the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 Nov 2023
How a warmer Mediterranean preconditions the upper-level environment for the development of Medicane Ianos
Abstract. Medicane Ianos in September 2020 was one of the strongest medicanes observed in the last 25 years. It was, like other medicanes, a very intense cyclone evolving from a baroclinic mid-latitude low into a tropical-like cyclone. The dynamical elements necessary to improve the predictability of Ianos are explored with the use of simulations with the Met Office Unified Model (MetUM) at 2.2 km grid spacing for five different initialisation times, from four to two days before Ianos's landfall. Simulations are also performed with the Sea Surface Temperature (SST) uniformly increased and decreased by 2 K from analysis to explore the impact of enhanced and reduced sea-surface surface fluxes on Ianos's evolution. All the simulations with +2 K SST are able to simulate medicane Ianos, albeit too intensely. The simulations with control SST initialised at the two earliest times fail to capture intense preceding precipitation events at the right locations, and the subsequent development of Ianos. Amongst the simulations with -2 K SST, only the one initialised at the latest time develops the medicane.
Links between sea-surface fluxes and upper-level baroclinic processes are investigated. We find (i) a bubble of low-valued potential vorticity (PV) formed within a trough above where Ianos developed, diabatic processes associated with a preceding precipitation event triggered a balanced divergent flow in the upper-levels which contributed to the creation and maintenance this low-PV bubble, as shown by results from a semi-geostrophic inversion tool, (ii) Upper-level geostrophic vorticity advection associated with the low-PV bubble forced quasi-geostrophic ascent during Ianos's cyclogenesis, and (iii) diabatic processes dominated by deep convection formed a vertical PV tower in Ianos and continued to produce diabatically-induced divergent outflow aloft, thus sustaining Ianos's development. Simulations missing any of these three elements do not develop medicane Ianos.
Our results imply that preceding convection was essential for the subsequent development of Ianos, highlighting the importance of the interactions between near-surface small-scale diabatic processes and the upper-level quasi-geostrophic flow. A warmer SST strengthens the processes and thus enables Ianos to be predicted in simulations initiated at the earlier times that failed to generate the medicane with control SSTs.
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-2431', Anonymous Referee #1, 12 Jan 2024
This study considers a suite of experiments to explore the evolution of “Medicane” Ianos. In particular, the focus is on the role of preceding unorganized convection in generating (“pre-condition”) an upper-level environment favorable for the development and intensification of Ianos. This general finding is well supported by the presented evidence from the phenomenological perspective and constitutes interesting new insight into the variety of processes that may be associated with the evolution of medicanes. The manuscript will thus be eventually suitable for publication in WCD. The presented evidence to establish the mechanistic link between the pre-conditioned environment and Ianos’ evolution is, however, insufficient. A few major comments and several further comments need to be addressed before final publication.
General commentI find the presentation to be mostly descriptive, despite the use of two quantitative diagnostics. Importantly, I was often left wondering about the significance of described observations. While most observations support the authors’ main point about the importance of the upper-level low-PV “bubble” well, the presentation of observations without an evident link to theory or conceptual models make for a rather lengthy read. Personally, I would have much appreciated more such links to conceptual or theoretical frameworks of TC-like storm dynamics. I am aware that this comment is very general, but it would take me an unwarranted amount of time to think more thoroughly about Ianos’ evolution to make more specific suggestions for improvement. I consider the presentation “ok” for the purpose of the present study, in the sense that the presentation does provide evidence for the main point of the study. I thus leave it to the authors’ discretion to improve their manuscript in this respect.
Major commentsQG omega analysis:
The importance of the low-PV bubble is well supported from a phenomenological perspective and is of interest in itself. The mechanistic link of how these upper-level changes impact Ianos’ evolution, however, is not established sufficiently well by the QG omega inversion analysis. I have several issues with the analysis. First, the signal that the authors present is very weak, and the associated discussion is partly confusing (L465: I do not see the absence of vertical motion described by the authors; L475: I am confused if the authors show omega or w. Equation 3 says omega, but here positive values seem to be referred to as ascent.) Only two out of the six simulations are contrasted. It is thus not possible to say if the weak signal is at least systematic. Second, there is no theoretical or conceptual justification of how the observed small differences would impact the evolution of Ianos. Note that arguments based on the dynamics of (fully) extratropical cyclones can be applied here only partially because Ianos has clearly tropical characteristics. Formation and intensification mechanisms of tropical-like storms thus need to be taken into account also. Third, the authors consider one part of the forcing only. Without evidence that the other forcing term is indeed negligible, I consider the analysis to be incomplete. Finally, the authors should acknowledge the intricacies of a vertical partitioning of forcing in the omega equation, as discussed by Morgan (1999).
