Lightning-intense deep convective transport of water vapour into the UTLS over the Third Pole region

Singh, Prashant; Ahrens, Bodo

doi:10.5194/egusphere-2025-1728

Preprints

https://doi.org/10.5194/egusphere-2025-1728

Preprints

15 May 2025

| 15 May 2025

Lightning-intense deep convective transport of water vapour into the UTLS over the Third Pole region

Prashant Singh and Bodo Ahrens

Abstract. The Himalayas are known to be prominent locations for lightning-intense deep convective systems. Deep convective systems can transport significant amounts of water vapour into the upper troposphere and lower stratosphere (UTLS). Lightning data from the TRMM-LIS observation over 10 years, along with water vapour data from ERA5 reanalysis and satellite observations (AIRS, MLS), point to a possible link between the lightning-intense deep convective systems and water vapour in the UTLS region. We used the ICON-CLM at km-scale to investigate the transport of water vapour by lightning-intense deep convective systems. A year-long simulation indicates an increase in water vapour concentration during lightning events in the upper troposphere (∼200 hPa). This finding is also supported by ERA5, AIRS, and MLS. Noticeably, ERA5 overestimates water vapour increases, especially during the monsoon period. A Lagrangian analysis of air parcels for over 1,600 lightning events, using ERA5 and ICON-CLM data, reveals that ERA5 transports significantly more air parcels to the upper troposphere than ICON-CLM over the Third Pole region. The air parcels in the coarser-meshed (∼30 km) convection-parameterized ERA5 data rise slowly, cross the Himalayas and reach the upper troposphere over the Tibetan Plateau. In contrast, the km-scale convection-permitting ICON-CLM shows fast vertical and less horizontal transport for the same events. In general, simulated lightning-intense deep convective events moisten the upper troposphere, but only a few instances result in direct moistening of the lower stratosphere over the Third Pole. Once an air parcel reaches the upper troposphere, its fate depends on synoptic circulation.

Received: 10 Apr 2025 – Discussion started: 15 May 2025

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Journal article(s) based on this preprint

08 Dec 2025

Lightning-intense deep convective transport of water vapour into the UTLS over the Third Pole region

Prashant Singh and Bodo Ahrens

Atmos. Chem. Phys., 25, 17869–17888, https://doi.org/10.5194/acp-25-17869-2025,https://doi.org/10.5194/acp-25-17869-2025, 2025

Short summary

Prashant Singh and Bodo Ahrens

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-1728', Anonymous Referee #1, 17 Jun 2025

