the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 19 Jan 2022
Downstream development associated with two types of ridging South Atlantic Ocean anticyclones over South Africa
Abstract. There are at least two types of ridging South Atlantic Ocean high pressure systems in the South African domain. Type-N events occur north of 40° S and Type-S occur south of this latitude line. This study shows that there is no evidence of surface downstream development in terms of the evolution of eddy kinetic energy and associated ageostrophic geopotential fluxes for both types of ridging high events. Rather, for these systems downstream development is an upper level process. The baroclinic waves associated with the ridging develop from baroclinic instability, by converting eddy available potential energy to eddy kinetic energy. The bulk of the conversion is located at the upstream end of the waves. The downstream trough, which is the part of the wave that influences upward motion over South Africa, develops from the transport of eddy kinetic energy across the trough axis by means of ageostrophic geopotential fluxes. These fluxes are stronger for Type-S events. The absence of downstream development at the surface and the presence of it aloft demonstrates that there are differences in the underlying dynamics in the evolutions of these systems in the vertical. The evolution of eddy kinetic energy associated with baroclinic waves that occur during the ridging events is different from what has been observed for cut-off low pressure systems in the South African domain.
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Interactive discussion
Status: closed
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RC1: 'Comment on wcd-2022-2', Anonymous Referee #1, 17 Feb 2022
The authors aim to build the understanding of the ridging process in the South Atlantic from a downstream development perspective, given the relationship between ridges in this region and extreme rainfall in South Africa. The manuscript builds on a body of existing work by the authors, but I was left somewhat unconvinced that this paper provides a significant step beyond what the authors have already published on this topic given their reliance on the results presented in previous papers.
Major comments:
1.
I was left wondering about the value of separating between Type-N & Type-S ridges. A key finding of the manuscript is that there is “no evidence of downstream development at the surface during the evolution of ridging highs”. This finding seems to apply to both ridge types, so I’m not sure what one has gained by providing separate analysis of the two ridge types. I note also that the findings discussed in the abstract are generalisable to both ridge types. Can the manuscript be simplified by focusing on ridges in general? Or can the authors provide stronger motivation, beyond that ridges in general are a source of moisture for South Africa, for the separation into ridge types? It wasn’t until I referred to Ndarana et al. (2022) that I understood that there might be differences in the severity of rainfall associated with each ridge type. Can the authors provide clearer justification for the separation into Type-N and Type-S ridges? (e.g. in the Introduction, around Line 56)
2.
The authors rely heavily on composite analysis in the manuscript yet provide no information about the sample size used to produce the composites nor present any significance testing, which makes it difficult to judge how representative the composites are of the dynamics at play. I referred to Ndarana et al. (2022) and note the use of the Brown & Hall (1999) t-value approach to significance. I assume this was applied in the case of this manuscript but see no evidence of that. I have some concern about the marked difference in sample size used for the Type-N and Type-S composites, assuming it is the same as presented in Ndarana et al. (2022). The composites for Type-S ridges appear more intense (e.g. more extreme values) than those for Type-N. Smaller samples can be more affected by extreme events in the sample pool. Can the authors provide evidence to assure the reader that the results for Type-S ridging are not an artefact of the smaller sample size?
3.
Where does blocking fit into the story? In my experience cutoff low pressure systems are associated with the presence of blocking highs. That is, the low pressure system is cutoff from the westerly stream by the blocking high. Could some of the ridges the authors identify be blocks? Can the authors comment on this?
Minor comments:
Line 33: “is” transported
Line 157: incorrect spelling of “shift”
Line 180: the authors note that the eddy kinetic energy associated with type-S events is stronger than that associated with Type-N events – Fig 2a suggests it is marginally stronger? Is this a significant increase between N & S events? Worthy of remarking on? Similar question for vertical motion (velocity).
Figure 2: I think there is some incorrect referencing of the figures. In the caption I think 2e and 2f need to be switched around
Figure 2h: Conversion spelled incorrectly
Figure 2i: incorrectly labelled as 2h
Figure 2i: can the authors make comment about the differences between Type N & Type S in this instance?
