the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 23 Jan 2025
Evidence of Tropospheric Uplift into the Stratosphere via the Tropical Western Pacific Cold Trap
Abstract. Understanding air mass sources and transport pathways in the Tropical Western Pacific (TWP) is crucial for determining the origins of atmospheric constituents in the stratosphere. This study uses lidar and ballon observations in Koror, Palau, and trajectory simulations to study the upward transport pathway over the TWP in the upper troposphere and lower stratosphere (UTLS). During northern hemisphere winter, the region experiences the highest relative humidity with respect to ice (RHi) and the lowest temperatures (<185 K) at 16–18 km, and is called the "cold trap". These conditions lead to water vapor condensation, forming thin cirrus clouds. Latent heat released during cloud formation drives the ascent of air masses.
A case study in December 2018 shows a subvisible cirrus cloud layer (optical depth < 0.03) measured by lidar, coinciding with high supersaturation (RHi > 150 %) observed by radiosonde. Trajectories initiated from the cloud layers confirm that air masses ascend slowly from the troposphere into the stratosphere primarily during northern hemisphere winter. In contrast, lidar measurements show similar cloud layers during a summer case (August 2022) with warmer temperatures and drier conditions, where air descends after cloud formation indicated by the forward trajectory. Among all cirrus clouds observed in December and August, 46 % of air masses rise above 380 K after cloud formation in December, compared to only 5 % in August, possibly influenced by the Asian summer monsoon. These findings underscore the importance of the cold trap in driving air mass transport and water vapor transformations in the UTLS.
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Interactive discussion
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RC1: 'Comment on egusphere-2024-3981', Anonymous Referee #1, 10 Feb 2025
This manuscript presents a well-structured and scientifically significant study on air mass transport pathways over the Tropical Western Pacific (TWP), with a particular focus on the role of the cold trap in the upper troposphere and lower stratosphere (UTLS). Using lidar and balloon-borne observations from Koror, Palau, combined with trajectory simulations, the authors provide valuable insights into seasonal variations in air mass ascent and dehydration processes.
The study is particularly relevant as it enhances our understanding of how air masses transition from the troposphere to the stratosphere, a process fundamental for stratospheric water vapor balance and global climate dynamics. The authors effectively demonstrate the stark contrast between winter (December 2018) and summer (August 2022) conditions, showing that upward transport through the cold trap occurs primarily in winter, while summer air masses tend to descend. The inclusion of quantitative metrics, such as the fraction of air masses reaching above 380 K potential temperature, strengthens the findings and makes a compelling case for the seasonal dependency of stratospheric entry pathways.
Overall, this is an important and well-executed study that deserves publication. I only have a few minor suggestions regarding clarity, phrasing, and data presentation that could further improve the manuscript. These are detailed below:
(151). The use of two case studies (December 2018 and August 2022) allows for a seasonal comparison, adding depth and clarity to the analysis. However, in both case studies there is no description of the meteorological conditions that would have provided additional information about the local context in which the measurements were taken.
(176). There is no information here on where precisely the backtrajectories are initiated. Midcloud maybe? One trajectory per cloud? Clusters? You may consider to shift here (if appropriate) lines 417-420 from appendix A.
Fig2. It would be nice that panel 2a, 2c and 2d could share the same vertical axes. This can be done by removing panel 2 b. The latter is cited in the text but not further discussed so it might be sufficient to state in the text the amount, variability and trends (if any) of the COD, and skip the figure. By the way, in the figure 2b there appear to be a lack of COD data between 11.5 and 12.5 and 13.15-14.15. Why? Moreover, in fig2a the colors are coded in a.u. Why? Can you plot explicitly the BR?
Fig.3. : See the remarks on Fig. 2
(205-207). In this sentence, it almost seems as if dehydration is a cause for upwelling, and it could be rephrased to avoid giving this impression. The Brewer-Dobson Circulation provides the dominant large-scale upwelling mechanism in the TTL, and deep convection and cold trap dehydration regulate the water vapor content, influencing the efficiency of air entering the stratosphere. While it is true that latent heat release from cirrus formation enhances local ascent, the radiative effects of cirrus can either amplify or dampen vertical motion depending on cloud properties. Therefore, the role of latent heat release by condensing cirrus in TTL ascent can be complex and not straightforward.
(229). The potential influence of Kelvin waves is a valuable addition to understanding the August case. Is there direct observational evidence, as temperature anomalies or reanalysis data that confirm the presence of such waves?
