the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Linking torrential events in the Northern French Alps to regional and local driving atmospheric conditions
Abstract. The Alpine region is strongly affected by torrential floods, sometimes leading to severe negative impacts on society, economy, and the environment. Understanding such natural hazards and their drivers is essential to mitigate related risks. In this article we propose a statistical method based on a mere discriminative index to objectively isolate the atmospheric variables associated with torrential events with long return periods. The method is applied to the Grenoble Metropolitan area in the Northern French Alps using a list of dates of damaging torrential events 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. Two spatial scales are considered – a local scale (the Grenoble conurbation) and a regional scale (the French Alps) –, in order to study the spatial variability of the atmospheric signature. 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. The results show that a simple discriminative index – the so-called silhouette index – is able 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. All atmospheric variables turn out to be less discriminant for torrential events before 1950 according to 20CRv2c – this is likely linked to 20CRv2c issues over the remote past. For the post-1950 period, in the North-West class – which is both the most frequent class generating torrential events and the best discriminated – humidity and particularly humidity transport (IVT) plays the greatest role. In the Southeast-Southwest class – the second most frequent class generating torrential events–, 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. The gain in prediction provided by the discriminant variables is substantial. 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). Although the results are likely to be region-dependent worldwide, the methodology used in this article is generic and could be used elsewhere to find the most discriminating atmospheric variables – provided a list of flooding dates is available. It is also remarkable that the same atmospheric variables with the same discriminative power are found whether we consider them at local or regional scale. This means that, although torrential events are triggered by very local precipitation, the atmospheric signature for such events is actually much wider. Thus, although the present study applies to a small region of the Northern French Alps, it is reasonable to presume that similar results would apply to other torrential catchments in the French Alps.
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RC1: 'Comment on hess-2023-197', Anonymous Referee #1, 30 Oct 2023
The current work submitted for review attempts to characterise and identify the critical atmospheric driving conditions of torrential (flash) floods at a local scale using an event dataset collected over the Grenoble Metropolitan area in France and then tries to extrapolate the same over a larger regional scale (Northern French Alps). While the methods applied seem robust enough and the results plausible, I have some significant concerns which should be considered before the paper can be considered for publication in HESS.
My main concern is related to whether the paper is the best fit for the overall scope and target audience of HESS. In my opinion, the current work does not significantly advance the current state of the art in research linked to such extremes and has some critical shortcomings.
Critical Comment
- In the beginning of Methods (203-204): the authors remark that “Our goal is to determine which atmospheric variables are very different from the climatology the days of the torrential event”. However, in the 70 events considered for the subsequent analysis (117-118), b) one or several torrents and no river (purely torrential events), c) one or several torrents and one or several rivers (mixed events) are considered. Maybe I missed something…wouldn’t this lead to a situation where flooding episodes in which riverine processes (saturation excess/snowmelt runoff) played a major role are also included in the torrent event classification? The authors seem to neglect the purely riverine events. In my opinion, the riverine events could have been utilised to overcome the limitations of including the mixed events, or I would have only used purely torrential events.
- The authors state that 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. However, they then proceed to Furthermore events in the HP class are quite discordant between 20CR and ERA5 - half of the HP events of 20CR over 1950-2014 are otherwise classified in ERA5. For these reasons, the HP class is excluded from the analysis. Isn’t it also worthwhile to check why the coarser scale 20CR and finer ERA5 has difference in the HP class (which from Fig 2 looks to be the only class having a high pressure system over the study area). It is at least expected that the authors provide more explanation on the discrepancy before just ignoring them in subsequent analysis. They also exclude the E-NE class stating it has very few events and is not usually associated with high precipitation; however, that only makes it more worthwhile to explore why such events occur and what are the driving unusual atmospheric conditions behind them.
- I am also a bit confused about the spatial scales and the averaging used in this study. Are the NEP calculated for each grid point for both 20CR and ERA5 or do you average out all the grids and then calculate the probabilities based on the spatial average time series? Maybe I missed this information; however, this should be emphasised together with the uncertainties involved.
- The paper lacks a good discussion section linking the results of the work to the overall research gaps in the field of both extreme events and hydrology. While interdisciplinary studies like this are indeed valuable for the hydrological community, it is expected that the authors will provide a strong discussion linking their atmospheric research to catchment scale and more local water cycle changes. I also had a feeling that some of the content of the manuscript seems very clear and straightforward to the authors but is not so clear to more general readers. For example, Figure 1 doesn’t follow the acceptable standards for publication in an international journal. The north arrow and scale bar are missing. It is not clear to me what a torrential unit means. Is it linked to the geomorphology of the study region, or is it linked to the actual event reporting convention followed by the agency?
