Polar lows – moist-baroclinic cyclones developing in four different vertical wind shear environments

Stoll, Patrick Johannes; Spengler, Thomas; Terpstra, Annick; Graversen, Rune Grand

doi:https://doi.org/10.5194/wcd-2-19-2021

Articles | Volume 2, issue 1

https://doi.org/10.5194/wcd-2-19-2021

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

https://doi.org/10.5194/wcd-2-19-2021

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

Articles | Volume 2, issue 1

Research article

|

15 Jan 2021

Research article |

| 15 Jan 2021

Polar lows – moist-baroclinic cyclones developing in four different vertical wind shear environments

Patrick Johannes Stoll, Thomas Spengler, Annick Terpstra, and Rune Grand Graversen

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Final revised paper (published on 15 Jan 2021)
Supplement to the final revised paper
Preprint (discussion started on 03 Sep 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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- Supplement

RC1: 'review of this paper - what else could it be ?', Anonymous Referee #1, 21 Sep 2020
- AC1: 'Response to Review', Patrick Stoll, 14 Oct 2020
RC2: 'Review of «Polar lows- Moist baroclinic Cyclones developing in four different vertical wind shear environments » by PJ Stoll et al.', Anonymous Referee #2, 21 Oct 2020
RC3: 'Review of wcd-2020-41', Anonymous Referee #3, 30 Oct 2020
RC4: 'The Review of Stoll et al., WCD-2020-41', Anonymous Referee #4, 02 Nov 2020

Peer-review completion

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

AR by Patrick Stoll on behalf of the Authors (23 Nov 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (24 Nov 2020) by Silvio Davolio

RR by Anonymous Referee #2 (03 Dec 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

Second Review of «Polar lows- Moist baroclinic Cyclones developing in four different vertical wind shear environments » by PJ Stoll et al.

In this study, a classification of polar lows (PLs), characterized by a large variety of cloud structures, large scale environment, and intensification mechanisms is proposed It is based on PLs detected in ERA-5 during the period 1999-2019, and makes use of Self-Organizing Maps (SOM). Such a method provides different patterns of variability which can be then connected to the vertical wind shear. Five different configurations are found, of which 4 correspond to a strong shear. The orientation of the shear is found to be determinant for the dynamics of the system (which justifies previous classifications in forward and reverse shear PLs). In addition, it is found that there is no evidence for the existence of hurricane-like PLs that would intensify mainly by latent heat release. Spiraliform clouds would rather correspond to secluded cyclones.
General comments :
This is an original study on PLs for at least two reasons : so far the ability of ERA-5 to represent PLs has not been assessed, and the use of the SOM method fot displaying typical patterns in high-dimensional data, widely used by the scientific community, is certainly promising.
The revised paper is clearly written and represents a significant improvement compared to the previous version. In particular, a number of clarifications have been brought to the part dealing with the detection of PLs in ERA-5.
There are nevertheless still points that need to be precised before the paper can be accepted for publication, they are listed below.

Regarding the PL detection and representation in ERA-5, the choice of the threshold values (tuned to have a good agreement with the Rojo list) remains a bit arbitrary. I do not understand what is meant by the authors when they mention that the results are QUALITATIVELY insensitive to the exact setting in the track matching an exclusion procedure. I am also wondering whether considering all the tracks within a distance of less that 150km to a PL for at least one time step to be matches is conservative enough. Said differently, I am afraid that systems which are not PLs are included in the analysis.
I do not agree with the explanation given to Reviewer 3 (Resp Line 116) that PLs in the Rojo list are seroiusly truncated because the observations which are uneven may miss the genesis and lysis periods (at least not to the point that the average duration should be 35,4 hours in ERA-5 while in Rojo et al, 2015 it is stated than most of the systems last less than 24 hours). This is a clearly a drawback in the representation in the reanalyses that should be mentioned (and this has an effect on the SOM analysis, see below).

Regarding the use of the SOM method, I have 2 main points :
- can the choice of a 3x3 nodes array be better justified ?
- would it make sense torun the method separately for genesis, mature ans lysis phases to better isolate the mechanisms (I agree with Reviewer 4 that it is not totally correct to speak of development for the lysis phase) ? Also this would counterbalance (at least to some extent) the fact that PLs with a longer lifetime have a larger effect on the analysis.

How do the results of this study relate to those which consider weather regimes to describe the synoptic conditions prevailing in PL cases ? In my opinion, this is something that is missing and should be discussed in the paper.

What is the degree of confidence which can be put in ERA-5 precipitation fields in case of PLs ? Using ERA-5 precipitation to assess the relative importance of convective processes in weak/strong shear classes is really questionable.

Last sentence : CARA should be described (at least in a general way), or omitted. Is it possible to add a reference ?

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ED: Publish subject to revisions (further review by editor and referees) (10 Dec 2020) by Silvio Davolio

AR by Patrick Stoll on behalf of the Authors (22 Dec 2020) Author's response Author's tracked changes Manuscript

ED: Publish as is (23 Dec 2020) by Silvio Davolio

AR by Patrick Stoll on behalf of the Authors (02 Jan 2021) Author's response Manuscript

Post-review adjustments

AA: Author's adjustment | EA: Editor approval

AA by Patrick Stoll on behalf of the Authors (13 Jan 2021) Author's adjustment Manuscript

EA: Adjustments approved (13 Jan 2021) by Silvio Davolio

Short summary

Polar lows are intense meso-scale cyclones occurring at high latitudes. The research community has not agreed on a conceptual model to describe polar-low development. Here, we apply self-organising maps to identify the typical ambient sub-synoptic environments of polar lows and find that they can be described as moist-baroclinic cyclones that develop in four different environments characterised by the vertical wind shear.