On the intensity and destructive potential of a past extreme, synoptic storm in a future warmer climate

Øelund, Johanne Kristine Haandbæk; Christensen, Jens Hesselbjerg; Zeitzen, Rune Magnus Koktvedgaard; Vedel, Henrik; Feddersen, Henrik

doi:https://doi.org/10.5194/egusphere-2025-3173

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https://doi.org/10.5194/egusphere-2025-3173

Preprints

04 Aug 2025

| 04 Aug 2025

On the intensity and destructive potential of a past extreme, synoptic storm in a future warmer climate

Johanne Kristine Haandbæk Øelund, Jens Hesselbjerg Christensen, Rune Magnus Koktvedgaard Zeitzen, Henrik Vedel, and Henrik Feddersen

Abstract. The global climate is undergoing significant changes, with rising greenhouse gas concentrations driving increased temperatures, altered precipitation patterns, and sea level rise. Yet, the full implications for mid-latitude storm systems remain an area of active investigation. This study examines how such storms may intensify under future warming, focusing on Storm Anatol, which severely impacted Denmark on 3 December 1999. Using the high-resolution weather prediction model HARMONIE-AROME and a pseudo-global warming (PGW) framework, the storm is simulated under a range of thermodynamic conditions representative of a warmer climate.

Results show a clear warming signal, with increased near-surface temperatures and atmospheric moisture contributing to stronger storm dynamics. Wind and gust speeds rise systematically with temperature, while the spatial extent and duration of damaging wind conditions also expand. To quantify these changes, we introduce the Cumulative Wind Exposure Index (CWEI), which captures the integrated spatial and temporal severity of wind exposure.

Application of CWEI reveals a marked increase in cumulative wind exposure in warmer scenarios relative to the historical case. When linked to established wind-damage relationships, this translates to substantially higher potential for structural damage and disruption. These findings suggest that storms like Anatol, already destructive in the past, are likely to become even more damaging under future climate conditions. This underscores the need for climate-resilient building standards, improved early warning systems, and long-term adaptation strategies across northern Europe.

Received: 02 Jul 2025 – Discussion started: 04 Aug 2025

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Johanne Kristine Haandbæk Øelund, Jens Hesselbjerg Christensen, Rune Magnus Koktvedgaard Zeitzen, Henrik Vedel, and Henrik Feddersen

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-3173', Anonymous Referee #1, 17 Sep 2025
Review of “On the intensity and destructive potential of a past extreme, synoptic storm in a future warmer climate” by Oelund et al. 2025 submitted to NHESS
This study applies a pseudo-global warming approach to the extreme storm Anatol (December 1999) to investigate its destructive potential in a warmer climate. To quantify wind damage potential, the authors introduce a cumulative wind exposure index that accounts for both spatial and temporal wind exposure. The study shows increased wind damage potential under some warming scenarios; however, further work is needed to improve the robustness of the results and the readability of the manuscript. Overall, the manuscript is very interesting, fits well within the scope of the journal, and has the potential to make a valuable contribution to the literature on storms and their damage potential in a warmer climate. I do, however, have several comments that should be addressed before publication.
General comment
Some reorganization would improve the overall storyline of the manuscript. The current Theory section could be condensed and integrated into the Introduction. I also recommend moving and shortening Section 4.4 Storm damages as well as part of Section 4.2 Pseudo-global warming and storyline approach into the Introduction. Lines 123–130 may also be placed in the corresponding sections of 4.1 and 4.2. In addition, Section 2 The storm Anatol, viewed from a Danish perspective would fit better either between the Methods and Results, or within the Results section.

The manuscript would benefit from greater consistency in terminology and notation. For example: extratropical low-pressure system (Line 17), extratropical cyclone (Line 330), and extratropical storm (Line 340) are all used;

‘threshold’ and ‘limits’ are used interchangeably, including in figures;

time units are given as GMT (Lines 74, 80) and UTC (from Line 235 onward and in figure captions);

the terms (mean) wind speed and (wind) velocity are not clearly distinguished (e.g. ‘maximum mean wind speed’ vs. ‘maximum wind speed’).

