the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Jul 2023
Comparing short term intensity fluctuations and an Eyewall replacement cycle in Hurricane Irma (2017) during a period of rapid intensification
Abstract. An eyewall replacement cycle that occured after a period of rapid intensification of Hurricane Irma (2017) between 07 September and 08 September was investigated in a detailed modelling study using Met Office Unified Model (MetUM) convection permitting ensemble forecasts. The eyewall replacement cycle was then compared to intensity fluctuations that occurred during the period of rapid intensification between 04 September and 06 September. Both the short term fluctuations and eyewall replacement cycle involved an initial dynamical response to localized convection from inner rainbands in the former case and outer rainbands in the later case. One key difference between the intensity fluctuations and the eyewall replacement cycle is that, in the case of the intensity fluctuations, the small radial distance between the eyewall and the inner rainbands meant that a moat region did not form and hence the intensity fluctuations did not lead to an eyewall replacement.
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RC1: 'Comment on egusphere-2023-1272', Anonymous Referee #1, 12 Nov 2023
General comments:
The study used two members from an ensemble simulations of Hurricane Irma to study the SEF mechanisms. Three old mechanisms are examined, but it is not clear that this paper provides a new and unique contribution to the topic of SEF. Furthermore, most of the results are relatively shallow, which makes it difficult to identify the potential new contribution of this work. The conclusions about SEF are drawn from only one member from ensemble, which is overstated.
Major comments:
-The story is not well told. The background of Irma, whether it’s influenced by the environment, how the TC intensity evolves in both observation and simulations, are all missing.
-The observation is completely not necessary. It is not used to validate the simulation, and the forecast result is different from the observation anyway.
-Justification of picking and comparing the two members is missing: are they similar in structure before SEF? It seems not according to Figure 13, the theta_e difference is huge. What’s the purpose of comparing these two ensemble members? What are the behaviours of other ensemble members? Why are the results in this case study unique from other case studies?
Minor comments:
-Figure 1, the time over each panel is very confusing, they are not the same as the other figures.
-Inaccurate or confusing expressions:
In the title: eye wall replacement cycle cannot happen ‘during’ a period of rapid intensification
L11: ‘typically increase over time’?
L11-L15: the description here only fits the situation with no hostile environmental conditions
L37-L38: not very sure what this sentence means
Citation: https://doi.org/10.5194/egusphere-2023-1272-RC1 -
CC1: 'Comment on egusphere-2023-1272', John Methven, 28 Nov 2023
This paper focuses on the distinction between two processes associated with tropical cyclone intensification. At the end of the Introduction the authors make clear that they attempt to answer whether the two processes are indeed distinct or actually manifestations of the same phenomenon at different amplitudes. The study examines one tropical cyclone case where both processes have been previously documented in the literature. This is interesting because many of the environmental conditions of the TC are similar for the intensity fluctuation and eyewall replacement cycle in this case. A recent paper by the same authors presents a modelling study of Hurricane Irma including the period of intensity fluctuations. In this new paper the authors need to make much clearer in their introduction what the novel aspects are in this study and how it relates to their last paper. This is not clear from lines 35-46 on p.2.
Overall, the paper was well presented and the figures were high quality. The diagnostic analysis was carefully conducted. The final schematic (Fig.14) presents an excellent summary of the distinction in structure and evolution of intensity fluctuations and the eyewall replacement cycle. The final paragraph of the conclusions is a good summary of the key findings. The main distinction highlighted is whether the most intense convection and latent heating occurs in inner rainbands near the eyewall or on outer rainbands far from the eyewall.
Although I don’t dispute this distinction it did seem a bit limited to me. The big question is then why an outer convective rainband is forming in one case and not in the other. I found the paper very descriptive, with many diagnostics presented, but it did not answer some of the fundamental questions about why the different processes occur – even on the same TC at different times. The only mechanism that seems to touch on the development of an outer rainband is the vortex Rossby wave (VRW) mechanism. I was not convinced by the authors’ arguments that the VRW outwards propagation was not a reason for the distinction between the two types of events. Figure 2 seemed to indicate VRW events preceding vertical velocity increase at the outer radius. I could not understand why figure 3 was truncated early without extending to T+85h, missing the vital period 80-85 hours when more VRWs reached the stagnation radius.
