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| 04 Apr 2024
Western disturbances and climate variability: a review of recent developments
Abstract. Western disturbances (WDs) are synoptic-scale weather systems embedded within the subtropical westerly jet. Manifesting as upper-level troughs often associated with a lower-tropospheric low over Western India, they share some dynamical features with extratropical cyclones. WDs are most common during the boreal winter (December to March), during which they bring the majority of precipitation – both rain and snow – to the Western Himalaya, as well as to surrounding areas of north India, Pakistan and the Tibetan Plateau. WDs are also associated with weather hazards such as heavy snowfall, hailstorms, fog, cloudbursts, avalanches, frost, and coldwaves.
In this paper, we review the recent understanding and development on WDs. Recent studies have collectively made use of novel data, novel analysis techniques, and the increasing availability of high-resolution weather and climate models. This review is separated into six main sections – structure and thermodynamics, precipitation and impacts, teleconnections, modelling experiments, forecasting at a range of scales, and paleoclimate and climate change – each motivated with a brief discussion of the accomplishments and limitations of previous research.
A number of step changes in understanding are synthesised. Use of new modelling frameworks and tracking algorithms has significantly improved knowledge of WD structure and variability, and a more frequentist approach can now be taken. Improved observation systems have helped quantification of water security over the Western Himalaya. Convection-permitting models have improved our understanding of how WDs interact with the Himalayas to trigger natural hazards. Improvements in paleoclimate and future climate modelling experiments have helped to explain how WDs and their impacts over the Himalaya respond to large-scale natural and anthropogenic forcings. We end by summarising unresolved questions and outlining key future WD research topics.
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RC1: 'Comment on egusphere-2024-820', Anonymous Referee #1, 30 May 2024
I have reviewed this article with my limitations, I hope authors may find it helpful for improving the readability and scientific credentials.
Title: Western disturbances and climate variability: a review of recent developments
Author(s): Kieran M. R. Hunt et al.
MS No.: egusphere-2024-820
MS type: Review articleRecommendation: Accept with revision
This is a comprehensive review of WDs, authors have put meticulous efforts in this review research work by including all the available relevant research studies. This is surely useful for researchers interested in this field. However, I strongly feel simplified description will be more beneficial for new researchers to get crucial interest in the subject. It is clear that this review is more focussed on boreal winter time WDs, having baroclinic structure, basically ‘frontal synoptic scale’ in nature? Here the dynamical processes are dominant over thermodynamical. Though it is more confined to Himalayan regions, it also extends over central and western Indian regions. In my opinion this review of the past and present studies can be better structured (like IPCC report) where any scientific argument is categorised with low, medium and high confidence level. This may help in simplifying the description, otherwise it very confusing at each stage. The simplified description will enhance the readability as well as its scientific credentials. In deed this article should be accepted in this journal but with revision. Kindly find the line by line comments below
Line 90: While describing western disturbances (in addition to it’s interaction with summer monsoon systems) it would be more appropriate to distinguish it from typical summer monsoon synoptic systems in which the complex thermodynamics as well as dynamics plays a crucial role.
Line 100: Along with Chevuturi and Dimri, 2016, you may like to refer Vellore et al. 2015/16
Caption of Figure 3: It is Cold and dry ‘air’ advection?
Line no. 136: Firstly, recent studies …… increasingly ?????? high-resolution models, ….
Line No. 140 and 145: Please Consider simplifying these statements.
Line No. 160: Kindly include Vellore et al. 2015/16
Line No. 191 and 192: These studies are ‘more recent analyses’???? Sentence may be corrected.
Line No. 271: How WDs are different from Frontal system?
Line 282,283: Sentence is not clear.
Line No. 344: Simplify the sentence for better readability describe how a negative correlation with ……?
Line No. 349 – 351 : Do you mean baoclinicity?
Line No. 378 : Please correct the sentence for better readability.
Line No. 436-439: Sentence not clear.
Line No. 440: Provide suitable references.
Line No. 445: flawed????
Line No. 455: How significant is Mediterranean moisture?? here when it is not a majority moisture source?
Line 475-485: In fact Section 2.4 is too confusing, you may kindly retain very relevant references?
Line No. 510: Figure 10: Caption- Is the percentile calculation based on entire time-seires or has been calculated on monthly basis.
Line No. 531: ‘….. associated with all winter WDs’ What about other seasons?
Line 535: why 350 hPa is being considered in analysis? please provide the supporting argument
Line no. 550: The difference between two studies is not understood here.
Line no. 570: dynamical characteristics and categories are two separate issues?
Line no. 642-643: This could be part of data and methodology?
Line no. 665: is it supported by back trajectories etc?
Line no. 670: any reference?
Line no. 702: This can be shifted to next section?
Line no. 753: ‘radiation fog’ – any reference?
Line no. 757: You mean blocking high?
Line no. 795: what about sub continental blocking?
Line no. 869-874: How these past and recent studies are connected?
Line no. 880: Please be clear what you want impress upon.
Line no. 992-994: I get lost between Agricultural applications and features over Indo-Gangetic plains
Line no. 944: What is fir tree? In this sentence
Line no. 946-949: How this connected with WDs?
Line no. 956: I am again lost here to connect with WDs.
Line no. 991: Sudden jump to stratosphere? when ENSO relation itself is not clear?
Line no. 994: What is SSW? In this sentence?
Line no. 1005: What is IWM in this sentence?
Line no. 1019: Needs more attention.
Line no. 1034: Is it region specific? As it is not seen in case of summer monsoon convection over Western Ghats?
Line no. 1045: This may be true when dynamics is dominant in the weather system?
Line no. 1064-1065: This statement is irrelevant here.
Line no. 1067-1069: repeated statement.
Line no. 1070-1074: This sentence is not clear.
Line no. 1076: Infact the local dynamics seems to play important role.
Line no. 1084-1089: very confusing statements, needs reformation.
Line no. 1100: How it is connected to WDs.
Line no. 1115: Is it connected to WDs?
Line no. 1150: In fact, these early studies explored the qualitative analysis.
Line no. 1164: This is a serious concern needs to be addressed appropriately.
Line no. 1180: ‘…..context of WDs is left an important ….. ‘ This is a serious concern needs to be addressed appropriately.
Line no. 1190: Which is tract 1 in Figure 17??
Line no. 1203: ‘….. sensitivity had to be reduced …..’ Needs to be elaborated here.
Line no. 1230: ‘….. winter precipitation there is brought by WDs.’ Sentence is not clear.
Line no.1234-1236: This statement is contrary to that of line no. 1230.
Line no. 1269: This is very confusing.
Line no. 1281:’ … Paleoclimate modelling’ It would be more appropriate to segregate observational and modelling studies
Line no. 1315: What is the confidence level here?
Line no. 1413: Section 7.2.1 Counting WDs - Very interesting section can be better presented - it is very complex at the moment
Line no. 1463: it is Krishnan et al. 2019?
Line no. 1471: No confidence?
Line no. 1480: Here - The impact of climate forcing over the trend would be very interesting? Though may not have confidence level.
Line no. 1495: ‘…interdecadal variability’ - There are lots of jumps from long-term trends to decadal scale trends?
Line no. 1506-1508: Very difficult to understand this content.
Line no. 1528: ‘….which attributed to WDs’ Is it the frequency of WDs?
Line no. 1530 : is it related to increased WD frequency?
Line no. 1544: ‘….. surface levation.’ What about lapse rate?
Line no. 1554: is it also supported by in-situ observations?
Line no. 1576: ‘…. Anomaly’ - you mean positive anomaly? If so mention it for better readability.
Line no. 1580-1581: most closely and mostly closely? Correct the sentence.
Line no. 1581-1582: ‘….. particularly as a result of changing WD activity.’ Please explain how?
Line no. 1588-1590: Please restructure the sentence for better clarity.
Line no. 1625: It would be more appropriate to summarise the contents here before proceeding further.
Line no. 1720: Though it is a comprehensive description of future projections, it would be more appropriate to classify this in near-future, mid-future and far-future. The uncertainty of near future projection say 2030 or 2040 could be very useful for various sectors.
Line no. 1721: Section 8 Future research questions and challenges:
This section is very well written.
Line no. 1819: In view of the above comments Section 9 Summary needs to be considerably improved for quantitative description and better readability.
Line no. 1822: Again to remind that WD over the region of interest is Importantly a synoptic frontal type of system having baroclinic structure and dominance of dynamics.
Line no. 1839: Indeed, Quantitative description may be more beneficial for readers.
Line no. 1866: Yes the future scope of this study is well defined in this manuscript.
Citation: https://doi.org/10.5194/egusphere-2024-820-RC1 -
AC2: 'Reply on RC1', Kieran Hunt, 14 Aug 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-820/egusphere-2024-820-AC2-supplement.pdf
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AC2: 'Reply on RC1', Kieran Hunt, 14 Aug 2024
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RC2: 'Comment on egusphere-2024-820', Anonymous Referee #2, 11 Jun 2024
Review of EGUsphere 2024-820 "Western disturbances and climate variability: a review of recent developments" by Kieran M. R. Hunt et al.
Synopsis:
This review paper by Hunt et al. aims at providing an overview of recent research on the topic of western disturbances (WDs) over the Indian subcontinent. The review addresses many aspects such as the structure and dynamics of WDs, natural hazards associated with them, their predictability and their response to climate change. Overall, the paper is well written and it covers many aspects. However, the paper appears to be overly detailed in places and it is difficult for the reader to identify the essential points. Accordingly, I suggest some points for revision before the paper can be published.Comments:
1) My major comment is that several subsections have a 'list' structure summarizing one result after another (e.g., 4.3, 4.2, 3.4.1, 7.1.3). Thus, the storyline of the review does not necessarily become clear and it may be helpful to decide on a consistent conceptual framework for each subsection early in the paper. For example, a brief statement at the beginning of each subsection could describe our current understanding of a certain aspect of WDs and indicate the confidence that the research community has. The statement could then be followed by a more detailed summary. Still, this summary should not simply list all studies but for example comment on the confidence that we have or explain why several aspects are still uncertain. It would also be helpful to provide a table in the paper where key points are listed together with a measure of confidence. I would expect that the paper will receive greater visibility if key points are directly visible instead of being hidden in the text. Overall, I would hope the paper to become more concise once the subsections with a 'list' structure have been revised.
2) Even for state-of-the-art reanalysis data sets, there are uncertainties on the fraction of precipitation that can be attributed to WDs. Accordingly, it seems to be even harder to quantify the fraction of precipitation associated with WDs for past or future climate states. Still, quite often the authors refer to studies establishing a link between precipitation and WDs. For such statements, it would be important to explain at least briefly how the authors come to the conclusion that a clear link between WDs and precipitation exists even if the WDs have not been identified objectively in some cases (e.g., l. 1568, l. 1581, l. 1664).
l. 92: Can you comment here or later on what processes lead to the WD cyclogenesis?
l. 136-142: In principle, I agree. But what about the fact that WDs travel along distance before they reach India/that they are embedded in the large-scale weather? Wouldn't this require global models?
l. 169: Please specify that it is cyclonic potential vorticity anomalies.
l. 175: What about diabatic processes? I assume these also play a crucial role in the development of mid-tropospheric PV anomalies of WDs.
l. 229 and elsewhere: Please double-check whether all acronyms have been introduced before their first use.
l. 230: Are you referring to the layer mean relative vorticity between 450 to 300 hPa.
l. 225 & l. 232: The study regions of WDs differ. Would it be an important future step to agree on one region across the the WD science community? Likewise, the minimum lifetime seems to be quite variable. Also concerning this aspect, a critical discussion of the different criteria would be well suited in a review paper (see also comment on Fig. 5).
l. 239-252: Though I appreciate the authors attempt to list existing techniques to approximate WD frequency and related statistics, it may be more important to the reader to understand what the implications of the different techniques are. For example, what fraction of uncertainties in WD statistics (number per year, speed etc.) can be attributed to different tracking techniques.
l. 277: Is it really the case that disturbances are blocked by the Tibetean Plateau? For example, taking relative vorticity at 400-300 hPa, I'd be surprised if the systems were blocked by the Tibetean Plateau. Is it not rather the case that flow configurations advecting disturbances southward do hardly occur? Or are you referring here to disturbances near the surface?
l. 292: Diabatic heating can also occur over a prolonged period and with considerable latent heat release in stratiform precipitation (e.g., in WCBs). So, is it a necessary condition for the intensification that WDs are associated with convective precipitation?
l. 304: Through which process does latent heat release increase the strength of the upper-level (cyclonic) PV anomaly? If the latent heat release occurs in the mid-troposphere, it would rather lead to an anticyclonic PV anomaly in the upper troposphere, i.e., reduce the strength of the upper-level (cyclonic) PV anomaly. Perhaps you can also refer to lower and upper troposphere, instead of lower and upper level.
l. 342: Are you referring to divergence in the upper- or lower troposphere? Also, I found it very difficult to follow the line of arguments here. If there is a negative correlation between propagation speed and cloud-top height, would this not mean that systems with low cloud-top height propagate faster, and those with high cloud-top heights propagate slower? This at least would be dynamically understandable: Systems with high cloud-top heights are associated with stronger upper-tropospheric divergence and a corresponding divergent outflow which would be directed against the eastward propagation of the WD. If it is the case that WDs with stronger convection tend to propagate more quickly: Is this because of diabatically generated lower-tropospheric cyclonic PV similar to diabatic Rossby waves/vortices?
l. 359: The quasi-geostrophic ascent could also occur before the moisture reaching the orography. Is this also observed?
l. 370: It reads as if there is "orographic instability" which is certaintly not meant here. Please clarify.
l. 408: Please clarify trough which process the transport of ozone is happening since a PV anomaly per-se does not necessarily lead to stratosphere-troposhere exchange.
l. 439: How exactly could a moisture flux analysis supplement the isotope analysis? Moisture flux analysis provides only a Eulerian viewpoint and does therefore not provide information on the actual moisture source.
l. 475: See previous comment.
l. 620: Is this a general statement or specific to the region affected by WDs?
l. 655: That the percentage of precipitation attributed to WDs varies substantially between studies calls for a consistent approach when matching WDs and rainfall. Have there been approaches where the distance at which rainfall is still attributed to a WD is based on objective criteria such as the Rossby radius of deformation? A critical discussion would be worthwhile here.
l. 730: I am wondering whether the heading of section 3.4 would be a better one for Section 3. Precipitation can lead to natural hazards so the separation between precipitation and other natural hazards seems a bit arbitrary.
l. 739: What is the reason for the cool ground?
l. 750: Are there already insights on why the boundary layer turbulence is suppressed in the rear sector of WDs? Is it due to descending air masses causing an inversion layer that prevents the downward mixing of momentum.
l. 758: Though I am not an expert in this field, I would expect that the increasing pollution is the most important factor.
l. 770: This somewhat confirms my previous statement.
l. 735-786: This section needs to be revised substantially. It currently reads as a collection of literature, but due to the partly opposing research results it is difficult to develop a conceptual picture. A different approach would be to rather summarize the findings about which we are certain and then to mention the uncertainties which still need to be quantified.
l. 805: Can you explain what is meant by "nor'westers"? It reads like a phenomenon associated with strong winds, but this would not fit to the section dealing with lightning and hailstorms.
l. 890: A further shortcoming might be that the role of lower-level PV maxima and upper-level PV maxima has not been quantified yet. The concept of piecewise potential vorticity inversion would be one diagnostic to assess the role of different PV anomalies.
l. 898: Though I agree that SAM, NAO rectify onto SSTs with similar patterns to those shown here, it is still questionable whether a physical link exists. For example, through which physical process would SAM be connected to the WD occurrence frequency? This aspect definitely needs some explanation.
l. 1012-1019: These lines should not be part of section 4.4 since they summarize Section 4 in total.
l. 1122: Could you explicitly state which land surface datasets were found to yield superior results? Or do you only want to state that the representation of WDs is more sensitive to the land surface dataset than to model parametrisations.
l. 1165: Are these short-range forecasts deterministic? If so, can you comment on the added value of ensemble forecasts and this is one way forward?
l. 1200: Extended range and subseasonal forecasts often use hybrid approaches combining statistical and dynamical models. Have there been insights on whether statistical models for the occurrence of WDs are useful? For example, though their connections to ENSO, NAO etc. there could be valuable predictors on this longer time scale.
Section 7: Overall, Section 7 needs to be shortened. On several occasions the link to WDs is not clear and it is difficult to synthesize all the given information to form a consistent picture. Further, it is not clearly explained how WDs are linked to precipitation in paleo-climate studies. Such explanation is necessary given the difficulty of identifying WDs even in state-of-the-art reanalysis data sets.
l. 1220: Could you include an initial hypothesis on why a response of WDs to climate change is expected?
l. 1541: The "elevation-dependent warming" can presumably be removed.
Fig. 2: Please include state borders.
Fig. 3: WDs are tilted northwestward with height. Is there a reason for not showing this tilt?
Fig. 5: To my understanding a "commonly used WD track capture region" does not really exist in literature and the definition of regions varies from paper to paper. Please reconsider the formulation.
Fig. 10: Given the stronger jet during winter, it is probably not too surprising that the WDs are stronger in winter. If the intensity was normalized with the seasonal mean vorticity, would this show intense WDs also during summer?
