Linking torrential events in the Northern French Alps to regional and local atmospheric conditions

Blanchet, Juliette; Reverdy, Alix; Blanc, Antoine; Creutin, Jean-Dominique; Kiennemann, Périne; Evin, Guillaume

doi:https://doi.org/10.5194/nhess-2022-276

Preprints

https://doi.org/10.5194/nhess-2022-276

Preprints

18 Jan 2023

| 18 Jan 2023

Status: this discussion paper is a preprint. It has been under review for the journal Natural Hazards and Earth System Sciences (NHESS). The manuscript was not accepted for further review after discussion.

Linking torrential events in the Northern French Alps to regional and local atmospheric conditions

Juliette Blanchet, Alix Reverdy, Antoine Blanc, Jean-Dominique Creutin, Périne Kiennemann, and Guillaume Evin

Abstract. In this article we study the atmospheric conditions at the origin of damaging torrential events in the Northern French Alps over the long run, using a database of reported occurrence of damaging torrential flooding in the Grenoble conurbation since 1851. We consider seven atmospheric variables that describe the nature of the air masses involved and the possible triggers of precipitation. Using both 20CRv2c and ERA5 reanalyses, we try to isolate the variables associated with torrential events, by objectively determining which of them differ particularly from the climatology at the dates of torrential events. This analysis is done conditionally on the main types of generating atmospheric circulation derived from Lamb weather classes, namely the North-West, Southeast-Southwest and Barometric Swamp classes. Furthermore, the atmospheric variables are considered over two spatial scales – a local scale (the Grenoble conurbation) and a regional scale (the French Alps), in order to study the spatial variability of the atmospheric signature. The results show that all atmospheric variables are less discriminant for torrential events before 1950 according to 20CRv2c – this is likely more linked to 20CRv2c limitations over the remote past than a consequence of climate change. For the post-1950 period, similar atmospheric signatures are found both at local and regional scales in the North-West and Southeast-Southwest classes and for both reanalyses. In the North-West class – which is the best discriminated – humidity and particularly humidity transport (IVT) plays the greatest role. In the Southeast-Southwest class, instability potential (CAPE) is mostly at play. In the Barometric Swamp class both humidity (PWAT) and instability (CAPE) are discriminant – and even more at the local scale –, showing more mixed situations generating torrential events in this class. In total, depending on the class, torrential events are 4 to 14 times more likely when the respective discriminant variables are extreme (typically above their 0.95-quantile).

Received: 19 Nov 2022 – Discussion started: 18 Jan 2023

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Juliette Blanchet, Alix Reverdy, Antoine Blanc, Jean-Dominique Creutin, Périne Kiennemann, and Guillaume Evin

Status: closed

RC1:
'Comment on nhess-2022-276', Anonymous Referee #1, 16 Feb 2023
The paper nhess-2022-276 “Linking torrential events in the Northern French Alps to regional and local atmospheric conditions” shows several weakness points and in my opinion should be rejected.
First of all, the title is not appealing: torrential events are “naturally” linked to climatic conditions.
By reading the abstract I guess that the paper is strictly related to the study area. No general conclusions are present, nothing that could be useful in places other than the study area, and this is not in line with the requirements of an international journal. Here, the aim of the paper is not well focused. Even the quotation of climate change seems not strictly related to the analysis performed in the paper.
In the introduction the problem analyzed seems the forecast of torrential events, but at the end of the introduction it seems that the authors work in a simple descriptive way, without finalize the work to something like a classification of severity levels of the effects according to the kind of conditions that triggered them or to something else.
The structure of the paper is not appropriate. In the sections DATA, the authors actually just describe us what they did, and not describe the kind of data required to perform the research. And in METHOD they describe their case study. Then the paper does not supply a clear vision of what someone needs to perform the same research in another geographical area
Moreover, there are no sections describing in a simple way the aim of the paper and the approach followed to reach it. Maybe a flow chart of DATA and STEPS of the METHODOLOGY could help to understand.
Some of the data used are actually NOT described. “20CRv2c (in short 20CR, Compo et al., 2011) covers the period 1851-2014 with a spatial resolution of 2◦. In this article, we use the mean member but results with members 1 and 2 (arbitrarily tested as they are independent) are very similar (not shown). In addition, in order to study the impact of the spatial resolution, the ERA5 reanalysis”. This is simple and clear for you but not for readers that never used this kind of data and could be interested in doing. If I would like to apply the same “approach” in another study area I don’t know what data I need and were could I find them!
Even the information that are obtained from the analysis are sparse all around the sections and it is difficult to identify what can be useful in the practical management of torrential events. Maybe something like a table of the main findings could be useful.
I think that the paper must be completely re-written, putting light more on the scientific question that the paper aims to face and less on the study area (that currently is the focus of the paper). As is the paper does not show interest for an international audience.
Conclusions lack to finalize the results of the research, because not give to the reader the explanation of how these results will help in the management of torrential events or in some other framework.
Further elements in the following:
Figure 1 could represent something everywhere. I imagine that the authors know very well their study area but why the reader should? Geographical maps in scientific journal must have a national map inside, depicting the country where it is located, and almost the north arrow…Moreover, in the caption, RTM are quoted, but the definition of this acronym is in the next page so the reader doesn’t know, at this page, what is the meaning.

