Review of 1st revision of Emanuel et al. (2025)
I sincerely thank the authors for addressing the comments outlined in my previous review in a punctual and effective manner. There are still just a few unclear points in their reply, that I discuss below. I will focus only on points I still find critical for conciseness, but would also like to commend the authors for all the other interesting and thoughtful replies. I believe that with relatively little additional explanation and correction the paper will be ready for final publication.
Given that the detailed lines where modifications have been brought in response to the previous comments have not been specified in the reply document, I apologize in advance if any of my current comments were to touch upon already made corrections. To avoid this, please indicate in your reply the detailed line numbers where changes have been made in response to each comment.
General comments
1) “We agree that in this particular context, stating that cyclones can be driven by one or both processes (baroclinic instability, WISHE) could be misleading, but we here are making a more general statement about cyclones. Hybrid storms can be driven by both processes at the same time, and we now provide a reference for that possibility. But we do not argue that both processes are at work at the same time in the CYCLOPs cases we examined, though in the December, 2005 medicane, warm advection did serve to deepen the synoptic scale precursor surface cyclone.”
I am not fully satisfied with this answer and with the related modifications brought to the manuscript. First of all, the reference to support the possibility of “double driving” has not been provided in the revised text (sorry if I missed it). Looking at the reply on the WCD website, I infer that this might be Fantini et al. (1990). However, that work focused on explaining -using “highly idealized” simulations (cf. Wernli and Gray 2024)- the problem of explosive intensification of extratropical cyclones in presence of latent heat release. I would argue this is a different problem than discerning between driving by baroclinic instability and driving by a positive feedback involving surface enthalpy fluxes and deep convection (or, said in short, by WISHE). It is true, as the authors say, that for explosive (and also non-explosive) cyclones latent heat release can be considered as part of the “driving” of the extratropical cyclone together with dry baroclinic processes, but this shifts the discussion on a whole different level with respect to the focus of this work about the nature of CYCLOPs, and substantially increases the risk to confuse the reader. The key point is that the “help” given by latent heat release in clouds to the explosive deepening of extratropical cyclones is not by WISHE because a) it is not a self-sustaining process, but lasts as long as the baroclinic wave provides forcing for ascent (e.g., by upgliding along fronts, by convective destabilization…) and b) it does not result in a system with “tropical” characteristics, but in an even stronger extratropical cyclone. I believe it is very important to keep distinct these two typologies of “latent-heating-assisted” cyclone intensification process.
In this regard, the examples brought by the authors to support their point are not completely convincing. The work by Fantini (1990) notices a first stage of moist baroclinic growth with strong contribution from latent heat release, which then transitions -I guess after occlusion- to a tropical cyclone-like structure given the favorable environmental conditions. The driving of the first part of the storm was by moist baroclinic processes (that resulted in a wave with vertical baroclinic structure, see their Fig. 1, and surface fronts), but as soon as they weaken WISHE or some sort of CISK could set in to intensify the system (the storm acquires a single, dominant updraft only 4 days after the beginning of the simulation, see their Fig. 7): so, again, indication of different driving processes at different times and not of co-occurrent driving. The example of the December 2005 medicane brought by the authors confirms indeed that warm advection was then a preconditioning to the development of the CYCLOP, and not part of the driving, because it helped to spin the surface low in which the CYCLOP would have developed. The example of extratropical transitioning storms might be the only one featuring a transient double driving, but as I discussed in the previous review it is the presence of baroclinic processes that fundamentally allows the intensification of the system in such unfavorable conditions because, otherwise, a system with tropical characteristics would have a hard time maintaining WISHE over relatively cold water and in presence of strong shear. Extratropical transition is also a very specific example that should not be used -in my opinion- to make a “more general statement about cyclones” as stated in the reply.
Finally, I fundamentally disagree that tropical transitioning storms can be driven by both processes at the same time as stated at line 69: in the best case, extratropical dynamics can create favorable conditions (e.g., via Rossby wave breaking, or via the formation of warm seclusions in cyclones) that allow WISHE to take over -and it is the fact that driving by surface enthalpy fluxes becomes important, the discriminating aspect of tropical transition. The fact that PI is present or not in the environment is again a matter of preconditioning that -as we are talking about the driving- is here of secondary importance. Although the authors specify in the following words “with relative proportion usually varying over the life of the storm”, I still find it potentially misleading because it does not exclude that the two mechanisms can be active at the same time (e.g., in a “50/50”, or “40/60” proportion).
As actionable items, I would again suggest to remove the “or both” at line 64, and to modify the statement at lines 69-70 “with the relative proportion usually varying over the life of the storm” to something along the line of “with distinct driving mechanisms over the life-cycle of the storm”. An alternative could be to remove the paragraph at lines 64-72 altogether, as it does not feature a single citation and is quite general with respect to the strong focus of the Introduction on CYCLOPs: instead, the third paragraph could seamlessly follow from the first. Also, the acronym “APE” introduced at line 65 is not used in the reminder of the text.