References:
Keller, J. H., and Coauthors, 2019: The Extratropical Transition of Tropical Cyclones. Part II: Interaction with the Midlatitude Flow, Downstream Impacts, and Implications for Predictability. Mon. Wea. Rev., 147, 1077–1106, https://doi.org/10.1175/MWR-D-17-0329.1.Citation: https://doi.org/10.5194/egusphere-2024-820-RC2 -
AC4: 'Reply on RC2', Kieran Hunt, 14 Aug 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-820/egusphere-2024-820-AC4-supplement.pdf
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AC4: 'Reply on RC2', Kieran Hunt, 14 Aug 2024
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RC3: 'Comment on egusphere-2024-820', Anonymous Referee #3, 15 Jun 2024
This manuscript presents an excellent and thorough review of the current state of understanding of western disturbances. It is well structured and quite easy to follow while including substantial insights. A similar review was carried out in 2015 and much research has been published in the intervening years. The manuscript describes this more recent research in detail and also briefly discusses what was understood in 2015 for context. The manuscript makes a substantial contribution mainly by bringing together existing knowledge to increase overall understanding of the subject and formulating coherent plans for future research; there is also new material and new presentation of previously published material. The manuscript is very long: my personal recommendation is that its length should not be reduced (subject to any formal length limits imposed by the journal) as all of the material is of interest and relevance and any repitition serves to improve its clarity; it is also fairly well packaged into sections for those who are interested primarily in one aspect of western disturbances research. I note that it is longer than all of the 5 existing published review articles in WCD but of a similar length to the longest one.
I recommend publication subject to satisfactorally addressing the following issues/questions. Numbers without other context refer to line numbers in the manuscript.
87: This is at first confusing as on line 94 they are described as moving eastwards. Do you mean westwards relative to the jet? In that case I suggest changing what is inside the parenthetical dashes to something like "a synoptic-scale trough moving westward relative to the subtropical westerly jet , in which it is embedded"
A general question that I thought might come up for a reader relatively new to western disturbances (more on reading Section 2 but perhaps could be addressed in Section 1) is whether WDs occur by definition in this particular part of South Asia (and perhaps similar phenomena occur elsewhere but have different names) or if they can only occur in this region (e.g., due to the unique orography of the Himalaya and what is to the west of it).
173-174: Can you explain how this is shown by Dimri (2004)?
I felt it might be worth clarifying that Figure 6 refers to all times rather than just during WDs.
344-345: Naively I'd associate a higher cloud top height with more convection -- can you explain how a negative correlation of speed with cloud top height implies that WDs associated with stronger convection tend to propagate more quickly?
Figure 8: is the mean for the anomaly 10 days either side of the WD?
401: It is not clear from Figure 8 how there is a northwestward tilt with height.
435: Where does Pfahl & Sodemann (2014) say that colder climates lead to higher d values? I could only find a positive relationship between SST and d.
Section 2.4: One question I had when reading this, was how much the methods rely on modelling, and whether they do extensive sampling of the isotopic ratios of collected precipitation. Obviously one can read the references but a sentence clarifying this in general might be interesting.568-569: Is it more correct to say that Javed et al. (2023) thresholds on vorticity? (I realise this is directly dependent on wind speed though!)
580-581: I don't know if you are trying to say that these proportions of active WDs are surprisingly low, but if so the manuscript itself earlier defines active as only the top quarter!
Section 3.4.1: Is it fair to say that this is particularly uncertain (compared with other topics discussed in the manuscript)? So the overall message is there is strong evidence that WDs affect fog, but in what way is as yet quite unclear?
596: Can you point out where Bamzai & Shukla (1999) and Liu & Yanai (2002) say this?
774-775: Does the reference to Patnaik et al. (2024) relate to their mention of increased PDNC during WDs (so less pollution during WDs?)?
809-810: Why does their box 1 not cover the high-strike-intensity area to the west?
Figure 15: Please define the acronyms and what the signs mean in the caption.
901,904: Roy (2006) reports negative correlations with PDO and ENSO over India as a whole so presumably Western Himalaya is an exception to this? Is this based on their Figure 4?
958: Is this based on the positive correlation between NAO and temperature during cold periods (their Figure 7b) and cold periods being linked with increased winter precipitation?
965: Are you suggesting that paleoclimate studies are less reliable because the historical climate is more difficult to observe?
1087-1088: Is the bias weak (as in small) or negative (as in winds being too weak)?
1131: Seems odd that heavy precipitation is not mentioned in the list of hazards, although I accept you want to emphasise the less commonly considered ones.
Figure 16: Should these be accessible from the website given in the caption? I was not able to find them easily.
1219: I would argue that the issue of how to forecast them (i.e., shorter range) is of similar importance (see also lines 1862-1866).
Figure 19: Does the horizontal axis increase into the future or into the past?
1329: Don't these studies look at somewhat later periods than 3500 to 1500 years ago?
1383: Wasn't the link to global warming in Munz et al. (2017) with the weakening IWM?
1396-1397: What makes these two different from the other studies in blue in Figure 20?
1416: "Earlier studies have suggested a decline in WD frequency": is this based on the two black minus signs (and no black plus signs) amongst the blue studies from Figure 20?
Figure 20: Presumably the different shades of blue/red/green are just to differentiate the studies and don't have any other meaning?
1494-1495: Looks like something has gone wrong with the text here so that the meaning is not clear.
Figure 22: What do the grey contours (that are not very clearly visible) represent? And can one tell from these panels where the gauges are for each dataset?
1519-1520: Does this mean that we know which of Pai et al. (2013) and Chauhan et al. (2022) provide the correct interpretation?
1541,1556: The formatting implied by the bold text is not clear here.
1692-1693: Meher & Das (2022): is this based on their Figure 5 (standard deviation)? Are you arguing that an increased standard deviation implies an increased mean?
Figure 23: It might be helpful to move this forward a bit, nearer to where it is first referred to in the text.
Figure 24: Which study are the solid coloured lines from?
1833: Does this imply that extreme WD precipitation rarely occurs in the core WD season? Or could this interaction occur over a timescale of a few months?
I would also like to make the following typographical recommendations/comments.
103: I would suggest changing "avalanches" --> "and avalanches" to improve comprehension of the sentence here.
150: "rather" --> "rather than"
251: The word "weather" appears twice: I think this is a mistake?
336: Could change "greater" --> "higher" just to make absolutely clear that you don't mean higher pressures (and thus lower altitudes).
432: One of the "delta"s has not been rendered correctly.
483: "occur" --> "occurring".
1005: Please define IWM here.
1333: "differential": do you mean "different"?
