More frequent flash flood events and extreme precipitation favouring atmospheric conditions in temperate regions of Europe
- 1Catchment and Ecohydrology Group (CAT), Environmental Research and Innovation, Luxembourg Institute of Science and Technology (LIST), Belvaux, 4422, Luxembourg
- 2Faculty of Science, Technology and Medicine (FSTM), University of Luxembourg, Esch-sur-Alzette, 4365, Luxembourg
- 3Institute of Water Resources and River Basin Management, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
- 4Air Navigation Administration, MeteoLux, Findel, Luxembourg
- 1Catchment and Ecohydrology Group (CAT), Environmental Research and Innovation, Luxembourg Institute of Science and Technology (LIST), Belvaux, 4422, Luxembourg
- 2Faculty of Science, Technology and Medicine (FSTM), University of Luxembourg, Esch-sur-Alzette, 4365, Luxembourg
- 3Institute of Water Resources and River Basin Management, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
- 4Air Navigation Administration, MeteoLux, Findel, Luxembourg
Abstract. In recent years, flash floods repeatedly occurred in temperate regions of central western Europe. Unlike in Mediterranean catchments, this flooding behaviour is unusual. In the past, and especially in the 1990s, floods were characterized by predictable, slowly rising water levels during winter and driven by westerly atmospheric fluxes (Pfister et al., 2004). The intention of this study is to link the recent occurrence of flash floods in central western Europe to extreme precipitation and specific atmospheric conditions to identify the cause for this apparent shift. Therefore, we hypothesise that an increase in extreme precipitation events has subsequently led to an increase in the occurrence of flash flood events in central western Europe and all that being caused by a change in the occurrence of flash flood favouring atmospheric conditions. To test this hypothesis, we compiled data on flash floods in central western Europe and selected precipitation events above 40 mm h−1 from radar data (RADOLAN, DWD). Moreover, we identified proxy parameters representative for flash flood favouring atmospheric conditions from the ERA5 reanalysis dataset. High specific humidity in the lower troposphere (q ≥ 0.004 kg kg−1), sufficient latent instability (CAPE ≥ 100 J kg−1) and weak deep-layer wind shear (DLS ≤ 10 m s−1) proved to be characteristic for long-lasting intense rainfall that can potentially trigger flash floods. These atmospheric parameters, as well as the flash flood and precipitation events were then analysed using linear models. Thereby we found significant increases in atmospheric moisture contents and increases in atmospheric instability. Parameters representing the motion and organisation of convective systems occurred slightly more often or remained unchanged in the time period from 1981–2020. Moreover, a trend in the occurrence of flash floods was confirmed. The number of precipitation events, their maximum 5-minute intensities as well as their hourly sums were however characterized by large inter-annual variations and no trends could be identified between 2002–2020. This study therefore shows that the link from atmospheric conditions via precipitation to flash floods cannot be traced down in an isolated way. The complexity of interactions is likely higher and future analyses should include other potentially relevant factors such as intra-annual precipitation patterns or catchment specific parameters.
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Judith Meyer et al.
Status: final response (author comments only)
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RC1: 'Comment on hess-2021-628', Anonymous Referee #1, 09 Jan 2022
Comments on “More frequent flash flood events and extreme precipitation favouring atmospheric conditions in temperate regions of Europe”, by Meyer et al., submitted to HESS for possible publication.
The authors examined the temporal trend in the occurrences of flash floods and extreme rainfall over the western Europe based on a compilation of flash flood dataset and radar rainfall retrieval. They link various meteorological parameters extracted from ERA5 reanalysis fields with each flash flood event. They claim that atmospheric conditions favoring extreme rainfall and flash floods are becoming more frequent. While I believe the question the authors are trying to address is absolutely important and is of great interest to the readers of HESS, their dataset and methods adopted in analysis are seriously flawed. The three hypothesis that they raised in the manuscript cannot be validated based on the existing analytical framework (see details below).
Aside from the technical issues, a key problem is that throughout the manuscript the authors do not specially define what is exactly a “flash flood” (in their perspective). We all know flash floods can be different from other types of riverine floods in various ways. However, it is never proper to simply classify floods during the summer months as flash floods (as distinguished from the winter floods). Without clarification of the basic concept, some of the sentences seem logistically biased. For instance, “The development of flash floos relies on long-lasting, extreme precipitation” (Line 108). This is not true, since extreme rainfall does not have to be “long-lasting” to generate a flash flood, although it is true for a subset of flash floods (not vice versa).
Another concern of mine is that the flash flood database is not consistent in space and time. Any trend analysis based on the dataset would not be able to generate true insights into the real world. The authors also admit that the database is non-exhaustive. I would suggest the authors to demonstrate their efforts in making the database at least consistent in time. Otherwise, people would argue whether the significant trend is due to sampling biases or not. This corresponds to their first hypothesis (Line 404-405).
In addition, the authors use cumulative statistics to quantify the occurrences of flash floods for each year. Since floods cluster in space and time, the authors need to be aware that the issue of repeated counting. This is relevant to their second hypothesis where they evaluate trend in the occurrences of extreme rainfall. It would be biased to count the number of grids with rain rate exceeding certain thresholds. The statistics thus reflect the combined effect of intensity and spatial coverage of rainfall, not changes in the frequency.
Lastly, I did not see significant increases in the proxy parameters for flash flood potential. This is mainly a concern with Fig. 5. Increases in moisture content are kind of expected according to the Clausius-Clapeyron relationships, but other than that, the other two proxy parameters show negligible significance (especially for DLS). In addition, flash floods are tied to comprehensive combinations of atmospheric conditions. By examining the trend in individual component of the comprehensive conditions as the authors did here offer limited insights into the real changes in flood potential. The threshold values are also chosen in a subjective way that needs further justification.
I would not go into any further details about the presentation of the manuscript. Some of the sections (like Introduction, Discussion) needs to be shortened and merged. These issues are relatively less important compared to the aforementioned concerns of mine.
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AC1: 'Reply on RC1', Judith Meyer, 16 Mar 2022
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2021-628/hess-2021-628-AC1-supplement.pdf
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AC1: 'Reply on RC1', Judith Meyer, 16 Mar 2022
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RC2: 'Comment on hess-2021-628', Ruben Imhoff, 10 Jan 2022
Dear authors,
Thanks for submitting this manuscript. I have attached my review.
Sincerely,
Ruben Imhoff
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AC2: 'Reply on RC2', Judith Meyer, 16 Mar 2022
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2021-628/hess-2021-628-AC2-supplement.pdf
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AC2: 'Reply on RC2', Judith Meyer, 16 Mar 2022
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RC3: 'Comment on hess-2021-628', Anonymous Referee #3, 18 Jan 2022
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AC3: 'Reply on RC3', Judith Meyer, 16 Mar 2022
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2021-628/hess-2021-628-AC3-supplement.pdf
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AC4: 'Reply on RC3', Judith Meyer, 16 Mar 2022
The comment was uploaded in the form of a supplement: https://hess.copernicus.org/preprints/hess-2021-628/hess-2021-628-AC4-supplement.pdf
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AC3: 'Reply on RC3', Judith Meyer, 16 Mar 2022
Judith Meyer et al.
Judith Meyer et al.
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