the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 29 Jun 2026
Extratropical interactions with the tropical moist margin: a case study and the role of latent heating
Abstract. The tropical moist margin occasionally extends into the extratropics, where it can lead to heavy precipitation in otherwise dry regions. Here, we examine the physical processes involved in these events and their interaction with the extratropical flow. Employing tools such as budget and trajectory analysis, we analyse a case study of an extratropical interaction with the moist margin in January 2018. This event produces a subtropical cyclone and results in damaging winds and precipitation over New Zealand. The event is then simulated with the ACCESS-rAM3 model, highlighting the critical role of latent heating in the development of the cyclone. Overall, the results suggest that interactions between extratropical PV and the moist margin occur in both directions. Deep cyclonic PV anomalies of upstream extratropical origin can result in poleward moisture advection leading to poleward displacement of the moist margin, while convection inside a perturbed moist margin can lead to upper-level ridge building through poleward advection of anticyclonic PV by the divergent wind. These results have implications for the diagnosis and prediction of severe weather, especially for extreme precipitation in the subtropics.
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Status: open (until 26 Sep 2026)
- RC1: 'Comment on egusphere-2026-3451', Anonymous Referee #1, 12 Aug 2026 reply
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RC2: 'Comment on egusphere-2026-3451', Anonymous Referee #2, 14 Sep 2026
reply
The manuscript explores the interaction between the tropical moist margin and the extratropical atmospheric circulation through the analysis of a case study involving a surface cyclone developing off the coast of eastern Australia in January 2018 which coincided with a poleward excursion of the moist margin. The authors first provide a overview of the synoptic evolution in reanalysis data, both through moisture budgets, potential vorticity budgets, cross-sections, and back-trajectory analysis, and then proceed onto assessing the role of latent heating by running model experiments where the latent heat release constant is modulated in order to emulate different intensities of latent heating.
The manuscript is overall fairly well written and structured. However, I am struggling to summarise its main new results, primarily because the authors do not properly present the research question/questions they aim to answer. There is a moist margin at the tropical-extratropical boundary, sometimes it travels a bit further polewards and this happens when there is a synoptic disturbance strong enough, or close enough, to perturb it. Why is the focus on the moist margin and not, say, on atmospheric rivers more generally? How are moist margin perturbations different from atmospheric rivers? I do not mean to say your results are trivial, but framed as they are now, it is hard to see what is the point the authors are trying to make and how it relates to previous literature.
Since this a rather major concern, I recommend major revisions. I believe there is merit in the work, so I am confident the authors can revise it and get it published in WCD. Below I provide more detailed comments which I hope will help in the revision of the manuscript.
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Detailed comments:
Lines 13-20: The first two paragraphs could be merged into one.
Lines 20-22: I am missing the context of your research, is the prediction of moist margin displacements particularly challenging? And if so, why and how is this study contributing? Even reading through the rest of the introduction, it remains somewhat unclear to me what the research question is.
Line 49: The citation should be in brackets (Hersbach et al., 2020)
Line 52: '27 levels between 100 and 1000 hPa' would that be all the available levels from the ECMWF? For reproducibility, I would just list which levels exactly you used.
Line 56: You have introduced MSLP for mean sea level pressure already, so I would be consistent and use this acronym here too.
Line 58: You have already introduced these acronyms (i.e. TCWV, PV), no need to repeat them. The same applies everywhere throughout the manuscript, figure captions included.
Line 66: Is this separation used in the rest of the study? If so, I would either give it an equation number (2) or add a passage in (1).
Line 82: In Equation 4, I suppose by P you mean PV? I would incorporate Eq. 4 into Eq. 3, and simply state what u_psi and u_chi are, and that you follow Teubler and Riemer (2016).
Lines 110-115: These lines read rather technical and I am not really sure what is the convective prognostic precipitation variable and why it is reset to a constant value? If this is important, it needs to be explained more clearly.
Line 115: There is a rogue "
Line 123: What do you mean by 'late on 18Z'? Perhaps this was meant to be the full date (2018-01-02) given that you only specify the year on line 127.