In addition, it should be clarified whether Ref and Control refer to the same experiments. Ensuring consistency throughout the manuscript would improve readability.

It is not clear why the authors sometimes use the entire model domain and other times only the land area of Denmark to compare results. For a holistic perspective on the storm, it would be helpful to also present the numbers for the entire storm, not just Denmark.

A limitation of your pseudo-global warming (PGW) approach is that it uniformly adds temperature, thereby neglecting projected changes in vertical temperature profiles and meridional temperature gradients. Is it possible to account for these in the experiments, or alternatively, could the authors explain why the PGW approach remains reasonable? In Line 167ff, it is stated that “a sensitivity test was conducted on vertical temperature lapse rates and the overall large-scale north-south temperature gradient.” It is not entirely clear what was done in this test. If some simulations with more realistic warming of vertical temperature profiles and north-south temperature gradients were conducted, this would be an interesting aspect and should be included or discussed in this study. A more detailed explanation of what is meant by sensitivity test would be helpful.

It is quite a strong statement to say that the overall intensity of the storm increases by using such a large domain’s maximum mean wind speed and gusts for 5 simulations (Line 242, Table 1 and Figure 3). As the author mentioned the linear relationship is not large. Is the result significant? In Table 1. It becomes evident that for the -2°C the maximum wind gust in the domain is lower than for the +1°C and almost similar to the Ref. Also, for the -1°C the max mean wind speed is higher than for the Ref and the +1°C wind gust is very similar to the +3°C. Can the author discuss this result and explain? It may also be helpful to include additional metrics for cyclone intensity for example minimum core pressure (only mentioned for Ref and +3°C) or maximum vorticity? Is the footprint of the storms changing for different warming levels?