Figure 3: label incorrect. The eddy kinetic energy centres are shown in (b) and (f) not (h)
Figure 3: I don’t understand the last sentence in the figure caption. There is no Figure 3l
Line 297: ridging not riding
Paragraph 310-327: the authors introduce “Life Cycle” with limited discussion around what this means. The authors have an expectation that the reader is familiar with concept. A brief description of what is meant by LC1 and LC2 would assist the reader.
Line 372: make use of
Citation: https://doi.org/10.5194/wcd-2022-2-RC1 -
RC2: 'Comment on wcd-2022-2', Anonymous Referee #2, 23 Feb 2022
Reviews
Submitted to wcd-2022-2
Downstream development associated with two types of ridging South Atlantic Ocean anticyclones over South Africa
Authors: T. Ndarana et al.
The authors investigate two types of ridging high pressure systems in South Africa and surroundings from an eddy kinetic energy perspective using the ERA5 reanalysis. Results show that ridging highs located in more poleward latitudes are stronger than those in lower latitudes. The study also presents a comparison between the ridging high and cutoff low development. Nevertheless, after reading the paper, several fundamental questions come to mind, and my feeling is that if the authors can address some of these questions the scientific content of the paper will be improved making this a more valuable contribution.
- The authors compare two types of ridging highs and while I do agree that they are dynamically distinct it seems to me that the categorization is based on subjective interpretation rather than to information gained by a physical basis. A related question, can these ridging building be classified by cyclonic or anticyclonically breaking waves? If so, perhaps a more objective approach could be done by applying the anticyclonic and cyclonic types (LC1 vs. LC2 paradigm). Maybe this is not the goal of the study, but then the motivation for examining the differences between the two types of ridging highs should be mentioned explicitly in the Introduction. Even the authors refer to their previous paper, the main differences between the two types of highs need to be clearly mentioned and why this classification is important.
- Another question is whether the ridging process may be induced by the South African topography. Presumably these types of ridging highs might be affected by topographic Rossby waves, and to me this could be mentioned in the manuscript.
- The paper does not discuss the possible contribution of diabatic processes to the ridging process. As mentioned by the authors, the vertical motion is significant in the type-S highs, in particular the downward motion, thus inducing eddy kinetic energy creation or destruction via baroclinic conversion. Diabatic processes are part of the eddy available potential energy (EAPE) budget, and EAPE is converted into eddy kinetic energy by baroclinic conversion. Thus, eddy kinetic energy can be indirectly enhanced by diabatic processes if these provide sufficient EAPE. Diabatic processes such as latent heating and radiative cooling are known to play an important for the evolution of mid- to upper-level atmospheric systems and these may contribute the development of ridging highs. Even though the accuracy of diabatic profiles is susceptible to errors in reanalysis, being more easily dealt in numerical models, I think this point should be discussed in the paper.
- Could quantify the relative contribution of each energetic term for the ridging phenomenon. More quantitative evidence of each mechanism (ageostrophic geopotential flux, baroclinic and barotropic conversions, etc) could provide a more comprehensive view of the contribution of each energetic term to the ridging highs through their life cycle. Also, it would be worth including upper-level winds or geopotential height fields to the composites for allowing a clear view of ridges and troughs.
Minor comments
Line 24: It should mention what level these quasi-stationary anticyclonic circulation occurs.
Line 60: Can explicitly state which “these waves” are.
Line 116: Add (Figure 1) after “During the first stage”
Line 167: This is not entirely true, other studies have already focused on the vertical profile of eddy kinetic energy, such as Rivière et al. (2015) and Pinheiro et al. (2021).
Rivière, G., Arbogast, P., & Joly, A. (2015). Eddy kinetic energy redistribution within windstorms Klaus and Friedhelm. Quarterly Journal of the Royal Meteorological Society, 141(688), 925-938.
