An attempt to explain recent changes in European snowfall extremes

Faranda, Davide

doi:https://doi.org/10.5194/wcd-1-445-2020

Articles | Volume 1, issue 2

https://doi.org/10.5194/wcd-1-445-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/wcd-1-445-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 1, issue 2

Research article

|

04 Sep 2020

Research article |

| 04 Sep 2020

An attempt to explain recent changes in European snowfall extremes

Davide Faranda

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Final revised paper (published on 04 Sep 2020)
Preprint (discussion started on 18 Dec 2019)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Recommend to resubmit after substantial revision', Anonymous Referee #1, 16 Jan 2020
- AC1: 'Answer to review 1', Davide Faranda, 13 Mar 2020
RC2: 'Review of « An attempts to explain recent trends in European snowfall extremes » by D. Faranda', Anonymous Referee #2, 06 Feb 2020
- AC2: 'Answer to review 2', Davide Faranda, 13 Mar 2020
EC1: 'Editorial comment', Christian M. Grams, 19 Mar 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Davide Faranda on behalf of the Authors (13 Apr 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (01 May 2020) by Christian Grams

RR by Anonymous Referee #1 (16 May 2020)

For final publication, the manuscript should be

accepted as is

accepted subject to technical corrections

accepted subject to minor revisions

reconsidered after major revisions

I would be willing to review the revised paper, if the editor considers it necessary.

I am not willing to review the revised paper.

rejected

Suggestions for revision or reasons for rejection

Comments to "An attempt to explain recent changes in European snowfall extremes" by Davide Faranda (previous Reviewer #1)

I would thank you for your substantial modification of the manuscript. Now I evaluate that this manuscript is greatly improved in quality and becomes suitable for the WCD paper, because there are rich data analyses and discussion for atmospheric fields that cause the increasing trends of local heavy snowfall. Although I would like to recommend minor revision for the manuscript, I think some questions or concerns are still remained to be addressed before the publication. I have the following comments and please consider for the revised manuscript.

1. Additional data analyses are nice for the thermodynamics and dynamics to explain the changes of heavy snowfall. However, I am wondering the reason (interpretation) why the more frequent AR pattern appears during the heavy snowfall events in the recent period? At first I thought this is because the frequency of the pattern increases during the recent period, yet the author writes "whereas the same evidence does not exist for the increase in Atlantic Ridge patterns" (L293-294). In addition, the discussion with Figure 11 seems not to answer the question, since it rather relates to the (synoptic) flow patterns for heavy snowfall events but not the (large-scale) low-frequency variability patterns. Please add more discussion in the revised manuscript.

2. It is difficult to agree to use snowfall in reanalysis as the basis to select the 4 countries where heavy snowfall events increase, though I agree that the author focuses on the 4 countries. Can you reconstruct the logic in L169-174 and discussion for Fig. 6, can you find indirect evidence in observation such as precipitation rather than snowfall itself to explain the increasing trend of maximum snowfall? Otherwise, please emphasize throughout the text, such as abstract, that the increase of heavy snowfall is only in the reanalysis dataset.

3. Time scales of atmospheric phenomena that drive the heavy snowfall events seem ambiguous. Although the author mentions to focus on synoptic and daily time scales (L38-39), synoptic phenomena and the low-frequency variability such as NAO or blocking are discussed without distinguishing. Low-frequency variability is not in synoptic time scale, yet there seems no description for the distinction. Therefore description in the paragraph L41-62 should be strengthened. Also, does the author use "weather regimes" same as low-
frequency variability? I cannot find their relation.

Minor comments:
- You may change the significant level for snowfall difference (Figs. 3-4) with 10% confidence level, since it is difficult to find out the trend of (extreme) snowfall.

- Significance tests is also needed for the data analysis of atmospheric fields (Figs 7-9) with typically 5 or 1% confidence level.

- Is it possible to comment what are origins of synoptic cyclones in Fig 9? Are they extratropical cyclones traveling from the Atlantic or local meso-scale cyclones appearing in specific regions?

- L242 "Following the approaches of Nakamura et al. (1997) and Kawase et al. (2016)": these citations here seem strange, because they carefully analyzed atmospheric fields and investigate mechanisms behind. The paragraph is rather related to introduce weather regimes or probably low-frequency variability that can modulate synoptic fields and local snowfall events.

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RR by Anonymous Referee #2 (25 May 2020)

For final publication, the manuscript should be

accepted as is

accepted subject to technical corrections

accepted subject to minor revisions

reconsidered after major revisions

I would be willing to review the revised paper, if the editor considers it necessary.

