the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Mar 2026
The role of the secondary circulation in the tropical transition of Hurricane Ophelia
Abstract. This study investigates the mesoscale dynamic and thermodynamic mechanisms governing the tropical transition (TT) of Hurricane Ophelia (2017). A fundamental aspect of this transition is the co-evolution of the primary and secondary circulations; specifically, the development of the secondary overturning circulation is what drives the structural evolution of the vortex. As the first high-resolution analysis of secondary circulation in a real TT, it broadens the scope of existing diagnostic frameworks, proving that methods originally developed for idealized tropical cyclones are also effective for quantifying the dynamics of transitioning systems. Using high-resolution numerical simulations, advanced energy-budget diagnostics and wind-tendency equations have been computed to assess the evolution of the secondary circulation. Results show that following an initial phase driven by an upper-level potential vorticity intrusion and baroclinic forcing, organized deep convection facilitates vorticity redistribution and core warming. During the transition phase, momentum and thermal forcings contribute nearly equally to the intensification of the secondary circulation. However, once the transition is complete, thermal forcing becomes the dominant mechanism. The equivalent potential temperature budget analysis reveals a fundamental shift in system energetics: while vertical diffusion, associated with surface fluxes and air-sea instability, dominates energy input during the transition, organized vertical advection within the eyewall sustains the system in its mature stage. The study also identifies a period of structural relaxation midway through the process, highlighting the non-linear nature of the tropical transition before achieving self-sustaining convective coupling. By clarifying currently debated TT behavior, this work establishes key signatures that facilitate the non-trivial characterization of these systems.
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Status: open (until 21 May 2026)
- RC1: 'Comment on egusphere-2026-1446', Anonymous Referee #1, 12 May 2026 reply
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RC2: 'Comment on egusphere-2026-1446', Anonymous Referee #2, 13 May 2026
reply
This study explores the dynamics of the secondary circulation associated with tropical transition (TT). While the analyses used in this study appear to be more suitable for diagnosing the dynamics of axisymmetric tropical cyclones than that of asymmetric baroclinic systems, they show some interesting dynamical differences among the stages before, during, and after TT. In particular, the analyses show changes in the intensification rate, the radius of maximum wind, and the structures and dynamics of the secondary circulation. However, I have several concerns regarding the current manuscript as shown below, and I am not confident that the interpretations and conclusions presented in the manuscript are appropriate. Therefore, I will be able to review this manuscript once the following issues particularly raised in my comments from 1 to 4 are addressed sufficiently. At this time, I recommend major revision of this manuscript.
Specific comments
1. It is unclear how well the WRF-ARW simulation agrees with the observations and the best track analysis. For example, are the differences in intensification rate between the stages in Fig. 4 found in the best track? The authors should also provide a more detailed explanation of the degree to which the timing and structures of cloud pattern are similar between the observation and the simulation (Figs. 3, 5).
2. The authors should explain the analyses in more detail. Regarding section 2.2, it would be easier to understand if the authors elaborated on the physical meanings of R and H where they first appear. In addition, when this analysis is applied to the output of the simulation, what values are used for R and H? Are horizontal winds at z=0 directly obtained from the model output? Are the assumptions regarding wind and pressure at the boundaries reasonable in the model output?
3. While this study focuses on TT processes, the roles of baroclinicity and a cut-off low are not clear. If the analyses of the secondary circulation indicate any influences of the cut-off low, it would be beneficial to discuss that point. As the authors mention baroclinicity many times, I also recommend adding an analysis of baroclinicity that demonstrates the differences among the stages. The secondary circulation is not directly linked to baroclinicity itself, although it can be affected by baroclinicity.
4. It would be easier to read if the description of what the figure shows were distinguished from the explanation of how it can be interpreted. I will elaborate on this point later in relation to Figs. 2, 3, and 5.
5. Line 109-110: The Coriolis force is also included.
6. Equation (12): I think the third equation should be divided by 2π.
7. Figure 1: What does θ denote?
8. Caption of Fig. 1: It is somewhat strange to explain that arrows denote mixing ratio.
9. Lines 149-150: What does “catalyst” mean? It sounds as if a cut-ff low is unchanged during TT processes.
10. Lines 162-180: This paragraph is difficult to read because the description of Fig. 2 and the explanation of previous studies are mixed together.
11. Lines 166-168: It is not clear which characteristics in this description can be identified in Fig. 2.
12. Line 177: When is the TT time (t0), and how is it defined? Why do the authors use relative time for the observations and actual time for the simulation? This makes it difficult to compare the two.
13. Lines 182-184: How can the authors conclude that the dominant perturbation energy source is a consequence of a baroclinic instability? While the cloud pattern in Fig. 3a may show a baroclinic signature, it is too strong to refer to the dominant energy source.
