How an uncertain short-wave perturbation on the North Atlantic wave guide affects the forecast of an intense Mediterranean cyclone (Medicane Zorbas)

Portmann, Raphael; González-Alemán, Juan Jesús; Sprenger, Michael; Wernli, Heini

doi:https://doi.org/10.5194/wcd-1-597-2020

Articles | Volume 1, issue 2

https://doi.org/10.5194/wcd-1-597-2020

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

https://doi.org/10.5194/wcd-1-597-2020

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

Articles | Volume 1, issue 2

Research article

|

19 Oct 2020

Research article |

| 19 Oct 2020

How an uncertain short-wave perturbation on the North Atlantic wave guide affects the forecast of an intense Mediterranean cyclone (Medicane Zorbas)

Raphael Portmann, Juan Jesús González-Alemán, Michael Sprenger, and Heini Wernli

Download

Final revised paper (published on 19 Oct 2020)
Supplement to the final revised paper
Preprint (discussion started on 28 Aug 2019)
Supplement to the preprint

Interactive discussion

Status: closed

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

- Printer-friendly version

- Supplement

RC1: 'Review', Ron McTaggart-Cowan, 01 Sep 2019
RC2: 'Review', Florian Pantillon, 14 Sep 2019
RC3: 'revision', Anonymous Referee #3, 17 Sep 2019
AC1: 'Replies to all reviewer comments', Raphael Portmann, 28 Nov 2019

Peer-review completion

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

AR by Raphael Portmann on behalf of the Authors (24 Dec 2019) Author's response Manuscript

ED: Referee Nomination & Report Request started (09 Jan 2020) by Michael Riemer

RR by Anonymous Referee #3 (21 Jan 2020)

RR by Florian Pantillon (31 Jan 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

Review of “Medicane Zorbas: Origin of an uncertain potential vorticity streamer position and impact on cyclone formation” by Raphael Portmann, Juan Jesús González-Alemán, Michael Sprenger, and Heini Wernli.

The paper quality has greatly increased since the first version, in particular its structure and clarity. Medicanes and forecast uncertainty are now well introduced (and early), and the results emphasize coherent scenarios from Rossby wave dynamics over the North Atlantic to Medicane formation over the Mediterranean thanks to new figures and a revised organization.

There are, however, several points that still need improvement. First of all, the paper’s broad topic remains unclear, from the title that is not really convincing, from the introduction that does not state what is new, and from the conclusions that do not reflect the topics discussed in the introduction. Furthermore, some choices appear arbitrary and give the feeling that results may not be representative: why look at 72h forecasts in particular? Why look at PV amplification from 12-18h only? Finally, the new analysis of terms in PV error growth (Figure 9) needs more work in both methods and discussion. Comments are listed below to help further improving the paper.

Specific comments

The abstract should somehow state that the paper focuses on medium-range predictability and synoptic to mesoscale dynamics, as higher-resolution model forecasts may struggle to accurately represent Medicanes even at short range.

l. 21 Why 15 years? Better be less precise if it has not started with a specific event.

l. 30-31 Tropical transition does not always occur through WISHE (see Davis and Bosart 2004).

l. 53-55 This sentence is unexpected here, as Medicane Zorbas has not been introduced yet.

l. 64, 71 The distinction between origin and amplification of model errors is unclear here, as model errors themselves are mentioned as origin (which is confusing), warm conveyor belts are mentioned for both (but likely belongs to the latter) and ensemble forecasts are mentioned for the latter only (but account for both).

l. 76-77 Proxy for the expected forecast uncertainty rather than error.

l. 92-93 It is unclear why these specific lead times are chosen (84 h and 72 h); either justify or omit.

l. 91-101 What is missing here is what is new in the paper and what goes beyond previous studies?

l. 114 Remove “accumulated”.

l. 123 and elsewhere It would be helpful to know where to look at in the supplementary material (S1,S2,…, Figs…).

l. 133 Non-frontal is indicated by B<10.

l. 152 Symbol sigma_PV in formula (1) is not consistent with sigma_delta_PV_AB in the text.

l. 185-186 Even though it is not the focus of the paper, the three-day-long deep warm core appears unprecedented, especially for a relatively shallow cyclone, and deserves a comment (see first review). Was the SST particularly high?

l. 196-200 The role of Leslie is unclear: did it contribute to the PV streamer formation or not?

l. 208-212 Fig. 3d,e rather suggests than low-theta_e air is advected into the cyclogenesis area.

l. 213, 217 Refer to Fig. 1b (warm/cold core).

l. 224 Why is it consistent with the cyclone path?

