Environmental Characteristics Associated with the Tropical Transition of Mediterranean Cyclones

Bernini, Lisa; Cavicchia, Leone; Desbiolles, Fabien; Scoccimarro, Enrico; Pasquero, Claudia

doi:https://doi.org/10.5194/egusphere-2025-3861

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https://doi.org/10.5194/egusphere-2025-3861

Preprints

20 Aug 2025

| 20 Aug 2025

Environmental Characteristics Associated with the Tropical Transition of Mediterranean Cyclones

Lisa Bernini, Leone Cavicchia, Fabien Desbiolles, Enrico Scoccimarro, and Claudia Pasquero

Abstract. Cyclonic perturbations in the Mediterranean region sometimes acquire characteristics typical of tropical cyclones, such as a deep inner warm core. In these cases, they become very intense structures that can cause large precipitations and significant damage. In this study, the environmental conditions during the intensification of cyclones are investigated using reanalysis data. A comparison of the conditions associated with the evolution of classical and intense cold-core extratropical cyclones and those associated with the development of tropical-like disturbances highlights the characteristic that favors the conversion: a much larger potential intensity and a weaker vertical wind shear. The larger potential intensity associated with Mediterranean tropical-like cyclones comes from both higher SST and a strong PV-intrusion that destabilizes the air column. Sea surface cooling induced by the cyclones is further shown to play a role in the dissipation of tropical-like cyclones. Future research should focus on the role of potential intensity as a precursor for Mediterranean tropical-like cyclone forecasting, improving predictive capabilities and risk mitigation strategies in the Mediterranean region.

Received: 08 Aug 2025 – Discussion started: 20 Aug 2025

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Lisa Bernini, Leone Cavicchia, Fabien Desbiolles, Enrico Scoccimarro, and Claudia Pasquero

Status: final response (author comments only)

RC1:
'Comment on egusphere-2025-3861', Anonymous Referee #1, 28 Aug 2025

The paper classifies Mediterranean cyclones into three categories and describes the mean and standard deviation of their characteristics 36 hours before their maximum intensity. The paper sorely lacks arguments to convince the reader that the three categories of cyclones have consistent characteristics and that the framework around the maximum intensity is relevant for analyzing such a diversity of cyclones.
Major comments
The title "Environmental Characteristics Associated with the Tropical Transition of Mediterranean Cyclones" is misleading. The paper compares ETC, intense ETC and MLTC over a 72-h period centered on the first minimum sea level pressure (SLP). It therefore deals with the intensification of the three types of cyclones, not the tropical transition of Mediterranean cyclones.
With regard to tropical transition (TT), the authors should refer to cyclones for which "a fundamental dynamic and thermodynamic transformation of an extratropical precursor (of baroclinic origin and initially considered a cold-core system) is required to create a warm-core tropical cyclone" (Davis and Bosart 2004). In other words, TT deals with ETCs that transform into MLTCs. These transformed cyclones correspond to your definition of hybrid cyclones, which are not included in the study. Under these conditions, I do not understand how the study can actually address TT.
Line 26, it should of interest to cite Miglietta et al. (2025) "A medicane is a mesoscale cyclone that develops over the Mediterranean Sea and displays tropical-like cyclone characteristics: a warm core extending into the upper troposphere, an eye-like feature in its center with spiral cloud bands around, an almost windless center surrounded by nearly-symmetric sea-surface wind circulation with maximum wind speed within a few tens of km from the center."
Line 95, Mediterranean Tropical-Like Cyclones (MTLC) ("also known as Medicanes", Line 19), are defined as cyclones that during part of their lifetime develop a deep warm core for at least six hours while they are over the sea. This corresponds to the first criterion in the definition by Miglietta et al. (2025). The other criteria are therefore discarded. This could partly explain the difference in the number of occurrence reported in Section 3.1, i.e. about 1.5 Medicanes per year compared to 3.4 cyclones per year of MLTC. It also means that the MLTC you have defined are not Medicanes. They should therefore be named "Deep Warm-Core Cyclones (DWCD)."
Section 2.2. The use of a different radius to calculate the B and V parameters (100 km versus 125 km) must be justified. Furthermore, these radii are well below 200 km, as used by Chaboureau et al. (2012), or 250 km, as used by Fita and Flaounas (2018). The use of a small radius must also be justified. As noted by Miglietta et al. (2025), these small radii "may be misleading for the diagnosis of symmetric or upper warm-core structures. These considerations need to be taken into account in future studies."
Section 3.1. The threshold of 20% chosen for the classification of intense ETC must be justified. In addition, this section (or the next one) should include a description of the distribution of SLP and track length for the three categories.
Figure 1, line 144, "About one-third of MTLC have a full warm core at the time of peak intensity." In other words, about two-thirds of MTLC do not have a full warm core at the time of peak intensity, meaning that they are not MTLC at that moment. This shows that the time of peak intensity is not the TT time for two third of the MLTC. This implies a broad TT spectrum for the MLTC set.
In most figures, the mean and standard deviation of several variables are shown. This suggests that these variables have a Gaussian distribution, which is certainly not the case. Instead, the median and interquartile values should be shown. The 5th and 95th percentiles and outliers should also be added to the lower panels of Figures 4, 5, 6, 9, 10, 13 and A5.
References
Chaboureau, J.-P., F. Pantillon, D. Lambert, E. Richard, E. and C. Claud, 2012: Tropical transition of a Mediterranean storm by jet crossing. Quart. J. Roy. Meteor. Soc., 138, 596–611, https://doi.org/10.1002/qj.960
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
Fita, L., and E. Flaounas, 2018: Medicanes as subtropical cyclones: The December 2005 case from the perspective of surface pressure tendency diagnostics and atmospheric water budget. Quart. J. Royal Meteor. Soc., 144, 1028–1044. https://doi.org/10.1002/qj.3273
Miglietta, M. M., and Coauthors, 2025: Defining Medicanes: Bridging the Knowledge Gap Between Tropical and Extratropical Cyclones in the Mediterranean. Bull. Amer. Meteor. Soc., in press, https://doi.org/10.1175/BAMS-D-24-0289.1