Specific comments
Line 7: The authors state ‘with increased near-surface temperatures and atmospheric moisture contributing to stronger storm dynamics.’ However, in this study not only near-surface temperature, but the temperature of the entire atmosphere was modified, which makes the statement somewhat misleading. Furthermore, it is not clearly demonstrated in this study how atmospheric moisture contributes to stronger storm dynamics.
Line 8: The term ‘systematically’ may be too strong, as the +2°C simulation also shows instances of lower wind speeds and gusts.
Line 26 and Line 32: It is not clear how the reported 24m/s value relates to the 24.5m/s used as the threshold in the subsequent analysis. Could the authors clarify the reasoning behind choosing 24.5m/s later in the study?
Line 33: A reference to the commonly used threshold for defining storm events is missing.
Line 34: Here a reference to the ‘storms of this magnitude are relatively rare’ is missing.
Line 51: How is the study different form the cited study investigating the same storm (Zeitzen et al. 2025)? Highlight the novelty of the study more.
Line 57: Could the authors give a brief explanation of the pseudo-global warming method, and the general idea of this approach? What are advantages of this method?
Line 59: ‘However, they pose certain challenges. Extreme storms are inherently unique, forming due to the precise alignment of multiple meteorological and climatological drivers (Ulbrich et al., 2009). When modifying atmospheric conditions to reflect a warmer climate, there is a risk of altering the storm’s fundamental dynamics, potentially reducing its extremity rather than amplifying it (see e.g. Matte et al. (2022)‘ may be moved to the discussion or justify why you still use this method.
Line 72: Here you write ‘temperature contrasts’, perhaps write ‘temperature gradient’?
Line 76: The phrase ‘based on the simulations from HARMONIE-AROME’ introduces the simulation experiment for the first time, which makes it appear abrupt.
Line 76: Which method was used to determine the storm track?
Line 68: What kind of ‘atmospheric disturbances’?
Line 81: Later in the results are you also using 10-minute mean wind speeds? Please clarify later, when explaining the data used.
Line 126: A reference to the ERA5 is missing here.
Line 133: On what grid is the model simulated?
Line 136: Please state here the exact extend of the domain.
Line 140: To what is ERA5 interpolated to?
Line 143: What time period is the simulation exactly? Pleases also include hours of the start and end.
Line 145: For better understanding move this sentence to Line 140.
Line 151: A reference is missing here.
Line 151 to 155: This part should be mentioned in the introduction already and can be removed here.
Line 154: As I understand Brogli et al. (2023) also adapt geopotential and winds?
Line 159: Please expand the description of adjustment of specific humidity with one sentence, as it is an important part of your method.
Line 160: ‘unmodified reference run’ refers to the ‘The control simulation uses unmodified ERA5 temperatures’ (Line 145)? Please keep consistency throughout the study to refer to the control simulation.
Line 164: This sentence needs further explanation to clarify its meaning. According to Matte et al. (2022), PGW-Uni differs from PGW-GCM. If I understand correctly, your study uses PGW-Uni. This distinction should be made clearer in order to avoid confusion.
Line 165: I don’t think you need to introduce the abbreviation for GCM.
Line 171 to 184: This section is rather long and would fit better in the Introduction. Regarding the phrase ‘and identify the mechanisms responsible can be explored’ this would indeed be valuable, but it is not actually carried out in this study. In addition, the meaning of ‘primary and secondary processes driving the storm’s behavior’ is unclear. Why is this statement included here, given that the manuscript does not provide a detailed discussion of different processes leading to more intense winds? I suggest that Lines 176–180 can be moved to Section 4.1 Harmonie Model and data.
Line 193: Here it is not quite clear what the difference between the predefined limit and the threshold is? Can you state the number of the threshold used in this study? It would be helpful to only use ‘threshold’ or ‘limit’ throughout the study and the figures (title, caption, ...). Where do you normalize in that equation?
Line 197 to 211: This part of the section is rather lengthy and could be shortened. Since it belongs to the Methods section, it currently contains too much discussion of literature.
Line 212 to 229: At present, this section contains too much discussion of literature. It should either be focused more clearly on the method, explicitly describing what was done, or moved to the Introduction. The use of the power damage could then be explained in the Results section accordingly.
Line 234: With intensity you refer to, wind speed? Storm intensity can also be determined with minimum core pressure or maximum vorticity. Please clarify what you mean.
Line 234: Why not include the other warming scenarios?