Review of "Lightning-intense deep convective transport of water vapour into the UTLS over the Third Pole region", by Prashant Singh and Bodo Ahrens, submitted to Atmospheric Chemistry and Physics (ACP)
This paper investigates the role of lightning-associated convection in transporting water vapour into the upper troposphere and lower stratosphere (UTLS) over the Himalayas and Tibetan Plateau ("Third Pole") region. The authors use lightning data from the Tropical Rainfall Measuring Mission (TRMM-LIS), along with forward trajectories derived from ERA5 reanalysis and high-resolution ICON-CLM simulations, to track moist air masses. The goal of the authors is to assess their contribution to the well-documented water vapour enhancement observed by MLS and ACE-FTS (which are more appropriate than AIRS) within the Asian Summer Monsoon (ASM) anticyclone.
It is well known that, in the tropical lower stratosphere, and also within monsoon systems, the stratospheric water vapour entry values are primarily controlled by the freeze-drying of moist tropospheric air at the cold point tropopause (CPT) (Brewer, 1949; Randel and Park, 2019; Smith et al., 2021; see also the introductions of the Ploeger et al. or Clemens et al. papers cited in your paper). Deep convection that directly crosses the tropopause is also under debate but remains much more difficult to quantify. As a result, all statements related to the stratosphere in this paper remain very qualitative; even the position of the WMO tropopause or the cold point tropopause is completely ignored. In my view, no robust conclusions can be drawn from this study regarding any impact on stratospheric water vapour.
Unfortunately, even regarding the upper troposphere, the findings are quite weak, especially when compared to earlier studies such as Price et al. or Singh and Ahrens (2023). The correlation between enhanced upper tropospheric water vapour and lightning counts is not new, and actually appears more clearly in daily data than in strongly averaged climatologies such as Fig. 1. Even the domain-averaged daily time series (Fig. 2) show large inconsistencies, with unexplained spikes between 15 March and 15 June. The correlation coefficients are actually weakest during the monsoon time (Table 3), the time period when intense thunderstorm activity is expected.
The only truly new contribution, in my view, is the comparison of trajectory behavior between ERA5 and ICON-CLM, as you also highlight in your abstract. While the coarser-meshed (~30 km) convection-parameterized ERA5 data show slow ascent, with air parcels crossing the Himalayas and reaching the upper troposphere over the Tibetan Plateau, the convection-permitting km-scale ICON-CLM model reveals faster vertical and more direct transport for the same events (Figs. 3, 4, and 5).
However, there is significant potential to improve the presentation and interpretation of these results. For example, the color bar in Fig. 5 is not readable, and the visual contrast makes interpretation difficult. Moreover, interpreting the highest trajectory points in Figs. 3d and 3g as being in the stratosphere seems, at best, an overinterpretation. Without proper reference to the cold point or WMO tropopause, such a claim cannot be supported with confidence.
In your conclusions, you attempt to link your findings to the recently identified significant wet bias in the lowermost stratosphere in climate models (Charlesworth et al., 2023; Ploeger et al., 2024). However, your results are strongly confined to the upper troposphere. Furthermore, the wet bias in ERA5 upper tropospheric water vapour, diagnosed in your paper by comparison with MLS and AIRS data, is also present in the high-resolution ICON-CLM model, which you otherwise describe as more physically realistic in terms of vertical transport along trajectories. This is really confusing.
Given these concerns, I can only recommend rejection of the current version. The manuscript would need to be fundamentally rewritten. Possibly, Figs. 3, 4, and 5 could serve as a starting point for a completely new and more focused version.
A few other important points:
**Introduction**

If you want to make claims about the stratosphere, large parts of the introduction would need to be rewritten to reflect the relevant processes and literature more accurately.
**Singh 2015**

This reference appears to be grey literature and, in my view, should not be used in a peer-reviewed journal submission.
**Lagrangian Tracking**

There is no proper citation of the Lagrangian trajectory tool used in the study. I strongly recommend using a well-established and widely cited tool such as FLEXPART or MPTRAC for this type of analysis.

Citation: https://doi.org/10.5194/egusphere-2025-1728-RC1
- AC2: 'Reply on RC1', Prashant Singh, 14 Aug 2025
  
  Thank you for your valuable comments and suggestions on our manuscript. We greatly appreciate your insights, which have been helpful in improving our work. Our detailed responses are provided in the attached PDF.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1728-AC2
RC2:
'Comment on egusphere-2025-1728', Anonymous Referee #2, 01 Jul 2025

The study focuses on water vapour transport mechanisms driven by lightning-induced deep convection in the Third Pole region, a global lightning hotspot where UTLS (upper troposphere and lower stratosphere) water vapour exchange significantly impacts climate. The research addresses critical regional and global climatic implications.The combination of multi-source data (TRMM-LIS, ERA5, AIRS, MLS) with km-scale ICON-CLM modeling and Lagrangian tracking creates a robust "observation-reanalysis-model" validation framework, demonstrating methodological advancement.The logical and the results are reasonable. The structure is clear, and the writing is well. However, there are some concerns about this study.

1.Lightning datasets (TRMM-LIS, ISS-LIS) span long periods, but the impact of temporal discrepancies (TRMM: 1995–2015; ISS-LIS: 2019–2020) on result consistency requires clarification.

2.Incorporate recent (post-2023) observational studies on deep convection in the Third Pole (e.g., ground-based radar or satellite retrievals) to enhance timeliness.