Citation: https://doi.org/10.5194/egusphere-2025-3861-RC1
- AC1: 'Reply on RC1', Lisa Bernini, 19 Sep 2025
  
  We thank the reviewer for their constructive feedback. The comments certainly contribute to making our study more robust and comprehensive. In the following, the reviewer’s comments are reproduced before each reply. The mentioned figures are available in the supplementary file of the answer.
  
  The paper classifies Mediterranean cyclones into three categories and describes the mean and standard deviation of their characteristics 36 hours before their maximum intensity. The paper sorely lacks arguments to convince the reader that the three categories of cyclones have consistent characteristics and that the framework around the maximum intensity is relevant for analyzing such a diversity of cyclones.
  We propose to integrate the manuscript in a manner that better describes those aspects. More details on how we plan on doing this are provided in the following, in response to the specific comments below.
  
  Major comments
  1) The title "Environmental Characteristics Associated with the Tropical Transition of Mediterranean Cyclones" is misleading. The paper compares ETC, intense ETC and MLTC over a 72-h period centered on the first minimum sea level pressure (SLP). It therefore deals with the intensification of the three types of cyclones, not the tropical transition of Mediterranean cyclones.
  We agree that the title might have been misleading and propose to change it to: “Environmental characteristics associated with the development of tropical-like features in Mediterranean Cyclones”. In this work, we identify the anomalous properties (with respect to climatologies) that characterize Mediterranean cyclones that develop a deep warm core. We then compare them with those that characterize cyclones that do not develop a deep warm core (ETC and intense ETC), highlighting similarities and differences to distinguish specific characteristics that are associated with the formation of a deep warm core. This will be better explained in a revised version of the manuscript. We will also clarify that, rather than focusing on the processes responsible for the TT, such as in Davis and Bosart 2004, we aim at identifying the synoptic scale (or environmental) conditions that are found in cyclones that develop a deep warm core.
  
  2) With regard to tropical transition (TT), the authors should refer to cyclones for which "a fundamental dynamic and thermodynamic transformation of an extratropical precursor (of baroclinic origin and initially considered a cold-core system) is required to create a warm-core tropical cyclone" (Davis and Bosart 2004). In other words, TT deals with ETCs that transform into MLTCs. These transformed cyclones correspond to your definition of hybrid cyclones, which are not included in the study. Under these conditions, I do not understand how the study can actually address TT.
  We thank the reviewer for this important remark. We would like to clarify that in our classification, hybrid cyclones are not equivalent to systems undergoing tropical transition. Hybrid cyclones, as we defined them, are systems that either exhibit a shallow warm core, only in part of the troposphere, or develop a deep warm core only for a very short time (less than 6 hours). The intent here is to discard cyclones that are not clearly fully cold core nor deep warm core for any substantial time. This distinction should have been stressed more clearly in the manuscript, and we propose to revise the text accordingly.
  In our study, we used the term tropical transition to describe the evolution of cyclones that begin their lifecycle in the cold-core phase of the Hart phase space diagram and subsequently evolve into a deep warm-core phase for at least 6 hr, in line with the Davis and Bosart (2004) definition cited above. We propose to clarify this important point in the revised manuscript.
  
  3) Line 26, it should of interest to cite Miglietta et al. (2025) "A medicane is a mesoscale cyclone that develops over the Mediterranean Sea and displays tropical-like cyclone characteristics: a warm core extending into the upper troposphere, an eye-like feature in its center with spiral cloud bands around, an almost windless center surrounded by nearly-symmetric sea-surface wind circulation with maximum wind speed within a few tens of km from the center."
  We thank the reviewer for the suggestion. We will quote the definition of Medicanes proposed by Miglietta et al. (2025) in the new version of the manuscript. We’ll also be more careful in the use of the Medicane term and in the MTLC term (see comment 4 below) throughout the manuscript.
  
  4) Line 95, Mediterranean Tropical-Like Cyclones (MTLC) ("also known as Medicanes", Line 19), are defined as cyclones that during part of their lifetime develop a deep warm core for at least six hours while they are over the sea. This corresponds to the first criterion in the definition by Miglietta et al. (2025). The other criteria are therefore discarded. This could partly explain the difference in the number of occurrence reported in Section 3.1, i.e. about 1.5 Medicanes per year compared to 3.4 cyclones per year of MLTC. It also means that the MLTC you have defined are not Medicanes. They should therefore be named "Deep Warm-Core Cyclones (DWCD)."
  We thank the reviewer for this comment. We agree that the use of the term Medicane would indeed be misleading to describe the structures analyzed in our study, as this label refers to a more specific set of criteria, and it is still debated across the community. For this reason, we initially chose the term Mediterranean Tropical-Like Cyclones (MTLC), to emphasize some features these systems share with tropical cyclones—namely the warm core and the symmetry—while avoiding the more restrictive definition of Medicane. It was our mistake to say the two were equivalent.
  We acknowledge that the term Deep Warm-Core Cyclones (DWCC), as suggested by the reviewer, may better reflect the definition we adopted. Our initial hesitation in using this phrasing was that these cyclones are not characterized by a deep warm core throughout their entire lifetime, but only during part of it. Nevertheless, considering this comment, we are open to adopting it in the revised version of the manuscript for greater clarity and consistency with the literature.
  
  5) Section 2.2. The use of a different radius to calculate the B and V parameters (100 km versus 125 km) must be justified. Furthermore, these radii are well below 200 km, as used by Chaboureau et al. (2012), or 250 km, as used by Fita and Flaounas (2018). The use of a small radius must also be justified. As noted by Miglietta et al. (2025), these small radii "may be misleading for the diagnosis of symmetric or upper warm-core structures. These considerations need to be taken into account in future studies."
  We thank the reviewer for this comment. There is a mistake in the manuscript: the radius is the same in all cases, and the correct value is 137.5 km (or 5 ERA5 grid points, i.e., a radius of 1.25°). This will be corrected.
  Regarding the choice of radius, several studies have shown that the original 500 km radius of Hart (2003) is too large for Mediterranean cyclones. For instance, Miglietta et al. (2013), Picornell et al. (2014), Cioni et al. (2016), and de la Vara et al. (2021) reduced the radius to 150 km, while Cavicchia et al. (2014) and Noyelle et al. (2019) used 100 km. Ragone et al. (2018) reviewed different approaches and performed sensitivity tests, reporting that choosing values between 70 km and 150 km did not make significant changes in the categorization. Gutiérrez-Fernández et al. (2024) also demonstrated that results obtained with 150 km are consistent with those obtained at 300 km. Importantly, we note that, also in Miglietta et al. (2025), despite their cautionary remarks, the CPS diagram presented in their work was computed using a 150 km radius.
  In this context, our choice of 137.5 km falls within the range commonly adopted in the literature and is consistent with the reduced spatial scale of Medicanes. We understand that using a small radius could be a limitation for the diagnosis of cyclone symmetry, but this issue is partly mitigated by the fact that most tracking algorithms used in Flaounas et al. (2023) already apply explicit symmetry criteria when identifying a cyclone. On the other hand, we are less certain about how the radius size would affect the determination of the upper-level core temperature.
  