l. 228 Which characteristics in particular?

l. 237 Why this specific initialization?

l. 239 Section 3 does not explicitly show that the streamer is crucial for the cyclogenesis.

l. 239-241 This sentence appears too early: it tells the result before explaining the method.

l. 253 Explicit “C” stands for central cluster.

l. 267-268 Red curve

l. 279-280 How do you know differences are not significant?

l. 280-283 This sounds very speculative: upper-level PV differences may also arise from initial differences in various tropospheric fields.

l. 285 What exactly is similar to Baumgart et al. (2018)?

l. 291-292 More details are needed here.

l. 296-305 The purpose of this paragraph is unclear.

l. 306-329 Large parts of this should be moved to the methods section.

l. 324-325 This is not obvious.

l. 329-330 Does the amplification really takes place during this 6h time interval only?

l. 331-349 This discussion is hard to follow altogether (see comment on Figure 9).

l. 374-375 Does a cyclone develops in all (or most) ensemble members? Refer to Table 1?

l. 381-382 Add the analysis MSLP to the box plots for comparison?

l. 389-390 The westward shift also appears to have a non-linear response, as the number of cyclones in W decreases from 12 UTC 27-28 Sep (vs. increases in C).

l. 400 Show the box on Figure 11?

l. 415 onward The LLTHE-ULPV diagram reminds of the coupling index for tropical transition based on the thetaE difference between tropopause and 850 hPa levels:
McTaggart-Cowan, R., E.L. Davies, J.G. Fairman, T.J. Galarneau, and D.M. Schultz, 2015: Revisiting the 26.5°C Sea Surface Temperature Threshold for Tropical Cyclone Development. Bull. Amer. Meteor. Soc., 96, 1929–1943, https://doi.org/10.1175/BAMS-D-13-00254.1

l. 418 What does LLTHE stand for?

l. 422-424 The definition of ULPV is unclear.

l. 427 Explicit the medicane criterion.

l. 450-459 There is a clear correlation between high/low PV and low/high thetaE but it does not tell which one is the cause or consequence.

l. 460-479 It is meaningful to remind the variety of systems that emerge from similar synoptic situations, but the discussion is too detailed to be followed easily (e.g. the vertical shear, which is not shown) and could thus be streamlined. In contrast, it would be interesting to mention the analysis, which does not seem to follow cluster C entirely.

l. 492-500 Please update according to comments above.

l. 501 “storm”

l. 519-521 Again, initial uncertainties relevant to the illustrated error growth are not necessarily located at the tropopause level.

l. 522-530 The cited papers are relatively old and could be complemented with more recent studies.

Figure 9: Amplification of PV differences along the upper-level wave guide: to compare the relative contribution of different terms (equations 2-3) you need to compute and show them explicitly. I cannot visually estimate PV advection from vectors and contours.

Hide

RR by Ron McTaggart-Cowan (10 Feb 2020)

ED: Reconsider after major revisions (11 Feb 2020) by Michael Riemer

AR by Raphael Portmann on behalf of the Authors (08 Jun 2020) Manuscript

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

RR by Anonymous Referee #3 (27 Jun 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 have already expressed my positive evaluation of this manuscript in the previous versions, since the results improve our understanding of the mechanisms of development of Medicanes. In the newer version, I repeat my appreciation for this work, evaluating positively the Authors’ efforts to further improve the manuscript (very clear the present version of Fig. 12!). I think the paper is really mature for publication, a few comments needed for better clarification are reported below.

Minor points:
- Line 40: please clarify what you mean with “far equatorward reaching PV streamers”;
- Line 56 and elsewhere: Libya instead of Lybia;
- Line 136: Lagrangian instead of Langrangian;
- Line 173 and elsewhere: (Wilks, 2011) instead of (Wilks, D. S., 2011);
- Line 218: acquired instead of aquired;
- Line 222: this suggests that the cyclone belongs to the first category of Medicanes, following Miglietta and Rotunno (2019);
- Lines 225-228: Include a reference to better explain this point, e.g. Fig. 9.10 in Holton, IV edition;
- Line 230: (average … of about 30K): do you mean that the maximum equivalent potential temperature difference between the parcel and the environment is about 30 K?
- Figure 1 caption: the values of wind speed (white contours) are not indicated;
- Figure 2 caption: the reason for the white areas in the lower panels is not clear;
- Figure 7 caption: the definition of PVav should be included in the caption;
- Figure 11 caption: the meaning of the small circles and of the bars in the insets is not clear;
- Figure 5S caption: the names of the clusters should be C, E, W not 1, 2, 3.