In addition, there is an alternative plausible hypothesis how the changes to the upper levels impact Ianos: by changes to the vertical wind shear. The authors note this mechanism in the discussion of preconditioning role of preceding convection during tropical transition in L542. The same mechanisms may be at work here, too, and analysis of this aspect would provide much benefit to the manuscript.Title:
I fully agree with the authors that convection preceding Ianos can be considered as a preconditioning of subsequent evolution. Warmer SST makes this preconditioning more robust in the model simulations of this specific case. I’d argue, however, that the preconditioning is still by the preceding convection and not by the warmer SST. It is hard to make a general case about the preconditioning role of SST from this single case. I think that it is thus warranted to adjust the title accordingly.
Reference to tropical transition:
At a few points in the manuscript the authors refer to the evolution of Ianos as tropical transition. Is there evidence that Ianos indeed started as a (distinct) midlatitude cyclone? The authors argument in L520 that baroclinic forcing alone was insufficient for strong cyclogenesis speaks against a distinct baroclinic origin of Ianos. An alternative interpretation could consider the observed evolution more akin to tropical cyclogenesis. I believe that clarification or a more explicit discussion of tropical transition vs. tropical cyclogenesis would be beneficial to the manuscript because the underlying mechanism differ distinctly between these pathways.
Minor commentsL118, single-moment scheme: Please clarify which hydrometeors are represented. Is there ice? The presentation suggests that there is rain only. More generally, please add a brief discussion on how the simplistic one-moment scheme affects the representation of convection.
L281: I do not see an indication of dipole structure in the figure and thus this explanation seems mere speculation. Alternatively, and more simply, the observed tropospheric PV values at upper-levels could simply be due to vertical transport (cross-isentropic as well as adiabatic) of PV from the lower troposphere.
L303; “horizontal gradients”: Did the authors evaluate different contributions to PV or is this speculation? Usually, diabatic heating generates PV anomalies by generating stability anomalies, i.e., changing the vertical gradient of theta.
Fig. 14: I am confused: Is this a Hovmoeller diagram for which the meridional location is adjusted at each time step to match the meridional position of Ianos? I have a hard time understanding this rather unconventional and complex plot and thus suggest further improving the description/ introduction of the format. Or is there a simpler way to depict the relevant information?
L526: The authors did not study the adjustment process and thus I recommend not emphasizing this process in the conclusions.
Editorial commentsL10: “bubble” I think it is fine to use this term in the main text (L285) in quotation marks and when supported by a figure. For the purpose of the abstract, I recommend rewording.
L15-16: The sentence reads as if the divergent outflow aloft sustains Ianos’ development. Is this the causality that the authors mean to imply. Or isn’t the outflow at this point during the development rather a consequence of Ianos’ sustained development? Please clarify.
L105: What is the medium-complexity model for surface fluxes? Or do you imply that surface fluxes fully control the diabatic processes analyzed with Semi-Geotriptic inversion tool? Please clarify.
L116: change “resolve” to “represent”
L116: The eye is not a process. Please reword.
L128: The use of the term “trajectory” is not entirely clear to me here. Please clarify.
L138: Does this sentence make sense? In addition, please note in the following discussion if the inversion is linear (e.g., around L151-152).
L155: What is your estimate of the Rossby radius of deformation? 0.32 deg would be a rather small estimate. Why do you choose this length scale?
L208: This statement is not strictly correct because the relation is not monotonic. Please revise.
L213 “trajectory” (here and elsewhere): “track” is the common term used for tropical-like storms. I suggest revising to avoid confusion.
Titles of subsections 4.3 and 4.4: These subsections discuss also the tropospheric-deep PV tower of Ianos, not only the upper-level structure. I thus suggest revising the titles.
L302: I have a hard time to identify the low PV values aloft. Please clarify or change color bar.
L405: suggest adding: “size of the” dots
Reference:
Morgan, M. C. (1999). Using piecewise potential vorticity inversion to diagnose frontogenesis. Part I: A partitioning of the Q vector applied to diagnosing surface frontogenesis and vertical motion. Monthly weather review, 127(12), 2796-2821.Citation: https://doi.org/10.5194/egusphere-2023-2431-RC1 - AC1: 'Reply on RC2', Claudio Sanchez, 10 Apr 2024
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AC2: 'Reply on RC1', Claudio Sanchez, 10 Apr 2024
Apologises, the comment above should read as "reply to reviewer RC1" rather than reviewer RC2. The attached file is the same for both reviewers.
Citation: https://doi.org/10.5194/egusphere-2023-2431-AC2
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RC2: 'Comment on egusphere-2023-2431', Anonymous Referee #2, 06 Feb 2024
Review of “How a warmer Mediterranean preconditions the upper-level environment for the development of Medicane Ianos” by Sanchez et al.
