the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Jan 2025
Bridging the polarimetric structure and lightning activity of an isolated thunderstorm during the cloud life cycle
Abstract. Cloud microphysics and dynamics produce lightning flashes, which can be detected as polarimetric structures by radar. Many studies have indicated that differential reflectivity (ZDR) and specific differential phase (KDP) columns, which serve as proxies for updraft strength, are related to lightning activity; moreover, the quantities of ice and supercooled liquid water strongly influence the occurrence of lightning flashes via noninductive charging. However, few studies have focused on clarifying the sequence or interactions among these factors from the perspective of the cloud life cycle. Here, we establish the ‘3D mapping columns’ method, which is based on the Cartesian grid datasets; this method is sensitive for identifying and quantifying the ZDR columns in the early phase of cloud formation. Our study bridges the polarimetric structure and lightning activity within an isolated thunderstorm during the cloud life cycle. The results indicate that i) the parameter most relevant to total flashes/cloud-to-ground flashes is the content of supercooled rainwater/graupel. ii) The onset of the ZDR column can be used to forecast lightning initiation in advance. iii) The signatures of the ZDR and KDP columns should be complementary and used to retrieve dynamic information instead of lightning activity. Notably, the variation in the ZH intensity within ZDR columns has high potential for predicting lightning activity during the cloud life cycle, which is valuable for exploration in the future. Our study improves the overall understanding of cloud microphysics and lightning activity, and suggestions for using these multiple polarimetric signatures to forecast severe weather are provided.
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Status: open (until 31 Mar 2025)
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RC1: 'Comment on egusphere-2024-4069', Eric Bruning, 09 Feb 2025
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The authors analyze a single case study of an isolated thunderstorm over land to the northeast of Guangzhou, China. Analysis of differential reflectivity and specific differential phase columns over the lifecycle of this storm allows the authors to analyze how the cloud microphysics lead to lightning, and the lead time that polarimetric radar allows in inferring the onset of lightning.
The case study largely repeats previous findings. There are a few valuable advancements in analysis methods (column identification methodology; inferring supercooled rain water content using a method from the late 1990s / early 2000s; lead time calculations by different methods). There are also some process inferences related to different pathways by which lightning might be produced that could be valuable and clarifying, but the universality of which is hard to judge on the basis of a single case study.
The authors have therefore engaged substantively in an ongoing tradition of analysis of polarimetric radar and lightning signals, with fair-to-good scientific significance, and good scientific and presentation quality. Below I note additional areas that could improve the manuscript, adding some missing information and clarifying the interpretation.
Major comments:
The authors do a nice job of reviewing the literature. I wanted to also mention our just-published paper, Bruning et al. (2024, 10.1175/MWR-D-24-0060.1), which pursues a very similar analysis on a large sample of storms. The authors’ detailed look at the time-series perspective here is valuable (and something we did not yet do), and I would be interested to see where this study fits in the distribution of lightning and polarimetry of storms sampled by Bruning et al., which were probably similar small, isolated, subtropical storms.
355-57: It is not clear to me that the Zh signal is better related to lightning. – for instance, the Zh signal is quite noisy, while there is a very clear max in high LWC values just before each of the peaks in lightning that is much less noisy – and the authors conclude later that the LWC signal is the most robust. So this claim confused me.
369: how does the collapse of the column result in an increase in lightning if graupel (which is thought to be necessary for electrification) is inferred as decreasing or absent in the column? Further discussion of the process would be valuable here; there are some hints in the discussion/conclusion section here, but I felt that further information and data was needed to verify the interpretation of the two different pathways to lightning the authors have identified.
393: note, however, that the correlation with Zdr is relatively large and increases (0.6) for about 20 min before the maximum in lightning, but falls off rapidly by 12 min after the lightning increases. From a practical point of view, the timing of the maximum correlation is less important than a trend toward confidence for lightning, and so in that sense the Zdr signal is more helpful.
423-6: These correlation coefficients do not seem different enough to allow the authors to say one is best, especially on the basis of a single case study. Values all >0.8 are quite high for each of these variables.
449: After studying the lead times and identifying and emphasizing a 6 min lead time in their results section, the authors return to quoting the 36 min lead time in their conclusions, which does not seem supported by the detailed analysis the authors undertook. Of course, the 36 min lead is there in the data, but it is not well-correlated to lightning. Many moderately vigorous storms will produce a small Zdr column without going on to produce lightning. Likewise on 476-477, I would be reluctant to forecast lightning on the basis of a 36 min lead - that cell is simply one to keep an eye on for future lightning.
Fig. 12: the authors indicate that no Kdp column was present in their data, but do not show Kdp in Fig. 6. I would like to see further data on this, as it may explain the relatively fewer cases in Bruning et al. (2024) that had Zdr columns and lightning but did not have a Kdp column.
Minor comments:
31: The grammar implies lightning flashes can be detected with polarimetric structures; this is not directly possible. The polarimetric signatures are proxies for lightning with some associated error. Please rephrase.
37: “establish” — this study is not the first to use this method, as many of the authors’ citations show. “Build on” or “improve” would be a better choice, since “establish” implies that the authors have made a pioneering advancement. There are some thoughtful adjustments to past methods here, but they are incremental refinements.
124: “later” - do the authors mean a time scale immediately following the Zdr column (~5 min) or subsequent updraft pulses in a multicellular sequence (~20-30 min per cell)?
125: “attempted to determine the constraints of“ should be “attempted to constrain”
140-141: “therefore the correlation coefficient … was not high.” What does “therefore” mean here? It typically indicates that a conclusion has been reached, so the facts supporting the conclusion need to be stated first. They seem to be in the sentence following “therefore”.
175-181: what Kdp calculation method was used? Kdp is a very noisy measurement, and so is very sensitive to algorithm design and configuration choices.
220: here and throughout the paper, melting level is preferable, since melting always begins at this level for any hydrometeor but freezing might not.
221: What are other parameters (CAPE, etc.) of this sounding? They would be helpful in placing this storm in the context of other environments globally.
227: “automatically” should be “automatic”
271: A new sentence should start after “(Figure 2e,f)”.
281: “resulted by” should be “resulting from”
447: I suggest dropping “inappropriate”. Any algorithm choice requires some judgment, and reflectivity thresholds have a sound physical basis and are in wide use. Of course, using fewer or improved variables and thresholds is also good, and in that way the authors have made a nice methodological contribution, but “inappropriate” is unnecessarily harsh.
Citation: https://doi.org/10.5194/egusphere-2024-4069-RC1
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