the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Oct 2023
Impact of upper-level circulation on upper troposphere and lower stratosphere ozone distribution over Northeast Asia
Abstract. Ozone (O3) in the upper troposphere and lower stratosphere (UTLS) is strongly regulated by upper-level circulation dynamics. Understanding the coupling between UTLS O3 distribution and upper-level circulation dynamics is important not only to understand synoptic processes governing O3 distribution and variability, but also to test the fidelity of chemistry transport models in simulating the stratosphere–troposphere exchange (STE) processes. This study presents the first systematic assessment of observationally constrained UTLS O3 variability associated with upper-level circulation patterns over the Northeast Asia region. By applying the self-organized mapping (SOM) technique to 500, 250, and 100 hPa geopotential height (GPH) data, 12 circulation patterns are quantified and then used to characterize the UTLS O3 distribution in the period 2000–2020 in both four-site (Beijing, Pohang, Tateno, and Sapporo) ozonesonde data and regional-scale satellite products. The underlying dynamic transport mechanism responsible for UTLS O3 responses to different circulation patterns are further explored through correlation analysis between O3 anomalies and transport indicators. The results indicate that although O3 at almost all altitudes shows statistically significant sensitivity to circulation patterns, lower-stratospheric O3 exhibits a far stronger sensitivity when compared with upper-tropospheric O3. Circulation patterns featuring the East Asian Trough (EAT) show clear enhancement of O3 southwest of the trough, and the enhancement zone moves with the eastward propagation of the EAT. Circulation patterns featuring eastward-shedding vortices of the Asia Summer Monsoon Anticyclone (ASMA) show the opposite signal, in which O3 concentrations are decreased, especially at Sapporo, and the negative O3 anomaly zone stretches from South Japan to Sakhalin Island. Each circulation pattern is characterized by distinct transport pathways, which play a determining role in the pattern-specific UTLS O3 response. Positive O3 anomalies are usually associated with post-trough downward and southward transport, whereas negative O3 anomalies are commonly associated with fore-trough upward and northward transport. In the lower stratosphere, the correlation between O3 anomalies and transport indicators is significantly stronger than that in the upper troposphere, and the strongest correlation occurs in the lower stratosphere of Beijing.
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RC1: 'Comment on egusphere-2023-1393', Anonymous Referee #2, 05 Dec 2023
Review of egusphere-2023-1393: ‘‘Impact of upper-level circulation on upper troposphere and lower stratosphere ozone distribution over Northeast Asia’‘ by Zhiheng Liao et al.
In the current study by Zhiheng Liao et al. reanalysis data is combined with a classification scheme to identify different synoptic situations over East Asia and the northern Western Pacific. Along with satellite and radio-sounding data a further evaluation is completed whether the ozone distribution in the upper troposphere and lower stratosphere differs between these different synoptic situations. Finally, two new transport metrics are introduced to determine whether the different ozone distributions can be related to meridional or vertical transport. With this the authors come to the conclusions that i) four different circulation patterns dominate over the target region with other circulation patterns representing transitional phases; the circulation pattern are a result of the varying interaction between the three dominating dynamical features; ii) O3 patterns in the UTLS reveal differences between all circulation patterns with the East Asian Trough being a major driver of these differences.
Ozone in the UTLS is a crucial factor for our climate system. With dynamical factors dominating the abundance of ozone in this region, it is well justified to assess the interrelation between dynamics and ozone distribution in a specific region. More so, the combination of reanalysis data, Eulerian and Lagrangian metrics along with observations can potentially reveal new insights into the topic of the ozone-dynamics relationship. Having said this, I was very keen on reading the paper and get new insights into the UTLS O3. This also means that this paper is in the scope of ACP and I would in principle support a publication. However, in the current form I have major concerns about the results and methodological aspects of the paper which are explained in more detail as major comments below. I hope that these aspects may be regarded as suggestions to make the paper content and structure more clear to the reader and to better highlight the major results.
Major comments
- Analysis of flow patterns and major goals of this study
I find the discussion of the results in the sections quite difficult to follow and especially to extract the relevant results. I think there are two reasons for this.
The first reason is that I do not really understand why 12 different flow patterns have to be discussed throughout the paper. The initial analysis reveals that there are four dominant flow regimes while all others are mostly transitional patterns between those four. I also felt a little bit left alone why there are 12 patterns, the text states only that this was based on a subjective conclusion but no reasons were given on which criteria this is based. The second reason is that the discussions are often based on giving numbers on how often a certain pattern is associated with a certain flow characteristics. Do not get me wrong a quantification is important, but having this for all 12 flow patterns makes it difficult to get the main message. In the text it is even stated that there are sometimes very little differences between the patterns and when looking at Figure 2, many of the patterns look very similar.
In principle there are four different flow patterns and I think the paper would significantly profit if the focus would be on those four flow patterns. This still means that a thorough and extensive analysis of the flow can be done in the beginning, highlighting that there are 12 different patterns, but with the first result that the ozone distribution is mainly varying between four patterns. This would strengthen the focus on the major results and would also make the discussions more straight forward. When doing this, I would highly appreciate to obtain more insight on why 12 nodes have been chosen. - Methods and data
My second major concern is about the data selection and methods. The initial flow pattern analysis is completed with ERA5 data on three isobaric surfaces. The trajectory analysis is done with HYSPLIT based on NCEP reanalysis data, with trajectories starting on two height levels. Later an analysis centered around ozone variability and stratosphere-troposphere exchange is completed without having defined a proper tropopause or transition region. This comes a bit arbitrary and if done like this needs more justification. It reads a bit as if there was an available data collection put together but not in a consistent manner. For example, there are differences within both used reanalyses data sets which are not negligible, especially in the UTLS, see e.g. the special issue of S-RIP (https://acp.copernicus.org/articles/special_issue829.html) or the S-RIP report (https://www.sparc-climate.org/activities/reanalysis-intercomparison/). I do not fully understand why the trajectories have not been calculated with ERA5, there are many open source trajectory models available which allow for such calculations. Or otherwise, why the NCEP data has not been used instead of ERA5 in the SOM analysis. More so, the definition of the UTLS is rather arbitrary. On page 4, line 140 ff, the UTLS height range is defined on a mean tropopause altitude taken from another study. From this the height range of the UTLS is defined to be within 6 and 16 km. (By the way, the UTLS definition for the extratropics with +/- 5 km around the local tropopause is to my knowledge defined in Gettelman et al., 2011). Just below this statement, Figure 1 (c panels) show the seasonal cycle of the ozone distribution at the four sonde stations. Taking a value around 100 – 120 ppbv ozone to mark the tropopause, it becomes clear that at all four stations the tropopause is above the UTLS definition. This leads to a strong systematic bias in the analysis. This definition of the UTLS could have been made much more tailored to this analysis. The tropopause altitudes could have been derived from the ERA5 data, either using the lapse rate or dynamical (potential vorticity based) tropopause definition. The ozonesonde data could have been used to define an ozonepause (see Bethan et al. (1996)) or, since the ozonesonde data usually comes with a temperature measurement, the lapse rate tropopause. The data to do such an analysis is available but not used to its full potential and I do not fully understand why. Defining a tropopause would also give the possibility to do this analysis in a tropopause relative framework. For instance the flow could be analyzed relative to the tropopause (three layers: one below, one at, one above the tropopause) and all ozone profiles could have been analyzed in such a coordinate system. This would significantly reduce the ozone variability and would strengthen the connection between ozone distribution and dynamics. I also wondered about the trajectories. First of all, why have the trajectories been started at 8 and 14 km? Why not at the altitudes of the SOM analysis (three layers) or covering all the altitudes in the UTLS (at least between 6 and 16 km)? Then I also wondered why the trajectories have only been calculated for three days backward in time. This does not cover the full synoptic time scale and no justification is given for why the authors have chosen a duration of three days.