This study performs an analysis of the development of Medicane Ianos over the Mediterranean Sea. By performing numerical simulations/forecasts and using different advanced analysis techniques, the authors conclude that the cyclone development was favored by baroclinic (QG) forcing thanks to a preceding convective burst event which altered the upper-level dynamics. Also, the authors state that with a warmer Mediterranean Sea, this process becomes clearer and, thus, models would have less problems with predictability. This is a very nice and substantial contribution to the community and thus, the study deserves publication but with some revisions.
In my opinion, this paper is potentially publishable, and I applaud the authors for analyzing this case study from this high-quality point of view. Also, the analysis is exhaustive and very well structured, with deep information on the case, although in some parts it is quite large (I would summarize it).
However, I think this paper, as it is now, must undergo major revision to robustly justify some of the conclusions and the line of thinking and, also, to address its weaknesses. Therefore, I’m afraid I cannot accept this study for publication for now.As I think that a major review or explanation to justify the paper for publishing (as it is now) is needed, I will only focus on the most important points not about text/grammatical issues. Once major issues are solved, I’ll later focus on these.
GENERAL POINTS
- The tittle does not totally represent the content of the study and the conclusions: It focus more on the mechanisms that favor the development of Ianos rather than on the interaction between a warm sea or warmer sea and the convective factors that provoke Ianos cyclogenesis. It deals with this issue, but it is not the main part of the study, in my opinion. Indeed, in the abstract we do not see any focus on this.
I would accept the title if the study had focused on the mechanisms and interaction between preceding convection and air-sea fluxes.
- The study argues about the importance of baroclinic forcing in the development of Ianos, i.e. from a midlatitude meteorology point of view, but given it is a tropical transition phenomenon, it lacks addressing its development for a tropical meteorology point of view, i.e., the dynamics of convection and its organization. Therefore, I would explicitly mention this concern about the partial view of this phenomenon in this study when discussing the results and conclusions.
SPECIFIC POINTS
- I would try to reduce the length of the article, especially Section 4.2, 4.3, 4.4 (which are not the main body of the line of arguments; or also merging 4.3 and 4.4 after summarizing) and Conclusions and just focus on the main important aspects for understanding the main message about the mechanisms involved in Ianos’ cyclogenesis and the role of the SST (although this is not mainly addressed).
Section 1
Line 35-40: I would argue that upper-level baroclinic forcing is essential for the development of precursors to medicanes, not for the intensification of medicanes, at least for those robust (tropical) ones.
Section 4
4.1
Line 197: What is the argument to say that it becomes a medicane at this time?
Figure 2: As the different initialization times are indicated by a different symbol, I would put just the same color for the CTL, -2K and +2K groups. In this way, the interpretation becomes clearer without too much noise.
Lines 215-225: It would be nice to make this analysis more complete by adding the CPS or at least the VTU values for each run, to provide a clearer idea of its tropical (or not) structure.
4.2
Line 233: Could you provide more evidence for this mechanism of triggering these thunderstorms.
4.3
Line 304-305: Why the authors compare this development with those of extratropical cyclones and not with those of tropical transition? I would argue that this is a typical development for tropical transition cyclones and the result of Ianos acquiring a more robust tropical structure.
Line 307: Talking about tropical-like transition here is not suitable as this is not a rubust definition in the literature. The best way to deal with this type of development is talking about the tropical transition process, to be more in line with the community that study cyclone transitions.
In this section and the rest of the sections, I miss discussions about the upper-level dynamics that considers the jet streak behavior. It is true that the PV field is related to the jet dynamics, but it would be interesting to also consider the analysis/explanations from the jet’s QG forcing perspective in the analysis of this section and the following ones.
4.5
Figure 13: MetUM means full solution?
Line 403: Is not the y-axis showing the dates? (Figure 14)
4.6
Line 467: By watching Figure 16 (a-b), one could argue that the forcing at lower levels is stronger in (a) and, therefore, conclusions derived from Figure 15 are subject to the level of choice (700 hPa). If we choose 800 hPa or 850 hPa, for instance, conclusions could be different. Indeed, the signal seems to decrease a lot at the 700 hPa level. Please, elaborate more on this. Why not choose 800 hPa (it would still avoid boundary layer effects)? This argument of similar low-level forcing appears to be quite weak.
Conclusions
Line 538-540: I see the idea of the authors here, but I don’t fully see the relationship between Ianos’ development and this mechanism in extratropical transitions and downstream cascade of events. The link is about the upper-level PV modification by a diabatic source, but just up to this point.
Line 541-544: As I mentioned earlier, an analysis focusing on this (from the tropical transition perspective) would make the article more complete.
Line 545-552: Again, I think this part is not very well connected with Ianos’ mechanisms and it could derive some confusion.
Line 562: I wouldn’t mention this hypothetical situation as (as you discuss later) the model is uncoupled and a robust analysis about model performance in this case has not been undergone. This scenario could be quite fictitious.
Citation: https://doi.org/10.5194/egusphere-2023-2431-RC2 - AC1: 'Reply on RC2', Claudio Sanchez, 10 Apr 2024
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