(243-261) The section nicely extends the previous case studies by performing trajectory analyses for all cirrus clouds in two seasons. The inclusion of potential temperature analysis strengthens the transport pathway discussion. However, there is no information on where the trajectories are initiated in case of geometrically thick clouds. Midcloud? Or if the geometrical thickness is large, more than a backtrajectory is used for the same cloud? And for long lasting cirrus observation, do you launch a trajectory every 3 hrs? The sentence at (248) “The trajectories are initialized at the observed time and altitude of cirrus clouds above Palau, consistent with the case study methodology.” should be expanded to provide such information explicitly. This has an impact on the understanding of the AEF afterward.
(265) How are different box regions (1-6) defined? In terms of continental/maritime convection? Presence of monsoon? Please explain in further detail the reason for this boxing choice.
(267). What does N represent? Total trajectory points? Please state that explicitly.
(275) The text presents a strong seasonal contrast (46% vs. 5%) but does not explicitly explain why December favors stratospheric entry. My suggestion: "The significantly higher AEF in December (46% reaching 380 K is consistent with stronger upwelling over the TWP during NH winter. This aligns with the seasonal phase of the Brewer-Dobson Circulation, which facilitates upward transport of TTL air into the stratosphere."
(279) The reference to the easterly upper-level winds and QBO connection need more clarity: The role of QBO-driven zonal wind patterns should be briefly explained. My suggestion: “The dominance of easterly winds in both months corresponds to the observed QBO phase during December 2018 and August 2022 (Diallo et al., 2018). Easterlies in the lower stratosphere favor upward transport by reducing mixing with mid-latitude air, enhancing tropical stratospheric entry."
(289). A better Explanation of the Chemical Equator (CE) is here needed. Add a brief sentence explaining what the CE represents and why it matters for tropical atmospheric composition
(330-337) Here consider to quote “Khaykin, S. M., Moyer, E., Krämer, M., Clouser, B., Bucci, S., Legras, B., Lykov, A., Afchine, A., Cairo, F., Formanyuk, I., Mitev, V., Matthey, R., Rolf, C., Singer, C. E., Spelten, N., Volkov, V., Yushkov, V., and Stroh, F.: Persistence of moist plumes from overshooting convection in the Asian monsoon anticyclone, Atmos. Chem. Phys., 22, 3169–3189, https://doi.org/10.5194/acp-22-3169-2022, 2022.” and the dual role of overshooting convection, which may lead to hydration or dehydration depending on the synoptic-scale tropopause temperatures.
In general, this paragraph needs more clarity on the role of overshooting tops: First, explain why overshooting convection bypasses the cold trap and directly injects air into the stratosphere. Then, explain the impact of short-lived species.Citation: https://doi.org/10.5194/egusphere-2024-3981-RC1 - AC1: 'Reply on RC1', Xiaoyu Sun, 19 Mar 2025
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RC2: 'Comment on egusphere-2024-3981', Anonymous Referee #2, 13 Feb 2025
Review of “Evidence of Tropospheric Uplift into the Stratosphere via the Tropical Western Pacific Cold Trap”by Sun et al.
The authors use lidar and balloon measurement data, along with the trajectory model, to show the transport pathway over the tropical western Pacific cold trap. By comparing two typical cases, they find that the winter case shows the lowest temperature, coinciding with the RHi >150%. Nearly half of the air particles in the December cirrus cloud rise above 380 K after cloud formation. In contrast, the summer case shows a warmer tropopause and a non-supersaturated environment (RHi <100%). Only 3% of the air particles in the August rise above the 380 K level. The authors provide evidence of the uplift of tropospheric air to the stratosphere via the cold trap during NH winter. Analysis of such simulations is valuable and it is crucial to highlight the importance of the cold trap in driving air mass transport. I propose accepting the paper after the minor revisions listed below:
Line 27-29: The following ref. Such as Vogel et al., (2019) maybe useful.
Ref. Vogel, B., Müller, R., Günther, G., Spang, R., Hanumanthu, S., Li, D., Riese, M., and Stiller, G. P.: Lagrangian simulations of the transport of young air masses to the top of the Asian monsoon anticyclone and into the tropical pipe, Atmos. Chem. Phys., 19, 6007–6034, https://doi.org/10.5194/acp-19-6007-2019, 2019.
Line 30: ...in the lower atmosphere... to ...in the lower stratosphere...
For table 1: How should the mean cloud base or top height be considered for double cirrus layers in the UTLS region? Such as the cases on 13 December 2018 and 1 August 2022.
Line 98: the mid-cloud temperature? Please give more information.
Line 136: Figure 1a and b... to Figures 1a and b...
Line 142: ...over the tropical region (±30°N)... to ...over the tropical region (30°S-30°N)...
Figure 1: why were reanalysis data from different periods used for Fig. 1a-b (1980-2019) and Fig. 1c-d (1992-2022)? The cold point temperature is usually not used in mid- and high-latitude regions.
Line 263:...the relevant/dominant transport...?