14. Lines 225-226: At which level is the cold core identified in Fig. 5b?
15. Figure 6: This analysis is interesting. If the authors could confirm that the sum of the three terms is consistent with the time derivative of the secondary circulation, it would enhance the robustness of this analysis.
16. Lines 249-250: I think that the small S(t) is not necessarily related to baroclinicity.
17. Figure 7: Although this figure may present interesting results, the authors should provide a more detailed explanation of the purpose of the analysis and calculation method. As the atmosphere is generally stratified, I expect that the maximum potential intensity at r=0 (vertical axis) occurs at the top of the analyzed domain. If so, at which level is the top? I would also like the authors to discuss what dynamics can change the potential temperature at the center of the cyclone. Adiabatic warming is a possible explanation, but it may disappear if the top of the analyzed domain is H according to the assumption.
18. Line 268: The peak maximum equivalent potential temperature in Fig. 7a appears to be about 350 K.
19. Line 322: Where are Figs. A1 and A2 presented?
20. Line 344-346: In relation to the calculation of eddy heat flux, how is the center of the cyclone determined?
21. Lines 377-384: Figure 11a shows complex evolution of radial wind at z=14 km. Does the radial wind at this level really reflect deep convection? I’m asking for several reasons. Figure 5f apparently indicates that strong divergent zonal flow occurs below z=14 km after the TT period. Figure 10 shows a positive vertical advection (downdraft) around z= 12 km implying the updraft at z = 14 km may not be connected to the updraft of deep convection. In addition, because the large vertical advection at z=14 km may be partly attributed to the large stratification, is the vertical velocity itself large at this level?
22. Figure 11: The gray shading becomes hard to see when it is overlaid on the color shading, particularly in Figs. 11a, 11b, 11c.
23. Lines 403-404: Which part of Fig. 8 indicate the TC eye warming rate?
Technical collections
24. Line 170: Replace “by (Hoskins et al., 1985)” with “by Hoskins et al. (1985)”
25. Line 316: “anan” appears to be a typo.
Citation: https://doi.org/10.5194/egusphere-2026-1446-RC2
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- 1
The study investigates the tropical transition of Hurricane Ophelia, focusing on the secondary circulation evolution. While I agree there is a gap in process-based studies of tropical transition at convection-allowing resolution, I have major concerns about the analysis and interpretation that lead me to recommend the study be rejected for publication. My most major concerns are summarized below. I hope the authors find these comments constructive to improve their investigation and presentation of results.
Choice of tropical-transition period. The simulation is initialized on 00 UTC 9 Oct 2017, six hours before Ophelia was designated “tropical” in IBTrACS. It is curious that the authors define the tropical transition period between ~13 UTC 9 Oct 2017 through ~13 UTC 10 Oct 2017. First, there isn’t any objective reasoning or justification as to why this period was chosen. Second, given the evidence presented, I would argue that much of this period is when Ophelia is tropical and what is investigated is more tropical cyclone intensification, rather than the true tropical transition period itself. Thus, what the authors have mostly discovered is the strengthening of the secondary circulation associated with intensification, which is not a novel finding.
Baroclinic support versus convective processes. In several places in the manuscript, the authors argue that there is a transition from baroclinic to convective support/processes. I agree that, conceptually, this is the crux of the tropical-transition process, but I do not think the authors present any convincing evidence of this transition. Specifically, how is “baroclinic support” quantified in this framework, and how is it tied to S(t)? Wouldn’t S(t) increase for any cyclone with a thermally direct circulation, tropical or non-tropical, that is undergoing cyclogenesis or intensifying?
Rather than the secondary circulation as the central metric, I think a PV framework would be a more promising avenue. The authors touch upon the relevance of PV redistribution themselves. I recommend looking at Bentley et al. (2016)[https://doi.org/10.1175/MWR-D-15-0251.1]. One idea could be to adapt their framework to analyze the high-resolution model simulation of Ophelia.
Uneven presentation of findings. There are numerous instances in the manuscript where I have trouble following the authors or am skeptical of the results.
A general criticism is that there are multiple instances of unsubstantiated claims where no evidence is provided or findings stated before evidence is presented, which results in confusion.
Budget closures are not assessed (e.g., Figs. 6 and 8) by showing the residual between the sum of forcing terms and actual tendencies. I appreciate budgets calculated from model output are challenging to close, especially thermodynamic ones, but it’s important to be transparent here. From my eye, it seems like the residual could be large, which implies there is uncertainty in interpreting individual terms.
Something also seems off in the budget presented in Fig. 6. Why wouldn’t the frictional term continue to increase in magnitude as the TC intensifies? Early in the defined TT period, the thermal forcing (red line) never gets larger than the blue line (dynamical forcing), and the increase in the frictional term seems important for the bump up in S(t). Why?
Some of the visualizations and variable choices are either confusing or not well explained/justified. Some examples: