Occurrence and transition probabilities of omega and high-over-low blocking in the Euro-Atlantic region

Detring, Carola; Müller, Annette; Schielicke, Lisa; Névir, Peter; Rust, Henning W.

doi:https://doi.org/10.5194/wcd-2-927-2021

Articles | Volume 2, issue 4

https://doi.org/10.5194/wcd-2-927-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/wcd-2-927-2021

© Author(s) 2021. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 2, issue 4

Research article

|

06 Oct 2021

Research article |

| 06 Oct 2021

Occurrence and transition probabilities of omega and high-over-low blocking in the Euro-Atlantic region

Carola Detring, Annette Müller, Lisa Schielicke, Peter Névir, and Henning W. Rust

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Final revised paper (published on 06 Oct 2021)
Supplement to the final revised paper
Preprint (discussion started on 28 Dec 2020)
Supplement to the preprint

Interactive discussion

Status: closed

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

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RC1: 'A novel perspective on blocking statistics', Anonymous Referee #1, 19 Jan 2021
- AC1: 'Reply on RC1', Carola Detring, 09 Mar 2021
RC2: 'Interactive comment on "Atmospheric blocking types: Frequencies and transitions" by C. Detring et al.', Anonymous Referee #2, 30 Jan 2021
- AC2: 'Reply on RC2', Carola Detring, 09 Mar 2021

Peer-review completion

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

AR by Carola Detring on behalf of the Authors (05 Apr 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (09 Apr 2021) by Peter Knippertz

RR by Anonymous Referee #2 (26 Apr 2021)

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

By addressing both reviewers' comments, the present study by Detring et al. has improved considerably. In particular, the analysis of the significance of observed trends supports the results importantly. Still, there are numerous minor issues where the text could be more specific or where the grammar and wording make the text difficult to understand. I have done my best to point out the numerous instances needing correction, and I have provided suggestions to help the authors. I therefore recommend minor revisions prior to acceptance in WCD.

Minor Comments:
Title: Please consider to specify the region for which occurrence and transition probabilities are investigated in this study (e.g., Europe or Atlantic-European region).

l. 3: "single low" sounds a bit awkward. What about "isolated low" or very specific "cut-off low"?

l. 6: Please introduce acronym "NCEP-DOE". If not overly important you could say more general "Based on reanalysis data in the...".

l. 13: "state" instead of "states".

l. 15: The statement "...and the previous time step neglecting all previous time steps" could be confusing to the reader. "...an the previous time step neglecting all further time steps before" could be an alternative. In line 72 of the manuscript this is explained much more clearly.

l. 19: Please insert "to be" between "and" and "more".

l. 25: The way High-over-Low and Omega blocks are described is valid only for the Northern Hemisphere. To clarify this you could state in this line "Northern Hemisphere blocks are characterized by a steady high pressure area...".

l. 35: Please include a reference to a study showing that Omega blocking caused the widespread temperature records in 2019.

l. 38: Please consider to delete this sentence. In its current form it is more or less a repetition of the sentence in lines 22 and 25.

l. 43: Many conceptual explanations of atmospheric blocking exist. Therefore, I would suggest to account for this by saying "One conceptual explanation of atmospheric blocking is...".

l. 51: What is "both speeds" referring to? Please clarify.

l. 54: Please consider to state that the point vortex theory allows "to classify and to distinguish the two blocking states High-over-Low and Omega". Also please, remove "blocking types" at the end of the sentence in l. 54.

l. 57: Please delete the sentence starting with "Using this method..." since it is redundant with the sentence in l. 52-53.

l. 59: Isn't the question already answered in l. 54? There you state "Furthermore, this leads to the possibility to classify and analyse the two blocking states...". In its current form it reads a bit awkward to raise a question that seems to be rhethorical in nature as it has been addressed by Müller et al. 2015 and Hirth et al. 2018.

l. 92: Depending on your response to my previous comment (l. 59), please consider to exclude this question as it seems to be answered already.

l. 106: Please introduce acronym "NCEP-DOE" if not already done in the abstract.

l. 120: Here and elsewhere in the manuscript: Please note that the term "blockings" does not really exist in the English language (plural of gerund). Write "blocks" instead.

l. 130: Better write "obtain" instead of "get".

l. 134: "Longitude" instead of "longitudes".

l. 136: Better write "Note, that two blocking events can exist at the same time...". In its current form this is quite colloquial.

l. 148: Please replace "over" with "of the".

