Review of egusphere-2025-918

Insights from hailstorm track analysis in European climate change simulations

by Killian P. Brennan et al.

Overview: This is an excellent article embedded in a series of articles based on convection-allowing simulations in a current and a warmer climate. This particular study nicely analyzes individual storm properties, mainly showing that hailstorms will produce more severe hail due to stronger updrafts, that the melting of hail does not have a large effect on the hail size in a given storm, and that reduced frequency of hail in some parts of Europe is likely attributed to a reduced frequency of storms and not a reduction in severity. These results are highly relevant.

The figures and scientific language are of high quality and the manuscript has a clear structure. I only have one general major comment. However, since this might affect the robustness of the whole methodology and all studies using these simulations, I recommend major revisions.

General comments:

1)    My main concern is that the simulated storms are insufficiently resolved to represent real hailstorms, so the study might yield (partially) misleading results. It has been shown that at ~2 km resolution as used here, peak updraft speeds are substantially reduced (e.g., Adlerman and Droegemeier 2002 and references therein). This issue might also affect other updraft characteristics like width and depth. Since these factors are extremely important for hail growth (e.g., Lin and Kumjian 2022 and references therein) a strong impact can be expected. I think Hailcast somewhat corrects for these factors (Adams-Selin and Ziegler 2016) but it is unclear how well it performs with the Cosmo model and at this particular resolution. 

How severe these limitations are is not discussed in the article and is thus difficult to say. However, there are two reasons that at least point towards a strong bias in the simulations. 

(a) The sister-study that goes more into the verification of the model (Cui et al. 2024) shows that the hail distribution in the tested cases is not well covered. As far as I know, this preprint is still under review, but I’m not a reviewer on it so I don’t know if it will be accepted. Since the methodology is the basis for all follow-up articles, like the present one, I’m somewhat hesitant to recommend publications, at least not without emphasizing these limitations. 

(b) Lines 323-326: the obvious discrepancy between CAPE and the maximum w in the simulations is not discussed. For example, Fig. 10 at least roughly suggests that average CAPE values of for example 1500 J/kg leads to maximum updrafts of 20 m/s in your simulations, while from the w_max equation (line 324) one would expect 55 m/s. Yes, the latter is a theoretic value not necessarily realized due to entrainment, but especially hailstorms (often supercells) can realize most of their CAPE (e.g., Peters et al. 2019,2020a). Also, vertical velocities >50 m/s are likely common in supercells (e.g., Peters et al. 2020b) while only extreme outliers reach this range in your simulations (your Fig. 10). This is consistent with the expected negative bias in updraft intensity at 2.2 km resolution (Adlerman and Droegemeier 2002).

So far, the authors only briefly discuss that “fine-scale processes influencing hailstorm development” might not be represented (line 460). I think this insufficiently describes the problems of the simulations for the reasons outlined above. A much deeper discussion is necessary and the reader should be made aware which results must be taken with a grain of salt because of the biases in updraft characteristics (see e.g., comment 12 below, but I could see that there are other less obvious implications). 

I think it is still worth to publish these important results but to me there is some uncertainty to all of them, which must be made clear. 

Specific comments:

1)    Lines 32-33: can you add a reference to your statement that CC scaling leads to stronger average updrafts and hail? I agree that it is so on average but as you mention further below these links are not 100% clear, so at least providing a reference here seems warranted.

2)    Line 33: Also here you could add a reference showing the rise in the 0°C lvl (e.g., Prein and Heymsfield 2020, Gensini et al. 2024).

3)    Line 35: Is the slower fall speed really the main reason why small hail is more affected from melting? I always thought it is that larger hail has a relatively smaller surface area and more mass, so the cooling from latent heat exchange can slow down melting more effectively. Can you perhaps add a reference?

4)    Line 46: Several other means come to mind: observational proxies like lightning and overshooting tops (e.g., Punge and Kunz 2017), hail pads (e.g., Manzato et al. 2022), hail reports and radiosonde soundings. You don’t need to mention all possible methods though. Perhaps just rephrase that the means you introduce here are the ones that have so far been used in the literature to study climate trends (perhaps adding hail pad studies).

5)    Lines 55-56: This might be confusing to some readers. The European domain is not larger than the US, no? I suggest rephrasing.

6)    Line 173: It is not immediately clear what “mean storm maximum hail diameter” is. Is it the average over all maximum diameters occurring within the storms at a gridpoint? Consider defining it more clearly once.

7)    Line 179: I’m assuming this refers to the area that is over the 10mm threshold? Consider mentioning this explicitly since area could also refer to other variables like precipitation.

8)    Line 257: Consider replacing “drawn in” with “generated”, which seems more accurate.

9)    Line 261: Can you add a reference or some more context for why you consider this "the inflow level"? Can’t the inflow be dominated by any layers in the lowest 3 km or so and vary substantially for example between elevated storms and supercells (e.g., Nowotarski et al. 2020)?

10)    Lines 326-327: It’s not clear to me what you mean by “sampling bias due to coarse vertical resolution”. Aren’t all model gridpoints used? Then the w_max in the simulated cell is what matters for the simulated hail production. So in other words, it’s not a sampling bias but more a model bias which might significantly impact the whole methodology (see general comment 1). Or did I misunderstand something?

11)    Lines 362-364: Agreed! One way to test this would be to repeat the analysis for the far-field environment. The appropriate distance from the storm could be determined based on where you stop seeing storm-induced perturbations in the pressure and wind field (Fig. 5) and on the literature (Coniglio and Parker 2020; in general this and other references within could be added to this paragraph). Would this feasible? 

12)    Lines 372-376: The underestimation of maximum updraft intensity outlined in general comment 1 could be the reason why no nonlinear behavior is seen in your simulations compared to Lin and Kumjian.

13)    Line 380: Here or later in the discussion it might be helpful to clarify what exactly “small” refers to. Gensini et al. argue that melting dominates for hail < 4 cm (their Fig. 1) while in your study “small” is used for much smaller diameters (e.g., lines 439, 426). I think this clarification is important to put your work into perspective because it emphasizes that your results point in a different direction.

14)    Section 5.2: I’ve never used HAILCAST but if I recall correctly, accurately representing melting of hail in such a model is not an easy task and subject to heigh uncertainty. If you agree, this point should be discussed, because it could be the reason why melting does not have a strong effect in your simulations compared to the other literature.

15)    Line 424: The last sentence here seems out of place and could be removed?

16)    Line 440: I’d suggest briefly mentioning what “uncertainties” you are referring to because the references alone leave room for interpretation.

17)    Lines 447-449 and 456: I fully agree and these are important results. 

18)    Line 460: I recommend also citing Adams-Selin (2025) here.

Technical corrections:

Line 153: So you mean “SON” for November?

Line 204: Consider replacing “different” with “spatially heterogeneous” and “similar” with “homogeneous” to be clearer.

Footnote 2: Consider shortening to “The second approach is arguably better as…”

References:

Adams-Selin, R. (2025). The Quasi-Stochastic Nature of Hail Growth: Hail Trajectory Clusters in Simulations of the Kingfisher, Oklahoma, Hailstorm. Monthly Weather Review, 153(1), 67–87. https://doi.org/10.1175/MWR-D-23-0233.1

Adlerman, E. J., and K. K. Droegemeier, 2002: The Sensitivity of Numerically Simulated Cyclic Mesocyclogenesis to Variations in Model Physical and Computational Parameters. Mon. Wea. Rev., 130, 2671–2691, https://doi.org/10.1175/1520-0493(2002)130<2671:TSONSC>2.0.CO;2.

Coniglio, M. C., & Parker, M. D. (2020). Insights into supercells and their environments from three decades of targeted radiosonde observations. Mon. Wea. Rev., 148, 4893–4916. https://doi.org/10.1175/mwr-d-20-0105.1

Punge, H. J., Bedka, K. M., Kunz, M., & Reinbold, A. (2017). Hail frequency estimation across Europe based on a combination of overshooting top detections and the ERA-INTERIM reanalysis. Atmospheric Research, 198(July), 34–43. https://doi.org/10.1016/j.atmosres.2017.07.025

Nowotarski, C. J., Peters, J. M., & Mulholland, J. P. (2020). Evaluating the effective inflow layer of simulated supercell updrafts. Mon. Wea. Rev., 148(8), 3507–3532. https://doi.org/10.1175/MWR-D-20-0013.1

Peters, J. M., Nowotarski, C. J., & Morrison, H. (2019). The role of vertical wind shear in modulating maximum supercell updraft velocities. Journal of the Atmospheric Sciences, 76(10), 3169–3189. https://doi.org/10.1175/JAS-D-19-0096.1

Peters, J. M., Nowotarski, C. J., & Mullendore, G. L. (2020a). Are Supercells Resistant to Entrainment because of Their Rotation? J. Atmos. Sci., 77(4), 1475–1495. https://doi.org/10.1175/jas-d-19-0316.1

Peters, J. M., Morrison, H., Nowotarski, C. J., Mulholland, J. P., & Thompson, R. L. (2020b). A formula for the maximum vertical velocity in supercell updrafts. J. Atmos. Sci., 77(9), 3033–3057. https://doi.org/10.1175/JAS-D-20-0103.1.

Prein, A. F., & Heymsfield, A. J. (2020). Increased melting level height impacts surface precipitation phase and intensity. Nature Climate Change, 10(8), 771–776. https://doi.org/10.1038/s41558-020-0825-x

This is a very interesting study looking at numerous aspects of hail in Europe under a warming climate (e.g. number of hailstorms, hail diameter and swath size). They nicely couple this with the inflow environment of the storms again in the current/future climate framework. The study is suitable for the journal and shows several novel findings, but the analysis could be adapted further, and some methods/datasets are based on unpublished papers. Therefore, I recommend major revisions before publishing.

Main comments:

1. In my view the authors try a bit too hard to match their findings with previous studies and cite some papers that are not available or unreviewed preprints. I’d suggest that they stick to primarily citing current peer-reviewed literature. The authors could also consider delaying the publishing of the manuscript until there are further papers available to support their conclusions and the datasets they use.
2. The authors mention the possible changes in convective initiation and how that may affect hailstorm frequency (e.g. L203-204). I think this is a plausible hypothesis and should be investigated in more detail. For example, the authors could set a reflectivity threshold as a proxy for convective initiation (CI) e.g. 40 dBZ and look at changes in CI frequency in Europe between current and future. Maybe this will show similarities to the plot shown in Fig. 2c.
3. I also think they could do more to consider kinematics, i.e. considering hail path trajectories, which is key in determining the hail size on the ground.

Inline comments:

L30-38: Whether convective initiation will become more or less frequent should already be mentioned in the introduction.

L70: Which simulations are the authors talking about?  Please be more specific which simulations are being used, on the previous lines both Cui et al. (2024) and Thurnherr et al. (2025) are cited.

Both Cui et al. (2024) and Thurnherr et al. (2025) are preprints. More specifics of the simulations in the methods and data section should be included if this study is to be published first.

L124-141: It would be useful to include information about the time component. Do the hailstones just need to meet these criteria for one timestep (i.e. 5 minutes)?

The hailstorm definition used for tracking could also be better summarized in a table.

L169: RE: reference to Feldmann (2025). I’m generally ok with authors citing personal correspondence if there is a good reason to do so such as communication with a forecaster about model biases because the forecaster will not have any publications. However, just a simple citation with no further details should not be included. Maybe the person has an upcoming publication on the topic, however no paper is given in the reference.

L202: By ‘hail track frequency’ I am assuming the authors are referring to Figures 2a,b,c which is labeled as ‘n storms per season’. I’d suggest using consistent terminology e.g. n hailstorms per season and use the term hailstorm throughout. In section 2.3 it is made clear that your hailstorm definition is based on the tracking algorithm.

Figure 6: One of the authors’ research questions is to investigate whether there are changes in the vertical wind shear surrounding hailstorms. The change in the vertical wind shear is not shown on Figures 6 and 7, only wind speed at 400 hPa in the current climate. It is shown in Figure 9 but I think it would already be useful to see it for the most intense storms.

I’m wondering if the authors have considered kinematics in more detail. For example, depending on the hailstone trajectory it could be ejected earlier and not grow as large (e.g. Allen et al. 2020). The rain rate and wind gust panels are nice complementary information but are not specific to the dynamics/environment of hailstorms.

L394-395. I don’t agree with this conclusion in the way it is written. The level of the zero-degree isotherm does play a key role, otherwise we’d see large hail more often in the tropics. I think a more plausible hypothesis is that in the future scenario increased melting is offset by the increase in the typical maximum size hailstones can reach. I’d suggest rewording to not disregard the importance of melting.

L396: I see now that in the subsequent lines the authors mention the hypothesis above and explain it in more detail. I’d suggest deleting lines L394-395 as it could be misleading.

L434: Convective available potential energy?

L444: Please remove unpublished studies. Furthermore, Wilhelm et al. (2024) showed an increase in hail days both north and south of the Alps whereas your results show a decrease in the number of hailstorms south of the Alps (Fig. 2c).

L445: Where is it shown that hail environments change similarly in all subregions? The only geographic maps are Figs. 2 and 3. I could not see this in the supplement either, only inflow environments for storms in different regions. If the authors are referring to Figs. S6 – S16 I suggest they be specific that this is for the top 10% cases inflow environments not general trends per region. 

As a general comment, I think the authors could be more precise what they are referring to when make their conclusions to avoid any confusion or misinterpretation of the results.

L458–466: It’s nice to see that the authors have openly discussed the limitations of this work. I think the fact that the changes in hail frequency/size/intensity etc will also be dependent on the actual level of warming is important to mention. In this study, the authors base all their results on 3K of global warming which is a real possibility. Nevertheless, I think it should be mentioned that the results are valid assuming a 3K warming and that the actual level of warming will depend on different factors (e.g. future emissions).

Reference

Allen, J. T., Giammanco, I. M., Kumjian, M. R., Punge, H. J., Zhang, Q., Groenemeijer, P., et al. (2020). Understanding hail in the Earth system. Reviews of Geophysics.