General response

We extend our gratitude to the two anonymous reviewers for their careful reading and constructive criticism of our article. We have carefully considered all reviewers' comments and we respond accordingly to each of those and plan to undertake further work and incorporate their suggestions and address their concerns. We believe the reviewers' comments will significantly enhance the manuscript. Detailed responses are provided below. The reviewers' comments are in italics, while our responses are in plain font.

Point-to-point response ro comments of Reviewer 1

The manuscript describes a set of specific analyses done to understand and characterize complex extremes, such as the ones occurred in 2018. The text is well written and results clearly discussed. Despite the great readability, methods should be better explained and more details provided. This would give all readers the possibility to better appreciate the main findings and the figures. This is especially true for sections 2.2, 2.3, 2.8. For instance: the EPI and the TPDM are barely described;  the choice of the ensemble size of the hydrological simulations is not discussed, neither is the experimental setting.

Dear Reviewer,

Thank you very much for your thorough and constructive feedback on our manuscript. We are pleased to hear that you found the text well-written and the results clearly discussed.

We appreciate your suggestion to provide more detailed explanations of the methods, as this will indeed enhance the overall clarity and allow readers to better understand and appreciate our findings. Please consider that for the submitted manuscript we have been worried about the length of the work and have kept short the methodological information that in cases may be described in dedicated papers. In response to your comments, we will ensure that sections 2.2, 2.3, and 2.8 are expanded with more detailed descriptions, particularly regarding the EPI and TPDM methodologies. We will also provide a clearer discussion of the rationale behind the ensemble size chosen for the hydrological simulations and give a more comprehensive explanation of the experimental setting.

Once again, thank you for your valuable input. We are confident that these revisions will strengthen the manuscript, and we look forward to presenting a more detailed and robust version.

General response

We extend our gratitude to the two anonymous reviewers for their careful reading and constructive criticism of our article. We have carefully considered all reviewers' comments and we respond accordingly to each of those and plan to undertake further work and incorporate their suggestions and address their concerns. We believe the reviewers' comments will significantly enhance the manuscript. Detailed responses are provided below. The reviewers' comments are in italics, while our responses are in plain font.

Point-to-point response ro comments of Reviewer 2

Review of “Compound events in Germany in 2018: drivers and case studies” by Elena Xoplaki and colleagues. 

This manuscript sets out to discuss compound events on the example of Germany in 2018. It is reasonably well-written and structured. A number of very long sentences make it hard to follow at times and the structure of the methods could be improved. 

I acknowledge the effort that went into this work and I’m reasonably sure that it will be a good resource to look up European extremes in 2018 and case studies. But I also feel that the authors could have done a better job at building a more concise (i.e., shorter) story and make the paper feel less like a loosely connected collection of results in particular given the topic of compound events. 

I do not expect the authors to overhaul the entire paper but I would encourage them to include some more connecting and structuring elements like tables or flow charts summarizing the methods and comparing the different results and their relation. 

My only major issue is the almost complete lack of conclusions in the conclusion section. One thing that might be good to address would be: What did we learn from this study that we did not already know (apart from collecting it in one place). 

Dear Reviewer,

Thank you very much for your valuable and insightful comments on our manuscript. We are grateful for your recognition of the effort that went into this work and your positive outlook on its potential as a resource for understanding European extremes in 2018.

We fully acknowledge your suggestions regarding the length and structure of the manuscript. We will carefully review the text to identify and shorten lengthy sections and complex sentences to improve readability. We will also work on making the narrative more concise and focused, ensuring that the paper presents a cohesive story rather than a collection of loosely connected results. To enhance clarity, we will consider incorporating more connecting and structuring elements, such as tables or flow charts, to better summarize the methods and compare the different results and their relationships.

Regarding your comment on the conclusion section, we agree that it could benefit from a more substantive discussion. We will revise this section to more clearly articulate the key takeaways from our study, emphasizing the new insights gained and their significance in the context of compound events.