In summary, I would like to see how well the reanalysis used here fit together and that the results do not depend on the various choices made for this paper. - Transport diagnostics and stratosphere troposphere exchange
My third major point is again related to methods and could have been part of the second point. However, this point left me thinking so much that I thought it might better to be an individual point. In section 3.3 transport indicators are defined to study the O3 variability and in particular stratosphere troposphere exchange (STE). The MTI indicator is based on the trajectory calculations to quantify meridional transport, but I think it could have been realized much easier. A mean trajectory with standard deviation could have been calculated for each flow pattern or a mean meridional displacement. However, the issue that I see here is that there is definitely no direct connection to STE with the MTI. The meridional displacement mostly shows the position of the Rossby wave relative to the observational point. The other transport metric is introduced due to the bad vertical transport representation in the trajectory calculation. However, instead satellite data is used which has a very limited vertical resolution in the UTLS. So I question if this is really a better choice. Since this indicator is used to evaluate STE, I wonder why the trajectories have not been used to assess STE (e.g., based on potential vorticity). If the resolution is the issue, then this would call for better data to calculate the trajectories (e.g., ERA5 which has the same vertical resolution as ECMWF forecast, roughly 300 m in the UTLS) which is much finer than what is available for the NCEP reanalysis data. In summary, I wonder why the trajectories have not been used more directly and more extensively to analyze transport pathways and where the benefit of the transport indicators is. If, however, the goal was to establish these transport indicators as novel diagnostics then I would wish for a more detailed introduction and discussion of these parameters.
Minor comments (in order of appearance)
- L2, L62-66: To my understanding the main convective outflow is around 360 K for tropical convection while the tropopause is located further upward between 370 and 380 K. In the subtropics the mentioned process might be of relevance and the references point also more to STE in subtropics (or even mid-latitudes).
- P2, L 71: I'd be careful. TST has many sources such as radiation (e.g., Zierl and Wirth, 1997), gravity wave dynamics along with KHI occurrence (e.g., Kunkel et al., 2019, Whiteway et al., 2004, Pepler et al., 1998) or convective overshoots (e.g., Homeyer et al., 2014, 2015, Tang et al., 2011). WCBs transport air from low levels to upper levels but seem not be too relevant for direct TST (see also Madonna et al., 2014ab).
- P5, L166 and L 172: the analysis areas do not match. Which did you use?
- P5, L192: What is meant with standard error? Is it the standard deviation in time and space?
- P6, L196: What is meant with self-organized circulation patterns? I tend to think of convective related circulations when associating self-organized with circulation.
- P11, L340: Is this potentially related to the different abundances of O3 in the UT and LS? What about some relative quantities like relative standard deviation? More so, looking at Figure 4, what is the point? Is there a real statistical difference between the O3 abundances per node? Has a statistical test been used to determine whether the medians differ statistically from each other?
- Figure 5: An analysis of potential vorticity similar to the O3 analysis could potentially reveal how the ozone distribution is linked to the dynamics here. Also this figure in a tropopause relative coordinate system would provide more insight in the differences of O3 between the various nodes. A lot of the the differences that seem to be apparent here might be due different tropopause heights.
- P 11, L 358: This would imply a significant transport of tropospheric air into the LS during WCB. However, other studies suggest that this transport pathway does not often occur (Madonna et al., 2014). See comment before.
- P13, L 413: Why anomaly? I would rather argue, this is a displacement, the jet is either shifted north or southward and thus is the sloped region of the tropopause.
- P13, L416: A tropospheric intrusion means a high tropopause at higher latitudes, initially, that does not affect the ozone in the LS but only the ozone at a given altitude. A proper tropopause definition would help here.
- P14, L 429-434: This is synoptic scale meteorology at play. Upstream of the trough axis, there is usually more upward transport in the troposphere (often associated with WCB) while downstream of the axis, downward motions prevail (often associated with a dry intrusion). The usage of upwelling and downwelling in this context should be avoided. Upwelling/downwelling occurs is more often used to characterize the residual circulation in the stratosphere with tropical upwelling and extratropical downwelling. This occurs on much slower time scales and has nothing to do with the vertical motions associated with Rossby wave dynamics in the extratropical troposphere.
- P15, L 451: In my eyes, this can not be generalized as such. ASMA shedding events do not necessarily interact with WCB nor do they necessarily subsequently occur.
- P15, L455: Depends on how the lower stratosphere is defined. This looks much more like a vertical displacement due to a high tropopause altitude.
- P15, L467: What is a climatological trajectory?
- P15, L467/468: MTI gives no information on the altitude.
- P17, L 511: Which statistical test has been used to test for significance?
