the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Mar 2024
Revisiting the evolution of downhill thunderstorms over Beijing: A new perspective from radar wind profiler mesonet
Abstract. Downhill thunderstorms frequently occur in Beijing during the rainy seasons, leading to substantial precipitation. The accurate intensity prediction of these events remains a challenge, partly attributed to insufficient observational studies that unveil the thermodynamic and dynamic structures along the vertical direction. This study provides a comprehensive methodology for identifying both enhanced and dissipated downhill thunderstorms. In addition, a radar wind profiler (RWP) mesonet has been built in Beijing to characterize the pre-storm environment downstream to the thunderstorms at the mountain foot. This involves deriving vertical distributions of high-resolution horizontal divergence and vertical motion from the horizontal wind profiles measured by the RWP mesonet. A case of enhanced downhill thunderstorm on 28 September 2018 is carried out, which support the idea that the enhanced southerly flow and corresponding convergence detected by the RWP mesonet could favorably support the development of thunderstorms in the afternoon. The results also indicate that low-level convergence is an effective signal in accounting for convective maintenance. Statistical analysis based on radar reflectivity from April to September 2018–2021 have shown that a total of 63 thunderstorm events tend to be enhanced after moving into downhill and urban areas, accounting for about 66 % of the total number of downhill thunderstorm events. A critical region for intensified thunderstorms lies on the downslope side of the mountains west to Beijing. The evolution of the downhill storm is associated with the dynamic conditions over the plain compared to its initial morphology. The lifting induced by stronger westerly winds and vertical shear in the low and midlevel troposphere exerts a critical influence on the enhancement of convection. The findings underscore the significant role of RWP network in elucidating the evolution of downhill storm.
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RC1: 'Comment on egusphere-2024-707', Anonymous Referee #1, 22 Mar 2024
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This paper provides an examination of the evolution of downhill thunderstorms over Beijing by using a radar wind profiler (RWP) mesonet. The results elucidate the storm's dynamic structures with a focus on both enhanced and dissipated events. The usage of high-resolution horizontal divergence and vertical motion data from the RWP mesonet to characterize the pre-storm environment represents a significant advancement in understanding these meteorological phenomena. The detailed case study, along with statistical analyses spanning the warm seasons of 2018 to 2021, contribute to our understanding of the factors influencing thunderstorm intensity and evolution in this region. Thus, I recommend the publication of this paper in Atmospheric Chemistry and Physics after some minor corrections for clarification.
Minor Comments:
1. The discussion on the implications of these findings for weather forecasting and model improvements may be further strengthened. Expanding this discussion could enhance the practical relevance of the research, suggesting pathways for incorporating these observations and methodologies into weather forecast models.
2. As suggested by the statistical analysis, urbanization effects may play a potential role in the enhancement of downhill thunderstorms. The authors may include a more detailed discussion of the process-level mechanism due to the importance of this effect.
3. The methodology for identifying downhill thunderstorms has been clearly described with a thorough explanation of its criteria. It may be beneficial to clarify on how this methodology compares or improves upon existing approaches.
4. The authors may strengthen the discussion regarding the pre-convective environment for the downhill thunderstorms. Some discussions for the statistical characterizations for the humidity and temperature profiles may be helpful.Citation: https://doi.org/10.5194/egusphere-2024-707-RC1 -
RC2: 'Comment on egusphere-2024-707', Anonymous Referee #2, 24 Apr 2024
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Summary:
The authors undertook a comparative analysis of intensified and dissipating downhill thunderstorms utilizing a radar wind profiler mesonet. This research holds considerable importance for forecasting precipitation in Beijing. The comprehensive vertical observations facilitated by the radar wind profiler mesonet offer a valuable opportunity to scrutinize the thermodynamic and dynamic evolution mechanisms of downhill thunderstorms. While the manuscript is generally well-written, I have identified several concerns detailed below. Therefore, I recommend this paper would be accepted after major revision.
Major comments and concerns:
- The primary emphasis of this study lies in the examination of a case study on intensified downhill thunderstorms. However, the statistical analysis section appears somewhat abbreviated, considering its significance as the main focus of the research. To address this concern, the reviewer suggests expanding the discussion on the statistical analysis. In addition, it would be beneficial to select two representative cases each for both intensified and dissipated downhill thunderstorms.
- The proximity of Beijing's topography undoubtedly plays a pivotal role in the dynamics of downhill thunderstorms. However, the present study appears to provide less emphasis on the terrain effect. It is essential to delve deeper into how the terrain influences the dynamics throughout the processes under investigation.
- It would be beneficial for the authors to present the trajectories and their moving directions of two types of downhill storms.
- The distinctions in divergence and vorticity appear subtle, as indicated by the overlapping ranges of blue and red shadings between their maximum and minimum values in Figs. 8e and 8f. Specifically, the vorticity preceding Enhanced Downhill Storms (EDSs) appears weaker compared to that preceding Dissipated Downhill Storms (DDSs). The authors attribute this difference to the mountain-valley wind breeze. However, this explanation is challenging to grasp.
- What factors contribute to the prolonged duration of downhill storms that reach plain areas in the late afternoon? Additionally, could you provide the number of cases that arrive at plain areas in the late afternoon and early morning, respectively?
- The representativeness of this case needs clarification. While low-level convergence is highlighted as an effective signal for convective maintenance in this instance (Lines 33-34), this finding is not observed in the statistical section.
- Lines254-256: A more comprehensive quantitative analysis is warranted to elucidate how cold-pool-induced horizontal vorticity overpowers over low-level wind shear before 1400 LST. Furthermore, could you provide an explanation for why this overpowering effect diminishes after 1400 LST?
Minor comments:
- The warm advection induced by veering of winds can also be observed in DDSs (Figs. 8c and 8d).
- The triangles 3-4 and the RWPs at BWO, PG, and SDZ are not utilized in the present analysis. Why then do authors reference the RWP mesonet?
- Line 332: "And" should be avoided at the beginning of a sentence in formal writing.
- Lines 368, 370, and 376: The “figure 8b”, “figure 8f”, and “figure 8a” should be “Figure”.
- Line 31: which support -> supporting.
- Figure 8: The legend of red and blue lines should be included in the figure.
Citation: https://doi.org/10.5194/egusphere-2024-707-RC2
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