We sincerely appreciate your constructive feedback and are confident that these revisions will strengthen the manuscript. Thank you once again for your thoughtful review.

49:  switch to past tense?

We appreciate your suggestion regarding the use of tense in our manuscript. We agree that consistency in tense is important for clarity and readability. Therefore, we will revise the sentence on line 49 to past tense to align with the rest of the section. The revised sentence will read:

"We shed light on the interannual influence of droughts on surface water and the impact of water scarcity and heatwaves on agriculture and forests."

71: “Zscheischler et al. (2018) further defined CEs as combinations of events that are individually not necessarily extreme, or multiple drivers and/or hazards, but in combination often lead to disproportionate impacts on people and ecosystems”

How is this different from line 68?

“3) a combination of events that are not themselves extremes but lead to an extreme event or impact when combined.”

Thank you for your careful reading of this section and for bringing the repetition to our attention. We will thoroughly revise this section to eliminate the repetition and enhance the clarity of the content.

74 “To quantify the probability of CEs in today’s and future climate is of great importance specifically adaptation planning for” → “for adaptation planning for”?

Thank you for suggesting a clearer and improved formulation. We will correct the phrasing. 

105: This is just an opinion and I appreciate that the authors also want to advertise their project with this paper but maybe they could tune down the references to climxtreme a bit? At least I can not see why it is relevant to the reader how climXtreme organized the investigation underlying this manuscript and the manuscript is already very long.

Also 134, 613, …

Thank you for pointing out the overemphasis on references to the ClimXtreme project. We appreciate your feedback and acknowledge that our enthusiasm for the project may have led to an excessive focus on it. We will take your advice and remove the sections related to internal organization and other irrelevant content to ensure a more concise and focused manuscript. 

Section 2: I am sorry but find this section quite hard to follow and at the same time it probably does not provide enough information to actually understand what was done in detail. I appreciate that it is not easy to write a concise method section for such an extensive paper but I think this could be done better. Currently the reader is thrown into more than 10 sub-sections mixing individual extreme definitions, compound events, and impacts. In addition, the level of detail somewhat differs between sections and there are many design choices that are not really reflected or compared. 

Thank you for your feedback on section 2. in our manuscript. We understand your concerns and will restructure this section to provide a better structure and a more consistent level of detail. We will further improve the organization of this section and focus on keeping it short and concise.

Just two examples: 

- the authors repeatedly state that the summer months JJA are used (lines 157, 199, 272) but in other sections they only refer to summer (145, 161) or other periods such as the hydrological year (161) or the warm season (282). Given that this study is about compounding it seems important to be clear on the properties of the different events considered and their relation.

- There are a range of relative thresholds used to define extremes. I fond some based on the 90, 95, and 98 percentiles. I’m not saying that this is inherently problematic but it should be made clear.

My suggestion would be to start with an overview section explaining the structure of this section. This could include a table summarizing the main features of each analysis to ensure a similar level of detail and ease comparing differences. 

Thank you for your very detailed remark and the examples. We will revisit the manuscript and improve the time definitions and produce a more consistent description of how extremes are defined across sub-projects. We appreciate your suggestion and will include an overview section with a table that explains the section better and also align the level of detail. 

150: Note that using a 15 day running window do define extreme thresholds (as done Fischer and Schär) of has been shown to be methodologically wrong and prone to biases recently: https://www.nature.com/articles/s41467-024-46349-x

Thank you for making us aware of this work! We will take into consideration the publication and results therein and update the analysis.

229: “The co-occurrence should be on the same day or the following day for precipitation, in the same grid cell and within a radius of 50 km, respectively.” I’m not sure I understand what it means for an event to be within the same grid cell and at the same time within 50km. If we assume the 0.25deg resolution corresponds to 25km, this would mean it can be a maximum of two grid cells away in the horizontal as an example? Is that correct? Can the authors clarify this?