Tab 1 and 2: there are no headings on the columns

Table A1 should be placed after Appendix A

The majority of formulas are not numbered

It is unclear how the torrential events are identified and selected

L 216: actually, these are not 4 DB, but instead 2 DB split in two according to the period

L 223: 20CR-1 e 20CR-2 have different length but can be compared because in the period 1930-1940 no events were recorded. Could they be comparable even if some events had occurred?
Citation: https://doi.org/10.5194/nhess-2022-276-RC1
- AC1: 'Reply on RC1', J. Blanchet, 17 Mar 2023
  
  We warmly thank the Reviewer for his/her comprehensive review. From our perspective, most of the comments made can be easily addressed. They mostly result from misunderstandings due to a lack of explanations of the main objectives of this study and of the possible use of the results. We answer these comments in the attached document.
  Best regards
  The authors.
  
  Citation: https://doi.org/10.5194/nhess-2022-276-AC1
RC2:
'Comment on nhess-2022-276', Anonymous Referee #2, 06 Mar 2023

I reviewed the paper “Linking torrential events in the Northern French Alps to regional and local atmospheric conditions”.
The paper shows a structure difficult to follow, strictly related to the study area and without an international general setting.
ABSTRACT. The abstract describes what authors did, but does not report why. And above all, it does not say what can be the possible use of the results of this analysis.
INTRODUCTION. In the introduction, the papers quoted as “state of the art” are mainly old or very old (the only recent references concern the effects of precipitations (Creutin 2022) and not the precipitations and atmospheric circulation).
At the end of this section, describing the aim of the paper:
“Together with the extremeness of the studied events - the torrential events correspond to return periods of order 2-3 years at the scale of the conurbation -, a benefit of our work in comparison to Turkington et al. (2014) is to study the driving atmospheric conditions with respect to the main types of atmospheric circulation”. It is a little unclear as aim of the paper. Figure 1 does not present reference elements, it is obscure where this area is.
DATA. Why a reader should search a figure published in another paper as suggested here? “The torrential units show varied geomorphological characteristics with surfaces of less than 1 km2 up to 170 km2 for the Gresse torrent, despite a majority of basins of less than 20 km2 (FIGURE 3 OF CREUTIN ET AL., 2022)”.
TORRENTIAL EVENTS. Here the description is verbose: a table could have been more explicative.
Here: “…the hydrometeorological events of Creutin et al. (2022) can last from one to several days and can involve ONE OR SEVERAL RIVERS AND/OR ONE OR SEVERAL TORRENTS. In total, the database presents 104 hydrometeorological events between 1851 and 2019. We extract from this database 70 events INVOLVING AT LEAST ONE TORRENT”. In my opinion this is contradictory or not properly explained…
Moreover: “. These torrential events have return periods of order 2-3 years at the scale of the conurbation but of order 6-60 years at the scale of the torrential units (Figure 1)”. This should be explained, and moreover in figure 1 there are no mention of return periods.
“Finally, each 3-day sequence between 1851 and 2019 can be flagged as either an event sequence - when centered around the reference day of a torrential event -, or a nonevent sequence – otherwise.” unclear…