  6) Section 3.1. The threshold of 20% chosen for the classification of intense ETC must be justified. In addition, this section (or the next one) should include a description of the distribution of SLP and track length for the three categories.
  We thank the reviewer for this insightful comment. The motivation for creating the “intense ETC” category was to obtain a sample of comparable size to that of the MTLC. We realize, thanks to this comment, that selecting the 15% most intense ETC (Figure R1) would have produced a sample size even closer to that of the MTLC (Figure R3).
  We therefore repeated the analysis using the 15% most intense ETC (see the last comment for the updated figures). The main message remains unchanged: in terms of potential impacts, MTLC (or DWCC) exhibit similar intensity to intense ETC in terms of wind speed, but they are associated with stronger precipitation. From a process-oriented perspective, MTLC are linked to the same type of PV intrusions as intense ETC, but they develop over much warmer SST and are therefore associated with a higher PI.
  Finally, the analysis of track-length distributions shows that, on average, MTLC last longer than both ETC and intense ETC (Figure R2).
  
  7) Figure 1, line 144, "About one-third of MTLC have a full warm core at the time of peak intensity." In other words, about two-thirds of MTLC do not have a full warm core at the time of peak intensity, meaning that they are not MTLC at that moment. This shows that the time of peak intensity is not the TT time for two-thirds of the MLTC. This implies a broad TT spectrum for the MLTC set.
  We thank the reviewer for this comment. We would like to clarify that we never intended to equate the time of peak intensity with the time of tropical transition. We agree that, for many MTLC, the development of a full warm core does not coincide with their peak intensity, and we will explicitly state this in the revised manuscript. Indeed, at the time of peak intensity, only 42% of cyclones exhibit—or have already exhibited—a deep warm core. We also note that the median of the first time a cyclone has a deep warm core is 1-3 hours before time 0 (see Figure 3, which will substitute Figure 1 in the manuscript). Our choice of peak intensity as the reference time (t = 0) was motivated by the need to construct composites with a consistent temporal reference across all cyclones. Using the minimum SLP provides such a common anchor point.
  We recognize that part of the confusion may stem from terminology, particularly around the use of “tropical transition.” Our focus was not to determine the exact TT timing of MTLC, but rather to investigate the environmental conditions associated with cyclones that eventually develop warm-core structures. We will clarify this point in the manuscript to avoid misunderstandings.
  
