the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 22 Sep 2026
How the North Atlantic storm track builds the moisture supply to warm conveyor belt ascent
Abstract. The moisture entering warm conveyor belt (WCB) ascent constrains the latent heat release that shapes extratropical cyclone development and the downstream circulation, yet the processes establishing this moisture remain poorly studied. Using 10-day backward trajectories initialized from North Atlantic WCB inflows in ERA5 over 43 winters (1980-2022), we identify four main dynamical histories of the inflowing air based on which environments shaped their moisture pick-up: cold air outbreak (CAO), fast descent, slow descent, and subtropical origin. WCB inflow is a superposition of these pathways, whose moisture is predominantly of recent extratropical origin and is acquired on strongly pathway-dependent timescales, ranging from about two days for the CAO and fast descent pathways to four to six days for air of subtropical and slow descent origin. We then ask why some WCBs are anomalously moist, while others remain relatively dry. We find that moist and dry inflows differ markedly in pathway composition, with subtropical origin air replacing CAO air for anomalously moist WCBs, but this shift contributes little to the moisture anomaly because the opposing compositional changes nearly cancel. The anomaly instead arises within the pathways: air arriving through every pathway is itself anomalously moist. Within each pathway, moist and dry inflows undergo similar thermodynamic evolution, but moist inflows progress farther toward warmer and more saturated conditions before ascent, while dry inflows are incorporated into the WCB at an earlier stage of this evolution. This difference is established well before WCB inflow: air feeding moist WCBs takes up moisture earlier, after an extended equatorward excursion on the flank of an antecedent anticyclone, whereas air feeding dry WCBs is moistened within a recent CAO shortly before ascent. The two groups are nonetheless embedded in nearly identical sea level pressure patterns at inflow, so their contrasting moisture is not apparent from the circulation at the onset of WCB ascent. How moist a WCB inflow becomes is therefore not determined by pathway identity alone, but by the preconditioning accumulated along that pathway. The antecedent circulation shapes both the route taken toward WCB ascent and how far air progresses along its associated thermodynamic evolution, thereby preconditioning the moisture available for latent heat release during WCB ascent.
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RC1: 'Comment on egusphere-2026-5486', Anonymous Referee #1, 19 Oct 2026
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2026/egusphere-2026-5486/egusphere-2026-5486-RC1-supplement.pdfReplyCitation: https://doi.org/
10.5194/egusphere-2026-5486-RC1 -
RC2: 'Comment on egusphere-2026-5486', Anonymous Referee #2, 30 Oct 2026
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This manuscript uses Lagrangian parcel trajectories to investigate the pathways and thermodynamic conditions along which warm conveyer belt (WCB) inflows gain their moisture. It thoroughly analyzes the different pathways through which air parcels become WCB inflows, their relative moisture contributions to the WCB inflows, the moisture uptake along these pathways, and how this moisture uptake occurs. I found this manuscript very interesting and well structured and support its publication. However, I suggest a round of revisions first, particularly focused on making the manuscript more readable for a broader audience that isn’t already very into the details of this topic, with a number of suggestions along these lines elaborated below.
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Overarching comments:
- What is learned from the slow descent category of back trajectories? It is always the least interesting one in the results. Is there any way you can better motivate why it is included separate from fast descent? I do understand that 4 makes for better figure symmetry than 3, so I wouldn’t push for getting rid of it, but it would be nice if you could motivate it better.
- It would be easier to combine insights from Fig. 2a and 2b if they had some common time reference points, for example including the time between maximum moisture uptake and WCB inflow in Fig. 2a and labeling the time of maximum moisture uptake on Fig. 2b. Similarly, it would be helpful to make Fig. 2b and Fig. 6 more comparable to one another in terms of which segment of the trajectories are shown (e.g., ending at WCB inflow time in both cases).
- The terminology is a bit convoluted and misleading in Section 4.1, referring to the effect of differences in moisture within pathways as a “mean effect”. I understand that this is a mean over multiple trajectories, but it is also a moisture anomaly between the tail WCB objects and the median WCB objects. I think this would be better described as a moisture anomaly effect, and I think the use of the term “mean” should be reconsidered throughout this section. At the very least, it should be specified what kind of mean is meant (no pun intended). Similarly, the “composition effect” would be more accurately specified as a “pathway composition effect”.
- In Section 4.2 / Figure 5, does it just happen to be the case that the inflow latitude is the same for moist and dry inflows or is this somehow imposed by the analysis? This has been hinted at earlier in the paper, but it should be mentioned again as a result, because it is surprising.