Thank you for your observation. You are correct in identifying the need for clarification. The intention was to specify that for co-occurrence of events, precipitation events should either occur on the same day or the following day, and while they should be recorded in the same grid cell, they can also be considered if they occur within a radius of 50 km from that grid cell. We will revise this section and aim for a clearer and more precise description, and ensure that the relationship between grid resolution and distance is accurately represented.

274: “The compound precipitation-wind events are defined on the winter (December to February) daily mean precipitation and daily maximum surface wind. The selection of events is based on the exceedance of the 98th percentile for the period 1975-2025…”

and then later

278: “Extreme compound wind and precipitation years exceed the 20-year return levels for precipitation and wind individually, defined as the 95th percentiles for the period 1975-2025.”

Sorry but this confuses me. Can the authors clarify the difference between “compound precipitation-wind events” and “ Extreme compound wind and precipitation years”?

Thank you for this comment. You are right it is confusing and also not correct as it is described. Extreme compound wind and precipitation years are defined by exceedance of the 20-yr return levels of their winter cumulative occurrences, while (extreme) compound precipitation-wind events are defined by exceeding the 98th percentile for the 1975-2025 period of each day (or the day after for the case of precipitation) and for each grid point. We will make sure that the new description will make clear the differences between these events/definitions.

303: “a large blocking system at 500 hPa, and a double jet stream configuration is visible in the 250mb zonal wind field”

Maybe settle for one unit?

Thank you for this observation. Of course we will rephrase the section using the same unit, here, and throughout the whole manuscript:

"a large blocking system at 500 hPa and a double jet stream configuration in the 250 hPa zonal wind field"

Also I read the sentence as saying that a 500hPa blocking is visible in the 250mb zonal wind field. I think I understand what the authors try to say here but you can not see 500hPa at 250hPa by definition…

Thank you for pointing to this wrong description. The blocking at 500 hPa information stems from the paper of Roussi et al. (2023), which we have wrongly cited a bit later in the sentence. We will of course correct this. In our figure we present only 250 hPa zonal wind and anomalies. We will also restructure this whole paragraph as you have noticed allows not only for misunderstandings but also wrong conclusions. 

Figure 12: what do the dashed lines represent?

Thank you for pointing out the missing information. The dashed lines denote the climate variable thresholds that define the extreme CEs.

Figure 13: Fontsize way too small

Thank you for that observation, we will adjust the font size and make the figure more accessible to the reader. 

(b) the title says CMIP5 1975-2021, the caption says historical CMIP5 ensemble. Maybe I’m not aware that the CMIP5 historical runs where extended otherwise this is wrong.

Thank you for this comment. The historical CMIP5 ensemble used in this figure has of course been extended by Aalbers et al. (2023). We will make sure adding this information also in the caption.

“Significant model ensemble grid points are shaded dark.” Unclear what that dark means here.

Thank you for your comment. The caption and description will define exactly the meaning of dark green and dark brown.

614: “The showcases presented in this paper include multivariate, pre-conditioning, temporally and spatial Ces” 

Again, a summarizing table would be nice to give an overview.

We appreciate your suggestion for a summarizing table. We agree that a table would enhance the clarity and provide a concise overview of the key aspects discussed. We will include a summarizing table in the revised manuscript to better present the information and facilitate a clearer understanding of the content.

611-648: Delete or shorten by a lot? This is not a conclusion but basically a reiteration of the results.

Thank you for your valuable feedback. We significantly shorten this section providing a thorough conclusions based on the findings in our results section.

655: “Further studies aim at expanding the current knowledge on the complex relationships between CEs and large-scale fields at different time horizons in order to improve the detection and thus the understanding of the climate system.”

Not sure what this sentence is trying to say? What further studies?

Thank you for pointing this out. We will clarify the reference to further studies by specifying the objectives and contributions of ongoing research to enhance the detection and understanding of compound events.

