This opinion paper reviews recent research on tropical cirrus clouds (anvil clouds and TTL cirrus), highlights remaining challenges, and suggests potential avenues for future investigations. The authors' perspective is undoubtedly of interest to ACP readership. I have a number of comments, mostly minor requests for clarification but also a few substantial points regarding the content. I recognize the special format of this paper as an ‘Opinion’ and, while I hope that the authors will consider my remarks, they should be taken as mere suggestions.

General Comments:

1) Availability of spaceborne observations:

I don't entirely share the authors' optimism regarding the "growing abundance of observational data.", since not all instruments have equal relevance for monitoring (thin) ice clouds. A significant reduction in available spaceborne observations has occurred since the ending of CALIOP on August 1st (https://www-calipso.larc.nasa.gov/). While future active spaceborne instruments (EarthCARE, etc.) are planned, the lack of overlap between upcoming spaceborne lidars and CALIOP might complicate the creation of long-term datasets necessary for trend analysis in the context of climate change. The authors might wish to mention this type of issue in the paper.

2) Balloon-borne observations

Both spaceborne and in situ aircraft measurements are reviewed, but somehow balloons are mostly omitted from the outlook section (Sect. 6), although a few studies based on data from this platform are cited elsewhere in the paper. Given the emphasis put on cloud tracking, life cycle and Lagrangian modeling techniques (Sect. 6.3-6.4 in particular), I am wondering whether the authors would like to provide a brief account of the possibilities offered by balloon soundings (radiosonde and long-duration balloons). Recently, a renewed effort in the development of lightweight instrumentation has been undertaken and balloons have been deployed in a few of campaigns (only to cite a few studies: Cirisan et al., 2014; Khaykin et al., 2016; Wolf et al., 2018, Ravetta et al., 2020, Kalnajs et al., 2021, Bramberger et al., 2022). Dynamical information inferred from quasi-Lagrangian balloons has also been used in a number of studies of TTL cirrus (Jensen et al., 2016; Corcos et al., 2023).

3) Presentation

To help the reader, some terms and concepts could be more clearly defined (e.g.,whether TTL cirrus are solely in situ or can be convectively detrained, what is meant by ‘convective origin’, ‘small scales’). Further details are provided below.

Specific Comments:

• Line 5-6: "a growing abundance of remotely sensed and in situ observations": I'm unsure to share this optimism (refer to main comment 1).

• Line 21-23: These sentences might be interpreted as considering all TTL cirrus to be in situ formed. Could you clarify?

• Line 25: The statement isn't supported by Fig. 2; you might include a reference here (Sassen et al., 2009 ?).

• Line 122: It would be useful provide a quantitative assessment of available data from aircraft campaigns (e.g., show the track of the flights from the Kramer et al Julia database in a figure, or provide the total number of flight hours)?

• Line 140: Were measurements of ice crystal isotopic composition indeed conducted during those campaigns? As far as I recall, there were only vapor phase measurements

• Line 147: What is the definition of “cirrus clouds originating from deep convection” in that paper ?

• Line 149: In my opinion (and also reading the following sentence), it would be more accurate to say that models bridge the gap between laboratory studies and observations.

• Line 163-164: Choose between accommodation coefficient and deposition coefficient.

• Line 172: Perhaps specify "numerical" models.

• Line 189: "removing bias…" – You might consider adding supporting references.

• Line 192-196: Introduce references for these points.

• Line 198-199: Does this not apply to midlatitude cirrus as well?

• Line 241-242: Please consider elaborating on this point further.

• Line 258: Suggest replacing 'drive' with 'be related to.'

• Line 280: An example of such phenomenon is provided by Ferlay et al., 2014.

• Line 333: Could you specify a range?

• Line 358-360: Would you say that the stability Iris mechanism is still disputed or widely accepted ? If a firm opinion emerges from the literature, it would be helpful stating it here.

• Lines 391-394: You might wish to expand upon this point.

• Line 400: Maybe refer to the paragraph above or mention again that the cited studies leverage SST variations related to different modes of variability (i.e. their conclusions may not apply to global warming).

• Line 429: Change "the level of neutral ..." to "their level of neutral…"

• Line 430: I unsure what is meant here. Isit that the correlation between TTL cirrus and convection is related to the detrainment of water more than to the cold TTL temperature anomalies in convective areas (cold traps)? If yes, please clarify, this may seem contradictory with the idea of a cold trap and a few observational studies (Kim et al. 2016, Randel et al., 2015).

• Line 451: I believe there is a typo and the citations ‘Podglajen et al., 2017; Karcher and Jensen (2017)’ should be replaced by 'Podglajen et al., 2016; Karcher and Podglajen (2019)’

• Line 475: It would be good to include a reference here.

• Line 494: I would recommend distinguishing between small scales and micro scales to avoid confusion.

• Line 500: "sharp threshold" sounds redundant to me.

• Line 509: Reference(s) for the controversy would be helpful.

• Line 530: Change 'ejected' to 'injected.'

• Line 542: maybe change vertical wind to temperature anomaly, since the waves considered in those studies mostly have a signature in temperature (not vertical wind)

• line 543: I am not sure that the Mace et al., 2006 reference is appropriate here. Maybe Alexander and Pfister (1995) ?

• Line 545: Maybe specify the type of scales which is referred to as ‘large scale’ – planetary or large-scale equatorial waves.