Line 235: msl is mean see level pressure (mslp) presumably, but please define.
Line 237: ‘Notably, the center of the low-pressure system shifts eastward with each degree of warming’, do you refer to the peak intensity timestep of the storm? Could you plot the storm tracks of all the simulations?
Line 238: In this sentence it is not quite clear, what is meant with ‘there is’, what do you refer to here? I assume you refer to the different simulations. Maybe rephrase this sentence: ‘The central pressure of the low-pressure system at peak intensity (?) is with 952hPa slightly lower in the +3°C warming simulation compared to the Ref simulation with 954hPa.’
Line 241: The model domain is not introduced, what is the extend of the model domain? Is the maximum mean wind speed from the complete simulation period or just one timestep? With mean wind components you mean 10 min averages and with gust instantaneous wind speed? How are the maximum wind gust values from the model? Are you referring to 10-minute wind speeds and 10m winds?
Line 245 ff: It is not quite clear what various methods described above you are exactly referring to? Maybe introduce the methods here shortly.
Line 249: Figure 4, why only show the time lapse for the Danish land and not the entire storm affected area?
Line 256: How would those numbers look for the entire area the storm is affecting?
Line 257: Is it possible to show the storm track for the different experiments?
Line 261: Over what time period with what resolution are the CWEI values calculated? How many grid points are taken into account?
Line 266: For what area is the maximum wind speed?
Line 268: You have already introduced the abbreviation for pre-industrial (PI) in Line 129.
Line 279: It would strengthen the manuscript to link the mechanisms that cause a storm intensification under a warmer climate. How are the different mechanisms influenced by warming, and what effects might this have on the storms? Can you provide insights into which mechanisms lead to stronger winds in your experiments? In Lines 269–271, you offer hypotheses explaining why stronger winds occur in the -1°C experiment. Extending this discussion to the other experiments, and showing which mechanisms cause the differences, would be highly informative.
Technical Corrections, Tables and Figures
Figure 1: Please, add the extend of the model domain in the caption.
Figure 2: Remove the title of the figure and state the date and time in the captions of (a) and (b). What are the black contours. At the colorbar of (a) what do you mean with ‘Reference Wind Speed’?
Table 1: What do you mean with maximum mean wind velocity (why not called speed anymore?) Mean over what time and what domain? Maximum over what time and domain?
Figure 3: Absolute maximum wind speed over what time and domain? .. wind = .., an equal sign is a bit misleading here. Typo in caption ‘pr’ → ‘per’. Table 1 mainly provides further support to the results shown in Figure 3 and can therefore be removed or integrated into Figure 3.
Figure 4 and Figure 5: The information in the title could be put in the captions. I am wondering why only the Danish part is shown. Would be nice to see the entire storm affected area. Figures 4 and 5 basically show the same information. Consider removing Figure 4. Same applies for Figure B1 and B2. Typo in ‘colourbar’ → ‘colorbar’.
Table 2: Percentage of what region land cover, Denmark? What is the total number of grid points used for the CWEI? Over what timeperiod was the CWEI calculated?
Figure 6: Remove the title of the figure and put information in the caption. Why is here the 24m/s threshold displayed and not the 24.5m/s? Again, the equal sign is misleading as you state the change per/°C warming? Please add the right unit to the slope of the curve. In the caption it would be great to state over what domain and time period the CWEI was calculated. Same applies to Figure C1.
Figure 7: This is a nice figure to see the temporal evolution. The maximum mean wind speed was taken over what area? Denmark or model domain? Mean wind speed refers to hourly averages? Could you also plot the CWEI value at each timestep?
Figure 8. What time period was used?
Line 52: Typo ‘Allan (2023) (von Storch et al., 2014)’ → ‘Allan (2023, von Storch et al., 2014)’
Line 55: Start a new sentence after ‘remain similar’
Line 62: In the citation on pair of () to much → ‘(e.g. Matte et al., 2022)’
Line 70: This citation should be in () -> ‘(e.g., Ulbrich et al. 2001)’
Line 80: In the citation one pair of () to much → ‘(e.g. Zeitzen et al., 2025)’
Line 122: ‘Data’ → ‘data’
Line 131: ‘Model’ -> ‘model’
Line 157: ‘Initial Conditions and Lateral Boundary Conditions’ → ‘initial conditions and lateral boundary conditions’
Line 167: A space missing between ‘... (Matte et al., 2022). In addition ...’
Line 178: The sentence: ‘As demonstrated in Zeitzen et al. (2025), in part due to the high resolution of the simulations.’ is incomplete.
Line 397: In the citation is a ‘]’ too much.
Citation: https://doi.org/10.5194/egusphere-2025-3173-RC1
RC2: 'Comment on egusphere-2025-3173', Anonymous Referee #2, 07 Oct 2025

