the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Oct 2025
On the Nationwide Variability of Low-Level Jets Prior to Warm-season Nocturnal Rainfall in China Revealed by Radar Wind Profilers
Abstract. Nocturnal rainfall initiation is closely linked to low-level jets (LLJs), but national-scale LLJ features over China—especially their evolution preceding warm-seasonal nocturnal rainfall—remain unknown due to scarce high-resolution vertical wind observations. Here, we reveal the multiscale responses of LLJs within 2 hours preceding the onset of nocturnal heavy rain (HR) and non-HR across four phases of rainy seasons in China during the warm season (April–October) of 2023–2024, utilizing data from a nationwide network of radar wind profilers (RWPs) in combination with surface observations and reanalysis data. Results show that nocturnal rainfall accounted for over 50 % of warm-season rainfall, with 56 % preceded by LLJs within 2 hours leading up to their onset. In monsoon regions, ~40 % of nocturnal HR (LLJ_HR) were LLJ-associated and had higher heavy rainfall probability than non-LLJ_HR. Critically, these LLJ_HR events depended on rapid, coupled minute-scale dynamical intensification, typically occurring 30 to 120 minutes before rainfall initiation. Specifically, coherent changes in LLJ wind profiles, frequency, vertical wind shear, and pronounced evolution in jet height were observed, operating in synergy with substantial thermodynamic instability. This behavior stood in sharp contrast to LLJ_non-HR events, which were characterized by relatively stable, weakening, or declining LLJ evolution and an inadequate thermodynamic response. These findings demonstrate that a minute-scale 'rapid reorganization' of dynamic and thermodynamic conditions driven by swift evolution of the LLJ is essential for nocturnal HR, providing observational constraints for regional model parameterizations and nowcasting.
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Status: open (until 27 Nov 2025)
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RC1: 'Comment on egusphere-2025-4939', Anonymous Referee #1, 08 Nov 2025
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The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-4939/egusphere-2025-4939-RC1-supplement.pdfReplyCitation: https://doi.org/
10.5194/egusphere-2025-4939-RC1 -
RC2: 'Comment on egusphere-2025-4939', Anonymous Referee #2, 23 Nov 2025
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Summary:
This study utilizing a nationwide network of radar wind profilers to reveal the different responses of LLJs within 2 hours preceding the onset of nocturnal HR and non-HR events during the warm season in China. Through the analysis of four Phases, the authors analyze the general characteristics, minute-scale and thermodynamic evolution between the nocturnal HR and non-HR associated with LLJs. In summary, this manuscript is well structured with detailed figures. Several statistical findings provide a comprehensive summary of the effect of LLJ in nocturnal HR and non-HR events, which offer a valuable insight and mechanism analysis for future nowcasting at the national scale. However, some critical issues remain unresolved. The manuscript would be significantly strengthened by a more in-depth analysis of the physical mechanisms underlying the distinct LLJ-rainfall relationships across different phases and regions. Furthermore, the statistical significance and physical meaning of some presented differences are not fully convincing and require further justification. The manuscript also has some typographical issues and typos, still need the authors to check carefully. I suggest major revisions before the manuscript can be accepted.
Major Comments:
- This work valuably extends the authors' previous research from North China to a national scale. To contextualize this new contribution, a dedicated discussion about differences and similarities with those from your previous regional study would be highly beneficial for readers.
- L141-145: The method of quality control in unclear. The authors mention that eliminated the observation during rainfall periods but don’t give an explanation. I recommend explaining it to help the readers to understand. And in the second step, the authors don’t clarify the standard of outliers, is it same as the standard in the third step? It is necessary to adjust the order to describe the definition of outliers.
- Is it reasonable to set the threshold of exceeding the 75th percentile of all rainfall events to define the HR events? The authors can supply that is it previous studies used this standard?
- Section 2.4: The identification of LLJs in this work doesn’t consider the BLJs and SLLJs. However, you investigated the BLJ (SLLJ)-induced rainfall event in your previous work. Why you just count the LLJs in this study? Additionally, the authors don’t consider the position of LLJs. More study focuses on the upstream areas of rainfall to study the effect of LLJs. Is it reasonable to identify rainfall events within a 25-km radius of RWP. Is it a reason that only a little more than half of HR preceded by LLJs.
- The authors emphasize the importance of LLJs prior to nocturnal rainfall. However, the statistics show that only 56% of HR preceded by LLJs and less than half (40%) of nocturnal HR were associated with LLJs. Moreover, some regions in Fig. 6 show that more HR event are not associated with LLJs, particularly in ROI-4 and ROI-1. The relationship between HR and LLJs seems not close, how the authors consider the contradictory between the importance of LLJs and the small proportion of LLJ_HR.
- The authors mention the nocturnal rainfall accounted for 51.6% of total warm-season rainfall in Fig. 2g (L210-211). However, the figure depicts the ratio of 50.9%. Moreover, the caption in Figs. 2g-2i show that it is the contribution ratios of heavy rainfall. So, I recommend the authors using nocturnal heavy rainfall in L215-217.