Figure 8: ...on the 10-d trajectory analysis (compare 6). (compare 6)? more details
Line 387: In August, only 3% of the air masses... In the discussion and sect. 3.4, the value is 5%?
For Figures A1-A4: The differences between the ATLAS and HYSPLIT trajectories are larger in December 2018 than in August 2022, please clarity?
For figure A5: title “Forward trajectories in 13 Dec14:00” to “Forward trajectories on 13 Dec14:00”
Citation: https://doi.org/10.5194/egusphere-2024-3981-RC2 - AC2: 'Reply on RC2', Xiaoyu Sun, 19 Mar 2025
Interactive discussion
Status: closed
-
RC1: 'Comment on egusphere-2024-3981', Anonymous Referee #1, 10 Feb 2025
This manuscript presents a well-structured and scientifically significant study on air mass transport pathways over the Tropical Western Pacific (TWP), with a particular focus on the role of the cold trap in the upper troposphere and lower stratosphere (UTLS). Using lidar and balloon-borne observations from Koror, Palau, combined with trajectory simulations, the authors provide valuable insights into seasonal variations in air mass ascent and dehydration processes.
The study is particularly relevant as it enhances our understanding of how air masses transition from the troposphere to the stratosphere, a process fundamental for stratospheric water vapor balance and global climate dynamics. The authors effectively demonstrate the stark contrast between winter (December 2018) and summer (August 2022) conditions, showing that upward transport through the cold trap occurs primarily in winter, while summer air masses tend to descend. The inclusion of quantitative metrics, such as the fraction of air masses reaching above 380 K potential temperature, strengthens the findings and makes a compelling case for the seasonal dependency of stratospheric entry pathways.
Overall, this is an important and well-executed study that deserves publication. I only have a few minor suggestions regarding clarity, phrasing, and data presentation that could further improve the manuscript. These are detailed below:
(151). The use of two case studies (December 2018 and August 2022) allows for a seasonal comparison, adding depth and clarity to the analysis. However, in both case studies there is no description of the meteorological conditions that would have provided additional information about the local context in which the measurements were taken.
(176). There is no information here on where precisely the backtrajectories are initiated. Midcloud maybe? One trajectory per cloud? Clusters? You may consider to shift here (if appropriate) lines 417-420 from appendix A.
Fig2. It would be nice that panel 2a, 2c and 2d could share the same vertical axes. This can be done by removing panel 2 b. The latter is cited in the text but not further discussed so it might be sufficient to state in the text the amount, variability and trends (if any) of the COD, and skip the figure. By the way, in the figure 2b there appear to be a lack of COD data between 11.5 and 12.5 and 13.15-14.15. Why? Moreover, in fig2a the colors are coded in a.u. Why? Can you plot explicitly the BR?
Fig.3. : See the remarks on Fig. 2
(205-207). In this sentence, it almost seems as if dehydration is a cause for upwelling, and it could be rephrased to avoid giving this impression. The Brewer-Dobson Circulation provides the dominant large-scale upwelling mechanism in the TTL, and deep convection and cold trap dehydration regulate the water vapor content, influencing the efficiency of air entering the stratosphere. While it is true that latent heat release from cirrus formation enhances local ascent, the radiative effects of cirrus can either amplify or dampen vertical motion depending on cloud properties. Therefore, the role of latent heat release by condensing cirrus in TTL ascent can be complex and not straightforward.
(229). The potential influence of Kelvin waves is a valuable addition to understanding the August case. Is there direct observational evidence, as temperature anomalies or reanalysis data that confirm the presence of such waves?
(243-261) The section nicely extends the previous case studies by performing trajectory analyses for all cirrus clouds in two seasons. The inclusion of potential temperature analysis strengthens the transport pathway discussion. However, there is no information on where the trajectories are initiated in case of geometrically thick clouds. Midcloud? Or if the geometrical thickness is large, more than a backtrajectory is used for the same cloud? And for long lasting cirrus observation, do you launch a trajectory every 3 hrs? The sentence at (248) “The trajectories are initialized at the observed time and altitude of cirrus clouds above Palau, consistent with the case study methodology.” should be expanded to provide such information explicitly. This has an impact on the understanding of the AEF afterward.
(265) How are different box regions (1-6) defined? In terms of continental/maritime convection? Presence of monsoon? Please explain in further detail the reason for this boxing choice.
(267). What does N represent? Total trajectory points? Please state that explicitly.
(275) The text presents a strong seasonal contrast (46% vs. 5%) but does not explicitly explain why December favors stratospheric entry. My suggestion: "The significantly higher AEF in December (46% reaching 380 K is consistent with stronger upwelling over the TWP during NH winter. This aligns with the seasonal phase of the Brewer-Dobson Circulation, which facilitates upward transport of TTL air into the stratosphere."
