Interactive 3-D visual analysis of ERA5 data: improving diagnostic indices for marine cold air outbreaks and polar lows

Meyer, Marcel; Polkova, Iuliia; Modali, Kameswar Rao; Schaffer, Laura; Baehr, Johanna; Olbrich, Stephan; Rautenhaus, Marc

doi:https://doi.org/10.5194/wcd-2-867-2021

Articles | Volume 2, issue 3

https://doi.org/10.5194/wcd-2-867-2021

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

https://doi.org/10.5194/wcd-2-867-2021

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

Articles | Volume 2, issue 3

Research article

|

15 Sep 2021

Research article |

| 15 Sep 2021

Interactive 3-D visual analysis of ERA5 data: improving diagnostic indices for marine cold air outbreaks and polar lows

Marcel Meyer, Iuliia Polkova, Kameswar Rao Modali, Laura Schaffer, Johanna Baehr, Stephan Olbrich, and Marc Rautenhaus

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Final revised paper (published on 15 Sep 2021)
Preprint (discussion started on 27 Apr 2021)

Interactive discussion

Status: closed

RC1: 'Comment on wcd-2021-20', Anonymous Referee #1, 14 May 2021

The comment was uploaded in the form of a supplement: https://wcd.copernicus.org/preprints/wcd-2021-20/wcd-2021-20-RC1-supplement.pdf

Citation: https://doi.org/10.5194/wcd-2021-20-RC1
RC2: 'Comment on wcd-2021-20', Anonymous Referee #2, 14 May 2021

The comment was uploaded in the form of a supplement: https://wcd.copernicus.org/preprints/wcd-2021-20/wcd-2021-20-RC2-supplement.pdf

Citation: https://doi.org/10.5194/wcd-2021-20-RC2
AC1: 'Comment on wcd-2021-20', Marcel Meyer, 02 Jul 2021

We would like to thank both reviewers for their detailed, constructive and encouraging comments. Please see the attached document for our detailed response.

Citation: https://doi.org/10.5194/wcd-2021-20-AC1

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Marcel Meyer on behalf of the Authors (02 Jul 2021) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (02 Jul 2021) by Heini Wernli

RR by Anonymous Referee #2 (09 Jul 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

Second review of the manuscript wcd-2021-20

I acknowledge the great effort of the authors in thoroughly answering and taking into account the comments of the first review but I still have a few more or less minor comments that should be addressed, some of them that I missed in the first version of the manuscript. My comments are listed here below and the lines refer to the non-tracked changes version of the manuscript.

Comments:
- Line 40: “sea surface potential temperature over the ocean”: it seems redundant to have sea surface and ocean here. Suggestion: potential temperature calculated using the sea surface temperature.
- Line 102 (and Appendix A): “137 vertical model levels, 37 pressure levels”. I am confused here: what are you actually using in Met.3D, model levels or pressure levels? Has the vertical interpolation onto pressure level to be performed before using Met.3D or does Met.3D interactively perform the vertical interpolation? Moreover, in Appendix A, only horizontal remapping is mentioned. Therefore, I suggest to explicitly write about the vertical interpolation somewhere, either in Appendix A if the vertical interpolation is performed before using Met.3D or in Sect. 2.1 if the vertical interpolation is performed in Met.3D. The authors can also maybe explicitly write the 37 pressure levels available as well.
- Line 105: I do not understand why a polar stereographic projection is required before using Met.3D (what does it do to the grid of the data?). Isn’t it Met.3D itself that performs the projection at the plotting stage? If yes, I suggest a sentence like “Met.3D displays fields using a polar stereographic projection.” or similar. Moreover, can the grid step in degrees be written here (for example 0.5ºx0.5º)?
- Line 120: 3-D features -> the 3-D structure
- Line 129: “on a large geographical domain (all longitudes; northern latitudes in the interval 25-90º” -> over the Northern Hemisphere (north of 25ºN)
- Lines 129-130: I suggest to remove “grid-dimension in lat-lon height: 261x1441x137”.
- Line 132: I suggest to also remove “on a smaller grid (440x440x137)”. The first “440” is not smaller than the “261” of line 130 so that is confusing. I suggest to replace “on a smaller grid….Nordic Seas” with “on a smaller domain covering the Barents and Nordic Seas”. The polar stereographic aspect is already mentioned in Sect. 2.1.
- Line 136: I do not understand “at which air aloft is considered for calculation of the MCAO index”. I suggest to remove as the rest of the sentence is clear without this part.
- Line 164: “without lower-level instability”: does it mean that the lowest-level potential temperature is larger than the potential skin temperature? If yes, maybe write it (with a formula).
- Line 171: remove “(in hPa)” as no units are written except in Table 1.
- Lines 214-216: if the hours with m_p=0 are not considered in the temporal average, then I suggest to remove “and setting the index to zero for all other hours” and instead write that the index is equal to 0 if all time steps have m_p=0. Or is there something I have misunderstood?
- Lines 319, 394, and caption of Fig. 6: I like the additional composites performed by the authors that I think really show the differences between the conventional and new MCAO indices. However, I do not think the term “long-term average” describes what is calculated. From my understanding, what is done here is a composite of the maps of the indices averaged over the polar lows duration.
- Section 3.3.2: would it make sense to switch the 3rd (“The number…”) and 4th (“To assess…”) paragraphs and maybe merge the new 4th paragraph with the 5th (“Results…”)? I think it would read better.
- Line 398: to help the reader, maybe point at the area between Iceland and Norway where there are more yellow colours in Fig. 6c than in Fig. 6d -> (e.g., compare the values in the area between Iceland and Norway in Fig. 6c,d).
- Line 421: does a true positive rate of 1 imply that the index always correctly detect a PL (100% match)? If yes, could it be mentioned somewhere (main body or caption of Fig. 7) to more easily understand the ROC curves?