Technical (in order of appearance)
- P2, L 53: climatologically instead of climatically
- P2, L 56: for STE there is a review from Stohl et al. (2003)
- P7, L252: troughs instead of roughs
- P16, L506: do you mean numerical instead of numerous
References:
S Bethan, G Vaughan, SJ Reid, A comparison of ozone and thermal tropopause heights and the impact of tropopause definition on quantifying the ozone content of the troposphere, Quarterly Journal of the Royal Meteorological Society, 1996
Gettelman, A., Hoor, P., Pan, L. L., Randel, W. J., Hegglin, M. I., and Birner, T. (2011), The extratropical upper troposphere and lower stratosphere, Rev. Geophys., 49, RG3003, doi:10.1029/2011RG000355.
Homeyer, C. R.: Numerical simulations of extratropical tropopause penetrating convection: Sensitivities to grid resolution, J. Geophys. Res., 120, 7174–7188, https://doi.org/10.1002/2015JD023356, 2015.
Homeyer, C. R., Pan, L. L., Dorsi, S. W., Avallone, L. M., Weinheimer, A. J., O’Brien, A. S., DiGangi, J. P., Zondlo, M. A., Ryerson, T. B., Diskin, G. S., and Campos, T. L.: Convective transport of water vapor into the lower stratosphere observed during double tropopause events, J. Geophys. Res.-Atmos., 119, 10941–10958, https://doi.org/10.1002/2014JD021485, 2014.
Kunkel, D., Hoor, P., Kaluza, T., Ungermann, J., Kluschat, B., Giez, A., Lachnitt, H.-C., Kaufmann, M., and Riese, M.: Evidence of small-scale quasi-isentropic mixing in ridges of extratropical baroclinic waves, Atmos. Chem. Phys., 19, 12607–12630, https://doi.org/10.5194/acp-19-12607-2019, 2019.
Madonna, E., H. Wernli, H. Joos, and O. Martius, 2014: Warm Conveyor Belts in the ERA-Interim Dataset (1979–2010). Part I: Climatology and Potential Vorticity Evolution. J. Climate, 27, 3–26, https://doi.org/10.1175/JCLI-D-12-00720.1.
Madonna, E., S. Limbach, C. Aebi, H. Joos, H. Wernli, and O. Martius, 2014: On the Co-Occurrence of Warm Conveyor Belt Outflows and PV Streamers. J. Atmos. Sci., 71, 3668–3673, https://doi.org/10.1175/JAS-D-14-0119.1.
Pepler, S. J., Vaughan, G., and Hooper, D. A.: Detection of turbulence around jet streams using a VHF radar, Q. J. Roy. Meteor. Soc., 124, 447–462, https://doi.org/10.1002/qj.49712454605, 1998.
Stohl, A., et al., Stratosphere-troposphere exchange: A review, and what we have learned from STACCATO, J. Geophys. Res., 108(D12), 8516, doi:10.1029/2002JD002490, 2003.
Tang, Q., Prather, M. J., and Hsu, J.: Stratosphere-troposphere exchange ozone flux related to deep convection, Geophys. Res. Lett., 38, 1–5, https://doi.org/10.1029/2010GL046039, 2011.
Whiteway, J. A., Klaassen, G. P., Bradshaw, N. G., and Hacker, J.: Transition to turbulence in shear above the tropopause, Geophys. Res. Lett., 31, 2–5, https://doi.org/10.1029/2003GL018509, 2004.
Zierl, B. and Wirth, V.: The influence of radiation on tropopause behavior and stratosphere-troposphere exchange in an upper tropospheric anticyclone, J. Geophys. Res., 102, 23883, https://doi.org/10.1029/97JD01667, 1997.
Citation: https://doi.org/10.5194/egusphere-2023-1393-RC1 - Analysis of flow patterns and major goals of this study
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RC2: 'Comment on egusphere-2023-1393', Anonymous Referee #1, 13 Dec 2023
Review of “Impact of upper-level circulation on upper troposphere and lower stratosphere ozone distribution over Northeast Asia” by Liao et al
This study primarily evaluates the prevailing synoptic patterns over Northeast Asia and their relationship to ozone anomalies in a layer that spans both troposphere and stratosphere. Using output from ERA5 reanalysis and self-organizing maps, the leading 12 upper-level circulation patterns are identified at pressure levels of 500, 250, and 100 hPa. Ozone data from satellite and ozonesonde observations are evaluated in the context of these 12 nodes based on the self-organizing map results. The ozone results largely mirror the prevailing differences in synoptic mode and appear to be largely related to variability in the altitude of the tropopause. While these synoptic patterns seem robustly evaluated, the narrative and analysis metrics used to interpret and assess the role of stratosphere-troposphere exchange for the observed co-variability in ozone is weakly justified. To this end, I provide detailed summaries of my impressions and suggested revisions below. A few suggested technical edits follow.