1345: Please define LIA here
1451: "if" --> "of"
1783-1784: MJO is listed twice.Citation: https://doi.org/10.5194/egusphere-2024-820-RC3 -
AC3: 'Reply on RC3', Kieran Hunt, 14 Aug 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-820/egusphere-2024-820-AC3-supplement.pdf
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AC3: 'Reply on RC3', Kieran Hunt, 14 Aug 2024
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RC4: 'Comment on egusphere-2024-820', Anonymous Referee #4, 19 Jun 2024
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AC1: 'Reply on RC4', Kieran Hunt, 14 Aug 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-820/egusphere-2024-820-AC1-supplement.pdf
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AC1: 'Reply on RC4', Kieran Hunt, 14 Aug 2024
Status: closed
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RC1: 'Comment on egusphere-2024-820', Anonymous Referee #1, 30 May 2024
I have reviewed this article with my limitations, I hope authors may find it helpful for improving the readability and scientific credentials.
Title: Western disturbances and climate variability: a review of recent developments
Author(s): Kieran M. R. Hunt et al.
MS No.: egusphere-2024-820
MS type: Review articleRecommendation: Accept with revision
This is a comprehensive review of WDs, authors have put meticulous efforts in this review research work by including all the available relevant research studies. This is surely useful for researchers interested in this field. However, I strongly feel simplified description will be more beneficial for new researchers to get crucial interest in the subject. It is clear that this review is more focussed on boreal winter time WDs, having baroclinic structure, basically ‘frontal synoptic scale’ in nature? Here the dynamical processes are dominant over thermodynamical. Though it is more confined to Himalayan regions, it also extends over central and western Indian regions. In my opinion this review of the past and present studies can be better structured (like IPCC report) where any scientific argument is categorised with low, medium and high confidence level. This may help in simplifying the description, otherwise it very confusing at each stage. The simplified description will enhance the readability as well as its scientific credentials. In deed this article should be accepted in this journal but with revision. Kindly find the line by line comments below
Line 90: While describing western disturbances (in addition to it’s interaction with summer monsoon systems) it would be more appropriate to distinguish it from typical summer monsoon synoptic systems in which the complex thermodynamics as well as dynamics plays a crucial role.
Line 100: Along with Chevuturi and Dimri, 2016, you may like to refer Vellore et al. 2015/16
Caption of Figure 3: It is Cold and dry ‘air’ advection?
Line no. 136: Firstly, recent studies …… increasingly ?????? high-resolution models, ….
Line No. 140 and 145: Please Consider simplifying these statements.
Line No. 160: Kindly include Vellore et al. 2015/16
Line No. 191 and 192: These studies are ‘more recent analyses’???? Sentence may be corrected.
Line No. 271: How WDs are different from Frontal system?
Line 282,283: Sentence is not clear.
Line No. 344: Simplify the sentence for better readability describe how a negative correlation with ……?
Line No. 349 – 351 : Do you mean baoclinicity?
Line No. 378 : Please correct the sentence for better readability.
Line No. 436-439: Sentence not clear.
Line No. 440: Provide suitable references.
Line No. 445: flawed????
Line No. 455: How significant is Mediterranean moisture?? here when it is not a majority moisture source?
Line 475-485: In fact Section 2.4 is too confusing, you may kindly retain very relevant references?
Line No. 510: Figure 10: Caption- Is the percentile calculation based on entire time-seires or has been calculated on monthly basis.
Line No. 531: ‘….. associated with all winter WDs’ What about other seasons?
Line 535: why 350 hPa is being considered in analysis? please provide the supporting argument
Line no. 550: The difference between two studies is not understood here.
Line no. 570: dynamical characteristics and categories are two separate issues?
Line no. 642-643: This could be part of data and methodology?
Line no. 665: is it supported by back trajectories etc?
Line no. 670: any reference?
Line no. 702: This can be shifted to next section?
Line no. 753: ‘radiation fog’ – any reference?
Line no. 757: You mean blocking high?
Line no. 795: what about sub continental blocking?
Line no. 869-874: How these past and recent studies are connected?
Line no. 880: Please be clear what you want impress upon.
Line no. 992-994: I get lost between Agricultural applications and features over Indo-Gangetic plains
Line no. 944: What is fir tree? In this sentence
Line no. 946-949: How this connected with WDs?
Line no. 956: I am again lost here to connect with WDs.
Line no. 991: Sudden jump to stratosphere? when ENSO relation itself is not clear?
Line no. 994: What is SSW? In this sentence?
Line no. 1005: What is IWM in this sentence?
Line no. 1019: Needs more attention.
Line no. 1034: Is it region specific? As it is not seen in case of summer monsoon convection over Western Ghats?
Line no. 1045: This may be true when dynamics is dominant in the weather system?
Line no. 1064-1065: This statement is irrelevant here.
Line no. 1067-1069: repeated statement.
Line no. 1070-1074: This sentence is not clear.
Line no. 1076: Infact the local dynamics seems to play important role.
Line no. 1084-1089: very confusing statements, needs reformation.
Line no. 1100: How it is connected to WDs.
Line no. 1115: Is it connected to WDs?
Line no. 1150: In fact, these early studies explored the qualitative analysis.
Line no. 1164: This is a serious concern needs to be addressed appropriately.
Line no. 1180: ‘…..context of WDs is left an important ….. ‘ This is a serious concern needs to be addressed appropriately.
Line no. 1190: Which is tract 1 in Figure 17??
Line no. 1203: ‘….. sensitivity had to be reduced …..’ Needs to be elaborated here.
Line no. 1230: ‘….. winter precipitation there is brought by WDs.’ Sentence is not clear.
Line no.1234-1236: This statement is contrary to that of line no. 1230.
Line no. 1269: This is very confusing.
Line no. 1281:’ … Paleoclimate modelling’ It would be more appropriate to segregate observational and modelling studies
Line no. 1315: What is the confidence level here?
Line no. 1413: Section 7.2.1 Counting WDs - Very interesting section can be better presented - it is very complex at the moment
Line no. 1463: it is Krishnan et al. 2019?