I am not willing to review the revised paper.

rejected

Suggestions for revision or reasons for rejection

General comments :
The article has been deeply rewritten, mainly following the Reviewers’ comments. In particular the analysis is now performed at country level rather than at regional level, which I definitively support. There are also new analyses on CAPE, 2m temperatures and weather regimes. However I’m still puzzled about several points:
- Although the author claims that the agreement between ERA5 and EOBS is “remarkable” (L. 121), I see in Figures 1-2 quite large differences, and not only for Turkey (the color scale doesn’t ease comparison but, e.g., there seems to be around 25-50% difference in all Scandinavia). It seems to me that ERA5 tends to overestimate EOBS.
- All the results of section 4 to explain positive trends in extreme snowfall are based on four countries : Albania (AL), Macedonia (ME), Switzerland (CH) and Turkey (TR). These countries are chosen because they show “the largest positive changes” in maximum snowfall in ERA 5 (L. 173). However according to Figure 4, three of these countries (namely CH, ME, TR) show strong negative trends in EOBS ! Therefore how much confidence can we have in the results for explaining positive trends? Note that the positive trend in ERA5 for Switzerland is in contradiction with several other studies based on snow stations (e.g. Scherrer et al. 2013, Marty and Blanchet, 2012)
- What the author calls “Macedonia” in all the article seems actually to be the Montenegro (which indeed shows a positive trend in both ERA5 and EOBS)
- According to Figure 4, there are only 3 countries with strong positive trends in both ERA5 and EOBS: Bosnia, Montenegro, Albania. All three are located on the eastern flank of the Adriatic Sea. Therefore I suggest focusing on these three countries to attempt explaining positive trends.
- I didn’t understand the results of the weather regime analysis. Whereas AL and ME show similar sea level pressure fields for the maxima, CH and TR show very different fields. Therefore isn’t it inconsistent that for the four countries AR and BLO are found to be by far the most frequent weather regimes during the days of maximum snowfalls?

Detailed comments:
- L 121 “is remarkable”: as said above it seems to me there are quite large differences for different countries. To ease comparison I suggest 1) using a color scale with larger gradient, 2) showing the ratio ERA5/EOBS.
- Figure 1c: why is part of Turkey missing ?
- L 162 “notably ion the Balkans”: I’d be more moderated on the Balkans because EOBS and ERA5 only agree on 3 countries.
- L 173: “the largest positive changes, namely Albania (AL), Macedonia (ME), Switzerland (CH) and Turkey (TR)”: as already said, among these countries actually ERA and EOBS only agree on AL…
- In all Section 4, don’t you mean Montenegro rather than Macedonia?
- Figure 6 and L. 181-184: the difference are so small that it seems to me to be only sampling variability.
- L 188 “for Switzerland maxima of snowfalls tend to occur in December rather than in January”: Yes, this is in accordance with Klein et al. 2016.
- Figures 7-8-9: the green dots hide where we are supposed to look at. Please consider replacing the dots by the country borders.
- L 207 “and on the Alps for Switzerland”: actually for CH, CAPE is large not only over the Alps, but rather over all eastern Europe.
- L 214-215 “the local temperature difference … is small” : what would be large? For AL the difference is of 3°C. Isn’t it large? For CH: is it red or white? The green dot is about the size of the country so it doesn’t allow us to see the values.
- L 235 “increase of CAPE over the central Med … Switzerland”: I don’t understand this interpretation. According to Figure 9, the maxima in CH are produced by Atlantic flows, so I don’t get the interpretation in terms of Mediterranean CAPE.
- L 245 “technique presented in Faranda et al. 2017”: do you take the same thresholds as therein to produce the classification (98% and 2%)?
- L 245 “five possible regimes” It would be helpful to show the composites of the five classes.
- Figure 10 and L 248 “prevalence of BLO and AR patterns”: as already said, I don’t understand how BLO and AR can prevail for CH and TR whereas Figure 9 shows completely different influences for these countries.
- L 251-254 “this patterns consists … Mediterranean basin”: according to Figure 9, this doesn’t seem to apply to CH.

Typos and clarifications:
- L 147: positive IN Switzerland
- L 185 Figure 6 → Figure 6b
- L 186 date → average date?
- L 207 SwitZerland

References:
Klein, G., Vitasse, Y., Rixen, C. et al. (2016) Shorter snow cover duration since 1970 in the Swiss Alps due to earlier snowmelt more than to later snow onset. Climatic Change 139, 637–649. https://doi.org/10.1007/s10584-016-1806-y
Marty, C., Blanchet, J. (2012) Long-term changes in annual maximum snow depth and snowfall in Switzerland based on extreme value statistics. Climatic Change 111, 705–721. https://doi.org/10.1007/s10584-011-0159-9
Scherrer, Simon & Wüthrich, Christian & Croci-Maspoli, Mischa & Weingartner, Rolf & Appenzeller, Christof (2013). Snow variability in the Swiss Alps 1864–2009. International Journal of Climatology, 33, 10.1002/joc.3653.

