the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Towards a diagnostic framework unifying different perspectives on blocking dynamics: insight into a major blocking in the North Atlantic-European region
Seraphine Hauser
Franziska Teubler
Michael Riemer
Peter Knippertz
Christian M. Grams
Abstract. Atmospheric blocking describes a situation in which a stationary and persistent anticyclone blocks the eastward propagation of weather systems in the midlatitudes and can lead to extreme weather events. In the North Atlantic-European region blocking contributes to life cycles of weather regimes, which are recurrent, quasi-stationary, and persistent patterns of the large-scale circulation. Despite progress in blocking theory over the last decades, we are still lacking a comprehensive, process-based conceptual understanding of blocking dynamics. Here we combine three different perspectives on blocking, namely the commonly used Eulerian and Lagrangian perspectives, complemented by a novel quasi-Lagrangian perspective. Within the established framework of mid-latitude potential vorticity (PV) thinking the joint consideration of the three perspectives enables a comprehensive picture of the dynamics and quantifies the importance of dry and moist processes during a blocking life cycle.
We apply the diagnostic framework to a European Blocking weather regime life cycle in March 2016, which was associated with a severe forecast bust in the Atlantic-European region. All three perspectives highlight the importance of moist processes during the onset and maintenance of the ‘blocked’ weather regime. The Eulerian perspective, which identifies the processes contributing to the onset and decay of the regime, indicates that dry quasi-barotropic wave dynamics and especially the eastward advection of PV anomalies (PVAs) into the North Atlantic-European sector are associated with the onset of the regime pattern. By tracking the negative upper-tropospheric PVA associated with the ‘block’, the quasi-Lagrangian view reveals, for the same period, abrupt amplification due to moist processes. This is in good agreement with the Lagrangian perspective indicating that a large fraction of air parcels that end up in the negative PVA experiences diabatic heating. Overall, the study shows that important contributions to the development take place outside of the region in which the blocked weather regime eventually establishes, and that a joint consideration of different perspectives is important in order not to miss processes, in particular moist-baroclinic dynamics, contributing to a blocking life cycle.
Seraphine Hauser et al.
Status: final response (author comments only)
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RC1: 'Comment on wcd-2022-44', Anonymous Referee #1, 08 Sep 2022
This is a nice paper which develops and applies three different perspectives to diagnose the development of a blocking event over Europe. This is a useful approach which helps to see how different processes and their associated theories relate to each other. The first two perspectives are linked through a common diagnostic framework centred on the PV equation (4), while the third is more distinct. The three methods are combined in a 'joint consideration' as opposed to a quantitative framework, but this nevertheless provides a fascinating, balanced and in-depth picture of the case study.
Overall, the paper is clear and well presented and certainly a useful contribution to the literature on blocking. The main conclusion that moist processes play an important role but that this is missed by some methods is sound and supported by the evidence. It is especially nice to see a focus on including and reconciling different viewpoints from the literature. The paper is a little long but I think this is unavoidable given the number of methods used, and in fact a few extra details in a few places might still be needed. I am supportive of publication after consideration of the following:
Major points:
1. Perhaps the most basic mechanism of forming an anticyclonic PV anomaly is through poleward advection and the beta effect. This is part of the barotropic term (v' dot grad q_0) but this QB term is mostly discussed in terms of the downstream advection of existing PV anomalies. The role of this term in generating, not just re-arranging, PV anomalies could be diagnosed through calculation of the beta term, and should be at least discussed if not diagnosed. It was interesting to see in fig 13 that both the heated and non-heated back trajectories originate at lower latitudes than the final anticyclone, so that beta must play some role in generating the PV anomalies for both sets of parcels.
2. The methods differ fundamentally in whether they consider the cyclonic anomaly to be part of the block as well as the anticyclonic anomaly. I think the cyclone should probably be included given i) it's part of the regime structure used to define blocking here, and ii) it contributes to obstructing the westerly flow and causing some of the impacts outlined in the introduction. I don't think the analysis of methods 2 and 3 should be extended here to include the cyclone, but this limitation needs to be clearly discussed. It could also be summarised in the conclusions what drives the cyclonic part in this analysis.
3. The rationale for tracking of PV anomalies could be more convincingly justified. PV is conserved in the absence of diabatic/frictional effects but not PV anomalies (as seen from the beta effect), so why track the anomalies? There is a threshold for PVA, so arguably this method still misses some information on the initial origin of anomalies as well. The method is fine for this paper, as the single event has been studied carefully. But for future use it would be nice to see more validation of this new method, including sensitivity to the choices and/or parameters used.
Minor points:
1. The authors do a good job of selectively covering the plethora of suggested blocking methods in the introduction. But I think a mention of the methods of Noboru Nakamura and Clare Huang would be a good addition, especially since the authors claim to have gone further than others in combining adiabatic and diabatic processes, something which the Nakamura/Huang theory also attempts.
2. A few more details on the decomposition are needed - eg how exactly are v_up and v_low defined?
3. More justification is needed that the (v_low dot grad q_0) term encapsulates the baroclinic effects. At face value, this seems a cruder definition than that of Martineau et al (https://doi.org/10.1029/2022GL097791)
4. Are all projections etc performed over the region shown in fig 2a?
5. line 237 - 10% sounds like a low tolerance here. What is the sensitivity to this?
6. Fig 5 was a bit too dense for me - could try fewer contours for the div term? And label that colorbar
7. It's interesting how the eddy fluxes come into these analyses, and not clear how closely the splitting/merging corresponds to the conventional picture of these. It might help to note for the eulerian analysis of this that the eddy fluxes are often diagnosed upstream of the existing block, to quantify their role in maintaining the blocking structure against the mean westerlies (Illari 1984). A crude quasi-lagrangian approach?