General response

We extend our gratitude to the two anonymous reviewers for their careful reading and constructive criticism of our article. We have carefully considered all reviewers' comments and we respond accordingly to each of those and plan to undertake further work and incorporate their suggestions and address their concerns. We believe the reviewers' comments will significantly enhance the manuscript. Detailed responses are provided below. The reviewers' comments are in italics, while our responses are in plain font.

Point-to-point response ro comments of Reviewer 1

The manuscript describes a set of specific analyses done to understand and characterize complex extremes, such as the ones occurred in 2018. The text is well written and results clearly discussed. Despite the great readability, methods should be better explained and more details provided. This would give all readers the possibility to better appreciate the main findings and the figures. This is especially true for sections 2.2, 2.3, 2.8. For instance: the EPI and the TPDM are barely described;  the choice of the ensemble size of the hydrological simulations is not discussed, neither is the experimental setting.

Dear Reviewer,

Thank you very much for your thorough and constructive feedback on our manuscript. We are pleased to hear that you found the text well-written and the results clearly discussed.

We appreciate your suggestion to provide more detailed explanations of the methods, as this will indeed enhance the overall clarity and allow readers to better understand and appreciate our findings. Please consider that for the submitted manuscript we have been worried about the length of the work and have kept short the methodological information that in cases may be described in dedicated papers. In response to your comments, we will ensure that sections 2.2, 2.3, and 2.8 are expanded with more detailed descriptions, particularly regarding the EPI and TPDM methodologies. We will also provide a clearer discussion of the rationale behind the ensemble size chosen for the hydrological simulations and give a more comprehensive explanation of the experimental setting.

Once again, thank you for your valuable input. We are confident that these revisions will strengthen the manuscript, and we look forward to presenting a more detailed and robust version.

General response

We extend our gratitude to the two anonymous reviewers for their careful reading and constructive criticism of our article. We have carefully considered all reviewers' comments and we respond accordingly to each of those and plan to undertake further work and incorporate their suggestions and address their concerns. We believe the reviewers' comments will significantly enhance the manuscript. Detailed responses are provided below. The reviewers' comments are in italics, while our responses are in plain font.

Point-to-point response ro comments of Reviewer 2

Review of “Compound events in Germany in 2018: drivers and case studies” by Elena Xoplaki and colleagues. 

This manuscript sets out to discuss compound events on the example of Germany in 2018. It is reasonably well-written and structured. A number of very long sentences make it hard to follow at times and the structure of the methods could be improved. 

I acknowledge the effort that went into this work and I’m reasonably sure that it will be a good resource to look up European extremes in 2018 and case studies. But I also feel that the authors could have done a better job at building a more concise (i.e., shorter) story and make the paper feel less like a loosely connected collection of results in particular given the topic of compound events. 

I do not expect the authors to overhaul the entire paper but I would encourage them to include some more connecting and structuring elements like tables or flow charts summarizing the methods and comparing the different results and their relation. 

My only major issue is the almost complete lack of conclusions in the conclusion section. One thing that might be good to address would be: What did we learn from this study that we did not already know (apart from collecting it in one place). 

Dear Reviewer,

Thank you very much for your valuable and insightful comments on our manuscript. We are grateful for your recognition of the effort that went into this work and your positive outlook on its potential as a resource for understanding European extremes in 2018.

We fully acknowledge your suggestions regarding the length and structure of the manuscript. We will carefully review the text to identify and shorten lengthy sections and complex sentences to improve readability. We will also work on making the narrative more concise and focused, ensuring that the paper presents a cohesive story rather than a collection of loosely connected results. To enhance clarity, we will consider incorporating more connecting and structuring elements, such as tables or flow charts, to better summarize the methods and compare the different results and their relationships.

Regarding your comment on the conclusion section, we agree that it could benefit from a more substantive discussion. We will revise this section to more clearly articulate the key takeaways from our study, emphasizing the new insights gained and their significance in the context of compound events.