Pinheiro, H. R., Hodges, K. I., Gan, M. A., Ferreira, S. H., & Andrade, K. M. (2021). Contributions of downstream baroclinic development to strong Southern Hemisphere cutâoff lows. Quarterly Journal of the Royal Meteorological Society.
Line 176: How is t = 0 defined, would it be when the condition is firstly satisfied in the domain? This needs to be described in the methods and/or in the beginning of the discussion. Also, it's a bit confusing to present positive and negative values together, are these observed at different times?
Line 181: Does “The latter” refer to meridional perturbation velocity? Please mention the referred variable.
Line 186: baroclinic conversion reaches its maximum at lower levels, thus it does not explain the maximum tendency of kinetic energy near 250 hPa.
Lines 190-191: Figures 2h shows that the negative baroclinic conversion (ascending) maximizes at a higher level than the positive counterparts (descending), is there a physical explanation for this?
Line 191-192: The sentence that starts with “The ageostrophic geopotential flux has been shown…”seems to be disconnected from the rest of the paragraph or perhaps something is missing.
Line 223: Are these eddy kinetic energy centers computed vertically or at 1000 hPa, why don’t these centers match the maximum 1000-hPa eddy kinetic energy shown in shaded in Fig. 3? The authors do not mention if the Ke centers (namely I, II and III) are vertically integrated quantities as sometimes the energetics are referred to a single pressure level. After reading the paper, I assume these are integrated average fields, but nowhere is mentioned which levels are used for.
Line 232: replace “The latter” with “The centre II”
Line 297: In Fig. 6 it is not clear to me if the time lags coincide for all kinetic energy centers, I mean, does T=0 for center occur at the same time as that for center II? This is important for the correct interpretation.
Line 376: Please check if all fields are at 250 hPa as the caption of Fig. 9 says there are fields at the surface.
Line 444: Replace “the latter” with “type-S ridging”
Line 649: In Figure 3, can the quality of the lines that indicate positive/negative 1000-hPa geopotential height perturbations be improved as they are not clearly distinctable.
Line 766: In Figure 10, the acronyms UJ and DJ are not described. In that figure, what does mean the thick solid black oval contours?
Typos
Line 157: Replace “shifte” with “shift”
Line 223: replace “centre” with “centres” and “Fig. 3b and f” with “Fig. 3b and 3f”
Citation: https://doi.org/10.5194/wcd-2022-2-RC2 -
RC3: 'Comment on wcd-2022-2', Anonymous Referee #3, 25 Feb 2022
The comment was uploaded in the form of a supplement: https://wcd.copernicus.org/preprints/wcd-2022-2/wcd-2022-2-RC3-supplement.pdf
Interactive discussion
Status: closed
-
RC1: 'Comment on wcd-2022-2', Anonymous Referee #1, 17 Feb 2022
The authors aim to build the understanding of the ridging process in the South Atlantic from a downstream development perspective, given the relationship between ridges in this region and extreme rainfall in South Africa. The manuscript builds on a body of existing work by the authors, but I was left somewhat unconvinced that this paper provides a significant step beyond what the authors have already published on this topic given their reliance on the results presented in previous papers.
Major comments:
1.
I was left wondering about the value of separating between Type-N & Type-S ridges. A key finding of the manuscript is that there is “no evidence of downstream development at the surface during the evolution of ridging highs”. This finding seems to apply to both ridge types, so I’m not sure what one has gained by providing separate analysis of the two ridge types. I note also that the findings discussed in the abstract are generalisable to both ridge types. Can the manuscript be simplified by focusing on ridges in general? Or can the authors provide stronger motivation, beyond that ridges in general are a source of moisture for South Africa, for the separation into ridge types? It wasn’t until I referred to Ndarana et al. (2022) that I understood that there might be differences in the severity of rainfall associated with each ridge type. Can the authors provide clearer justification for the separation into Type-N and Type-S ridges? (e.g. in the Introduction, around Line 56)
2.
