions/parentheses-are-are-not-for-references-and-clarification-saving-space

Both the datasets and code for the analysis should be better deposited in long-term repositories with DOI (e.g., PANGAEA, Zenodo). I know it is not a journal requirement, but it is good practice for the assets that the authors can do with reasonable effort.

This is a well-focused, well-constructed, and well-executed study evaluating the use of various reference coordinates for examining upper troposphere lower stratosphere ozone distributions. Numerous datasets are leveraged to carry out the analysis and the results are consequently very robust. While it is lean on new discoveries, the study is nevertheless a worthwhile precursor to more extensive efforts expected in the future. I have nothing but a handful of minor suggested revisions below. 

The Abstract: some text should be added to the abstract to capture the main conclusions of the work. As it stands now, the abstract is a bit too vague and descriptive of the effort rather than the outcomes. A synthesis of the bulleted items from Section 5 or at the very least the most important elements of them would suffice. 

In several places within Section 3 & 4, the text would benefit from a discussion of the tropopause break, tropopause errors, and some other common features. The WMO tropopause would be impacted most by some of these challenges and it is important to emphasize why such is problematic and why (physically) alternatives such as PV or simply potential temperature would/should/could perform better. 

Line 106: "effective resolution" should be "effective vertical resolution"

Line 114: Add "2008" before the open paren as there was also a START05 that preceded this mission.

Line 184: suggest adding Tian & Homeyer 2019 here too.

Line 331: recommend deleted "sub-" as "categories" seems sufficient

Line 332: "coordinates" should be "coordinate"

Review of « Exploring ozone variability in the upper troposphere and lower stratosphere using dynamical coordinates » by Millan et al., submitted to ACP

This manuscript aims to assess the usefulness of different transport-relevant coordinate systems (altitude, pressure, potential temperature, equivalent latitude, distance to the subtropical jet and distance to the tropopause) for dividing the measurements into bins affected by different atmospheric regimes. Then, the overall objective is to combine measurements from different platforms with different sampling characteristics for assessing the ozone trends and attributing them to changing atmospheric dynamics. This study is definitely an important milestone of the SPARC OCTAV-UTLS activity and follows a previous analysis by Millan et al, AMT 2023 (with almost the same co-authors), which was dedicated to the presentation of such dynamical diagnostics to describe the meteorological context for multi-decadal observations in the UTLS by ozonesondes, lidars, aircraft, and satellite.

The manuscript is well organized and well written. The figures are good and support the analysis. I recommend the publication after addressing some comments and suggestions, in order to improve its impact and make it useful for other data sets.

General comments :

In order to clarify some aspects of the methodology, of the results and therefore increase the impact (the usefulness) of these dynamical coordinates, I propose the following suggestions.