We sincerely appreciate your constructive feedback and are confident that these revisions will strengthen the manuscript. Thank you once again for your thoughtful review.

49:  switch to past tense?

We appreciate your suggestion regarding the use of tense in our manuscript. We agree that consistency in tense is important for clarity and readability. Therefore, we will revise the sentence on line 49 to past tense to align with the rest of the section. The revised sentence will read:

"We shed light on the interannual influence of droughts on surface water and the impact of water scarcity and heatwaves on agriculture and forests."

71: “Zscheischler et al. (2018) further defined CEs as combinations of events that are individually not necessarily extreme, or multiple drivers and/or hazards, but in combination often lead to disproportionate impacts on people and ecosystems”

How is this different from line 68?

“3) a combination of events that are not themselves extremes but lead to an extreme event or impact when combined.”

Thank you for your careful reading of this section and for bringing the repetition to our attention. We will thoroughly revise this section to eliminate the repetition and enhance the clarity of the content.

74 “To quantify the probability of CEs in today’s and future climate is of great importance specifically adaptation planning for” → “for adaptation planning for”?

Thank you for suggesting a clearer and improved formulation. We will correct the phrasing. 

105: This is just an opinion and I appreciate that the authors also want to advertise their project with this paper but maybe they could tune down the references to climxtreme a bit? At least I can not see why it is relevant to the reader how climXtreme organized the investigation underlying this manuscript and the manuscript is already very long.

Also 134, 613, …

Thank you for pointing out the overemphasis on references to the ClimXtreme project. We appreciate your feedback and acknowledge that our enthusiasm for the project may have led to an excessive focus on it. We will take your advice and remove the sections related to internal organization and other irrelevant content to ensure a more concise and focused manuscript. 

Section 2: I am sorry but find this section quite hard to follow and at the same time it probably does not provide enough information to actually understand what was done in detail. I appreciate that it is not easy to write a concise method section for such an extensive paper but I think this could be done better. Currently the reader is thrown into more than 10 sub-sections mixing individual extreme definitions, compound events, and impacts. In addition, the level of detail somewhat differs between sections and there are many design choices that are not really reflected or compared. 

Thank you for your feedback on section 2. in our manuscript. We understand your concerns and will restructure this section to provide a better structure and a more consistent level of detail. We will further improve the organization of this section and focus on keeping it short and concise.

Just two examples: 

- the authors repeatedly state that the summer months JJA are used (lines 157, 199, 272) but in other sections they only refer to summer (145, 161) or other periods such as the hydrological year (161) or the warm season (282). Given that this study is about compounding it seems important to be clear on the properties of the different events considered and their relation.

- There are a range of relative thresholds used to define extremes. I fond some based on the 90, 95, and 98 percentiles. I’m not saying that this is inherently problematic but it should be made clear.

My suggestion would be to start with an overview section explaining the structure of this section. This could include a table summarizing the main features of each analysis to ensure a similar level of detail and ease comparing differences. 

Thank you for your very detailed remark and the examples. We will revisit the manuscript and improve the time definitions and produce a more consistent description of how extremes are defined across sub-projects. We appreciate your suggestion and will include an overview section with a table that explains the section better and also align the level of detail. 

150: Note that using a 15 day running window do define extreme thresholds (as done Fischer and Schär) of has been shown to be methodologically wrong and prone to biases recently: https://www.nature.com/articles/s41467-024-46349-x

Thank you for making us aware of this work! We will take into consideration the publication and results therein and update the analysis.

229: “The co-occurrence should be on the same day or the following day for precipitation, in the same grid cell and within a radius of 50 km, respectively.” I’m not sure I understand what it means for an event to be within the same grid cell and at the same time within 50km. If we assume the 0.25deg resolution corresponds to 25km, this would mean it can be a maximum of two grid cells away in the horizontal as an example? Is that correct? Can the authors clarify this?

