the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Sep 2026
Spatiotemporal Characteristics and Environments Controls of Non-Typhoon Mesoscale Convective Systems during the Post-flood Rainy Season over South China
Abstract. South China is one of China’s rainiest regions, where mesoscale convective systems (MCSs) frequently produce heavy rainfall during the post-flood rainy season (July-September). Using 1-km, 6-min radar mosaics and ERA5 reanalysis, this study identified 123 MCSs from 2019 to 2022. MCS activity is concentrated over southern Guangxi (S-GX) and the Pearl River Delta (PRD), with marked regional contrasts in diurnal evolution and environmental conditions. Variations in the relative positions of the South Asian High (SAH) and the western North Pacific subtropical high (WNPSH) produce distinct circulation regimes over the two regions. S-GX MCSs over the S-GX mainly form from midnight to early morning and are associated with an eastward-extended SAH and a westward-displaced WNPSH, which strengthen the southerly monsoonal flow and produce a deeper moisture transport layer extending to approximately 750 hPa. Together with larger convective instability (mean CAPE ≈ 1843 J kg⁻¹), these conditions allow convection to overcome the relatively larger convective inhibition (CIN) and support nocturnal MCS development. In contrast, PRD MCSs over the PRD mainly form from afternoon to evening under weaker large-scale forcing, shallower moisture transport largely confined below 900 hPa, lower but localized instability (mean CAPE ≈ 1523 J kg⁻¹) and smaller inhibition. These conditions increased the relative importance of daytime boundary-layer heating and sea-breeze convergence, consistent with the afternoon to evening MCS maximum. The results identify a synoptic-dynamical mode over S-GX and a boundary-layer thermodynamic mode over the PRD, providing a physical framework for understanding post-flood-season MCS development and heavy rainfall over South China.
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Status: open (until 26 Nov 2026)
- RC1: 'Comment on egusphere-2026-5422', Anonymous Referee #1, 15 Oct 2026 reply
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RC2: 'Comment on egusphere-2026-5422', Anonymous Referee #2, 17 Oct 2026
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Overall Assessment
This manuscript uses 1-km, 6-min radar mosaics and ERA5 reanalysis for 2019–2022 to identify 123 non-typhoon MCSs during the post-flood rainy season over South China. It shows that MCS activity is concentrated over southern Guangxi (S-GX) and the Pearl River Delta (PRD), and proposes two environmental modes: a synoptic-dynamical mode over S-GX and a boundary-layer thermodynamic mode over the PRD. The topic is relevant and the radar-based climatology is a useful contribution. However, the objectivity of the classification, the strength of the evidence for the proposed environmental mechanisms, and the generality of the conclusions need to be strengthened.
Major Comments
1. Robustness of the classification and matching method. The division at 111.2°E into S-GX and PRD MCSs lacks an objective basis and sensitivity testing; the authors should vary the dividing longitude or apply a clustering method to confirm that the two groups remain robust. The matching of non-MCS days using the 925 hPa geopotential height field, with thresholds of the lowest 40% Euclidean distance and highest 60% spatial correlation, is subjective and relies on a single level. Sensitivity tests with different thresholds and levels (e.g., 500 hPa) should be added, together with statistics showing the actual differences in the matching variables between MCS and non-MCS days.
2. Insufficient evidence for the causal interpretation of environmental differences. The boxplot comparisons in Fig. 9 lack significance testing; Mann–Whitney U tests and effect sizes should be provided. S-GX events show both larger CAPE and larger CIN, which the authors explain as stronger dynamical forcing overcoming greater inhibition, but no direct evidence (e.g., low-level convergence, terrain lifting, or cold-pool processes breaking the CIN) is given. The PRD mode emphasizes daytime boundary-layer heating and sea-breeze convergence, yet no direct diagnosis of sea-breeze circulation or boundary-layer diurnal evolution is presented. Diurnally stratified composites of low-level wind and divergence, a sea-breeze index, or boundary-layer height analyses should be added.