3. Correlation analysis (e.g., Table 2) shows the highest lightning-UT water vapour correlation over the Tibetan Plateau (r = 0.78). Whether the region’s low lightning frequency (<4 flashes/km²/year) affects statistical significance?

Citation: https://doi.org/10.5194/egusphere-2025-1728-RC2
- AC1: 'Reply on RC2', Prashant Singh, 14 Aug 2025
  
  Thank you for your valuable comments and suggestions on our manuscript. We greatly appreciate your insights, which have been helpful in improving our work. Our detailed responses are provided in the attached PDF.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1728-AC1

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2025-1728', Anonymous Referee #1, 17 Jun 2025

Review of "Lightning-intense deep convective transport of water vapour into the UTLS over the Third Pole region", by Prashant Singh and Bodo Ahrens, submitted to Atmospheric Chemistry and Physics (ACP)
This paper investigates the role of lightning-associated convection in transporting water vapour into the upper troposphere and lower stratosphere (UTLS) over the Himalayas and Tibetan Plateau ("Third Pole") region. The authors use lightning data from the Tropical Rainfall Measuring Mission (TRMM-LIS), along with forward trajectories derived from ERA5 reanalysis and high-resolution ICON-CLM simulations, to track moist air masses. The goal of the authors is to assess their contribution to the well-documented water vapour enhancement observed by MLS and ACE-FTS (which are more appropriate than AIRS) within the Asian Summer Monsoon (ASM) anticyclone.
It is well known that, in the tropical lower stratosphere, and also within monsoon systems, the stratospheric water vapour entry values are primarily controlled by the freeze-drying of moist tropospheric air at the cold point tropopause (CPT) (Brewer, 1949; Randel and Park, 2019; Smith et al., 2021; see also the introductions of the Ploeger et al. or Clemens et al. papers cited in your paper). Deep convection that directly crosses the tropopause is also under debate but remains much more difficult to quantify. As a result, all statements related to the stratosphere in this paper remain very qualitative; even the position of the WMO tropopause or the cold point tropopause is completely ignored. In my view, no robust conclusions can be drawn from this study regarding any impact on stratospheric water vapour.
Unfortunately, even regarding the upper troposphere, the findings are quite weak, especially when compared to earlier studies such as Price et al. or Singh and Ahrens (2023). The correlation between enhanced upper tropospheric water vapour and lightning counts is not new, and actually appears more clearly in daily data than in strongly averaged climatologies such as Fig. 1. Even the domain-averaged daily time series (Fig. 2) show large inconsistencies, with unexplained spikes between 15 March and 15 June. The correlation coefficients are actually weakest during the monsoon time (Table 3), the time period when intense thunderstorm activity is expected.
The only truly new contribution, in my view, is the comparison of trajectory behavior between ERA5 and ICON-CLM, as you also highlight in your abstract. While the coarser-meshed (~30 km) convection-parameterized ERA5 data show slow ascent, with air parcels crossing the Himalayas and reaching the upper troposphere over the Tibetan Plateau, the convection-permitting km-scale ICON-CLM model reveals faster vertical and more direct transport for the same events (Figs. 3, 4, and 5).
However, there is significant potential to improve the presentation and interpretation of these results. For example, the color bar in Fig. 5 is not readable, and the visual contrast makes interpretation difficult. Moreover, interpreting the highest trajectory points in Figs. 3d and 3g as being in the stratosphere seems, at best, an overinterpretation. Without proper reference to the cold point or WMO tropopause, such a claim cannot be supported with confidence.
In your conclusions, you attempt to link your findings to the recently identified significant wet bias in the lowermost stratosphere in climate models (Charlesworth et al., 2023; Ploeger et al., 2024). However, your results are strongly confined to the upper troposphere. Furthermore, the wet bias in ERA5 upper tropospheric water vapour, diagnosed in your paper by comparison with MLS and AIRS data, is also present in the high-resolution ICON-CLM model, which you otherwise describe as more physically realistic in terms of vertical transport along trajectories. This is really confusing.
Given these concerns, I can only recommend rejection of the current version. The manuscript would need to be fundamentally rewritten. Possibly, Figs. 3, 4, and 5 could serve as a starting point for a completely new and more focused version.
A few other important points:
**Introduction**