- In the conclusions, the authors noted that – “A remarkable result of our study….. the discriminant atmospheric variables are as discriminant whether we consider them at local or regional”. Are these results really remarkable? The authors themselves note that (185-190), as Grenoble is at the border between 2 pixels of 20CR, we cannot consider a smaller region than the whole French Alps for 20CR. Doesn’t this mean we move ahead with the assumption that local-scale site specific atmospheric conditions are indeed related to the dynamics captured by the coarser scale product? I would recommend defining the local and regional scale more strictly here to close these inconsistencies. Furthermore, spatially averaging (which is not well defined in this paper) at the so-called local and regional scales would add significant uncertainties to this methodology. By default, the spatial averages at both scales should indeed contain information complimentary to each other.
- The authors end the Introduction by stating – “Our work makes three contributions to the work of Turkington et al. (2014).” I expected a much more concise presentation about the work and its proposed relevant outcomes for understanding hydrological functioning rather than simply stating how this work is different from another similar study carried out around nine years ago. Furthermore, the first advancement is confusing (As far as we know, no study applied such an approach at torrential scale before the 1950s), as it is later stated that the period before 1950 is not being investigated fully due to inconsistencies in the 20CR model (Given these discrepancies, we were unfortunately unable to study the nonstationarity of the driving atmospheric conditions. Thus, the rest of the paper focuses on the post-1950 period)
Minor comments:
- In conclusion, the authors again state The fact that we were able to find discriminant signatures at regional scale is a good hope that global climate models such as CMIP6. To my understanding, the grid resolution of CMIP6 is at least comparable to the resolution of the 20CR model used here. Why not use the historical CMIP6 model ensemble directly at the so-called regional scale?
- The abstract needs to be restructured for better readability. I would omit the part about applying the method elsewhere..
- The terms ERA5-3 and ERA5-4 is actually confusing because you also talk about 3 day windows.. I would recommend naming them as ERA5-A for Alps and ERA5-G for Grenoble.
- 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 – It is not clear to me what is meant here.
- In Figures 6-8, the authors only show one atmospheric variable (IVT, CAPE & PWAT) for each class (NW, SE-SW and BS respectively), however, it would be interesting to see the three variables for all the three events so that a ready comparison is possible. This could help strengthen the argument about different drivers for different events.
- I could also find quite some missing words and grammatical inconsistencies. The draft could benefit from a fresh reading to smoothen out such errors.Overall, in view of the critical shortcomings, I recommend the paper to be rejected.
Thank You!Citation: https://doi.org/10.5194/hess-2023-197-RC1 - AC1: 'Reply on RC1', J. Blanchet, 10 Jan 2024
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RC2: 'Comment on hess-2023-197', Anonymous Referee #2, 03 Nov 2023
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2023-197/hess-2023-197-RC2-supplement.pdf
- AC2: 'Reply on RC2', J. Blanchet, 10 Jan 2024
Status: closed
-
RC1: 'Comment on hess-2023-197', Anonymous Referee #1, 30 Oct 2023
The current work submitted for review attempts to characterise and identify the critical atmospheric driving conditions of torrential (flash) floods at a local scale using an event dataset collected over the Grenoble Metropolitan area in France and then tries to extrapolate the same over a larger regional scale (Northern French Alps). While the methods applied seem robust enough and the results plausible, I have some significant concerns which should be considered before the paper can be considered for publication in HESS.
My main concern is related to whether the paper is the best fit for the overall scope and target audience of HESS. In my opinion, the current work does not significantly advance the current state of the art in research linked to such extremes and has some critical shortcomings.
Critical Comment
- In the beginning of Methods (203-204): the authors remark that “Our goal is to determine which atmospheric variables are very different from the climatology the days of the torrential event”. However, in the 70 events considered for the subsequent analysis (117-118), b) one or several torrents and no river (purely torrential events), c) one or several torrents and one or several rivers (mixed events) are considered. Maybe I missed something…wouldn’t this lead to a situation where flooding episodes in which riverine processes (saturation excess/snowmelt runoff) played a major role are also included in the torrent event classification? The authors seem to neglect the purely riverine events. In my opinion, the riverine events could have been utilised to overcome the limitations of including the mixed events, or I would have only used purely torrential events.