l. 153: Since you are dealing with data on a regular latitude-longitude grid: Have you used the corresponding equations in spherical coordinates? If yes, please provide the equations in spherical and not cartesian coordinates.

l. 166: What is the motivation for choosing a radius of 1500 km around P_max,neg? Is this related to the Rossby radius of deformation? As the reader might get the impression that the radius was chosen quite subjectively, please explain why a radius of 1500 km is reasonable. Could you also calculate the circulation as the sum of all grid points in an object which encloses only grid points with negative circulation?

l. 174: "Latitude" instead of "latitudes".

l. 170-184: Many parameters need to be chosen. Please include some discussion on the sensitivity of the results on the choice of the parameters (not necessarily here but somewhere in the manuscript).

l. 193: This sentence needs clarification/simplification. The usage of 4x "of" makes the sentence difficult to understand.

l. 203: Just to be sure: Do you mean "breached" or "reached"?

l. 211: Other types of blocking identification methods exist that are not solely based on indices. E.g., Schwierz et al. 2004. Hence, I suggest to explicitly reference the studies which use an index for blocking identification. Otherwise, the current statement is too general.

l. 256: Please replace exclamation mark with full stop.

l. 312: In its current form it reads that in theory you can describe time-varying transition probabilities. Please rewrite the sentence so that it becomes clear that this is what you are actually doing.

l. 337: Insert blank between full stop and "For".

l. 344: Here and elsewhere in the manuscript: Please consider to write "propagation" instead of "travel".

l. 353: Replace "sensible" with "sensitive".

l. 360: Please merge this paragraph with the previous sentence since these seem to be related.

l. 388: "This" instead of "his".

l. 404: What is the reason that the trend is highly significant in JJA when considering an entire season but not significant when considering individual months?

l. 426: "about" instead of "About".

l. 442/443: What is the reasoning for distinguishing between decrease/slight decrease and increase/slight increase. Is there any objective criterion to make this distinction? If not I would suggest to only use increase and decrease to describe the trends. The quantitative information of the significance is more informative than the qualitative distinction between "slight" and "not slight".

l. 444: Similar to the previous comment, the distinction between "strongly" and "pronounced" seems to be a bit arbitrary.

l. 453: Are you sure that the trend in February is significant for HoL (solid) but not for Omega (dashed). This is striking given the marked increase of Omega blocks in February. Please double check.

l. 456: "as" instead of "than".

l. 461: The final sentence is a bit colloquial. I guess what you want to say is that "All in all, the net trend is zero".

l. 463: Please specify at the beginning of this subsection that the analysis is only performed/shown for the Euro-Atlantic region.

l. 467: Please remove "of" between "and" and "three".

l. 472 and after: Please correct quotation marks.

l. 508: "become" instead of "becomes".

l. 520: Depending on whether the first question is kept in the introduction or not, this line needs to be changed.

l. 537: "On the one hand" needs to be followed by "on the other hand" somewhere in the sentence. So, I guess you can delete "on the one hand" from the sentence.

l. 547: This sentence needs reordering. My suggestion: As shown in Fig. 7b the probabilities of blocks increase slightly, but significantly in summer.

l. 548: This statement is not supported by results of this study. Of course, significant trends have been identified based on historical data, but no climate projections have been investigated. Therefore, the statement is speculative and I would suggest to delete it from the manuscript. As an alternative you could write "If this significant increase continues in a warmer climate, the occurrences of exceptional droughts could occur more often in the future".

l. 559: Replace "in" with "on".

l. 576: To ease the comparison with Drouard and Woolings (2018): Why are you not showing Fig. 15 for June to August only? Since you are analysing some of the results for each season separately, it should be fairly straightforward to show Fig. 15 for June to August. So, please revise the figure so that it is better comparable to the results of Drouard and Woolings (2018).

l. 606: "relatively" instead of "relative".

l. 630: Consider to replace "decision" with "identification". Also, to my understanding the method is a continuation/refinement of the method introduced by Hirth et al. (2018). Therefore, I'd suggest to cite their work also in the concluding section.

l. 648: "find" instead of "found" (present tense).

l. 651: Please specify that this is the subjective impression of the authors (or reference a paper which confirms the impression).

l. 656: In what sense could the analysis of temperature and wind shear help to better understand blocking? What about other processes such as diabatic processes, Rossby wave breaking etc?

l. 658: What is the purpose of "however" at the beginning of the sentence?

l. 663: Replace "development" with "trends" or "long-term changes".