The authors rely heavily on composite analysis in the manuscript yet provide no information about the sample size used to produce the composites nor present any significance testing, which makes it difficult to judge how representative the composites are of the dynamics at play. I referred to Ndarana et al. (2022) and note the use of the Brown & Hall (1999) t-value approach to significance. I assume this was applied in the case of this manuscript but see no evidence of that. I have some concern about the marked difference in sample size used for the Type-N and Type-S composites, assuming it is the same as presented in Ndarana et al. (2022). The composites for Type-S ridges appear more intense (e.g. more extreme values) than those for Type-N. Smaller samples can be more affected by extreme events in the sample pool. Can the authors provide evidence to assure the reader that the results for Type-S ridging are not an artefact of the smaller sample size?
3.
Where does blocking fit into the story? In my experience cutoff low pressure systems are associated with the presence of blocking highs. That is, the low pressure system is cutoff from the westerly stream by the blocking high. Could some of the ridges the authors identify be blocks? Can the authors comment on this?
Minor comments:
Line 33: “is” transported
Line 157: incorrect spelling of “shift”
Line 180: the authors note that the eddy kinetic energy associated with type-S events is stronger than that associated with Type-N events – Fig 2a suggests it is marginally stronger? Is this a significant increase between N & S events? Worthy of remarking on? Similar question for vertical motion (velocity).
Figure 2: I think there is some incorrect referencing of the figures. In the caption I think 2e and 2f need to be switched around
Figure 2h: Conversion spelled incorrectly
Figure 2i: incorrectly labelled as 2h
Figure 2i: can the authors make comment about the differences between Type N & Type S in this instance?
Figure 3: label incorrect. The eddy kinetic energy centres are shown in (b) and (f) not (h)
Figure 3: I don’t understand the last sentence in the figure caption. There is no Figure 3l
Line 297: ridging not riding
Paragraph 310-327: the authors introduce “Life Cycle” with limited discussion around what this means. The authors have an expectation that the reader is familiar with concept. A brief description of what is meant by LC1 and LC2 would assist the reader.
Line 372: make use of
Citation: https://doi.org/10.5194/wcd-2022-2-RC1 -
RC2: 'Comment on wcd-2022-2', Anonymous Referee #2, 23 Feb 2022
Reviews
Submitted to wcd-2022-2
Downstream development associated with two types of ridging South Atlantic Ocean anticyclones over South Africa
Authors: T. Ndarana et al.
The authors investigate two types of ridging high pressure systems in South Africa and surroundings from an eddy kinetic energy perspective using the ERA5 reanalysis. Results show that ridging highs located in more poleward latitudes are stronger than those in lower latitudes. The study also presents a comparison between the ridging high and cutoff low development. Nevertheless, after reading the paper, several fundamental questions come to mind, and my feeling is that if the authors can address some of these questions the scientific content of the paper will be improved making this a more valuable contribution.
- The authors compare two types of ridging highs and while I do agree that they are dynamically distinct it seems to me that the categorization is based on subjective interpretation rather than to information gained by a physical basis. A related question, can these ridging building be classified by cyclonic or anticyclonically breaking waves? If so, perhaps a more objective approach could be done by applying the anticyclonic and cyclonic types (LC1 vs. LC2 paradigm). Maybe this is not the goal of the study, but then the motivation for examining the differences between the two types of ridging highs should be mentioned explicitly in the Introduction. Even the authors refer to their previous paper, the main differences between the two types of highs need to be clearly mentioned and why this classification is important.
- Another question is whether the ridging process may be induced by the South African topography. Presumably these types of ridging highs might be affected by topographic Rossby waves, and to me this could be mentioned in the manuscript.
- The paper does not discuss the possible contribution of diabatic processes to the ridging process. As mentioned by the authors, the vertical motion is significant in the type-S highs, in particular the downward motion, thus inducing eddy kinetic energy creation or destruction via baroclinic conversion. Diabatic processes are part of the eddy available potential energy (EAPE) budget, and EAPE is converted into eddy kinetic energy by baroclinic conversion. Thus, eddy kinetic energy can be indirectly enhanced by diabatic processes if these provide sufficient EAPE. Diabatic processes such as latent heating and radiative cooling are known to play an important for the evolution of mid- to upper-level atmospheric systems and these may contribute the development of ridging highs. Even though the accuracy of diabatic profiles is susceptible to errors in reanalysis, being more easily dealt in numerical models, I think this point should be discussed in the paper.