2) In the revised manuscript, we make it clear that it is the local (in space and time) generation of PI that distinguishes CYCLOPs from other developments, including tropical transition. In our view, this is an important distinction, from both theoretical and practical standpoints. If we could magically remove the locally generated PI, a warm-core, TC-like cyclone would not happen and in that case we would not even make the mistake of mis-identifying the event as tropical transition, since in the end no TC-like entity would form. From a practical point of view, identifying the generation of modified PI in guidance would become an important consideration, which it is certainly not today.
Although I agree with the considerations of the authors, I cannot help wondering how they can apply to CYCLOPs and not, in the same way, to “pure” tropical transitions. The authors also say later in the reply document that indeed “there is a gray area between tropical transition and what we are calling CYCLOP development”. I feel the manuscript can be read in both senses now, because statements to support both perspectives are present. I agree that this is not the right place to potentially unify the two concepts, but I think the common aspects could be emphasized more in the manuscript, rather than insisting about a fundamental difference between pure “tropical transition” and “CYCLOP transition” (e.g., line 623). I would ask the authors to further reconsider this aspect to clarify the differences and the analogies between tropical transition and “CYCLOP transition”.
Detailed comments
Line 99-103: I appreciated the clarification by the authors that the cancellation is not common, but if this is the case why including it in a schematic that would like to be as generalizable as possible? What is the added value? I also thought of another potential issue: we know from PV dynamics that potential temperature (i.e., θ) anomalies at the surface can induce surface-based circulations, but here the lifting below the upper-level PV anomaly is an adiabatic process that generates only a temperature anomaly. And indeed, tropopause polar vortices -corresponding to very strong upper-level PV anomalies- are usually associated with cyclonic circulations at the surface (e.g., Cavallo and Hakim 2010). Could the authors further clarify this inconsistency, or maybe consider dropping this detail?
Lines 243-245: could a reference be provided to support this statement and direct the interested reader to additional information?
Bibliography
Cavallo, S. M., and G. J. Hakim, 2010: Composite Structure of Tropopause Polar Cyclones. Mon. Wea. Rev., 138, 3840–3857, https://doi.org/10.1175/2010MWR3371.1.
Fantini, M., 1990: The Influence of Heat and Moisture Fluxes from the Ocean on the Development of Baroclinic Waves. J. Atmos. Sci., 47, 840–855, https://doi.org/10.1175/1520-0469(1990)047<0840:TIOHAM>2.0.CO;2.
Wernli, H. and Gray, S. L.: The importance of diabatic processes for the dynamics of synoptic-scale extratropical weather systems – a review, Weather Clim. Dynam., 5, 1299–1408, https://doi.org/10.5194/wcd-5-1299-2024, 2024. |
Review of "A Unified Framework for Surface Flux-Driven Cyclones Outside the Tropics", by Emanuel et al. (submitted to Weather and Climate Dynamics)
Summary and overall assessment
Through a compelling series of case studies, Emanuel and co-authors propose a novel framework to explain the occurrence of low-pressure systems with "tropical" characteristics in extratropical regions where conditions are in principle unfavorable for their genesis – or in short, CYCLOPs. The "favorability" to the development of tropical features is quantified using a modified version of the potential intensity (PI) metric by Emanuel (2005) : the discriminant between TCs and CYCLOPs is the local and transient character of generation of PI by the large-scale flow, compared to the typical situation in the tropics and subtropics where the availability of PI is usually not a limiting factor. This piece of research is a bold attempt to advance scientific understanding in the field, attempting to bring wildly different weather systems such as polar lows, medicanes and Kona lows together under the same "umbrella". The CYCLOP concept is understandable for readers familiar with the PI framework already in the first section, while the rest of the paper is a series of case studies to back up the hypothesis from several different sources.
Although the concept of CYCLOPs is an innovative and useful heuristics, however, their deepening is mostly driven by a "spatio-temporally localized wind-induced surface heat exchange" (WISHE) and this makes them much more akin to tropical cyclones than to extratropical ones (driven by baroclinic instability). From this point of view, they would not deserve their own equidistant "corner" in the cyclone space (Fig. 30), and they could be more easily assimilated to the "trough-supported" tropical transition cases over cold water discussed by McTaggart-Cowan et al. (2015). Explaining the several unexpected tropical transitions observed over "cold" waters using the CYCLOP concept is already a very valuable contribution that this research work brings to the literature.