1. The major comment concerns the improvement in providing a further clarification of the objectives and of the results. This manuscript should better address the complementarity and/or the difference with the previous Millan et al., published in AMT in 2023, in the introduction. At the end, the reader misses a clear opinion on the advantages of using these coordinates and a further understanding of the ozone variability in the UTLS, or at least a further discussion on the gain in consistency within the different data sets. It is quite frustrating to read that interesting results will be published in two future studies without giving more information in this one. The last sentence of this manuscript “Another study will analyze how differences in sampling patterns and resolution (both vertical and horizontal) can affect the representation of the datasets as well as the trend quantification” is giving the negative impression that this manuscript is not going far enough to be really useful. It reveals that differences in sampling patterns and resolution are not addressed here. Therefore, the conclusions are somewhat weakened.
2. Regarding the data sets used in this analysis, it is important to further argue about the selection of these data sets. Why are there not the same as in Millan et al., 2023 ? Why is IAGOS-CORE not used here in addition to IAGOS-CARIBIC ? Why is the number of ozonesondes so limited ? Why not use the ones from the SHADOZ network with the advantage of sampling the tropical regions ? My suggestion would be to consider adding, as a result of this analysis, a list of recommendations for using such dynamical coordinates with other datasets. That would be valuable for the scientific community focused on providing ozone data sets and would increase the impact of this study.
3. Regarding the sampling patterns, the manuscript would be improved by adding a discussion on the impact (or not) of the differences in measurements locations. This comment is indeed linked to the one on the selection of the used data sets. MLS and ACE data sets are clearly “global” data sets but all the others are not and cannot be considered “symmetric zonally” like the satellite data sets. The sondes and lidars used in this study are only or mostly located in the “Western Hemisphere”, on contrary of the CARIBIC aircraft data sets which spans a wider range of longitudes. What is the impact of such differences in discussing consistency in terms of zonal averages? Also a brief discussion or a simple pedagogic explanation on the use of zonal averages for presenting these transport-relevant coordinates would be a valuable addition to the manuscript. A few comments in the manuscript mention some characteristics varying with longitudes (e.g., double tropopause, strength and sharpness of the subtropical jet). These differences in the representativeness of the different data sets should be addressed or the differences (if any) between the two hemispheres (western vs eastern) should be discussed using the data sets providing the full range of longitudes. For example, Cohen et al., 2028, showed that the IAGOS-CORE data sets have different levels of ozone between the North America – Atlantic and the Eurasian sectors in the UTLS, when the tropopause is defined as the 2 PVU. Providing zonal averages have clear advantages, but when it comes to reducing and analysing the ozone variabilities, this longitudinal dependence has to be clearly discussed.
4. Regarding the sampling period: What is the rationale to cover the 2005-2018 period while some of the data sets (i.e. aircraft) cover only a few years, and not the same for all (according to Table 1) ? A further discussion on this choice and on the impact (or not) of merging data sets from 2008 with those from 2015-2016 and those apparently equally distributed over the long 2005-2018 period would be valuable to add.
5. Regarding the differences in the vertical resolution: What is the impact (or not) of different vertical resolutions, among the data sets themselves (i.e. from 3 km for MLS to 100 m for the ozonesondes and probably less for aircraft) and with the vertical spacing of theMERRA-2 products (1.2 km) ? I recommend that table 1 includes the information on the vertical resolution and for aircraft, the detailed “Range” instead of flight levels which is not very informative as research aircraft may fly higher or lower than passenger aircraft.

Specific comments:

1. The abstract should better highlight the main findings by adding a few sentences from the Summary section.
2. Line 120 : A more general and recent reference to IAGOS should be added here, or at least the web site, e.g. htpp://www.iagos.org; Petzold, et al., 2015; Thouret et al., 2022.
3. Line 155 : Can you further explain this gridding ? Is such a 100 m gridding to reduce computing power applied to other data sets ? It is quite surprising as the ozone data set is probably not the “heaviest”. In general, it would be nice to have the same types of details in all sections describing the different data sets.
4. Line 158 : year is missing in the reference Smit and Thompson, as well as in the references list, line 607. It has been published in 2021.
5. Lines 164-166 : The question is then “why not using more ozone sondes stations in this analysis ?”
6. Lines 204-206 : Further details are necessary here regarding the vertical resolutions, from the native ones to the gridded ones.
7. Line 220 : “than” should be replaced by “that”.
8. Line 223-224 : Would it be because the range of sampled longitudes with these data sets is restricted to the “western hemisphere” (se also General Comment #3) ?

References:

Cohen, Y., Petetin, H., Thouret, V., Marécal, V., Josse, B., Clark, H., Sauvage, B., Fontaine, A., Athier, G., Blot, R., Boulanger, D., Cousin, J.-M., and Nédélec, P.: Climatology and long-term evolution of ozone and carbon monoxide in the upper troposphere–lower stratosphere (UTLS) at northern midlatitudes, as seen by IAGOS from 1995 to 2013, Atmos. Chem. Phys., 18, 5415–5453, https://doi.org/10.5194/acp-18-5415-2018, 2018.

Petzold, A., Thouret, V., Gerbig, C., Zahn, A., Brenninkmeijer, C.A.M., Gallagher, M., Hermann, M., Pontaud, M., Ziereis, H., Boulanger, D., Marshall, J., Nédélec, P., Smit, H.G.J., Friess, U., Flaud, J.-M., Wahner, A., Cammas, J.-P., Volz-Thomas, A. & Team, I. (2015). Global-scale atmosphere monitoring by in-service aircraft – current achievements and future prospects of the European Research Infrastructure IAGOS. (Vol. 67, pp. 28452). https://doi.org/10.3402/tellusb.v67.28452  

Thouret, V., Clark, H., Petzold, A., Nédélec, P. & Zahn, A. (2022). IAGOS: Monitoring atmospheric composition for air quality and climate by passenger aircraft. (pp. 1-14). https://doi.org/10.1007/978-981-15-2527-8_57-1