I recommend revisions focusing on tightening the discussion relating to dynamical mechanisms and the conclusions that are drawn from the study. In particular, Section 3.3 contains a lot of discussion and description of different diagnostics shown but I did not think the discussion was rigorous enough to be very compelling in terms of distinguishing mechanisms at play. I would recommend trying to reduce the length of this sub-section and diversity of diagnostics shown and homing in on the key results.Detailed comments
Section 2.2: This section lacked detail about the nature of the ensemble perturbations in the initial conditions and also the use of lateral BCs in the LAM. Presumably the ICs are interpolated from the parent global model onto the LAM grid and are therefore relatively smooth? Are any perturbations applied on smaller scales? Is there is a stochastic parametrization scheme operating?
Section 3.1: “Ensemble member that best matched observations”. Seems like a valid approach but in what measures was it best?l.130: “eyewall replacement cycle of Irma is captured well”. How can you say that? On what basis?
l.154-155: This seems to be crucial to the whole paper. You state that the azimuthal phase velocity of the VRW was consistent with the dispersion relation. This is really important to demonstrate that the theory is relevant and that the disturbances in the model can be described as vortex Rossby waves. Please show the evidence. Similarly, you indicate the radial propagation of a VRW packet on Fig.2 for one event. How did you calculate the curve? Why not do the same for the later events at T+75, 77, 79, 81, 83? They all seem to be similar in terms of PV disturbance and its radial propagation rate, even though in the text you write that “they do not propagate in a way consistent with the dispersion relation”. It looks to me that they are very similar. Also, I would make the dashed line yellow or some other colour that would stand out on red-blue shading.
Figure 3: This really needs to extend to the same point in the forecast as Fig.2. It seems to be missing all the action when the stagnation radius jumps outwards. It looks to me that there is tangential wind deceleration coinciding with the approach of VRWs towards the stagnation radius. It is hard to understand why it looks similar to me, but you state that it is not similar. Also, it seems that there is periodicity in the tangential wind acceleration events near the stagnation radius that is similar to the interval between the VRW events even if the phasing seems variable. I am sure some other readers will be picking up on the same things as me.
Figures 4 and 5. It would be better if the panels were at the same times as the other terms in the tangential wind budget?
l.166-167: I don’t follow how you deduce that the VRW is not the direct cause of the SEF.
l.183: “inertial parameter” -> “absolute vorticity of the azimuthally averaged state”
l.196: What do you mean by “broadening of the wind field”? You need to be more precise here.
l.199: Define agradient wind and supergradient wind, perhaps with aid of some equations.
Section 3.2.3: I don’t follow the mechanism outlined at the end of this section. How can “increased agradient outward forces” “promote convection in the SEF region”? Why does “broadening of the tangential wind occurring above the boundary layer” result from “balanced dynamical adjustment to outer-rainband activity”? You would need to explain more clearly how these mechanisms work to convince the reader that they describe the evolution observed. As far as I am concerned there is not enough evidence to rule out the VRW mechanism as the reason for initiating secondary eyewall formation.
Section 3.3.1: Missing some rationale here. Why look at PV structure and what is the hypothesised behaviour in terms of PV in the case of a PV ring or PV monopole? For example, one aspect is that a PV ring can be unstable with respect to waves around the ring (e.g., Schubert et al, 1999, JAS) while a monopole can support waves but is stable.
Section 3.3.2 and Figure 11. How do all the things mentioned relate? It feels like equations are needed to make this more comprehensible.
Section 3.3.3: If mass ventilation was shown in Torgerson et al (2023) why show it here? I was lost as to the purpose of figures 11, 12 and 13 and the related discussion. What does it all tell us? Basically I could follow Section 3.2, but I found it very hard to follow Section 3.3 and what the diagnostics tell us. The diagnostics were described but the interpretation and discussion was not rigorous enough to be very convincing. If you have equations in mind when you write the interpretation, then put them in the paper and sharpen the arguments rather than trying to say everything in words alone.
Appendix A: This is fairly standard and it would be sufficient to refer to the associated papers with the definitions.
Citation: https://doi.org/10.5194/egusphere-2023-1272-CC1 - RC2: 'Comment on egusphere-2023-1272', Anonymous Referee #2, 07 Dec 2023
- AC1: 'Comment on egusphere-2023-1272', William Torgerson, 30 Mar 2024
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