Major Comments
- The foundation of the composition analysis direction in the paper is an assumption that one can take the average tropopause altitude over the study region to represent a distinction between upper troposphere and lower stratosphere as a basis for transport analysis and interpretation. Unfortunately, this assumption is immediately invalidated by the fact that approximately half of the domain (the southern portion) is typically characterized by tropical tropopause heights uniformly near 16 km and the other half (the northern portion) is typically characterized by midlatitude tropopause heights nearer the stated 11 km level. The transition between these two modes in tropopause altitude is referred to as the tropopause break and there are both vertical and mostly lateral stratosphere-troposphere exchange processes that take place in this region. Ultimately, this makes a constant-altitude analysis like that included in the present study intrinsically flawed for assessing the role of stratosphere-troposphere exchange to the UTLS ozone budget (or composition of other gases) unless the altitude of the tropopause is considered. Furthermore, none of the pressure levels considered lie fully within the stratosphere and one likely lies fully within the middle-to-upper troposphere (500 hPa). The 100 hPa surface, in particular, lies at the approximate level of the tropical tropopause, which is demonstrated well in Figure 7. The select altitude layers in Figure 4 also do not reliably fall within (or are suitable characterizations of) the upper troposphere or lower stratosphere. Ultimately, this means that all the composition analysis is dominated by variability in the height of the tropopause that follows latitudinal meandering of the subtropical jet and tropopause break. The ozonesonde stations used straddle this tropopause break, as evidenced by the annual variability in ozone composition shown in Figure 1. As such, assessing the role of cross-tropopause transport in accomplishing the observed composition will be unreliable unless the analysis is completed in relative altitude to a reference tropopause. Revisions to address this will be required for much of the paper.
- Using NCEP reanalysis winds to drive the trajectory calculations introduces unnecessary inconsistencies with the remainder of the analysis built upon ERA5. The trajectory analyses should instead be driven by ERA5 winds. Moreover, the transport indicators used could be made more resilient to some of the issues above if a diagnosed potential vorticity change along the trajectory is used to increase confidence in stratosphere-troposphere exchange. Several prior studies have done such with great success.
Specific Comments
Line 17: “simulating the stratosphere” should be “simulating stratosphere”
Section 2.2: need to specify the vertical grid resolution of this dataset
Section 2.3: similarly here, what vertical resolution data do you use? Tropopause definition?
Line 187: these are levels, not layers. See above major comment #1 about the inappropriate labeling/interpretation of these levels representing upper troposphere and/or lower stratosphere.
Line 232: “tracer-to-tracer” should be “tracer-tracer”
Line 245: Add “The” to the start of this sentence.
Figure 2: Since this is showing anomalies, it would be more appropriate to use a diverging color scale with distinct hues for positive and negative values (e.g., blue-red).
Line 276: should add citations to support this here.
Citation: https://doi.org/10.5194/egusphere-2023-1393-RC2 - The foundation of the composition analysis direction in the paper is an assumption that one can take the average tropopause altitude over the study region to represent a distinction between upper troposphere and lower stratosphere as a basis for transport analysis and interpretation. Unfortunately, this assumption is immediately invalidated by the fact that approximately half of the domain (the southern portion) is typically characterized by tropical tropopause heights uniformly near 16 km and the other half (the northern portion) is typically characterized by midlatitude tropopause heights nearer the stated 11 km level. The transition between these two modes in tropopause altitude is referred to as the tropopause break and there are both vertical and mostly lateral stratosphere-troposphere exchange processes that take place in this region. Ultimately, this makes a constant-altitude analysis like that included in the present study intrinsically flawed for assessing the role of stratosphere-troposphere exchange to the UTLS ozone budget (or composition of other gases) unless the altitude of the tropopause is considered. Furthermore, none of the pressure levels considered lie fully within the stratosphere and one likely lies fully within the middle-to-upper troposphere (500 hPa). The 100 hPa surface, in particular, lies at the approximate level of the tropical tropopause, which is demonstrated well in Figure 7. The select altitude layers in Figure 4 also do not reliably fall within (or are suitable characterizations of) the upper troposphere or lower stratosphere. Ultimately, this means that all the composition analysis is dominated by variability in the height of the tropopause that follows latitudinal meandering of the subtropical jet and tropopause break. The ozonesonde stations used straddle this tropopause break, as evidenced by the annual variability in ozone composition shown in Figure 1. As such, assessing the role of cross-tropopause transport in accomplishing the observed composition will be unreliable unless the analysis is completed in relative altitude to a reference tropopause. Revisions to address this will be required for much of the paper.