MAIN TYPES OF ATMOSPHERIC CIRCULATION: “…we consider two atmospheric reanalyses which are spatial and temporal interpolations of past meteorological measurements using data assimilation techniques and a meteorological model. 20CRv2c (in short 20CR, Compo et al., 2011) covers the period 1851-2014 with a spatial resolution of 2◦. In this article, we use the mean member but results with members 1 and 2 (arbitrarily tested as they are independent) are very similar (not shown)”.
This is the only (not sufficient) explanation about data used, that are “described” in a section concerning atmospheric circulation, not DATA USED. So in this section there is a confusion between DATA, DESCRIPTION OF STUDY AREA CHARACTERISTICS and RESULTS OF THE ANALYSIS, with also literature quotations concerning approaches to study atmospheric circulation… it is very complex and not in line with the classical way in which scientific paper are structured (dividing data from method and results).
METHODOLOGY. The methodology is not clearly described as well as the data sources and data types used. The very large number of acronyms not reported in a list make very difficult to follow the paper.
I understand that the authors are treating a large amount of data and it is difficult to summarize results of the analysis but they lack either clarity or consistency in presentation.
The aim of the paper, the research question/s and the possible uses (especially outside the study area) are not clearly stated.
The method starts with: “OUR GOAL IS TO DETERMINE WHICH ATMOSPHERIC VARIABLES ARE VERY DIFFERENT FROM THE CLIMATOLOGY THE DAYS OF THE TORRENTIAL EVENTS.” Maybe there is something missing because I cannot understand this aim. Climatology for me is a discipline but maybe I’m wrong.
After this, the methodology is explained in a confuse manner, presented more like a description of what the authors did then like “how to do to perform the same experiment in another area”. The steps of the methodology are not described, it seems made of a series of actions having no linear path, without a chronological sequence to follow.
In the method sections, parts of data elaboration are included and the reader is unable to understand if these questions are related to the methodology or depends on the specific data used (“We note that the two periods have different lengths (99 versus 65 years), however considering two equal periods almost does not change the results due to the absence of events in the 1930-1940s (see Figure 3). Comparison of 20CR-2 and ERA5-3 allows assessing whether different reanalyses see the same driving atmospheric variables over the recent period and the same region. Note that we have also compared over the common period 1950-2014 and the results are very similar”).
Conclusions do not explain the way in which the results obtained can be practically or theoretically applied.
In my opinion this paper cannot be accepted.

Citation: https://doi.org/10.5194/nhess-2022-276-RC2
- AC2: 'Reply on RC2', J. Blanchet, 17 Mar 2023
  
  We warmly thank the Reviewer for his/her comprehensive review. From our perspective, most of the comments made can be easily addressed. They mostly result from misunderstandings due to a lack of explanations of the main objectives of this study and of the possible use of the results. We answer these comments in the attached document.
  Best regards,
  The authors.
  