The authors present an analysis of Storm Anatol and how it may change under future warming using perturbed NWP experiments from the HARMONIE model. The results are of interest and build upon the array of literature on counterfactual historical storms under different climate conditions. The results are consistent with this literature, but i have several issues relating to the writing and content of this manuscript which would need to be addressed prior to publication. I have only few scientific/methodological issues, with most relating to clarity, the written content, and style of the manucsript.

My main comments are as follows:
1 - Section 3 (Theory) feels somewhat out of place and unnecessary in a manuscript of this type. Explaining theory of storm formation is well established, i was expecting something more on how climate change alters storm formation as this would directly relate to this studys findings. Consider re-writing with this information.
2 - Section 4 - you should mention that you also include SST perturbations as i initially read this as just atmospheric perturbations, which raised initial concerns for me until i saw this later in the manuscript.
3 - L179/180 - please provide some evidence for the reference run of HARMONIE being consistent with observations. It not being shown concerns me that the model has significant biases
4 - Section 4.4 - storm damages - what is the purpose of this section? You do not introduce any metric or mention anything that is used in the study (although i suspect it is for the analysis L245-248. If you are not using any storm damage metrics then this can be deleted, if you are from L245-248 then these need stating explicitly here in this section
Minor comments:
L26 - what is "basic wind speed value"? Is this a defined quantity? Could you please say more about that this is, is it a resilience measure/threshold?
L31 - i'd like to see more info on storm anatol here, or merge in the later section on the storm. Furthermore there should be some information on ETCs and how these extreme winds are associated with cyclonic systems.
L34 - how rare is "relatively rare"? This is quite a vague statement.
L38 - this section should come before the posing of the question on L36-37 as there should be initial discussion on why the climate makes storms worse before this question.
L46 - the North/South contrast is more of a tri-polar pattern. See papers by Severino et al. (2024; NHESS), Little et al. (2023; NCOMMS), Priestley et al. (2024; QJRMS)
L50 - here would be a good place for the question posed by L36-37
L59 - also the study of Ermis et al. (2024; 10.1088/2752-5295/ad4200) should be cited here and acknowledged more by the authors throughout.
L79 - are the quoted sustained winds and gusts local/station observations? if so please state this and where they occurred
Fig 1. - i think it would be good to have a panel here showing the pressure evolution and you can also then overlay the HARMONIE control and the different GWL experiments to see the impact on pressure evolution
L127 - +2°C - also here i was expecting to see that you also apply SST perturbations - modify this in the text as you mention it further along but not here
Section 4.3 - is this the first time that the CWEI has been used/introduced. If so please state that, if not please include a reference
L198 - please include a reference for the 98th percentile of wind speed statement
L203/204 - also include a citation here for the 24.5 m/s wind speed justification. My feeling is that 20m/s is more commonly used as a damage threshold so i'd be interested to know where this has come from.
Figure 2 - it would also be good to have a difference panel in (c).
L231/232 - this entire sentence can be shortened/deleted as you already have this information several times throughout the manuscript.
L234/236 - put brackets for your figure referencing
L235 - i would argue that the cyclone has already made landfall at the time due to the centre being over land. Please state that you are referring to the associated wind field with this.
L243/244 - are these r^2 values significant? Due to their being so few points I would not be sure if that is the case or not?
L236-238 - which methods are you referring to here and how did you obtain these numbers? There is no explanation as to how these values were obtained. See also major comment 4 above.
L253 - "time lapse" is this the correct wording here? Do you just mean "difference plot"?
L289/290 - the trend you quote here must surely be very uncertain due to the small number of experiments you have looked at? Does this mean that a comparison with the Hawkins et al. study is fair or valid?
L291-297 - see also Priestley et al. (2024; 10.1002/qj.4849) where changes in area were assessed and found increases of 1-2% per degC for central Europe

Citation: https://doi.org/10.5194/egusphere-2025-3173-RC2

Johanne Kristine Haandbæk Øelund, Jens Hesselbjerg Christensen, Rune Magnus Koktvedgaard Zeitzen, Henrik Vedel, and Henrik Feddersen

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Short summary

This study explores how a powerful storm like Anatol, which hit Denmark in 1999, could change in a warmer future climate. Using a weather model, the storm was simulated under future temperature conditions. Results show stronger winds affecting larger areas for longer periods. A new index was introduced to measure storm severity. The findings highlight the growing risks to infrastructure and the need for better storm preparedness.


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