• Line 553: On turbulence in the vicinity of convection, you might also cite earlier studies by Lane et al., 2003; Podglajen et al., 2017; Barber et al. 2018

• Line 613-614: I would replace ‘sources of artificiality’ by ‘arbitrary thresholds and limiters’ or something along those lines

• Line 615: ‘numerical’ → ‘artificial’

• Line 617-622: The whole paragraph needs to be rephrased.

• Line 653: ‘evolution perspective’: perhaps consider replacing ‘evolution’ by ‘life cycle’

• Line 781: ‘both’? (there are 3 citations)

• line 802: Lagrangian sampling of models: I am not quite sure what is meant here. The citation is in any case not appropriate (neither ‘Lagrangian’ nor ‘trajectory’ appear in the cited paper)

• Line 802: Could you elaborate onthe microphysical uncertainty you are referring to ?

Please make sure that the references are listed in alphabetical order.

References:

Alexander, M. J. and Pfister, L.: Gravity wave momentum flux in the lower stratosphere over convection, Geophys. Res. Lett., 22, 2029–2032, https://doi.org/10.1029/95GL01984, 1995.

Barber, K. A., G. L. Mullendore, and M. J. Alexander, 2018: Out-of-Cloud Convective Turbulence: Estimation Method and Impacts of Model Resolution. J. Appl. Meteor. Climatol., 57, 121–136, https://doi.org/10.1175/JAMC-D-17-0174.1.

Ferlay, N., T. J. Garrett, and F. Minvielle, 2014: Satellite Observations of an Unusual Cloud Formation near the Tropopause. J. Atmos. Sci., 71, 3801–3815, https://doi.org/10.1175/JAS-D-13-0361.1.

Lane, T. P., R. D. Sharman, T. L. Clark, and H.-M. Hsu, 2003: An investigation of turbulence generation mechanisms above deep convection. J. Atmos. Sci., 60, 1297–1321, https://doi.org/10.1175/1520-0469(2003)60<1297:AIOTGM>2.0.CO;2.

Podglajen, A., Hertzog, A., Plougonven, R., and Legras, B. (2016), Lagrangian temperature and vertical velocity fluctuations due to gravity waves in the lower stratosphere, Geophys. Res. Lett., 43, 3543–3553, doi:10.1002/2016GL068148.

Podglajen, A., Bui, T. P., Dean-Day, J. M., Pfister, L., Jensen, E. J., Alexander, M. J., Hertzog, A., Kärcher, B., Plougonven, R., and Randel, W. J.: Small-scale wind fluctuations in the tropical tropopause layer from aircraft measurements: Occurrence, nature and impact on vertical mixing, J. Atmos. Sci., 74, doi:10.1175/JAS–D–17–0010.1, 2017.

Randel, W., Zhang, K., & Fu, R. (2015). What controls stratospheric water vapor in the NH summer monsoon regions? Journal Of Geophysical Research-Atmospheres, 120, 7988-8001. doi:10.1002/2015JD023622

Additional references of studies using balloon-borne data

Cirisan, A., Luo, B. P., Engel, I., Wienhold, F. G., Sprenger, M., Krieger, U. K., Weers, U., Romanens, G., Levrat, G., Jeannet, P., Ruffieux, D., Philipona, R., Calpini, B., Spichtinger, P., and Peter, T.: Balloon-borne match measurements of midlatitude cirrus clouds, Atmos. Chem. Phys., 14, 7341–7365, https://doi.org/10.5194/acp-14-7341-2014, 2014

Corcos, M., Hertzog, A., Plougonven, R., and Podglajen, A.: A simple model to assess the impact of gravity waves on ice-crystal populations in the tropical tropopause layer, Atmos. Chem. Phys., 23, 6923–6939, https://doi.org/10.5194/acp-23-6923-2023, 2023

Jensen, E. J.; Ueyama, R.; Pfister, L.; Bui, T. V.; Alexander, M. J.; Podglajen, A.; Hertzog, A.; Woods, S.; Lawson, R. P.; Kim, J. E. and Schoeberl, M. R., High-frequency gravity waves and homogeneous ice nucleation in tropical tropopause layer cirrus Geophysical Research Letters, 43, 6629-6635 (2016) 10.1002/2016GL069426

Kalnajs, L. E., Davis, S. M., Goetz, J. D., Deshler, T., Khaykin, S., St. Clair, A., Hertzog, A., Bordereau, J., and Lykov, A.: A reel-down instrument system for profile measurements of water vapor, temperature, clouds, and aerosol beneath constant-altitude scientific balloons, Atmos. Meas. Tech., 14, 2635–2648, https://doi.org/10.5194/amt-14-2635-2021, 2021

Khaykin, S. M., Pommereau, J.-P., Riviere, E. D., Held, G., Ploeger, F., Ghysels, M., Amarouche, N., Vernier, J.-P., Wienhold, F. G., and Ionov, D.: Evidence of horizontal and vertical transport of water in the Southern Hemisphere tropical tropopause layer (TTL) from high-resolution balloon observations, Atmos. Chem. Phys., 16, 12273–12286, https://doi.org/10.5194/acp-16-12273-2016, 2016

Ravetta, F., Vincent Mariage, Emmanuel Brousse, Eric d´Almeida, Frédéric Ferreira , Jacques Pelon, and Stéphane Victori: BeCOOL: A Balloon-Borne Microlidar System Designed for Cirrus and Convective Overshoot Monitoring, EPJ Web Conf., 237, 07 003, https://doi.org/10.1051/epjconf/202023707003, 2020