  8) In most figures, the mean and standard deviation of several variables are shown. This suggests that these variables have a Gaussian distribution, which is certainly not the case. Instead, the median and interquartile values should be shown. The 5th and 95th percentiles and outliers should also be added to the lower panels of Figures 4, 5, 6, 9, 10, 13 and A5.
  We thank the reviewer for raising this important point. We realize that in the submitted version, it was not clearly indicated what the error bars represent. These are not standard deviations, but rather the standard error of the mean. They therefore quantify the confidence in the mean values, but they do not represent the spread of values across cyclones.
  Consequently, as suggested by the reviewer, we produced boxplots for the different variables. For each cyclone, we computed the mean over the 10% highest values over the 10°x10° box shown in the top panels. The selection of the 10% highest values is motivated by the fact that the 10°x10° box is very large, and it includes areas that are not representative of the cyclone conditions; selecting a smaller area at a fix location with respect to the cyclone center however is not appropriate as different variables express anomalous values at different places (eg wind speed is typically larger in the southwest quadrant while precipitation maxima are on average larger to the north of the center and PV intrusion is in the northwest quadrant). For those reasons, we use a fixed-size area (covering 10% of the 10°x10° box) defined by selecting the largest values, regardless of their position in the box. [We also performed a sensitivity analysis over the used area fraction (5% and 25%, other than the 10% reported here), reaching similar conclusions to what is reported in the following.]
  Boxplots are then created for those mean values over all cyclones in each of the three classes and shown in the figures below. As foreseen by the reviewer, these boxplots indicate that the intraclass spread is large and show substantial overlap among the different classes. However, important signals emerge, as detailed in the following.
  Regarding 10m wind speed at the time of peak intensity (fig. R5), in 90% of MTLC/DWCC the value is larger than the median value of ETC. Also, the mean values for MTLC/DWCC and intense ETC are not significantly different (all statistical outcomes in this note, obtained with a two-sided t-test, are defined at the 95% confidence level).
  For hourly rainfall at the time of peak intensity (fig. R6), we looked into the values of all percentiles from the 1st to the 99th, and they are all larger for MTLC/DWCC than for both intense ETC and ETC. The mean values of the three classes are significantly different, indicating that, on average, rainfall is smaller for ETC, higher for intense ETC, and highest for MTLC/DWCC. Still, very intense rainfall can be present in some cold-core cyclones, with 30% of intense ETC and 15% of ETC having values larger than the MTLC/DWCC median value.
  In comparing intense ETC and MTLC/DWCC at time -36hr prior to the time of peak intensity, we report that their mean PV anomaly values (fig. R7) do not significantly differ, but their SST (fig. R4), PI and climatological PI (fig. R8, R9), and wind shear (fig. R10) do. Clearly, given the large intraclass spread, none of those variables is sufficient per se to unambiguously differentiate MTLC/DWCC from intense ETC, but they shed light on the environmental characteristics that favor the development of a deep warm core.
  This supports our main conclusion: while MTLC/DWCC and intense ETC share similar mean surface wind speeds at their peak intensity and are associated with PV intrusions of the same strength, MTLC/DWCC are favored by warm SSTs, low vertical wind shear, high PI, and typically produce more intense precipitation.
  We also note that the SST (fig. R4), PI (Fig. R8), and wind shear (fig. R10) distributions of MTLC/DWCC and ETC are quite similar. This, together with the fact that ETC are too weak to activate WISHE, might be responsible for the different evolution of ETC and MTLC (a mechanism discussed in Davis and Bosart 2004 to differentiate between tropical transitions in strong extratropical cyclones and in weak extratropical cyclones).
  
  Citation: https://doi.org/10.5194/egusphere-2025-3861-AC1
RC2: 'Comment on egusphere-2025-3861', Anonymous Referee #2, 29 Sep 2025

In this paper, the authors highlight the large-scale environmental conditions that are specific to the formation of warm-core Mediterranean Tropical-Like Cyclones (MTLCs) compared to the more typical cold-core Extra-Tropical Cyclones (ETCs) that form over the same basin. To do so, they composite the structure and environment of ETCs, intense ETCs and MLTCs from ERA5. This paper offers a good and comprehensive analysis of environmental factors related to MTLCs in the Mediterranean, utilizing a systematic climatological analysis that has been largely lacking in the Mediterranean Cyclones literature to date. As such, this is an important paper that fits within the scope of WCD. Should my main concerns be addressed, I would recommend it for publication.

NB: Please see the attached document for detailed comments.

Citation: https://doi.org/10.5194/egusphere-2025-3861-RC2

Lisa Bernini, Leone Cavicchia, Fabien Desbiolles, Enrico Scoccimarro, and Claudia Pasquero

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Short summary

We studied rare but dangerous Mediterranean storms that can resemble tropical cyclones, bringing heavy rain, strong winds, and severe damage. By analyzing weather data from many past storms, we found that they form more often in seasons and locations where the long-term climate provides warmer seas and favorable atmospheric conditions. These patterns help explain why such storms intensify, and they can guide better forecasts and early warnings to protect coastal communities.


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