Line 129: Here you refer to the moist margin, but you have never properly introduced it apart from a brief definition in the first paragraph of the introduction. Which contours are you using for it and why? The only thresholds I could find in this study are on line 203 (45 kg/m2), though it is used for selecting which grid-points to run the back-trajectories from, and then again on line 346, perhaps more clearly outlined but way too far down the manuscript. Please state this clearly in the methods.
Figure 2: What does the green contour line in the right panels represent? Also, there's a typo on the second line, 'clue' instead of 'blue'.
Line 140: to the west of what? Of the low-pressure system?
Line 146: I cannot really follow your argument. How does 'precipitative drying' (drying due to condensation and ensuing precipitation?) lead to 'precipitation outside the moist margin'? Are we not talking about the same thing, i.e. precipitation?
Figure 4: Can you mark where the two cross sections intersect? It would help relate the top and bottoms panels.
Line 168: What composite analysis are you referring to here?
Lines 170-173: I am somewhat confused by what you mean by interaction. To me it sounds like you are primarily referring to the co-occurrence of the moist margin and the jet? What is the expected structure and what is the expected interaction? This traces back to the lack of a clear research question in the introduction. Paragraphs above describe this particular synoptic event, but so far I struggle to see where this is going beyond a succession of observations.
Line 175: 'A' moisture budget implies there is more than one?
Line 177: 'similar conclusions' not shown? Please specify if you actually checked this.
Figure 5: I would refer to the equation these terms are coming from, namely Eq. 1 and its decomposition into convergence and advection, which currently does not have an equation number.
Line 181: The strong drying appears to me to be mostly due to the advection of the moist margin filament, consistent with Fig. 5 f. Why do you emphasise the dry intrusion here?
Lines 184-185: I cannot really see much from Fig. 5b, so it is hard to tell what you are referring to here and how significant that is.
Lines 198-207: Perhaps 'area' is not the best word here, since area 1 and 2 are the same except for a time difference. Maybe something like 'back-trajectory experiments' or 'cases'? It would also help to have the three different experiments summarised in a table, with area, time, and level at which back-trajectories are launched.
Line 199: The closing bracket is missing.
Figure 6: I would probably add the time reference in panels a-c in Figure 6. Also, what are the violin plots showing? Binned distributions and extrema? I guess it is a Python in-built function, but it should be specified somewhere nonetheless.
Line 208: 'Fig. 6, left panels'? Also, 'origins of air' sounds weird, you mean source regions?
Lines 208-214: I would not have said there are three different source regions. The latter two you mention are likely the same as I cannot really see a third spike in the pressure distributions in Fig. 6d. You do not even discuss further this alleged third, minor branch on lines 215-220, so I would just stick to the first two. Also, I am not sure how you are plotting the trajectories in Fig. 6a-c, but perhaps some overlap and are hidden/highlighted? It is clear this happens in Fig. 6b for instance, where the low-laying trajectories are plotted below the high-laying ones. Please clarify.
Line 219: How can you tell that the westerly air parcels moisten in the final 48 hours? From Figure 6 it is hard to tell, perhaps some not-shown analysis? If so, please specify.
Lines 225-227: Given nearly-constant specific humidity and increasing pressure, the decrease in relative humidity is expected, so how is it interesting?
Line 236: From the way you plot PV time series, it is quite hard to see any trend, apart from the extreme values, which even exceed the plotting range. This applies also to panels s and t. Reducing the plotting range might help visualise the results.
Lines 264-267: It is quite hard to follow this given the coarse quality of plots in Fig. 8. Isn't precipitation in control also concentrated along a NW-SE line?
Figure 8: It is not so easy to read precipitation off panels a-c. I understand it is relatively concentrated compared to the plotting domain, though it is also ofter covered by the MSLP contours and rather coarsely resolved when zooming in. Please consider improving these panels.
Line 272-274: Either fully discuss/elaborate on this statement here, or remove it and properly discuss it where relevant.
Line 294: Did you mean Figure 9d?