Thank you for your observation. You are correct in identifying the need for clarification. The intention was to specify that for co-occurrence of events, precipitation events should either occur on the same day or the following day, and while they should be recorded in the same grid cell, they can also be considered if they occur within a radius of 50 km from that grid cell. We will revise this section and aim for a clearer and more precise description, and ensure that the relationship between grid resolution and distance is accurately represented.

274: “The compound precipitation-wind events are defined on the winter (December to February) daily mean precipitation and daily maximum surface wind. The selection of events is based on the exceedance of the 98th percentile for the period 1975-2025…”

and then later

278: “Extreme compound wind and precipitation years exceed the 20-year return levels for precipitation and wind individually, defined as the 95th percentiles for the period 1975-2025.”

Sorry but this confuses me. Can the authors clarify the difference between “compound precipitation-wind events” and “ Extreme compound wind and precipitation years”?

Thank you for this comment. You are right it is confusing and also not correct as it is described. Extreme compound wind and precipitation years are defined by exceedance of the 20-yr return levels of their winter cumulative occurrences, while (extreme) compound precipitation-wind events are defined by exceeding the 98th percentile for the 1975-2025 period of each day (or the day after for the case of precipitation) and for each grid point. We will make sure that the new description will make clear the differences between these events/definitions.

303: “a large blocking system at 500 hPa, and a double jet stream configuration is visible in the 250mb zonal wind field”

Maybe settle for one unit?

Thank you for this observation. Of course we will rephrase the section using the same unit, here, and throughout the whole manuscript:

"a large blocking system at 500 hPa and a double jet stream configuration in the 250 hPa zonal wind field"

Also I read the sentence as saying that a 500hPa blocking is visible in the 250mb zonal wind field. I think I understand what the authors try to say here but you can not see 500hPa at 250hPa by definition…

Thank you for pointing to this wrong description. The blocking at 500 hPa information stems from the paper of Roussi et al. (2023), which we have wrongly cited a bit later in the sentence. We will of course correct this. In our figure we present only 250 hPa zonal wind and anomalies. We will also restructure this whole paragraph as you have noticed allows not only for misunderstandings but also wrong conclusions. 

Figure 12: what do the dashed lines represent?

Thank you for pointing out the missing information. The dashed lines denote the climate variable thresholds that define the extreme CEs.

Figure 13: Fontsize way too small

Thank you for that observation, we will adjust the font size and make the figure more accessible to the reader. 

(b) the title says CMIP5 1975-2021, the caption says historical CMIP5 ensemble. Maybe I’m not aware that the CMIP5 historical runs where extended otherwise this is wrong.

Thank you for this comment. The historical CMIP5 ensemble used in this figure has of course been extended by Aalbers et al. (2023). We will make sure adding this information also in the caption.

“Significant model ensemble grid points are shaded dark.” Unclear what that dark means here.

Thank you for your comment. The caption and description will define exactly the meaning of dark green and dark brown.

614: “The showcases presented in this paper include multivariate, pre-conditioning, temporally and spatial Ces” 

Again, a summarizing table would be nice to give an overview.

We appreciate your suggestion for a summarizing table. We agree that a table would enhance the clarity and provide a concise overview of the key aspects discussed. We will include a summarizing table in the revised manuscript to better present the information and facilitate a clearer understanding of the content.

611-648: Delete or shorten by a lot? This is not a conclusion but basically a reiteration of the results.

Thank you for your valuable feedback. We significantly shorten this section providing a thorough conclusions based on the findings in our results section.

655: “Further studies aim at expanding the current knowledge on the complex relationships between CEs and large-scale fields at different time horizons in order to improve the detection and thus the understanding of the climate system.”

Not sure what this sentence is trying to say? What further studies?

Thank you for pointing this out. We will clarify the reference to further studies by specifying the objectives and contributions of ongoing research to enhance the detection and understanding of compound events.