- The legend of all events in Fig 4a is easy for readers to misunderstand. I recommend modifying it to all nocturnal events. In L240-244, the ratio of non-LLJ and LLJ in HR events are lower than all nocturnal events, both the four ROIs occur the same reduction. Why the authors state the statistics demonstrate the crucial role of LLJs in nocturnal rainfall?
- L247-249: The number of nocturnal HR events are mistake. The authors plus LLJ_HR, LLJ_non-HR, and non-LLJ_HR. Actually, the LLJ_non-HR is redundant, and the number of HR events are smaller than the article described. Additionally, the red column of LLJ_HR in Fig. 4b is smallest, more events are LLJ_non-HR and non-LLJ_HR, how to explain this phenomenon.
- How do the authors define the heavier rainfall in L259? Why the authors consider the differences were less pronounced only during Phases 2 and 4.
- Fig 5f. shows the Phase 2 with non-LLJ_HR events. The high values of rainfall intensity appear at the North China, rather than Yangtze River Basin. How this explain?
11.The authors just describe some figures’ features in Section 3.2 but don’t explain its possible reasons and mechanisms. How are the differences in the relationship between different phases and LLJs? And why the statistics show these results? For example, why LLJs featured a bimodal vertical distribution with frequent occurrence layers and just a single peak in non-HR events? Why the height of LLJ_HR events is lower than non-HR events in Phase 2?
12. The Jet height in ROI-2 experienced a process that initially declined followed by a rise in Fig. 7b. However, why this trend doesn’t occur in the wind profiles in Fig.8b? It depicts a modest decrease in jet height as time moves. Does the averaging process in Fig. 8b smooth out the signal of this vertical redistribution? And wind speed decreased with the time close to the rainfall occur, so why the LLJs didn’t strengthen before rainfall?
13. In Fig. 10c, the wind blow from high values of equivalent potential temperature to low values. Why the authors consider it’s the interacting with cold air advection from westerly troughs in L376?
14. The wind rotates counterclockwise near the Sichuan Basin. Why the authors describe a stronger anticyclonic circulation in L383? Additionally, the easterly wind encounter the orographic forcing will be uplifted to promote nocturnal HR, but why this happen on the eastern lee slope rather than windward slope?
15. The differences in thermodynamic fields and MFD between LLJ_HR and LLJ_non-HR events in Figs. 10 and 11 appear subtle. The authors should more explicitly guide the reader to the key differentiating features in these figures.
16. The authors don’t clarify why they introduce the LLJ index and VWS in L390-391. And these two variables need to be obtained through calculation. Thus, I recommend the authors to explain not only the calculation method but also the reason to select these two variables. A more robust rationale for this index will be favorable for readers to understand.
17. The difference in LLJ index is not significant in Fig. 12. Compared to your previous study in the North China, LLJ index doesn’t surge before rainfall occur in this study. Thus, why the curve shows a small difference between HR and non-HR events and why it’s gentle before rainfall? I did not see significant difference in LLJ behaviors between HR and non-HR events. Their behaviors are also varied in different regions and phases, which are not explained.
Minor Comments:
- L122: The meaning of MFC is unclear. I suggest using the whole name of “moisture flux convergence (MFC)”.
- L180-181: The time in the North China Rainy Season is inverted. It should be July 15 to August 31, 2023, and July 22 to August 31, 2024, which are continuous in time.
- L212: The font is different in “In contrast,”.
- L335: Is it the higher probability above 1.7 km? The peak of LLJ_non-HR seems at nearly 1.5 km in Fig. 9g.
- L378: kg m-2s-1 -> kg m-2 s-1
- L427: 1.5 m s-1 -> 1.5 s-1
- L435: Phase 4. -> Phase 4
- Some mistakes occur in the caption of Fig. 1: There are two ROI-3 in Line 739; The rain gauges depicted with blue dots and the RWP seems a red circle rather than red star in Fig. 1b.
- I recommend adding the title in each column in Fig. 3. For example, “Daytime” over the Fig. 3a, and “Nighttime” over the Fig. 3e.
- I do not think that the Fig. 4b is same as panel (a) in L778, and (b) is not for nocturnal HR events, the authors also count the non-LLJ_non-HR events.
- L796: I suggest adding color description of pie chart in caption. LLJ-HR (red) and non-LLJ_HR (blue).
- The reference vector in Fig. 10 at the lower-left corner is incorrect. It should be 10 m s-1.
- L827: 850 Pa -> 850 hPa
- L831: unit: kg m-2 s-1; 1 hour -> 1-hour
- The authors use “LLJ-HR” in the caption of some Figures to describe the LLJs associated with HR. I recommend using the same abbreviation as in the main text. For example, in L781, 792, 796, 802, 808, 815, 824, 838, and 841. Modifying the “LLJ-HR” to “LLJ_HR”.
Citation: https://doi.org/10.5194/egusphere-2025-4939-RC2
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