Hide

RR by Ron McTaggart-Cowan (13 Jul 2020)

RR by Florian Pantillon (20 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

“How an uncertain short-wave perturbation on the North Atlantic wave guide affects the forecast of an intense Mediterranean cyclone (Medicane Zorbas)” by Raphael Portmann, Juan Jesús González-Alemán, Michael Sprenger, and Heini Wernli

The paper has changed substantially since the last round of reviews. Its scope is now well introduced and its limitations explained, while discussions have gained in clarity and are supported by very illustrative figures. Altogether, the paper highlights the role of remote uncertainties along the upper-level wave guide in the medicane forecast and presents an interesting hypothesis of upper-level PV erosion by latent heating that allows tropical transition to take place. I thus recommend the paper for publication pending minor corrections listed below to further improve clarity.

Specific comments

l. 23–24 could you give a more general statement or perspective to conclude the abstract?

l. 27 “the main meteorological threat”: exaggerated, as local high-impact weather is not necessarily related with cyclones (see HyMeX program)

l. 45 remove “for instance”

l. 52 typo: “causal link s”

l. 54 “whose formation due to an upper-level PV streamer is the subject of this
study”: too much information for this sentence (and not supported by Wikipedia); better keep for the end of the paragraph?

l. 79 “similar to Maier-Gerber et al., 2019”: explicit “for a tropical transition over the North Atlantic”?

l. 84–87 this sentence is disconnected from the previous one (no ET in Di Muzio et al. 2019)

l. 92–96 unclear (it becomes clearer in Section 4)

l. 228–230 do you mean that more than half of those parcels actually reach the upper levels? giving the average ascent in km or hPa rather than in K would be easier

l. 230–231 “they contributed substantially”: too strong statement

l. 231 “Despite limitations of trajectories in convective situations”: detail (convection has not been mentioned yet)

l. 232 “as a result”: why?

l. 233–235 the link with the previous sentence (PV erosion) is not straightforward

l. 255 typo: “precentile”

l. 278 “only 21 ensemble members predicted a medicane” that’s not bad!

l. 299 refer to black contour in Fig. 7 for the analysis

l. 337 unclear in what the region is “clearly stratospheric”: is it associated with PV>2pvu? It also appears to contradict the “upper-tropospheric” dynamics and forecast uncertainties mentioned below

l. 350 a westward shift of the wave pattern is apparent rather than a superimposed short-wave pattern

l. 353–355 emphasize that the negative (positive) difference becomes more and more (less and less) significant?

l. 367 “large drop” seems exaggerated for a decrease that is steep but continuous

l. 379 “in these regions”: which ones? Refer to white contours?

l. 394 Fig 11g

l. 402 84h and 106h lead times are not mentioned before

l. 407–408 refer to Fig. 6 again?

l. 421 rephrase: the stratospheric PV anomaly is not equivalent to the cyclone thermal structure

l. 427ff remind which color corresponds to which cluster to help the reader follow the discussion

l. 465 “they”: the significant differences?

l. 496 better “relevance of a PV streamer position for medicane formation” (without uncertain)?

l. 497 indeed, climatological studies have shown that some regions are more prone to medicane formation than others, but what is meant here exactly?

l. 515ff many DOI links are broken due to unwanted characters

l. 651 The Wikipedia reference is incomplete (I let the Editor judge whether it is appropriate): https://en.wikipedia.org/wiki/Mediterranean_tropical-like_cyclone#Zorbas_(27_Sep_%E2%80%93_1_Oct_2018)

Figure 1 Explain labeled features, or refer to the text?

Figures 8–9 Labeling relevant features as in Figure 1 would be helpful and would strengthen the link between the different Sections

Figure S5 do clusters 1, 2, 3 refer to C, W, E? The figure should be mentioned somewhere in the text or in the caption of Fig. 11 (or removed otherwise)

Hide

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

AR by Raphael Portmann on behalf of the Authors (15 Sep 2020) Author's response Manuscript

ED: Publish as is (25 Sep 2020) by Michael Riemer

AR by Raphael Portmann on behalf of the Authors (28 Sep 2020) Manuscript

Short summary

In September 2018 an intense Mediterranean cyclone with structural similarities to a hurricane, a so-called medicane, caused severe damage in Greece. Its development was uncertain, even just a few days in advance. The reason for this was uncertainties in the jet stream over the North Atlantic 3 d prior to cyclogenesis that propagated into the Mediterranean. They led to an uncertain position of the upper-level disturbance and, as a result, of the position and thermal structure of the cyclone.