Lines 297-298: I am confused as to what box you actually used, as Figure 8 shows one box and then Figure 10 shows another box which is located further northwest. Also, why showing the red box in Figure 8 if you then show it again in Figure 10?
Lines 304-305: You have been referring to PV until now without the need to specify that we are in the Southern Hemisphere, perhaps have this reminder earlier on in the methods?
Line 310: Please state the full reference, e.g., Fig. 10d. This applies also to lines 312, 322, 335, etc.
Line 328: Can you elaborate on this? Not clear to me what you mean.
Line 340: Why would different L_fact affect surface wind speed?
Line 345: Please add a reference for this definition, which I would also include in the methods, together with the specific threshold you used in this study, as already noted in another comment.
Figure 11: Title of panel b should read 'mean \theta profile minus control'.
Lines 426-454: Not sure a section on future research is relevant to a WCD paper. Maybe for a review, but not for a case study paper. I would rather reframe this section as caveats, remaining questions, all under the same concluding section.
Lines 455-464: The conclusion subsection as it stands is rather short and fails to contextualise your results (possibly because much of this is done in a previous subsection), which makes it in my view rather superfluous. Consider restructuring the entire section 5 in a more organic and concise way.
Citation: https://doi.org/10.5194/egusphere-2026-3451-RC2 -
EC1: 'Comment on egusphere-2026-3451', Shira Raveh-Rubin, 16 Sep 2026
reply
Dear Corey Robinson and coauthors,
We have now received detailed reviews of your submitted manuscript from two expert referees. As you can see, both referees share major concerns about the novelty of the work and quality of the manuscript and provide specific references to where the issues arise in your text, with potential ways forward.
I share these concerns, particularly on the too-vague aim of the manuscript to "examine the physical processes during interactions between the moist margin and extratropical PV anomalies". It is therefore unclear why modifying latent heating in the model experiments is a suitable approach to address the aim. Indeed, currently the results confirm the well-understood role of latent heating in extratropical cyclones for ridging and downstream development. The current design of the methodological approach does not fully allow novel understanding of the bidirectional connection between the tropical moist margin and the sub/extratropical cyclone.
At this stage, I invite you to submit final author comments with responses to the general concerns and a strategy plan for how you would address these in a revision. In your response, please consider the short time given for revision (~2 months), and whether it is feasible for you to substantially improve the manuscript. Alternatively, it is possible to withdraw the manuscript at this point and resubmit at a later time as a new submission to WCD.
I am looking forward to receiving your response and moving forward with your manuscript.
Best wishes,
Shira
Citation: https://doi.org/10.5194/egusphere-2026-3451-EC1
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The manuscript by Robinson et al. examines the physical processes associated with the extratropical transition of a tropical-origin low-pressure system that subsequently intensified in the midlatitudes, with particular emphasis on latent heating. The authors describe the synoptic conditions during the cyclone’s intensification and analyze the observed position of the tropical moist margin and the evolution of the upper-level potential vorticity (PV) structure.
However, the analyses and observations presented in the manuscript provide only limited, or indirect, support for the claims made in the abstract, introduction, and conclusions. Moreover, the different components of the analysis are not sufficiently integrated, making it difficult to understand how they collectively address the paper’s central research question.
The manuscript’s key analysis concerns latent heating within cyclones (and warm conveyor belts) and demonstrates its well-established role in cyclone intensification and in modifying upper-level PV structure (Davis et al. 1993, Schemm et al. 2013, Steinfeld et al. 2020, Huaser et al. 2023). This is investigated through numerical experiments with the ACCESS-rAM3 model, using different prescribed latent-heating values. However, this analysis is not convincingly connected to the tropical moist margin. Consequently, the manuscript does not yet establish the specific role of the moist margin in the event or clearly demonstrate the novelty of its central contribution.
Given the potential relevance of this topic to the Weather and Climate Dynamics Journal, the manuscript could be given further consideration if the authors restructure the storyline and either reformulate the key research questions to align with the current analysis or revise the analysis to address the stated questions. Therefore, I recommend a major revision and restructuring.
Best wishes!