3. Limited sample and climatic background restrict the generality of the conclusions. With only four years and 123 MCSs, the sample is small and may be influenced by particular interannual modes. The authors should provide year-by-year MCS frequencies and the proportions of the two types to show whether the results are dominated by any single year, discuss the representativeness of the 2019–2022 background (e.g., WNPSH intensity and position, South China Sea SST), and compare more quantitatively with the pre-summer MCS climatology. In addition, statistics on MCS duration, translation speed, and organizational mode should be added to characterize the differences between the two types more completely.Citation: https://doi.org/10.5194/egusphere-2026-5422-RC2 -
RC3: 'Comment on egusphere-2026-5422', Anonymous Referee #3, 20 Oct 2026
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General Comments
This study combines high-resolution radar mosaics and ERA5 reanalysis data to investigate non-TC mesoscale convective systems (MCSs) during the post-flood rainy season over South China. The regional comparison provides useful insights into the spatial distribution of MCSs, their associated dynamical and thermodynamic environments, and how these characteristics differ from those observed during the pre-summer rainy season. This work contributes to a more comprehensive understanding of convective activity over South China, with potential implications for forecasting convective weather and associated heavy rainfall. The manuscript is generally well organized, and the topic fits well within the scope of NHESS.
My main suggestions concern the criteria for classifying MCSs into different groups and the interpretation of the associated comparisons. These issues can be addressed through clarification of the existing results. I support the publication of this manuscript in NHESS after revision.
Major Comments
- The manuscript discusses regional differences in MCS characteristics and development mechanisms, but the criteria for assigning MCSs to the S-GX and PRD subregions need clarification. Since an MCS may initiate in one subregion and subsequently move into the other, please explicitly state how each MCS event is classified, particularly for systems crossing the 111.2°E boundary. In addition, please clarify the distinction between these analysis subregions and the geographical boundaries of the PRD shown in Fig. 1.
- The selection of non-MCS days with circulation patterns similar to those on MCS days is described in Section 2.3. Please clarify at the beginning of Section 3.2 that the environmental comparison is conducted between MCS and non-MCS days under similar large-scale circulation patterns. The qualification “selected non-MCS days with similar circulation” should also be retained in conclusion (2). to avoid generalizing the findings to all non-MCS days.
- The statement in Section 3.3.1 that S-GX MCSs exhibit stronger mid-level ascent would benefit from a more explicit comparison using Figs. 8–9. When each MCS category is evaluated within its respective occurrence region, the mean 500-hPa ω in Fig. 9 appears more negative for PRD MCS days over the PRD than for S-GX MCS days over S-GX. Please clarify whether the intended comparison is between event categories within a fixed region or between their respective occurrence regions, and reconcile the text with that comparison.
- The proposed development modes should be presented more cautiously in the Abstract and conclusion (3). While the composite analysis reveals differences in circulation, moisture transport, and instability, it does not directly quantify the contributions of daytime heating and sea-breeze convergence. I suggest using more qualified wording, such as “consistent with a greater role for local boundary-layer processes,” to ensure that the interpretations are adequately supported by the evidence.
Minor Comments
- Please simplify repeated regional labels in the Abstract, such as “S-GX MCSs over the S-GX” and “PRD MCSs over the PRD,” to improve readability.
- In the PRD discussion in Section 3.3.1, please correct figure references for ascent and low-level convergence. The citation to Fig. 7b, e follows a sentence discussing both quantities, although vertical velocity is shown in panels (a, d, g) and divergence in panels (b, e, h).
- Please check consistency between in-text citations and the reference list. Haberlie and Ashley is cited as 2022 in the Introduction, whereas the paper listed in the references is dated 2019. Please verify the intended source and make the citation and reference entry consistent.
Citation: https://doi.org/10.5194/egusphere-2026-5422-RC3 -
RC4: 'Comment on egusphere-2026-5422', Anonymous Referee #4, 24 Oct 2026
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General Comments
This manuscript uses high-resolution radar observations to investigate non-TC MCSs during the post-flood rainy season over South China. The study reveals interesting regional differences in convective timing and organization, while the environmental comparisons provide useful insights into the associated dynamical and thermodynamic conditions.
The manuscript is relevant to the scope of NHESS and has potential for publication after revision. My main suggestions concern the consistency between the MCS life-cycle descriptions and timing results, as well as the clarity of the statistical analyses and figure presentations. I support the publication of this manuscript in NHESS after these issues are addressed.
Major Comments
- Please ensure that the descriptions of MCS initiation and formation in the Abstract, Section 3.1, and conclusion (1) are consistent with the diurnal distributions in Fig. 3. In particular, Fig. 3a appears to show a pronounced midday-to-afternoon initiation peak over the PRD, which does not support the statement that PRD MCSs generally initiate from late night to morning. Please verify these statements and clearly distinguish the timing of convection initiation from subsequent MCS formation.