  Citation: https://doi.org/10.5194/nhess-2022-276-AC2

Status: closed

RC1:
'Comment on nhess-2022-276', Anonymous Referee #1, 16 Feb 2023
The paper nhess-2022-276 “Linking torrential events in the Northern French Alps to regional and local atmospheric conditions” shows several weakness points and in my opinion should be rejected.
First of all, the title is not appealing: torrential events are “naturally” linked to climatic conditions.
By reading the abstract I guess that the paper is strictly related to the study area. No general conclusions are present, nothing that could be useful in places other than the study area, and this is not in line with the requirements of an international journal. Here, the aim of the paper is not well focused. Even the quotation of climate change seems not strictly related to the analysis performed in the paper.
In the introduction the problem analyzed seems the forecast of torrential events, but at the end of the introduction it seems that the authors work in a simple descriptive way, without finalize the work to something like a classification of severity levels of the effects according to the kind of conditions that triggered them or to something else.
The structure of the paper is not appropriate. In the sections DATA, the authors actually just describe us what they did, and not describe the kind of data required to perform the research. And in METHOD they describe their case study. Then the paper does not supply a clear vision of what someone needs to perform the same research in another geographical area
Moreover, there are no sections describing in a simple way the aim of the paper and the approach followed to reach it. Maybe a flow chart of DATA and STEPS of the METHODOLOGY could help to understand.
Some of the data used are actually NOT described. “20CRv2c (in short 20CR, Compo et al., 2011) covers the period 1851-2014 with a spatial resolution of 2◦. In this article, we use the mean member but results with members 1 and 2 (arbitrarily tested as they are independent) are very similar (not shown). In addition, in order to study the impact of the spatial resolution, the ERA5 reanalysis”. This is simple and clear for you but not for readers that never used this kind of data and could be interested in doing. If I would like to apply the same “approach” in another study area I don’t know what data I need and were could I find them!
Even the information that are obtained from the analysis are sparse all around the sections and it is difficult to identify what can be useful in the practical management of torrential events. Maybe something like a table of the main findings could be useful.
I think that the paper must be completely re-written, putting light more on the scientific question that the paper aims to face and less on the study area (that currently is the focus of the paper). As is the paper does not show interest for an international audience.
Conclusions lack to finalize the results of the research, because not give to the reader the explanation of how these results will help in the management of torrential events or in some other framework.
Further elements in the following:
Figure 1 could represent something everywhere. I imagine that the authors know very well their study area but why the reader should? Geographical maps in scientific journal must have a national map inside, depicting the country where it is located, and almost the north arrow…Moreover, in the caption, RTM are quoted, but the definition of this acronym is in the next page so the reader doesn’t know, at this page, what is the meaning.

Tab 1 and 2: there are no headings on the columns

Table A1 should be placed after Appendix A

The majority of formulas are not numbered

It is unclear how the torrential events are identified and selected

L 216: actually, these are not 4 DB, but instead 2 DB split in two according to the period

L 223: 20CR-1 e 20CR-2 have different length but can be compared because in the period 1930-1940 no events were recorded. Could they be comparable even if some events had occurred?
Citation: https://doi.org/10.5194/nhess-2022-276-RC1
- AC1: 'Reply on RC1', J. Blanchet, 17 Mar 2023
  
  We warmly thank the Reviewer for his/her comprehensive review. From our perspective, most of the comments made can be easily addressed. They mostly result from misunderstandings due to a lack of explanations of the main objectives of this study and of the possible use of the results. We answer these comments in the attached document.
  Best regards
  The authors.
  
  Citation: https://doi.org/10.5194/nhess-2022-276-AC1
RC2:
'Comment on nhess-2022-276', Anonymous Referee #2, 06 Mar 2023

