the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 11 Apr 2024
The movement of atmospheric blocking systems: can we still assume quasi-stationarity?
Abstract. The quasi-stationary behaviour of atmospheric blocking is studied using a Lagrangian framework that enables the tracking of blocks in space and time. By combining a blocking index based on geopotential height with a lagrangian tracking algorithm, we investigate the characteristics of atmospheric blocking events for different zonal propagation velocities and their impacts on surface temperatures within the retuned EC-Earth3 global climate model. We observe that blocking events 5 can portray a large variety of propagation velocities. Distinct differences are found between the behaviour of eastward-moving blocks and westward-moving blocks, both in size and in spatial distribution. Although the size of blocks is of bigger importance for the temperature anomalies, the propagation velocity has an influence on the strength of the temperature anomalies in winter, due to the slower mechanism of air advection in winter, compared to diabatic heating in summer. In summer, the propagation velocity primarily influences the positioning of temperature anomalies relative to the centre of the blocking system. These 10 findings highlight the complex interactions between size, propagation velocity, and other blocking attributes, and their influence on temperature anomalies. Further research is warranted to explore regional differences in blocking behaviour and impact, as well as how atmospheric blocking and associated temperature anomalies may evolve under future climate conditions.
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RC1: 'Comment on egusphere-2024-999', Anonymous Referee #1, 21 May 2024
Overall recommendation: Major revision or rejection.
The topic of this manuscript is interesting. However, there are many ambiguous issues in this manuscript. In this manuscript, the authors emphasized the difference of the impact between eastward- and westward-moving ones. In fact, these differences are obvious and natural. In this study, the authors repeated previous studies and results. Thus, no enough new results are found in this manuscript. The authors should further discuss the relationship among the size, movement speed and strength of blocking because they are not independent each other in a blocking system. The authors also ignored many previous similar studies in the introduction and text in this manuscript so that the authors said “To our knowledge, no studies have considered the effect that the propagation velocity of atmospheric blockings has on our weather” in the introduction. Such a description is completely misleading. Moreover, I do not think that the author’s 2D Cell-tracking Algorithm on the zonal propagation velocity of atmospheric blocking is correct. Thus, I recommend a major revision or even rejection.
Please see the attached file for a detailed review
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RC2: 'Comment on egusphere-2024-999', Anonymous Referee #2, 21 Jun 2024
The paper investigates the climatology of eastward and westward moving atmospheric blocking. A 2d blocking index is used in combination with a Lagrangian tracking algorithm to follow the blocking (highs) in time and space and to derive properties such as propagation speed, size and intensity. Furthermore, the authors study the (boreal) summer and winter season. The authors also present the impact of blocking speed and size on the temperature anomalies in the vicinity of the blocking high. I have some remarks regarding the study.
The authors state that the propagation velocity has not been studied in detail so far, but there is a lot of literature that looked at this topic, too. For example Hirt et al. (2018) uses point vortex theory to compare the velocities of high-over-low and Omega blocks with the mean weaterly flow. They found east- and westward propagating blocks in the Europe-Atlantic region with differences for different block types (and method). Cheung et al. (2013) investigated regional differences in boreal winter blocking and found that an eastward propagation is favored over Eurasia while a westward propagation is favored for blocking over the Western and Central Pacific. Hence, I recommend that the authors revise the literature review. Additionally a short section revising Rossby wave theory would be beneficial.
Overall, I think that the results are in line with what is expected from theory and former literature, e.g. larger size for westward moving systems. The differences between block properties in summer and winter seem to be very small, except of the size. The impact on the temperature field is different in the different seasons (as is also shown in Kautz et al, 2022). However the differences for systems within the same season of different size and speed do not seem to be too large (Fig. 8)? I think that the authors should state more clearly what is different and new in their work compared to previous ones, especially in the conclusions. Please state more clearly, what the additional value of the study for the scientific community is.
More specific comments and questions
Introduction: The authors derive average velocities of the blocks of about 3.5 m/s (300 km/day) which is still much smaller than the typical synoptic scale wind speed of U=10 m/s. From the introduction, the reader gets the impression that the velocities of the block are much higher so that the quasistationarity assumption does not hold. Please clarify.
Section 2.1 and 2.2: Out of curiosity from my side: What method do you use to regrid the data? Is smoothing applied, too? What is your motivation to use these two datasets?
Section 2.3: Your method has a lot of thresholds at several places. How much are the results depending on these settings? Please check that you explain all variables. In the equations you sometimes use t as a coordinate and in other equations d, I assume that it is time in both cases. Why do you change the nomenclature?
l. 113/eq. (7): Please explain in more detail the meaning of BI? How can wie interpret the values of BI
l.117: Relating to my former comment: What is the meaning of the different thresholds? Can you give idealized examples?
l.119/fig. 2: You speak of blocking intensites, but the figure shows frequencies. Please explain.
Section 2.5: How would your results change if you used maximum or minimum temperature instead of mean temperature?
Section 3.1: How do you define the size of the blocking high? By summing up the grid points? How are winter and summer defined?
Table 1: I would recommend to additionally give the velocities in m/s since it then could be more easily compared to the standard assumption of U=10m/s on the synoptic scale.
Section 3.2: How do you define westward/eastward propagating and quasistationary systems. Please give a clear definition. I am very confused how many systems are in which category and if you define these by certain thresholds or by percentiles. If you use percentiles, please also add the according velocities per category.
Fig. 4: Can you please add a contour to the frequency shading that represents the percentage of systems: for example does the second blue shading already contain 50% of all systems? If this is not possible at all, please add the probability density distribution or a histogramm of the velocities. I still wonder how many systems are slowly moving and how many systems are faster moving.
Fig. 5: Some of the data is cut of in the left figure. The 15 day rolling mean is impossible to see, please use a different color, e.g. black, for these lines.
L.235ff: Are these the values of the 15 day rolling mean?
Fig. 8: the 40 degree times 80 degree is latitude-longitude, correct? Can you please add composite geopotential height lines here. This could also give you some information on blocking type.
Table 2: For the velocities there seem to be very long tails. I would recommend to either add more percentiles (5%,10%, 90%,95%) or add a figure showing the probability density function. See also my comment regarding Fig. 4.
l. 351: over 2x
References:
Cheung, H. N., Zhou, W., Mok, H. Y., Wu, M. C., & Shao, Y. (2013). Revisiting the climatology of atmospheric blocking in the Northern Hemisphere. Advances in Atmospheric Sciences, 30, 397-410.Hirt, M., Schielicke, L., Müller, A., & Névir, P. (2018). Statistics and dynamics of blockings with a point vortex model. Tellus A: Dynamic Meteorology and Oceanography, 70(1), 1-20.
Kautz, L.-A., Martius, O., Pfahl, S., Pinto, J. G., Ramos, A. M., Sousa, P. M., and Woollings, T.: Atmospheric blocking and weather extremes over the Euro-Atlantic sector – a review, Weather Clim. Dynam., 3, 305–336, https://doi.org/10.5194/wcd-3-305-2022, 2022.
Citation: https://doi.org/10.5194/egusphere-2024-999-RC2 - AC1: 'Comment on egusphere-2024-999', Jonna van Mourik, 19 Jul 2024
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