Technical comments:
Line 50: MCOA -> MCAO
Line 65: “that maximizes the link to observed PLs” -> that maximizes this link.
Line 212: shouldn't it be t*start and t*stop in the parenthesis (with the stars)?
Line 213: end -> lysis
Lines 219, 302: upper level -> upper-level
Line 239: Movies 1-2 -> Movies 1, 2
Line 289: remove one “in”
Line 321: remove comma after “areas”
Line 338: Fig. 5-h -> Fig. 5h
Caption of Fig. 5: “(>=95-th percentile)” is correct for the MCAO indices (panels c and f) but shouldn't it be the 5th percentile for the polar low index (panel i)?
Line 404: remove comma after “Sect. 2.4)”
Line 406: Fig. 6-e -> Fig. 6e
Eq. 6: columns (:) after ifs are not needed
Line 433: Fig. 7 -> Fig. 7a
Line 463: add a full stop (.) at the end of the sentence after “Table 2)”
Line 576: proof -> prove

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RR by Anonymous Referee #1 (17 Jul 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

rejected

Were a revised manuscript to be sent for another round of reviews:

I would be willing to review the revised manuscript.

I would not be willing to review the revised manuscript.

Suggestions for revision or reasons for rejection

The authors put a lot of effort into the revision of the paper, whose presentation is now much improved, and they responded well to my earlier comments. The intention of the paper now comes out clearly and the methodology is very well explained. The paper provides an important step forward in terms of diagnostic indices for MCAOs and identifying regions favourable for PL development. And it is an excellent showcase for the potential of 3D interactive analysis in meteorology. Hence, I am convinced it provides a valuable contribution to WCD.

I have one main comment and a few minor comments, as detailed in the following:

The collocation of the maximum MCAO depth and the occurrence of a PL is perhaps not too surprising given that PLs often require for their genesis some upper-level forcing in the form of a positive potential vorticity anomaly, as you outline in 3.2.3. Such upper-level positive potential vorticity anomalies are generally associated with an upward doming of the underlying isentropic surfaces (e.g., Hoskins et al. 1985) and, hence, also of the zero-isosurface m_theta_p, leading to reduced stability (and lifting). This relationship is the likely reason why the new MCAO index performs well in predicting PL occurrence; via the upward doming of the upper boundary of the MCAO, it contains implicit information on upper-level forcing. The conventional MCAO index does not contain such information and instead it is purely a measure of the coldness of the CAO air relative to the sea surface. The coldness of the air is likely not the most critical parameter for PL development as long as upper-level forcing is absent. I think the authors should discuss this aspect.

- L50: MCOA -> MCAO
- L80: Even though 3D depiction of atmospheric fields is not the standard, meteorologist are well aware of the potential of 3D visualization since a long time, see for example Figs. 6.12 and 6.14 in Uccellini (1990).
- L102: No need to mention the 37 standard pressure levels on which ERA5 output is available since you are anyway using data on model levels.
- Caption Table 1: Briefly mention in the caption the meaning of the different symbols (e.g., p_tr, p0, p* etc...)
- L173: Maybe rephrase as "... if the lower-level instability extends all the way to the tropopause." or similar. I don't think an MCAO induces an instability, but the instability is rather a defining characteristic of an MCAO.
- L195: Fig. B1 suggests that the lysis point is selected randomly within a circle with a radius corresponding to the mean track distance around the genesis point. Is this correct? The way it is written now, one may think that several such increments are computed.
- L238: Suggest to remove "more realistic insights". I don't think one way or the other of depicting meteorological data is more realistic, but the 3D approach is certainly useful for an interactive exploration.
- L289: delete "in"

References:

Hoskins, B. J., M. E. McIntyre, and A. W. Robertson, 1985: On the use and significance of isentropic potential vorticity maps.
Quart. J. Roy. Meteor. Soc., 111, 877–946, doi:10.1002/qj.49711147002.

Uccellini, L. W., 1990: Processes contributing to the rapid development of extratropical cyclones. Extratropical Cyclones. The Erik Palmen Memorial Volume, C. W. Newton, and E. O. Holopainen, Eds., American Meteorological Society, Boston, USA, 81–105.

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ED: Publish subject to minor revisions (review by editor) (23 Jul 2021) by Heini Wernli

AR by Marcel Meyer on behalf of the Authors (02 Aug 2021) Author's response Author's tracked changes Manuscript

ED: Publish subject to technical corrections (06 Aug 2021) by Heini Wernli

AR by Marcel Meyer on behalf of the Authors (10 Aug 2021) Author's response Manuscript

Short summary

Novel techniques from computer science are used to study extreme weather events. Inspired by the interactive 3-D visual analysis of the recently released ERA5 reanalysis data, we improve commonly used metrics for measuring polar winter storms and outbreaks of cold air. The software (Met.3D) that we have extended and applied as part of this study is freely available and can be used generically for 3-D visualization of a broad variety of atmospheric processes in weather and climate data.