I believe that part of the problem lays in a misunderstanding between processes setting up the preconditioning of the storm environment and the actual energetic driving of cyclones: while tropical cyclones are usually distinguished from extratropical cyclones through their energy source driving their deepening (i.e., WISHE vs baroclinic instability), in this paper CYCLOPs are distinguished from "classic" tropical cyclones through a difference in their preconditioning (the availability -or not- of PI in the environment where the low develops). This fundamental difference is not always clearly expressed in the manuscript, leading to some ambiguities that I tried to pinpoint in the comments below.
The other critique that could be moved to the paper is that it does not go the final mile in emphasizing the broad implications of the performed analysis. The title "A Unified Framework for Surface Flux-Driven Cyclones Outside the Tropics" suggests the attempt to "unify" all storms driven by "surface fluxes" (or to be more precise, by the feedback between surface fluxes, deep convection and wind speed conceptualized by WISHE), but the outcome of the paper is a rather divisive operation, that attempts to separate the CYCLOP category from supposedly more "pure" surface-flux driven cyclones (as in Sec.4, but also Sec. 5 and 1). The presence or the absence of PI in the environment is very interesting to discuss in order to understand "special" cases such as Medicanes or polar lows, but does not result in storms with fundamentally different properties than tropical cyclones: the final result of the process is still a -more or less transient- warm core, surface-flux driven storm. On the other hand, this piece of work provides a powerful and still unexploited unifying platform, because the extension of PI concept to the extratropics is shown to encompass virtually every known category of surface-flux driven cyclones, even allowing to assess whether a given storm can or cannot be surface-flux driven (e.g., at ll. 612-615).
I enjoying reading this paper and tried to read it and understand its implications and context as carefully as possible. The comments below are long, and I apologize for it, but I wanted to be as clear as possible in outlining my suggestions for improvement. The review is also complicated by the terminology used in the field, because terms as "tropical cyclone" or "tropical transition" bear a strong connotation and can be misleading when describing what might happen inside, e.g., a high-latitude polar low. Clarifying the used terminology makes up, I believe, half of the length of this review.
Despite the length of the review, however, I firmly believe this work has a great potential to enrich this field of research and see it as perfeclty fit for Weather and Climate Dynamics. I ask the authors to consider my comments, detailed below, and hope that they can be helpful in viewing the "CYCLOP's myth" from a complementary perspective.
Main comments
Line-by-line comments
l. 55: is the development of CYCLOPs actually more rapid than the one of extratropical cyclones? The organization of convection can be at times very slow (>24h).
l. 68: in which sense "baroclinic" processes? Rossby wave breaking and cut-off formation are rather considered as "barotropic" processes (probably also a not-so-appropriate term...). Could the authors be more specific? Otherwise it might sound like the generation of PI could be tied to baroclinic instability, which is not possible (see comment 1).
l. 90 “moistened”: I guess the authors are here referring to something like "humidified" or “brought closer to saturation”, because cooling alters the relative humidity rather than the specific moisture.
l. 91-94: two comments on this sentence, asking for further clarification/contextualization: 1) while the PV argument is elegant and correct, it might not immediately be clear to the reader why such a cancellation between upper PV and lower theta contribution is needed in a minimal conceptual model of CYCLOPs; 2) surface cyclones are often observed at the leading edge of a moving cut-off/PV streamer – e.g., to the east of a streamer moving from west to east- so one would not automatically expect a vertical alignment as in Fig. 1a.
l. 97-98: this is an interesting observation: do the authors observe that the scale of the PV anomaly and the scale of the CYCLOP are correlated across their range of case studies?
l. 118: could a legend table be added -maybe in the appendix- to show the units and the explanation of each symbol?
ll. 131-132: please specify that the surface fluxes are able to destroy the upper-level cold low thanks to the crucial effect of deep moist convection - otherwise their effect would not be "felt" outside the boundary layer. While the link to deep convection is more obvious in the Tropics, it is important to remind it for systems occurring away from warm tropical waters.
Section 2: this "Motivation section" actually extends only until line 171, while the remaining part is more an intro to Sec. 3 and the authors could consider moving it between line 203 and line 204. Furthermore, I would suggest the authors to add another motivation item besides predictability and ocean interaction, concerning the role that surface flux-driven cyclones will play in the future climate. In a warmer climate with higher SSTs, areas favorable to TC genesis will extend poleward and surface-flux driven cyclones will likely become more frequent and familiar to the extratropics. The approach outlined in this paper allows to objectively diagnose the presence of surface-flux driving in dubious cases, and even break it down in different contributions to PI. It provides a useful framework to talk about such systems and distinguish them from "usual" cyclones, and provides us with useful tools to quantify and study this effect of climate change onto the extratropical circulation.
l. 226: "(geopotential) height" instead of "pressure"? Pressure is constant on the 950hPa iso-surface.
l. 227: where is the "cold pool" visible?
l. 233: the wording raises the question: at least it should be shown in some way somewhere, before it can be claimed with "no doubt" (thanks for specifying here the role of deep convection!).