Line no. 1471: No confidence?
Line no. 1480: Here - The impact of climate forcing over the trend would be very interesting? Though may not have confidence level.
Line no. 1495: ‘…interdecadal variability’ - There are lots of jumps from long-term trends to decadal scale trends?
Line no. 1506-1508: Very difficult to understand this content.
Line no. 1528: ‘….which attributed to WDs’ Is it the frequency of WDs?
Line no. 1530 : is it related to increased WD frequency?
Line no. 1544: ‘….. surface levation.’ What about lapse rate?
Line no. 1554: is it also supported by in-situ observations?
Line no. 1576: ‘…. Anomaly’ - you mean positive anomaly? If so mention it for better readability.
Line no. 1580-1581: most closely and mostly closely? Correct the sentence.
Line no. 1581-1582: ‘….. particularly as a result of changing WD activity.’ Please explain how?
Line no. 1588-1590: Please restructure the sentence for better clarity.
Line no. 1625: It would be more appropriate to summarise the contents here before proceeding further.
Line no. 1720: Though it is a comprehensive description of future projections, it would be more appropriate to classify this in near-future, mid-future and far-future. The uncertainty of near future projection say 2030 or 2040 could be very useful for various sectors.
Line no. 1721: Section 8 Future research questions and challenges:
This section is very well written.
Line no. 1819: In view of the above comments Section 9 Summary needs to be considerably improved for quantitative description and better readability.
Line no. 1822: Again to remind that WD over the region of interest is Importantly a synoptic frontal type of system having baroclinic structure and dominance of dynamics.
Line no. 1839: Indeed, Quantitative description may be more beneficial for readers.
Line no. 1866: Yes the future scope of this study is well defined in this manuscript.
Citation: https://doi.org/10.5194/egusphere-2024-820-RC1 -
AC2: 'Reply on RC1', Kieran Hunt, 14 Aug 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-820/egusphere-2024-820-AC2-supplement.pdf
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AC2: 'Reply on RC1', Kieran Hunt, 14 Aug 2024
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RC2: 'Comment on egusphere-2024-820', Anonymous Referee #2, 11 Jun 2024
Review of EGUsphere 2024-820 "Western disturbances and climate variability: a review of recent developments" by Kieran M. R. Hunt et al.
Synopsis:
This review paper by Hunt et al. aims at providing an overview of recent research on the topic of western disturbances (WDs) over the Indian subcontinent. The review addresses many aspects such as the structure and dynamics of WDs, natural hazards associated with them, their predictability and their response to climate change. Overall, the paper is well written and it covers many aspects. However, the paper appears to be overly detailed in places and it is difficult for the reader to identify the essential points. Accordingly, I suggest some points for revision before the paper can be published.Comments:
1) My major comment is that several subsections have a 'list' structure summarizing one result after another (e.g., 4.3, 4.2, 3.4.1, 7.1.3). Thus, the storyline of the review does not necessarily become clear and it may be helpful to decide on a consistent conceptual framework for each subsection early in the paper. For example, a brief statement at the beginning of each subsection could describe our current understanding of a certain aspect of WDs and indicate the confidence that the research community has. The statement could then be followed by a more detailed summary. Still, this summary should not simply list all studies but for example comment on the confidence that we have or explain why several aspects are still uncertain. It would also be helpful to provide a table in the paper where key points are listed together with a measure of confidence. I would expect that the paper will receive greater visibility if key points are directly visible instead of being hidden in the text. Overall, I would hope the paper to become more concise once the subsections with a 'list' structure have been revised.
2) Even for state-of-the-art reanalysis data sets, there are uncertainties on the fraction of precipitation that can be attributed to WDs. Accordingly, it seems to be even harder to quantify the fraction of precipitation associated with WDs for past or future climate states. Still, quite often the authors refer to studies establishing a link between precipitation and WDs. For such statements, it would be important to explain at least briefly how the authors come to the conclusion that a clear link between WDs and precipitation exists even if the WDs have not been identified objectively in some cases (e.g., l. 1568, l. 1581, l. 1664).
l. 92: Can you comment here or later on what processes lead to the WD cyclogenesis?
l. 136-142: In principle, I agree. But what about the fact that WDs travel along distance before they reach India/that they are embedded in the large-scale weather? Wouldn't this require global models?
l. 169: Please specify that it is cyclonic potential vorticity anomalies.
l. 175: What about diabatic processes? I assume these also play a crucial role in the development of mid-tropospheric PV anomalies of WDs.
l. 229 and elsewhere: Please double-check whether all acronyms have been introduced before their first use.
l. 230: Are you referring to the layer mean relative vorticity between 450 to 300 hPa.
l. 225 & l. 232: The study regions of WDs differ. Would it be an important future step to agree on one region across the the WD science community? Likewise, the minimum lifetime seems to be quite variable. Also concerning this aspect, a critical discussion of the different criteria would be well suited in a review paper (see also comment on Fig. 5).
l. 239-252: Though I appreciate the authors attempt to list existing techniques to approximate WD frequency and related statistics, it may be more important to the reader to understand what the implications of the different techniques are. For example, what fraction of uncertainties in WD statistics (number per year, speed etc.) can be attributed to different tracking techniques.
l. 277: Is it really the case that disturbances are blocked by the Tibetean Plateau? For example, taking relative vorticity at 400-300 hPa, I'd be surprised if the systems were blocked by the Tibetean Plateau. Is it not rather the case that flow configurations advecting disturbances southward do hardly occur? Or are you referring here to disturbances near the surface?
l. 292: Diabatic heating can also occur over a prolonged period and with considerable latent heat release in stratiform precipitation (e.g., in WCBs). So, is it a necessary condition for the intensification that WDs are associated with convective precipitation?
l. 304: Through which process does latent heat release increase the strength of the upper-level (cyclonic) PV anomaly? If the latent heat release occurs in the mid-troposphere, it would rather lead to an anticyclonic PV anomaly in the upper troposphere, i.e., reduce the strength of the upper-level (cyclonic) PV anomaly. Perhaps you can also refer to lower and upper troposphere, instead of lower and upper level.