- Doesn’t defining the trajectories with respect to the max moisture uptake time (vs. the WCB uptake time) change the results of Figure 4 in an important way? Because then CAOs can go pick up their moisture and slowly reach back to the midlatitudes along a subtropical pathway, being labelled as a CAO based on what occurs before their maximum moisture uptake but ultimately approaching though more of a subtropical path to make up a moist WCB. That’s what seems to happen based on Figure 7 anyway. I think this doesn’t change the overall understanding that one comes away with from this study, but it’s worth thinking about whether this is the clearest way to present the results, because the results of Figure 4 are confusing and don’t necessarily contribute much to the overall understanding.
- Please consider what data (e.g., back trajectories) and/or analysis code might be helpful for readers to have access too. If at all possible, the code and data availability statement should say something about where to get the back trajectory code.
- As noted by the other reviewer, the text does feel a bit long for the amount of results. This is partially justified by thorough and interesting discussion, but I would still recommend a round of editing focusing on cutting tangential details so that readers can more easily find the new insights.
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Line comments:
- L10: I don’t find “this shift contributes little to the moisture anomaly because the opposing compositional changes nearly cancel” very clear, especially on a first read. Replacing “this shift” with “the shift in source air mass” might make it clearer.
- L12: The contrast in sentence construction between “moist and dry inflows” here and “…every pathway is itself anomalously moist” in the previous sentence is confusing, i.e., focusing on moist in one case but a comparison in the other. I found in particularly confusing on a first read, whereas it was clearer after reading the rest of the paper. Try to simplify the explanation here so that it is clear without knowing the details already.
- L25: “high-impact weather downstream” might fit better at the end of the list (after “shape the midlatitude storm tracks”)
- L26: add word “latent”
- L41: “and rapidly warm and moisten initially cold and dry air” is difficult to follow because it is hard to track down the subject (CAOs). I suggest “where initially cod and dry air rapidly warms and moistens”.
- L45: I’m not convinced “storm track circulation” is a helpful term, because the storm track is composed of many types of circulation. Consider different wording.
- L50: I find “rearward” challenging to interpret. Is there another way to express this?
- L66: missing “is known” after “less”
- L67: erroneous comma before “and”
- L68: this is a confusing list because several of the terms are mutually overlapping
- L85: Note that while the introduction has made the other categories clear, it hasn’t been discussed what “slow descent” corresponds to at this point.
- L96-98: It is challenging for a reader to understand at this point what is different about “the amount ultimately delivered depends on how the circulation preconditions the air along that route before ascent” than a route and timescale. Either this should be left out for now, or it needs a bit more explanation.
- L104: It’s not clear whether 1980-2022 refers to the D’s included or the JF’s
- L178: This is maybe a stylistic preference, but ideally you would be more specific about “this classification” on first mention, or reformulate to say that the “… goal is to classify according to…”
- Table 1: The results from Table 1 are not discussed in the text. Additionally, more details should be given about how the contribution % is calculated.
- L214: If I understand correctly, you are referring to the excursion in potential temperature, in which case the word “diabatic” is missing
- L268-273: It’s not clear why a window size larger than the 2° resolution used for the climatology is chosen and what role that might play in the results.
- L315: “meridional transport” -> “latitude”
- L330: It is probably worth saying that these two factors are closely related / not independent
- L377-378: “allows” and “leaves” implies a causality that hasn’t clearly been shown to be the case
- 7b: The first time I read the caption for panel (b), I thought the black contours were the trajectory density for the previous 10 days, but that can’t be true given the black contours in (c) and (d). Please clarify this.
- L411: I think “west” would be more accurate than “northwest”
- L500: The 4 pathways are defined by your analysis, so something like “We consider Wintertime North Atlantic WCB inflow as a superposition…” would be more appropriate
- L504: What is a “memory timescale”?
Citation: https://doi.org/10.5194/egusphere-2026-5486-RC2 -
EC1: 'Comment on egusphere-2026-5486', David Battisti, 01 Nov 2026
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To the authors.
I concur with the all of the general/overall/overarching comments by the two anonymous reviewers. I trust that you will address these, as well as, their minor/specific comments, in your reply and revised manuscript. David BattistiÂ
Citation: https://doi.org/10.5194/egusphere-2026-5486-EC1
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Talia Tamarin-Brodsky
Water vapor fuels the release of heat that can strengthen storms and influence weather far downstream, but we know less about how this moisture is supplied. By tracing air backward for 43 winters over the North Atlantic, we show that its moisture is built up over several preceding days as air moves through earlier weather systems. Storms therefore inherit a memory of past weather that helps determine their potential to release heat and affect the atmosphere.
Water vapor fuels the release of heat that can strengthen storms and influence weather far...