I reviewed the paper “Linking torrential events in the Northern French Alps to regional and local atmospheric conditions”.
The paper shows a structure difficult to follow, strictly related to the study area and without an international general setting.
ABSTRACT. The abstract describes what authors did, but does not report why. And above all, it does not say what can be the possible use of the results of this analysis.
INTRODUCTION. In the introduction, the papers quoted as “state of the art” are mainly old or very old (the only recent references concern the effects of precipitations (Creutin 2022) and not the precipitations and atmospheric circulation).
At the end of this section, describing the aim of the paper:
“Together with the extremeness of the studied events - the torrential events correspond to return periods of order 2-3 years at the scale of the conurbation -, a benefit of our work in comparison to Turkington et al. (2014) is to study the driving atmospheric conditions with respect to the main types of atmospheric circulation”. It is a little unclear as aim of the paper. Figure 1 does not present reference elements, it is obscure where this area is.
DATA. Why a reader should search a figure published in another paper as suggested here? “The torrential units show varied geomorphological characteristics with surfaces of less than 1 km2 up to 170 km2 for the Gresse torrent, despite a majority of basins of less than 20 km2 (FIGURE 3 OF CREUTIN ET AL., 2022)”.
TORRENTIAL EVENTS. Here the description is verbose: a table could have been more explicative.
Here: “…the hydrometeorological events of Creutin et al. (2022) can last from one to several days and can involve ONE OR SEVERAL RIVERS AND/OR ONE OR SEVERAL TORRENTS. In total, the database presents 104 hydrometeorological events between 1851 and 2019. We extract from this database 70 events INVOLVING AT LEAST ONE TORRENT”. In my opinion this is contradictory or not properly explained…
Moreover: “. These torrential events have return periods of order 2-3 years at the scale of the conurbation but of order 6-60 years at the scale of the torrential units (Figure 1)”. This should be explained, and moreover in figure 1 there are no mention of return periods.
“Finally, each 3-day sequence between 1851 and 2019 can be flagged as either an event sequence - when centered around the reference day of a torrential event -, or a nonevent sequence – otherwise.” unclear…
MAIN TYPES OF ATMOSPHERIC CIRCULATION: “…we consider two atmospheric reanalyses which are spatial and temporal interpolations of past meteorological measurements using data assimilation techniques and a meteorological model. 20CRv2c (in short 20CR, Compo et al., 2011) covers the period 1851-2014 with a spatial resolution of 2◦. In this article, we use the mean member but results with members 1 and 2 (arbitrarily tested as they are independent) are very similar (not shown)”.
This is the only (not sufficient) explanation about data used, that are “described” in a section concerning atmospheric circulation, not DATA USED. So in this section there is a confusion between DATA, DESCRIPTION OF STUDY AREA CHARACTERISTICS and RESULTS OF THE ANALYSIS, with also literature quotations concerning approaches to study atmospheric circulation… it is very complex and not in line with the classical way in which scientific paper are structured (dividing data from method and results).
METHODOLOGY. The methodology is not clearly described as well as the data sources and data types used. The very large number of acronyms not reported in a list make very difficult to follow the paper.
I understand that the authors are treating a large amount of data and it is difficult to summarize results of the analysis but they lack either clarity or consistency in presentation.
The aim of the paper, the research question/s and the possible uses (especially outside the study area) are not clearly stated.
The method starts with: “OUR GOAL IS TO DETERMINE WHICH ATMOSPHERIC VARIABLES ARE VERY DIFFERENT FROM THE CLIMATOLOGY THE DAYS OF THE TORRENTIAL EVENTS.” Maybe there is something missing because I cannot understand this aim. Climatology for me is a discipline but maybe I’m wrong.
After this, the methodology is explained in a confuse manner, presented more like a description of what the authors did then like “how to do to perform the same experiment in another area”. The steps of the methodology are not described, it seems made of a series of actions having no linear path, without a chronological sequence to follow.
In the method sections, parts of data elaboration are included and the reader is unable to understand if these questions are related to the methodology or depends on the specific data used (“We note that the two periods have different lengths (99 versus 65 years), however considering two equal periods almost does not change the results due to the absence of events in the 1930-1940s (see Figure 3). Comparison of 20CR-2 and ERA5-3 allows assessing whether different reanalyses see the same driving atmospheric variables over the recent period and the same region. Note that we have also compared over the common period 1950-2014 and the results are very similar”).
Conclusions do not explain the way in which the results obtained can be practically or theoretically applied.
In my opinion this paper cannot be accepted.

Citation: https://doi.org/10.5194/nhess-2022-276-RC2
- AC2: 'Reply on RC2', J. Blanchet, 17 Mar 2023
  
  We warmly thank the Reviewer for his/her comprehensive review. From our perspective, most of the comments made can be easily addressed. They mostly result from misunderstandings due to a lack of explanations of the main objectives of this study and of the possible use of the results. We answer these comments in the attached document.
  Best regards,
  The authors.
  
  Citation: https://doi.org/10.5194/nhess-2022-276-AC2

Juliette Blanchet, Alix Reverdy, Antoine Blanc, Jean-Dominique Creutin, Périne Kiennemann, and Guillaume Evin

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Short summary

We study the atmospheric conditions at the origin of damaging torrential events in the Northern French Alps over the long run. We consider seven atmospheric variables that describe the nature of the air masses involved and the possible triggers of precipitation and we try to isolate the most discriminating variables. The results show that humidity and particularly humidity transport plays the greatest role under westerly flows while instability potential is mostly at play under southerly flows.


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