l. 342: Are you referring to divergence in the upper- or lower troposphere? Also, I found it very difficult to follow the line of arguments here. If there is a negative correlation between propagation speed and cloud-top height, would this not mean that systems with low cloud-top height propagate faster, and those with high cloud-top heights propagate slower? This at least would be dynamically understandable: Systems with high cloud-top heights are associated with stronger upper-tropospheric divergence and a corresponding divergent outflow which would be directed against the eastward propagation of the WD. If it is the case that WDs with stronger convection tend to propagate more quickly: Is this because of diabatically generated lower-tropospheric cyclonic PV similar to diabatic Rossby waves/vortices?
l. 359: The quasi-geostrophic ascent could also occur before the moisture reaching the orography. Is this also observed?
l. 370: It reads as if there is "orographic instability" which is certaintly not meant here. Please clarify.
l. 408: Please clarify trough which process the transport of ozone is happening since a PV anomaly per-se does not necessarily lead to stratosphere-troposhere exchange.
l. 439: How exactly could a moisture flux analysis supplement the isotope analysis? Moisture flux analysis provides only a Eulerian viewpoint and does therefore not provide information on the actual moisture source.
l. 475: See previous comment.
l. 620: Is this a general statement or specific to the region affected by WDs?
l. 655: That the percentage of precipitation attributed to WDs varies substantially between studies calls for a consistent approach when matching WDs and rainfall. Have there been approaches where the distance at which rainfall is still attributed to a WD is based on objective criteria such as the Rossby radius of deformation? A critical discussion would be worthwhile here.
l. 730: I am wondering whether the heading of section 3.4 would be a better one for Section 3. Precipitation can lead to natural hazards so the separation between precipitation and other natural hazards seems a bit arbitrary.
l. 739: What is the reason for the cool ground?
l. 750: Are there already insights on why the boundary layer turbulence is suppressed in the rear sector of WDs? Is it due to descending air masses causing an inversion layer that prevents the downward mixing of momentum.
l. 758: Though I am not an expert in this field, I would expect that the increasing pollution is the most important factor.
l. 770: This somewhat confirms my previous statement.
l. 735-786: This section needs to be revised substantially. It currently reads as a collection of literature, but due to the partly opposing research results it is difficult to develop a conceptual picture. A different approach would be to rather summarize the findings about which we are certain and then to mention the uncertainties which still need to be quantified.
l. 805: Can you explain what is meant by "nor'westers"? It reads like a phenomenon associated with strong winds, but this would not fit to the section dealing with lightning and hailstorms.
l. 890: A further shortcoming might be that the role of lower-level PV maxima and upper-level PV maxima has not been quantified yet. The concept of piecewise potential vorticity inversion would be one diagnostic to assess the role of different PV anomalies.
l. 898: Though I agree that SAM, NAO rectify onto SSTs with similar patterns to those shown here, it is still questionable whether a physical link exists. For example, through which physical process would SAM be connected to the WD occurrence frequency? This aspect definitely needs some explanation.
l. 1012-1019: These lines should not be part of section 4.4 since they summarize Section 4 in total.
l. 1122: Could you explicitly state which land surface datasets were found to yield superior results? Or do you only want to state that the representation of WDs is more sensitive to the land surface dataset than to model parametrisations.
l. 1165: Are these short-range forecasts deterministic? If so, can you comment on the added value of ensemble forecasts and this is one way forward?
l. 1200: Extended range and subseasonal forecasts often use hybrid approaches combining statistical and dynamical models. Have there been insights on whether statistical models for the occurrence of WDs are useful? For example, though their connections to ENSO, NAO etc. there could be valuable predictors on this longer time scale.
Section 7: Overall, Section 7 needs to be shortened. On several occasions the link to WDs is not clear and it is difficult to synthesize all the given information to form a consistent picture. Further, it is not clearly explained how WDs are linked to precipitation in paleo-climate studies. Such explanation is necessary given the difficulty of identifying WDs even in state-of-the-art reanalysis data sets.
l. 1220: Could you include an initial hypothesis on why a response of WDs to climate change is expected?
l. 1541: The "elevation-dependent warming" can presumably be removed.
Fig. 2: Please include state borders.
Fig. 3: WDs are tilted northwestward with height. Is there a reason for not showing this tilt?
Fig. 5: To my understanding a "commonly used WD track capture region" does not really exist in literature and the definition of regions varies from paper to paper. Please reconsider the formulation.
Fig. 10: Given the stronger jet during winter, it is probably not too surprising that the WDs are stronger in winter. If the intensity was normalized with the seasonal mean vorticity, would this show intense WDs also during summer?
References:
Keller, J. H., and Coauthors, 2019: The Extratropical Transition of Tropical Cyclones. Part II: Interaction with the Midlatitude Flow, Downstream Impacts, and Implications for Predictability. Mon. Wea. Rev., 147, 1077–1106, https://doi.org/10.1175/MWR-D-17-0329.1.Citation: https://doi.org/10.5194/egusphere-2024-820-RC2 -
AC4: 'Reply on RC2', Kieran Hunt, 14 Aug 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-820/egusphere-2024-820-AC4-supplement.pdf
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AC4: 'Reply on RC2', Kieran Hunt, 14 Aug 2024
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RC3: 'Comment on egusphere-2024-820', Anonymous Referee #3, 15 Jun 2024
This manuscript presents an excellent and thorough review of the current state of understanding of western disturbances. It is well structured and quite easy to follow while including substantial insights. A similar review was carried out in 2015 and much research has been published in the intervening years. The manuscript describes this more recent research in detail and also briefly discusses what was understood in 2015 for context. The manuscript makes a substantial contribution mainly by bringing together existing knowledge to increase overall understanding of the subject and formulating coherent plans for future research; there is also new material and new presentation of previously published material. The manuscript is very long: my personal recommendation is that its length should not be reduced (subject to any formal length limits imposed by the journal) as all of the material is of interest and relevance and any repitition serves to improve its clarity; it is also fairly well packaged into sections for those who are interested primarily in one aspect of western disturbances research. I note that it is longer than all of the 5 existing published review articles in WCD but of a similar length to the longest one.