- The authors state that 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. However, they then proceed to Furthermore events in the HP class are quite discordant between 20CR and ERA5 - half of the HP events of 20CR over 1950-2014 are otherwise classified in ERA5. For these reasons, the HP class is excluded from the analysis. Isn’t it also worthwhile to check why the coarser scale 20CR and finer ERA5 has difference in the HP class (which from Fig 2 looks to be the only class having a high pressure system over the study area). It is at least expected that the authors provide more explanation on the discrepancy before just ignoring them in subsequent analysis. They also exclude the E-NE class stating it has very few events and is not usually associated with high precipitation; however, that only makes it more worthwhile to explore why such events occur and what are the driving unusual atmospheric conditions behind them.
- I am also a bit confused about the spatial scales and the averaging used in this study. Are the NEP calculated for each grid point for both 20CR and ERA5 or do you average out all the grids and then calculate the probabilities based on the spatial average time series? Maybe I missed this information; however, this should be emphasised together with the uncertainties involved.
- The paper lacks a good discussion section linking the results of the work to the overall research gaps in the field of both extreme events and hydrology. While interdisciplinary studies like this are indeed valuable for the hydrological community, it is expected that the authors will provide a strong discussion linking their atmospheric research to catchment scale and more local water cycle changes. I also had a feeling that some of the content of the manuscript seems very clear and straightforward to the authors but is not so clear to more general readers. For example, Figure 1 doesn’t follow the acceptable standards for publication in an international journal. The north arrow and scale bar are missing. It is not clear to me what a torrential unit means. Is it linked to the geomorphology of the study region, or is it linked to the actual event reporting convention followed by the agency?
- In the conclusions, the authors noted that – “A remarkable result of our study….. the discriminant atmospheric variables are as discriminant whether we consider them at local or regional”. Are these results really remarkable? The authors themselves note that (185-190), as Grenoble is at the border between 2 pixels of 20CR, we cannot consider a smaller region than the whole French Alps for 20CR. Doesn’t this mean we move ahead with the assumption that local-scale site specific atmospheric conditions are indeed related to the dynamics captured by the coarser scale product? I would recommend defining the local and regional scale more strictly here to close these inconsistencies. Furthermore, spatially averaging (which is not well defined in this paper) at the so-called local and regional scales would add significant uncertainties to this methodology. By default, the spatial averages at both scales should indeed contain information complimentary to each other.
- The authors end the Introduction by stating – “Our work makes three contributions to the work of Turkington et al. (2014).” I expected a much more concise presentation about the work and its proposed relevant outcomes for understanding hydrological functioning rather than simply stating how this work is different from another similar study carried out around nine years ago. Furthermore, the first advancement is confusing (As far as we know, no study applied such an approach at torrential scale before the 1950s), as it is later stated that the period before 1950 is not being investigated fully due to inconsistencies in the 20CR model (Given these discrepancies, we were unfortunately unable to study the nonstationarity of the driving atmospheric conditions. Thus, the rest of the paper focuses on the post-1950 period)
Minor comments:
- In conclusion, the authors again state The fact that we were able to find discriminant signatures at regional scale is a good hope that global climate models such as CMIP6. To my understanding, the grid resolution of CMIP6 is at least comparable to the resolution of the 20CR model used here. Why not use the historical CMIP6 model ensemble directly at the so-called regional scale?
- The abstract needs to be restructured for better readability. I would omit the part about applying the method elsewhere..
- The terms ERA5-3 and ERA5-4 is actually confusing because you also talk about 3 day windows.. I would recommend naming them as ERA5-A for Alps and ERA5-G for Grenoble.
- 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 – It is not clear to me what is meant here.
- In Figures 6-8, the authors only show one atmospheric variable (IVT, CAPE & PWAT) for each class (NW, SE-SW and BS respectively), however, it would be interesting to see the three variables for all the three events so that a ready comparison is possible. This could help strengthen the argument about different drivers for different events.
- I could also find quite some missing words and grammatical inconsistencies. The draft could benefit from a fresh reading to smoothen out such errors.Overall, in view of the critical shortcomings, I recommend the paper to be rejected.
Thank You!Citation: https://doi.org/10.5194/hess-2023-197-RC1 - AC1: 'Reply on RC1', J. Blanchet, 10 Jan 2024
-
RC2: 'Comment on hess-2023-197', Anonymous Referee #2, 03 Nov 2023
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2023-197/hess-2023-197-RC2-supplement.pdf
- AC2: 'Reply on RC2', J. Blanchet, 10 Jan 2024
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