- Please clarify the sampling and grouping methods used in Fig. 9, particularly whether each boxplot sample represents an individual MCS event or a daily regional mean. Given that 72 non-TC MCSs occurred on 62 MCS days, please explain how days with multiple MCS events were handled. Also, ensure that the group labels are consistent with the regional classification used in the text.
- The Conclusions mention implications for operational forecasting, but these remain general. Please briefly discuss how the regional contrasts in circulation, moisture-transport depth, and preferred formation time could inform the monitoring and forecasting of MCS development. Potential applications should be distinguished from demonstrated improvements in forecast skill.
- Please further discuss how seasonal changes in large-scale circulation between the pre-summer and post-flood rainy seasons may influence MCS initiation, organization, and propagation over South China. Previous studies over the U.S. Great Plains have demonstrated pronounced seasonal differences in MCS environments and propagation characteristics (Song et al., 2019), as well as the importance of large-scale environmental conditions and propagating disturbances for MCS development (Feng et al., 2019; Song et al., 2021). A comparison with these studies would help strengthen the physical interpretation of seasonal differences in MCS activity over South China.
- Song, F., et al. (2019). Contrasting Spring and Summer Large-Scale Environments Associated with Mesoscale Convective Systems over the U.S. Great Plains. Journal of Climate, 32, 6749–6767. DOI: 10.1175/JCLI-D-18-0839.1
- Feng, Z., et al. (2019), Spatiotemporal Characteristics and Large-Scale Environments of Mesoscale Convective Systems East of the Rocky Mountains, Journal of Climate，DOI: 10.1175/JCLI-D-19-0137.1
- Song, F., et al. (2021). Crucial Roles of Eastward Propagating Environments in the Summer MCS Initiation Over the U.S. Great Plains. Journal of Geophysical Research: Atmospheres, 126(16), e2021JD034991. DOI: 10.1029/2021JD034991
Minor Comments
- In the Abstract, please clarify that the 123 identified MCSs include both TC-related and non-TC events, of which 72 are classified as non-TC MCSs and analyzed in this study.
- In Section 3.3.1, please revise the sentence beginning “Compared with PRD MCSs days...” as its current wording is grammatically unclear. Also, use “MCS days” rather than “MCSs days” consistently throughout the manuscript.
- Please introduce the abbreviation as “precipitable water (PW)” at its first use in the Section 3.2 discussion. It currently appears as PW before the full expression is introduced later in the same paragraph.
- In Section 3.3.2, the sentence beginning “Combined with the afternoon peak of PRD MCS formation...” is repeated consecutively. Please remove the duplication.
- The caption of Fig. 11 refers to cross sections in Fig. 2c, but these are not clearly identified in Fig. 2c or its caption. Please verify the reference and explicitly indicate the locations of the two fixed-longitude cross sections.
- Please make conclusion (3) more concise by reducing repeated numerical details and emphasizing the key findings supported by Figs. 7–10. Also, correct the incomplete sentence beginning “Together with larger convective instabilityin conclusion (3).
- In the Code and Data Availability section, please revise the sentence beginning “All relevant codes used in this study and the processed MCS dataset...” to correct its grammatical structure and clarify the availability of the processed dataset.
Citation: https://doi.org/10.5194/egusphere-2026-5422-RC4 -
RC5: 'Comment on egusphere-2026-5422', Anonymous Referee #5, 29 Oct 2026
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The comments are described in the supplement.
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General comments
The manuscript is well written, logically organized, and easy to follow. The authors present a clear research idea and develop it coherently and comprehensively throughout the paper. The topic is explored in sufficient depth, with appropriate discussion and analysis that support the study's aim. The overall structure contributes to the manuscript's readability, and the arguments are presented clearly and systematically. Overall, this is a well-prepared paper that thoroughly examines the research problem and makes a valuable contribution to the subject area. However, the manuscript requires minor revisions, including language editing, before it can be accepted for publication.
Specific comments and technical corrections:
Line 29: Avoid using words that already appear in the title as keywords (Non-typhoon mesoscale convective systems, Post-flood rainy season, South China)
Line 139: It is unclear what the authors intend to convey by the expression “no MCS”.
Line 370-371: The statement is incomplete and requires further clarification.