I recommend publication subject to satisfactorally addressing the following issues/questions. Numbers without other context refer to line numbers in the manuscript.
87: This is at first confusing as on line 94 they are described as moving eastwards. Do you mean westwards relative to the jet? In that case I suggest changing what is inside the parenthetical dashes to something like "a synoptic-scale trough moving westward relative to the subtropical westerly jet , in which it is embedded"
A general question that I thought might come up for a reader relatively new to western disturbances (more on reading Section 2 but perhaps could be addressed in Section 1) is whether WDs occur by definition in this particular part of South Asia (and perhaps similar phenomena occur elsewhere but have different names) or if they can only occur in this region (e.g., due to the unique orography of the Himalaya and what is to the west of it).
173-174: Can you explain how this is shown by Dimri (2004)?
I felt it might be worth clarifying that Figure 6 refers to all times rather than just during WDs.
344-345: Naively I'd associate a higher cloud top height with more convection -- can you explain how a negative correlation of speed with cloud top height implies that WDs associated with stronger convection tend to propagate more quickly?
Figure 8: is the mean for the anomaly 10 days either side of the WD?
401: It is not clear from Figure 8 how there is a northwestward tilt with height.
435: Where does Pfahl & Sodemann (2014) say that colder climates lead to higher d values? I could only find a positive relationship between SST and d.
Section 2.4: One question I had when reading this, was how much the methods rely on modelling, and whether they do extensive sampling of the isotopic ratios of collected precipitation. Obviously one can read the references but a sentence clarifying this in general might be interesting.568-569: Is it more correct to say that Javed et al. (2023) thresholds on vorticity? (I realise this is directly dependent on wind speed though!)
580-581: I don't know if you are trying to say that these proportions of active WDs are surprisingly low, but if so the manuscript itself earlier defines active as only the top quarter!
Section 3.4.1: Is it fair to say that this is particularly uncertain (compared with other topics discussed in the manuscript)? So the overall message is there is strong evidence that WDs affect fog, but in what way is as yet quite unclear?
596: Can you point out where Bamzai & Shukla (1999) and Liu & Yanai (2002) say this?
774-775: Does the reference to Patnaik et al. (2024) relate to their mention of increased PDNC during WDs (so less pollution during WDs?)?
809-810: Why does their box 1 not cover the high-strike-intensity area to the west?
Figure 15: Please define the acronyms and what the signs mean in the caption.
901,904: Roy (2006) reports negative correlations with PDO and ENSO over India as a whole so presumably Western Himalaya is an exception to this? Is this based on their Figure 4?
958: Is this based on the positive correlation between NAO and temperature during cold periods (their Figure 7b) and cold periods being linked with increased winter precipitation?
965: Are you suggesting that paleoclimate studies are less reliable because the historical climate is more difficult to observe?
1087-1088: Is the bias weak (as in small) or negative (as in winds being too weak)?
1131: Seems odd that heavy precipitation is not mentioned in the list of hazards, although I accept you want to emphasise the less commonly considered ones.
Figure 16: Should these be accessible from the website given in the caption? I was not able to find them easily.
1219: I would argue that the issue of how to forecast them (i.e., shorter range) is of similar importance (see also lines 1862-1866).
Figure 19: Does the horizontal axis increase into the future or into the past?
1329: Don't these studies look at somewhat later periods than 3500 to 1500 years ago?
1383: Wasn't the link to global warming in Munz et al. (2017) with the weakening IWM?
1396-1397: What makes these two different from the other studies in blue in Figure 20?
1416: "Earlier studies have suggested a decline in WD frequency": is this based on the two black minus signs (and no black plus signs) amongst the blue studies from Figure 20?
Figure 20: Presumably the different shades of blue/red/green are just to differentiate the studies and don't have any other meaning?
1494-1495: Looks like something has gone wrong with the text here so that the meaning is not clear.
Figure 22: What do the grey contours (that are not very clearly visible) represent? And can one tell from these panels where the gauges are for each dataset?
1519-1520: Does this mean that we know which of Pai et al. (2013) and Chauhan et al. (2022) provide the correct interpretation?
1541,1556: The formatting implied by the bold text is not clear here.
1692-1693: Meher & Das (2022): is this based on their Figure 5 (standard deviation)? Are you arguing that an increased standard deviation implies an increased mean?
Figure 23: It might be helpful to move this forward a bit, nearer to where it is first referred to in the text.
Figure 24: Which study are the solid coloured lines from?
1833: Does this imply that extreme WD precipitation rarely occurs in the core WD season? Or could this interaction occur over a timescale of a few months?
I would also like to make the following typographical recommendations/comments.
103: I would suggest changing "avalanches" --> "and avalanches" to improve comprehension of the sentence here.
150: "rather" --> "rather than"
251: The word "weather" appears twice: I think this is a mistake?
336: Could change "greater" --> "higher" just to make absolutely clear that you don't mean higher pressures (and thus lower altitudes).
432: One of the "delta"s has not been rendered correctly.
483: "occur" --> "occurring".
1005: Please define IWM here.
1333: "differential": do you mean "different"?
1345: Please define LIA here
1451: "if" --> "of"
1783-1784: MJO is listed twice.Citation: https://doi.org/10.5194/egusphere-2024-820-RC3 -
AC3: 'Reply on RC3', Kieran Hunt, 14 Aug 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-820/egusphere-2024-820-AC3-supplement.pdf
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AC3: 'Reply on RC3', Kieran Hunt, 14 Aug 2024
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RC4: 'Comment on egusphere-2024-820', Anonymous Referee #4, 19 Jun 2024
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AC1: 'Reply on RC4', Kieran Hunt, 14 Aug 2024
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2024/egusphere-2024-820/egusphere-2024-820-AC1-supplement.pdf
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AC1: 'Reply on RC4', Kieran Hunt, 14 Aug 2024
Data sets
Tracks of western disturbances (1950-2022) impacting South Asia Kieran M. R. Hunt https://zenodo.org/records/8208019
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