- Could quantify the relative contribution of each energetic term for the ridging phenomenon. More quantitative evidence of each mechanism (ageostrophic geopotential flux, baroclinic and barotropic conversions, etc) could provide a more comprehensive view of the contribution of each energetic term to the ridging highs through their life cycle. Also, it would be worth including upper-level winds or geopotential height fields to the composites for allowing a clear view of ridges and troughs.
Minor comments
Line 24: It should mention what level these quasi-stationary anticyclonic circulation occurs.
Line 60: Can explicitly state which “these waves” are.
Line 116: Add (Figure 1) after “During the first stage”
Line 167: This is not entirely true, other studies have already focused on the vertical profile of eddy kinetic energy, such as Rivière et al. (2015) and Pinheiro et al. (2021).
Rivière, G., Arbogast, P., & Joly, A. (2015). Eddy kinetic energy redistribution within windstorms Klaus and Friedhelm. Quarterly Journal of the Royal Meteorological Society, 141(688), 925-938.
Pinheiro, H. R., Hodges, K. I., Gan, M. A., Ferreira, S. H., & Andrade, K. M. (2021). Contributions of downstream baroclinic development to strong Southern Hemisphere cutâoff lows. Quarterly Journal of the Royal Meteorological Society.
Line 176: How is t = 0 defined, would it be when the condition is firstly satisfied in the domain? This needs to be described in the methods and/or in the beginning of the discussion. Also, it's a bit confusing to present positive and negative values together, are these observed at different times?
Line 181: Does “The latter” refer to meridional perturbation velocity? Please mention the referred variable.
Line 186: baroclinic conversion reaches its maximum at lower levels, thus it does not explain the maximum tendency of kinetic energy near 250 hPa.
Lines 190-191: Figures 2h shows that the negative baroclinic conversion (ascending) maximizes at a higher level than the positive counterparts (descending), is there a physical explanation for this?
Line 191-192: The sentence that starts with “The ageostrophic geopotential flux has been shown…”seems to be disconnected from the rest of the paragraph or perhaps something is missing.
Line 223: Are these eddy kinetic energy centers computed vertically or at 1000 hPa, why don’t these centers match the maximum 1000-hPa eddy kinetic energy shown in shaded in Fig. 3? The authors do not mention if the Ke centers (namely I, II and III) are vertically integrated quantities as sometimes the energetics are referred to a single pressure level. After reading the paper, I assume these are integrated average fields, but nowhere is mentioned which levels are used for.
Line 232: replace “The latter” with “The centre II”
Line 297: In Fig. 6 it is not clear to me if the time lags coincide for all kinetic energy centers, I mean, does T=0 for center occur at the same time as that for center II? This is important for the correct interpretation.
Line 376: Please check if all fields are at 250 hPa as the caption of Fig. 9 says there are fields at the surface.
Line 444: Replace “the latter” with “type-S ridging”
Line 649: In Figure 3, can the quality of the lines that indicate positive/negative 1000-hPa geopotential height perturbations be improved as they are not clearly distinctable.
Line 766: In Figure 10, the acronyms UJ and DJ are not described. In that figure, what does mean the thick solid black oval contours?
Typos
Line 157: Replace “shifte” with “shift”
Line 223: replace “centre” with “centres” and “Fig. 3b and f” with “Fig. 3b and 3f”
Citation: https://doi.org/10.5194/wcd-2022-2-RC2 -
RC3: 'Comment on wcd-2022-2', Anonymous Referee #3, 25 Feb 2022
The comment was uploaded in the form of a supplement: https://wcd.copernicus.org/preprints/wcd-2022-2/wcd-2022-2-RC3-supplement.pdf
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