The life cycle of upper-level troughs and ridges: a novel detection method, climatologies and Lagrangian characteristics

Schemm, Sebastian; Rüdisühli, Stefan; Sprenger, Michael

doi:https://doi.org/10.5194/wcd-1-459-2020

Articles | Volume 1, issue 2

https://doi.org/10.5194/wcd-1-459-2020

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

https://doi.org/10.5194/wcd-1-459-2020

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

Articles | Volume 1, issue 2

Research article

|

10 Sep 2020

Research article |

| 10 Sep 2020

The life cycle of upper-level troughs and ridges: a novel detection method, climatologies and Lagrangian characteristics

Sebastian Schemm, Stefan Rüdisühli, and Michael Sprenger

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Final revised paper (published on 10 Sep 2020)
Supplement to the final revised paper
Preprint (discussion started on 03 Apr 2020)

Interactive discussion

Status: closed

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

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- Supplement

RC1: 'Initial review', Anonymous Referee #1, 17 Apr 2020
RC2: 'Review', Irina Rudeva, 28 Apr 2020
RC3: 'review', Anonymous Referee #3, 10 May 2020
AC1: 'Reply to: Initial review', Sebastian Schemm, 05 Jun 2020
AC2: 'Reply to RC2', Sebastian Schemm, 05 Jun 2020
AC3: 'Reply to RC3', Sebastian Schemm, 05 Jun 2020

Peer-review completion

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

AR by Lorena Grabowski on behalf of the Authors (19 Jun 2020) Author's response

ED: Referee Nomination & Report Request started (21 Jun 2020) by Michael Riemer

RR by Anonymous Referee #3 (06 Jul 2020)

RR by Irina Rudeva (09 Jul 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

The authors have done an excellent job reviewing the manuscript. They addressed all major questions raised by reviewers. The new manuscript reads better and new figures help understand the method description. I find the new section on the relationship to the ENSO to be very valuable.

I am happy to recommend the revised version of the manuscript for publication subject to very minor revision.

Minor:

p.1, l.19: change to ‘oriented’

p.1, l.24: for a good reason; equatorward moving cold air

p2, l.15: ‘Under anticyclonic shear, the trough deforms into a narrow band, called a streamer, which extends equatorward to the mean jet position.’ It should probably say ‘which crosses the jet and extends equatorward of the mean jet position.’ This type of wave breaking often results into formation of cut-off lows which may be mentioned here.

p.4, l.29: suggest identifying

Fig.2 (p.24, caption): (4) a 5 km step along the geopotential height isolines in the direction of vector (3) vector and a new tangent vector (green) derived at the new position

p.6, l.11: two consecutive time steps, ti and tj, are found

p.6, l.14: a splitting if tj > ti and a merging if ti > tj

P.7,l.30: Along with the low frequency of troughs next to Greenland, there is a relatively low number of ridges downstream of Rocky Mountains. Schemm et al (2018) suggests a similar number of troughs and ridges in both areas.

p.9, l. 8:remains at a relatively high level

P10, l.12: for such synoptic situation

P.10,l.25: a particular progress

P11,l.13: Can you better define what you mean by ENSO-affected winters? Is there, e.g., any threshold on the index value or a time lag?

P.12, l.11: “The vertical eddy structure 10 – which is more poleward when the eddy efficiency decreases during midwinter” - what do you mean by more poleward structure of eddies?

P.12, l.15: “The equatorward shift of the Pacific jet changes the large-scale environment in which synoptic systems grow because more systems will grow on the poleward flank of the jet.” How do you know that it is not the other way round, i.e. more frequent cyclonic wave breaking shifts the jet equatorward?

Fig. S.3: ‘500-hPa’

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RR by Anonymous Referee #1 (15 Jul 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

I think the authors have responded reasonably to most of my comments, except perhaps one point which I don't quite agree with their response. Other than that, I only have a couple of minor comments.

Major comment:
1) In my previous comments, I commented on the missing troughs and ridges along the subtropical jet south of the Tibetan Plateau, which are clearly present in previous studies (e.g. Fig. 14 of Hoskins and Hodges 2002, Chang and Yu 1999, and Chang 2005). In fact, if one compares the schematic in Hoskins and Hodges Fig. 14, all upper level principal tracks shown there can be found here, with the only exception being the southern branch over Eurasia. In this revision, the authors also showed results using 300 hPa height in the supplement, but those troughs/ridges are still absent at 300 hPa. In their response, the authors claimed that the waves in the southern branch are best seen in band pass filtered data, but in fact that's not the case. Chang and Yu (1999), and Chang (2005) both used unfiltered data, not band-pass filtered data as claimed by the authors in their response. Chang and Yu (1999) showed that these waves generally have longer periods (8 days or longer) than waves in the oceanic basins, and the composites shown in Chang (2005) also showed that these waves have low phase speed and longer periods. Hoskins and Hodges (2002) did use spatially filtered data (but not band-pass filtered data), but my guess is that since Hoskins and Hodges used vorticity at 250 hPa, even vorticity data that is not spatially filtered would show these systems. So the absence of these systems, even at 300 hPa, in the authors' results is still a mystery. In their response, the authors also stated that "This subcategory of waves however is still part of our data" and showed an example of that. The question then is why they don't show up in the climatology, while they showed up clearly in climatologies of previous studies (e.g. Fig. 12f and 13b of Hoskins and Hodges 2002)? The authors stated in their response that "In the revised manuscript, we placed a comment on this in the corresponding section". I might have missed that but I don't think I saw that comment in the revised manuscript. Can the authors point out where that comment is? In my opinion, as this seems to be the main difference between results of this study to those of previous studies for winter systems, this should definitely be pointed out and discussed in the paper.

Other comments:
2) p. 10, lines 16-17: Could that be related to the authors' choice of not including areas within closed contours as part of troughs?

3) p. 12, line 24: "bandpass-filtered eddies". This is also related to the authors' response to one of my previous comments. If I am not mistaken, Penny et al (2010) did not use bandpass-filtered data. They did remove the large spatial scale and long time scale (90-day high pass), but I don't think characterizing their data as bandpass-filtered is accurate.

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ED: Publish subject to minor revisions (review by editor) (02 Aug 2020) by Michael Riemer

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

ED: Publish as is (26 Aug 2020) by Michael Riemer

Short summary

Troughs and ridges are ubiquitous flow features in the upper troposphere and are centerpiece elements of weather and climate research. A novel method is introduced to identify and track the life cycle of troughs and ridges and their orientation. The aim is to close the existing gap between methods that detect the initiation phase and methods that detect the decaying phase of Rossby wave development. Global climatologies, the influence of ENSO and Lagrangian characteristics are discussed.