Figure 1: Please specify in the figure caption that relative vorticity is shown.

Figure 3: Please specify in the figure caption that relative vorticity is shown. Also, in line 2 of the caption it should be "vorticity" instead of "vorticty". Here and elsewhere: Please use date formatting as described in the Submission guidelines of WCD: https://www.weather-climate-dynamics.net/submission.html. Further, the units for geopotential height are different between Figs. 1 and 3 (gpm vs dm). I would suggest to use units consistently throughout the study.

Figure 10: Please consider to rewrite the Fig. caption in order to avoid 4x "for".

Reference:
C. Schwierz, M. Croci-Maspoli, and H. C. Davies, “Perspicacious indicators of atmospheric blocking,” Geophys. Res. Lett., vol. 31, no. 6, p. L06125, Mar. 2004, doi: 10.1029/2003gl019341.

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RR by Anonymous Referee #1 (24 May 2021)

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

In response to the original reviews, the authors have done considerable revisions to the manuscript; in fact, large sections have been completely rewritten. On a superficial level, the authors have addressed a good fraction of my earlier issues. Nevertheless, I feel that the paper still lacks something to be desired in order to make it truly convincing. I encourage the authors to do further revisions and I try to give suggestions below.

1. Provide better guidance to the reader

I cannot really judge the technique used to determine statistical significance. Having said this and assuming this technique to be sound, the paper is probably OK on a micro-level, i.e., by evaluating each sentence individually. However, I was sometimes missing guidance from the authors as to what the results really mean, providing a broader sense of interpretation and putting everything into the larger context. For instance, in the summary and concluding section I would expect less of a detailed repetition of what has been said before, but rather a discussion on a higher level of abstraction.

This issue is particularly relevant regarding the monthly trends (see next issue).

2. What do the detected trends mean?

The authors made an effort to say more about the statistical significance of their results and they are now more careful when quoting results that are lacking statistical significance. I am not an expert in statistics, but I simply trust that their method to quantify significance is sound. Yet, I still find some of the results puzzling, and it would be desirable if the authors could say a little more about how they interpret their own results.

As it turns out, there are no significant trends on a year-by-year basis; instead, some trends emerge only if one does the analysis on a seasonal or even month-by-month basis. At first sight this may appear surprising, because the monthly analysis is based on a much smaller data basis suggesting a lower degree of statistical significance than the year-to-year analysis. Especially those instances when a monthly trend reverses as one proceeds from one month to the next appear somewhat suspicious to me. Could you possibly comment in order to make me feel less uneasy? Is there anything that I have overlooked?

My broader question is whether those monthly trends (which, allegedly, are statistically significant) have to be considered as a signal that goes beyond the noise of natural variability. In other words: if we had 40 realizations of the three decades 1990 to 2019 that are subject to the same external “forcing” and that only differ in their natural variability, would you expect those monthly trends to be more or less identical in all 40 realizations (see Deser et al., 2020)? If this were the case, these monthly trends could be attributed to the “forcing” from, e.g., increased greenhouse gases --- otherwise they would be just noise. If the latter were true, I wonder what we have learned. I think the reader would appreciate more guidance from the authors.

3. Improvements to quality of the English

I think that the level of the English could and should be improved in numerous places. I have returned an annotated version of the revised manuscript where I give a few examples. I trust that the senior authors can help towards this goal. Alternatively, it might be beneficial to consult, e.g., the excellent book of David M. Schultz (“Eloquent Science”, published by the American Meteorological Society).

Reference:

C. Deser, F. Lehner, K. B. Rodgers, and collaborators, 2020: Insights from earth system model initial-condition large ensembles and future prospects. Nature Climate Change, 10, 277–286

Referee Report: PDF

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ED: Publish subject to minor revisions (review by editor) (01 Jun 2021) by Peter Knippertz

AR by Carola Detring on behalf of the Authors (20 Jul 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (30 Jul 2021) by Peter Knippertz

AR by Carola Detring on behalf of the Authors (07 Aug 2021) Author's response Manuscript

Short summary

Stationary, long-lasting blocked weather patterns can lead to extreme conditions. Within this study the temporal evolution of the occurrence probability is analyzed, and the onset, decay and transition probabilities of blocking within the past 30 years are modeled. Using Markov models combined with logistic regression, we found large changes in summer, where the probability of transitions to so-called Omega blocks increases strongly, while the unblocked state becomes less probable.