If you want to make claims about the stratosphere, large parts of the introduction would need to be rewritten to reflect the relevant processes and literature more accurately.
**Singh 2015**

This reference appears to be grey literature and, in my view, should not be used in a peer-reviewed journal submission.
**Lagrangian Tracking**

There is no proper citation of the Lagrangian trajectory tool used in the study. I strongly recommend using a well-established and widely cited tool such as FLEXPART or MPTRAC for this type of analysis.

Citation: https://doi.org/10.5194/egusphere-2025-1728-RC1
- AC2: 'Reply on RC1', Prashant Singh, 14 Aug 2025
  
  Thank you for your valuable comments and suggestions on our manuscript. We greatly appreciate your insights, which have been helpful in improving our work. Our detailed responses are provided in the attached PDF.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1728-AC2
RC2:
'Comment on egusphere-2025-1728', Anonymous Referee #2, 01 Jul 2025

The study focuses on water vapour transport mechanisms driven by lightning-induced deep convection in the Third Pole region, a global lightning hotspot where UTLS (upper troposphere and lower stratosphere) water vapour exchange significantly impacts climate. The research addresses critical regional and global climatic implications.The combination of multi-source data (TRMM-LIS, ERA5, AIRS, MLS) with km-scale ICON-CLM modeling and Lagrangian tracking creates a robust "observation-reanalysis-model" validation framework, demonstrating methodological advancement.The logical and the results are reasonable. The structure is clear, and the writing is well. However, there are some concerns about this study.

1.Lightning datasets (TRMM-LIS, ISS-LIS) span long periods, but the impact of temporal discrepancies (TRMM: 1995–2015; ISS-LIS: 2019–2020) on result consistency requires clarification.

2.Incorporate recent (post-2023) observational studies on deep convection in the Third Pole (e.g., ground-based radar or satellite retrievals) to enhance timeliness.

3. Correlation analysis (e.g., Table 2) shows the highest lightning-UT water vapour correlation over the Tibetan Plateau (r = 0.78). Whether the region’s low lightning frequency (<4 flashes/km²/year) affects statistical significance?

Citation: https://doi.org/10.5194/egusphere-2025-1728-RC2
- AC1: 'Reply on RC2', Prashant Singh, 14 Aug 2025
  
  Thank you for your valuable comments and suggestions on our manuscript. We greatly appreciate your insights, which have been helpful in improving our work. Our detailed responses are provided in the attached PDF.
  
  Citation: https://doi.org/10.5194/egusphere-2025-1728-AC1

Peer review completion

AR – Author's response | RR – Referee report | ED – Editor decision | EF – Editorial file upload

AR by Prashant Singh on behalf of the Authors (14 Aug 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (15 Aug 2025) by Marc von Hobe

RR by Anonymous Referee #1 (01 Sep 2025)

Suggestions for revision or reasons for rejection

Second Review of "Lightning-intense deep convective transport of water vapour into the UTLS over the Third Pole region", by Prashant Singh and Bodo Ahrens, submitted to Atmospheric Chemistry and Physics (ACP)

The paper has improved significantly. In particular, the focus on the advantages gained through the ICON-CLM simulations—especially in comparison to ERA5—is convincing. However, it is still not a proof that Lagrangian trajectories driven by ICON-CLM winds result in moistening of the stratosphere. The freeze-drying effect along the trajectories is not considered, so only qualitative statements are possible.

Another issue remains with the application of the Lagrangian method and, in my opinion, an insufficient description of how it is used to reconstruct stratospheric water vapour transport. Specifically, the freeze-drying process and the ascent of trajectories into the stratosphere above the level of zero convective heating are key to our understanding of water vapour entry into the lower stratosphere. I do not suggest that the authors must implement all these calculations; however, without them, no quantitative statements about moistening of the lower stratosphere can be supported.