(279) The reference to the easterly upper-level winds and QBO connection need more clarity: The role of QBO-driven zonal wind patterns should be briefly explained. My suggestion: “The dominance of easterly winds in both months corresponds to the observed QBO phase during December 2018 and August 2022 (Diallo et al., 2018). Easterlies in the lower stratosphere favor upward transport by reducing mixing with mid-latitude air, enhancing tropical stratospheric entry."
(289). A better Explanation of the Chemical Equator (CE) is here needed. Add a brief sentence explaining what the CE represents and why it matters for tropical atmospheric composition
(330-337) Here consider to quote “Khaykin, S. M., Moyer, E., Krämer, M., Clouser, B., Bucci, S., Legras, B., Lykov, A., Afchine, A., Cairo, F., Formanyuk, I., Mitev, V., Matthey, R., Rolf, C., Singer, C. E., Spelten, N., Volkov, V., Yushkov, V., and Stroh, F.: Persistence of moist plumes from overshooting convection in the Asian monsoon anticyclone, Atmos. Chem. Phys., 22, 3169–3189, https://doi.org/10.5194/acp-22-3169-2022, 2022.” and the dual role of overshooting convection, which may lead to hydration or dehydration depending on the synoptic-scale tropopause temperatures.
In general, this paragraph needs more clarity on the role of overshooting tops: First, explain why overshooting convection bypasses the cold trap and directly injects air into the stratosphere. Then, explain the impact of short-lived species.Citation: https://doi.org/10.5194/egusphere-2024-3981-RC1 - AC1: 'Reply on RC1', Xiaoyu Sun, 19 Mar 2025
-
RC2: 'Comment on egusphere-2024-3981', Anonymous Referee #2, 13 Feb 2025
Review of “Evidence of Tropospheric Uplift into the Stratosphere via the Tropical Western Pacific Cold Trap”by Sun et al.
The authors use lidar and balloon measurement data, along with the trajectory model, to show the transport pathway over the tropical western Pacific cold trap. By comparing two typical cases, they find that the winter case shows the lowest temperature, coinciding with the RHi >150%. Nearly half of the air particles in the December cirrus cloud rise above 380 K after cloud formation. In contrast, the summer case shows a warmer tropopause and a non-supersaturated environment (RHi <100%). Only 3% of the air particles in the August rise above the 380 K level. The authors provide evidence of the uplift of tropospheric air to the stratosphere via the cold trap during NH winter. Analysis of such simulations is valuable and it is crucial to highlight the importance of the cold trap in driving air mass transport. I propose accepting the paper after the minor revisions listed below:
Line 27-29: The following ref. Such as Vogel et al., (2019) maybe useful.
Ref. Vogel, B., Müller, R., Günther, G., Spang, R., Hanumanthu, S., Li, D., Riese, M., and Stiller, G. P.: Lagrangian simulations of the transport of young air masses to the top of the Asian monsoon anticyclone and into the tropical pipe, Atmos. Chem. Phys., 19, 6007–6034, https://doi.org/10.5194/acp-19-6007-2019, 2019.
Line 30: ...in the lower atmosphere... to ...in the lower stratosphere...
For table 1: How should the mean cloud base or top height be considered for double cirrus layers in the UTLS region? Such as the cases on 13 December 2018 and 1 August 2022.
Line 98: the mid-cloud temperature? Please give more information.
Line 136: Figure 1a and b... to Figures 1a and b...
Line 142: ...over the tropical region (±30°N)... to ...over the tropical region (30°S-30°N)...
Figure 1: why were reanalysis data from different periods used for Fig. 1a-b (1980-2019) and Fig. 1c-d (1992-2022)? The cold point temperature is usually not used in mid- and high-latitude regions.
Line 263:...the relevant/dominant transport...?
Figure 8: ...on the 10-d trajectory analysis (compare 6). (compare 6)? more details
Line 387: In August, only 3% of the air masses... In the discussion and sect. 3.4, the value is 5%?
For Figures A1-A4: The differences between the ATLAS and HYSPLIT trajectories are larger in December 2018 than in August 2022, please clarity?
For figure A5: title “Forward trajectories in 13 Dec14:00” to “Forward trajectories on 13 Dec14:00”
Citation: https://doi.org/10.5194/egusphere-2024-3981-RC2 - AC2: 'Reply on RC2', Xiaoyu Sun, 19 Mar 2025
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Xiaoyu Sun
Katrin Müller
Mathias Palm
Christoph Ritter
Denghui Ji
Tim Balthasar Röpke
Justus Notholt
The requested preprint has a corresponding peer-reviewed final revised paper. You are encouraged to refer to the final revised version.
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(9065 KB) - Metadata XML
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Supplement
(11225 KB) - BibTeX
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- Final revised paper