8. I struggled to understand the role of the radiation, especially as it seems to strengthen the trough in fig 6b, contrary to the idea of radiative damping. It seems to be a relatively uniform cyclonic influence across the domain. Is it due to bottom-amplified LW cooling acting to reduce stratification everywhere, or something else?
9. Could unify the names for the four periods between fig 11 and the text, for clarity.
10. Is the direct diabatic effect (section 4.2) only seen because the lower troposphere is excluded? (heating should give a negative PV anomaly above and a positive anomaly below)
11. p24: The case that the divergent PV tendencies are a moist impact seems a bit overstated. It seems to imply that all divergent tendencies reflect this, but the correlations are only consistent with 20-40% of the variance being shared.
12. Top of p28: could you make a link between the methods here, between the heated trajectories (lagrangian) and the divergence term (quasi-lag)? Seems consistent with the theory of Methven (https://doi.org/10.1002/qj.2393) that the role of the heating is not direct but to enhance ascent of low-level, low-PV air up the warm conveyor belt into the block.
Citation: https://doi.org/10.5194/wcd-2022-44-RC1 -
RC2: 'Comment on wcd-2022-44', Anonymous Referee #2, 18 Oct 2022
This work is a case study that aims to get a holistic view of different dynamics of blocking.
This is a very important and meaningful direction to work on.
Three perspectives (Eulerian, quasi-Lagrangian and Lagrangian) are used.
The case study highlights the importance of moist processes (warm conveyor belt), which drive divergent outflow aloft and PV tendency.The paper is well written overall.
It will likely be well suited for publication after addressing the following comments (and more importantly, comments from the other reviewer):(Reviewer 1 raised excellent points and I try not to repeat what they said.)
Major comments
1. Title: I think the main selling point of this study is that it gives a holistic view of different dynamics of blocking. The three perspectives (Eulerian, quasi-Lagrangian and Lagrangian) are not as attractive. Consider revising the title.
I am also unsure about the word “unifying” in the title. The three perspectives are presented (metaphorically) like three separate dishes, not as one. “Unify” might not be the appropriate word.
2. The Eulerian perspective seems to be inferior, because it “misses the processes associated with the development of PVAs advected into the region” (line 379). Does it have any advantage over the other perspectives?
3. I think this work lays out very good foundation where different proposed dynamics of blocking can be compared together. Right now, the direct latent heat release, indirect moist effect through divergent outflow and selective absorption (Yamazaki and Itoh, 2009) are considered. Many other proposed mechanisms, like the well-known eddy-straining idea (Shutts 1983), does not seem to be sufficiently discussed in results. Would be good to explicitly discuss them in results.
4. This work considers blocking from the perspective of weather regimes. I could be biased against weather regimes, but I feel like the perspective of weather regimes here brings few benefits but more burden. For example, amplification to the secondary ridge over the US East Coast might be confused with the block (line 360).
Maybe it is too much to ask you to give up on weather regime and redo the analysis, or to give up the phrase “blocking dynamics” and instead say “regime dynamics”. But I still think it takes up too much words and figures, and some of them can be moved to appendix/supplement, as it is not the key or a selling point.
5. The quasi-Lagrangian analysis might be able to explain why the block becomes strong and large, but not why it is stationary. Stationarity is also a key aspect to create extreme weather events. Dynamics to make a block stationary should at least be included as one of the future directions.
6. Line 242: The amplitude metric is spatial integral of q’ over the area A. Since the threshold of q’ is not zero but -0.8 PVU (line 229), would it be better to choose the amplitude metric instead to be the spatial integral of (q’+0.8) over the area A? In this way, whether marginal points cross the threshold or not would not make a big difference.
Minor comments
7. Section 4.1: Related to reviewer 1 major point 2, when separately considering the cyclonic and anticyclonic anomalies, please say that the main cyclonic part of the regime pattern does not contribute to obstructing the westerly flow (Figure 2).
8. Line 163: “eddy flux convergence may change PVAs locally but may neither generate new nor amplify existing PVAs in a globally averaged sense. Furthermore, eddy flux convergence may not change the area-integrated amplitude of PVAs that are defined by a boundary at which q’=0” I’m not sure about if these statements are true.
9. Line 642: “we are able to close our q’ budget…”. But this requires taking Delta-A from observed area change? If so, is this “cheating”? Around line 260, you should briefly say that Bnd is taking from observed area change (not just in appendix).
10. Fig. 11: How is the effect of splitting/merging events on PVA quantified? Does this require knowing the observed area change?
11. Line 10: “All three perspectives highlight the importance of moist processes…” Does the Eulerian perspective highlight the importance of moist processes?
12. Line 355: There no Fig. 6d. Please fix the typo.
13. Line 554: “… the quasi-Lagrangian perspective reveals an amplitude strengthening of the main PVA over Europe by the merging of further PVAs…” This statement in section 5 (synopsis) don’t seem to be supported in section 4.2 (quasi-Lagrangian perspective), especially Figure 11 finds merging and splitting to have *weakening* effect.
Citation: https://doi.org/10.5194/wcd-2022-44-RC2 -
AC1: 'Reviewer response letter', Seraphine Hauser, 16 Nov 2022
The comment was uploaded in the form of a supplement: https://wcd.copernicus.org/preprints/wcd-2022-44/wcd-2022-44-AC1-supplement.pdf
Seraphine Hauser et al.
Seraphine Hauser et al.
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