A minimum requirement, in my opinion, is to at least **describe these processes** in the introduction, because they form the basis of our understanding of water vapour transport into the stratosphere, and the authors rely on Lagrangian methods for their most important conclusions.

Therefore, I would still recommend **major revision**, due to the following points:

**Abstract:**
Please replace: "result in direct moistening" with "may lead to direct moistening" to better reflect the uncertainty in the findings.

**Introduction:**
I still miss a section describing the Lagrangian methods used to quantify the transport of water vapour into the stratosphere, particularly by identifying the coldest temperatures encountered along troposphere-to-stratosphere trajectories (Lagrangian cold points, LCPs). See, for example: Fueglistaler et al.; Fueglistaler and Haynes, 2005; Liu et al., 2010; Schoeberl and Dessler, 2011; Smith et al., 2021. This is our current understanding of how moistening of the stratosphere happens.

Here, you could also explain the concept of the level of zero radiative heating—only above this level do trajectories typically begin their slow ascent into the stratosphere, and after freeze-drying at the LCP, air parcels acquire their final amount of water vapour for entry into the stratosphere. Based on this framework, overshooting convection may indeed play a role (see, e.g., Avery et al., 2017; Ueyama et al., 2020; Jensen et al., 2020; Ueyama et al., 2023; Homeyer et al., 2023), and perhaps with ICON-CLM it is possible to represent this process in the model.

**A few other minor but important points:**

– **Figure 1:** Certainly an improvement. However, the cold-point tropopause appears to be closer to 90 hPa than 100 hPa. Also, when using Lagrangian modeling, the Lagrangian cold point is more relevant than the Eulerian cold-point tropopause. This further supports the point that your statements regarding transport into the stratosphere remain rather qualitative.

– **Figure 4:** This figure is not well explained. Panels (b) and (e) cannot be the initial points—they are clearly not the same. I think these panels show trajectory positions after some time has passed. Here, by the way, the results of ICON-CLM (localized convection, strong vertical motion, and short timescales), in comparison to ERA5-driven calculations, are presumably more physically realistic. In my opinion, this is the strongest part of your paper.

---

**Final remarks:**
I think the lightning-based approach is certainly innovative and shows potential as a new source of information related to the UTLS water vapour budget. Because of this, the work is worth publishing. The points mentioned above can certainly be clarified with some effort.

Hide

ED: Reconsider after major revisions (04 Sep 2025) by Marc von Hobe

AR by Prashant Singh on behalf of the Authors (16 Oct 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (21 Oct 2025) by Marc von Hobe

RR by Anonymous Referee #1 (27 Oct 2025)

ED: Publish as is (27 Oct 2025) by Marc von Hobe

AR by Prashant Singh on behalf of the Authors (03 Nov 2025) Manuscript

Journal article(s) based on this preprint

08 Dec 2025

Lightning-intense deep convective transport of water vapour into the UTLS over the Third Pole region

Prashant Singh and Bodo Ahrens

Atmos. Chem. Phys., 25, 17869–17888, https://doi.org/10.5194/acp-25-17869-2025,https://doi.org/10.5194/acp-25-17869-2025, 2025

Short summary

Prashant Singh and Bodo Ahrens

Interactive computing environment

Water Vapour Transport in the Upper Troposphere and Lower Stratosphere via Lightning-Intense Deep Convective Systems in the Third Pole Region Prashant Singh and Bodo Ahrens https://doi.org/10.5281/zenodo.15090109

Prashant Singh and Bodo Ahrens

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Short summary

Intense deep convective clouds (e.g. lightning events) can rapidly move water vapour and other gases into the upper troposphere. The Third Pole region, especially the Himalayas, frequently experiences such storms. ICON-CLM (3.3 km) and ERA5 reanalysis data (30 km), these convective events can lift water vapour into the upper troposphere but rarely into the lower stratosphere in the Third Pole. After reaching the upper troposphere, the water vapour tends to move horizontally away from the region.


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