Fig. 4 (and others): I see the challenge in finding a common unifying theme for the many figures across the manuscript, but nevertheless there are some aspects that could be improved:
l. 247: 750hPa
ll. 274-276, and also ll.600-608: tropical transition is sub-optimal as a TC genesis mechanism, and results in weaker TCs (McTaggart-Cowan et al. 2015, Sec.5), that might even go unnamed such as the one discussed by Bentley and Metz (2016) -which bears resemblance to the case discussed here in Sec. 3.5 . So, the capability or not of the cyclone to survive after the transition depends on the changing large-scale environment around each storm, and should not be regarded as a condition to define or not the presence or the quality of tropical transition.
ll. 312-313: couldn't it be because the upper-level low has also weakened (see comparison between Fig. 8a,b)? If this is the effect of deep convection, it would be nice to show it or to specify the hypothesized chain of processes.
Fig. 8b: label b) is missing
Sec. 3.3 is very fascinating, both figure and discussion!
Fig. 15: the description at lines 404-412 would greatly profit of labels or symbols added to the figures so that the track of the parent upper-level low can be followed. In addition, the authors could consider whether the chosen domain could be made smaller and some time steps skipped.
Fig. 16: the location of development with the upper-level cut-off low to the south-west of the surface low reminds me of the favorable TC-trough interaction by Fischer et al. (2019). Do also the other cases tend to develop with the upper-level cyclone to the SW of the low?
ll. 448-453: please refer to specific sub-plots that best depict the anticyclonic wave breaking (by the way, shouldn't it be cyclonic, and why does it matter?), the presence of the cold pool, and the deep trough.
ll. 515-519 and Fig. 22: could the authors explain what the additional discussion of the Kona low depicted in Fig. 22 adds to the -already quite heavy- manuscript?
ll. 541-545: the humidifying effect of the middle troposphere is actually consistent with the CYCLOP paradigm, as well as the effect of the Rossby wave breaking in bring the initial disturbance to trigger the storm. If it were not for the pre-existing PI, Zorbas would have been classified as a "classic" CYCLOP: a surface-flux driven storm that owes it existence to the preconditioning of an upper-level trough. I do not see why this should not be identified as a CYCLOP, although it is certainly interesting to point out the pre-existing PI in the Mediterranean, a condition that is -I guess- relatively common in September and becoming more and more common with anthropogenic global warming.
ll 627-628: An additional point of ambiguity is the comparison between "pure" tropical transition and "pure" CYCLOPs, because tropical transition is a process, while CYCLOPs are weather systems.
Fig. 24: would it be possible to show other time steps of PI to better depict the connection between PI and the upper-level flow?
ll. 666-667: could the authors discuss a bit more the appropriateness of this assumption in the extratropics or in the dry-convecting environments of polar lows?
ll. 673-676: it is not immediately clear which temperature is referred to here by saying "under the cut-off cyclone": the one at the tropopause, or the one at low-levels?
Bibliography
Bentley, A. M., and N. D. Metz, 2016: Tropical Transition of an Unnamed, High-Latitude, Tropical Cyclone over the Eastern North Pacific. Mon. Wea. Rev., 144, 713–736, https://doi.org/10.1175/MWR-D-15-0213.1.
Davis, C. A., and L. F. Bosart, 2003: Baroclinically Induced Tropical Cyclogenesis. Mon. Wea. Rev., 131, 2730–2747, 2.0.CO;2" target="_blank" class="Lexical__link">https://doi.org/10.1175/1520-0493(2003)131<2730:BITC>2.0.CO;2.
Davis, C. A., and L. F. Bosart, 2004: The TT Problem: Forecasting the Tropical Transition of Cyclones. Bull. Amer. Meteor. Soc., 85, 1657–1662, https://doi.org/10.1175/BAMS-85-11-1657.
Emanuel, K.A., 1988: Toward a general theory of hurricanes. Amer. Sci., 76, 370-379.
Fischer, M. S., B. H. Tang, and K. L. Corbosiero, 2019: A Climatological Analysis of Tropical Cyclone Rapid Intensification in Environments of Upper-Tropospheric Troughs. Mon. Wea. Rev., 147, 3693–3719, https://doi.org/10.1175/MWR-D-19-0013.1.
McTaggart-Cowan, R., T. J. Galarneau, L. F. Bosart, R. W. Moore, and O. Martius, 2013: A Global Climatology of Baroclinically Influenced Tropical Cyclogenesis. Mon. Wea. Rev., 141, 1963–1989, https://doi.org/10.1175/MWR-D-12-00186.1.