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ED: Reconsider after major revisions (02 Jun 2020) by Christian Grams

I thank the author for a major revision of the manuscript. The reviewers and I appreciate your effort in substantially improving the manuscript. However there are still some critical issues that require major revision before the paper could be considered for final publication in WCD. For instance, reviewer 2 points towards critical discrepancies between the snow rates in the EOBS data and ERA-5 which lead to statements not justified by both data sets and contradict findings related to snow-fall trends in CH in earlier literature. A work around could be to focus on the consistent trends in both data sets e.g. in maximum snowfall at the Adriatic Coast as suggested by R2. Another issue raised by reviewer 1 is the need for a clearer distinction of the different time scales involved or at least a clearer introduction to different time scales is needed. I hope that these second reviews encourage you for another major revision.
Kind regards

Minor editorial comments:

Section 1

Paragraph starting in l40: It would be good to introduces jet variability also in terms of more classical weather regime literature. Suitable references are for instance: Vautard 1990, Michaelangeli et al. 1995, Yiou and Nogaj 2004 , Woollings et al. 2010, Madonna et al. 2017, ....

l42: "confined north stream" -> Please check the formulation.

paragraph starting in l64: Here would be a place to briefly discuss Kawase et al. 2016

Section 2

l85 / 87 / 115: Abbreviations - also common ones - need to be introduced with full words: ERA5, E-OBS, C3S, NUTS, NUST. Please provide a reference where NUTS-2 regions are defined & taken from.

Section 4

l206, discussion of Figures 7&8, it would help to be more quantitative now and state the actual values. Also Could you compute the country average CAPE & 2mT during the max SF events in both periods and show it as box-and-whisker plots as Figure 6a (or included there)? This would help a lot contrast cases/countries.

discussion of Figures 7&8&9, it would help to have a few more references to the subfigures, as you jump a lot through the panels...

l221, delete "fully"

l228 you mean Figure 9

l235: Not sure what is meant here, the initially northerly flow on the western flank of the Genoa Low and then cyclonically around back to the Alps in southerly flow? Or the anomalous so southerly flow in 9f? Please check and clarify.

l272: Clarify that this is only true for the Mediterranean countries, not CH.

Discussion in line 275: The results of Santos et al. 2016 might help to discuss your results more in the light of weather regimes.

l291: check spelling / citationstyle.

References:

Madonna, E., C. Li, C. M. Grams, and T. Woollings, 2017: The link between eddy-driven jet variability and weather regimes in the North Atlantic-European sector. Q.J.R. Meteorol. Soc, 143, 2960–2972, doi:10.1002/qj.3155.

Michelangeli, P.-A., R. Vautard, and B. Legras, 1995: Weather Regimes: Recurrence and Quasi Stationarity. J. Atmos. Sci., 52, 1237–1256, doi:10.1175/1520-0469(1995)052<1237:WRRAQS>2.0.CO;2.

Santos, J. A., M. Belo-Pereira, H. Fraga, and J. G. Pinto, 2016: Understanding climate change projections for precipitation over western Europe with a weather typing approach. J. Geophys. Res. Atmos., 121, 2015JD024399, doi:10.1002/2015JD024399.

Vautard, R., 1990: Multiple weather regimes over the North Atlantic: analysis of precursors and successors. Mon. Wea. Rev., 118, 2056–2081, doi:10.1175/1520-0493(1990)118<2056:MWROTN>2.0.CO;2.

Woollings, T., A. Hannachi, and B. Hoskins, 2010: Variability of the North Atlantic eddy-driven jet stream. Q.J.R. Meteorol. Soc., 136, 856–868, doi:10.1002/qj.625.

Yiou, P., and M. Nogaj, 2004: Extreme climatic events and weather regimes over the North Atlantic: When and where? Geophys. Res. Lett., 31, doi:10.1029/2003GL019119. https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1029/2003GL019119.

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AR by Davide Faranda on behalf of the Authors (03 Jul 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (23 Jul 2020) by Christian Grams

RR by Anonymous Referee #2 (24 Jul 2020)

RR by Anonymous Referee #1 (28 Jul 2020)

ED: Publish subject to minor revisions (review by editor) (31 Jul 2020) by Christian Grams

AR by Davide Faranda on behalf of the Authors (04 Aug 2020) Author's response Manuscript

ED: Publish as is (17 Aug 2020) by Christian Grams

AR by Davide Faranda on behalf of the Authors (24 Aug 2020) Manuscript

Short summary

Despite the global temperature rise caused by anthropogenic emissions, we still observe heavy snowfalls that cause casualties, transport disruptions and energy supply problems. The goal of this paper is to investigate recent trends in snowfalls from reanalysis and observational datasets. The analysis shows an evident discrepancy between trends in average and extreme snowfalls. The latter can only be explained by looking at atmospheric circulation.