Interactive discussion
Status: closed
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RC1: 'Comment on egusphere-2023-1393', Anonymous Referee #2, 05 Dec 2023
Review of egusphere-2023-1393: ‘‘Impact of upper-level circulation on upper troposphere and lower stratosphere ozone distribution over Northeast Asia’‘ by Zhiheng Liao et al.
In the current study by Zhiheng Liao et al. reanalysis data is combined with a classification scheme to identify different synoptic situations over East Asia and the northern Western Pacific. Along with satellite and radio-sounding data a further evaluation is completed whether the ozone distribution in the upper troposphere and lower stratosphere differs between these different synoptic situations. Finally, two new transport metrics are introduced to determine whether the different ozone distributions can be related to meridional or vertical transport. With this the authors come to the conclusions that i) four different circulation patterns dominate over the target region with other circulation patterns representing transitional phases; the circulation pattern are a result of the varying interaction between the three dominating dynamical features; ii) O3 patterns in the UTLS reveal differences between all circulation patterns with the East Asian Trough being a major driver of these differences.
Ozone in the UTLS is a crucial factor for our climate system. With dynamical factors dominating the abundance of ozone in this region, it is well justified to assess the interrelation between dynamics and ozone distribution in a specific region. More so, the combination of reanalysis data, Eulerian and Lagrangian metrics along with observations can potentially reveal new insights into the topic of the ozone-dynamics relationship. Having said this, I was very keen on reading the paper and get new insights into the UTLS O3. This also means that this paper is in the scope of ACP and I would in principle support a publication. However, in the current form I have major concerns about the results and methodological aspects of the paper which are explained in more detail as major comments below. I hope that these aspects may be regarded as suggestions to make the paper content and structure more clear to the reader and to better highlight the major results.
Major comments
- Analysis of flow patterns and major goals of this study
I find the discussion of the results in the sections quite difficult to follow and especially to extract the relevant results. I think there are two reasons for this.
The first reason is that I do not really understand why 12 different flow patterns have to be discussed throughout the paper. The initial analysis reveals that there are four dominant flow regimes while all others are mostly transitional patterns between those four. I also felt a little bit left alone why there are 12 patterns, the text states only that this was based on a subjective conclusion but no reasons were given on which criteria this is based. The second reason is that the discussions are often based on giving numbers on how often a certain pattern is associated with a certain flow characteristics. Do not get me wrong a quantification is important, but having this for all 12 flow patterns makes it difficult to get the main message. In the text it is even stated that there are sometimes very little differences between the patterns and when looking at Figure 2, many of the patterns look very similar.
In principle there are four different flow patterns and I think the paper would significantly profit if the focus would be on those four flow patterns. This still means that a thorough and extensive analysis of the flow can be done in the beginning, highlighting that there are 12 different patterns, but with the first result that the ozone distribution is mainly varying between four patterns. This would strengthen the focus on the major results and would also make the discussions more straight forward. When doing this, I would highly appreciate to obtain more insight on why 12 nodes have been chosen. - Methods and data
My second major concern is about the data selection and methods. The initial flow pattern analysis is completed with ERA5 data on three isobaric surfaces. The trajectory analysis is done with HYSPLIT based on NCEP reanalysis data, with trajectories starting on two height levels. Later an analysis centered around ozone variability and stratosphere-troposphere exchange is completed without having defined a proper tropopause or transition region. This comes a bit arbitrary and if done like this needs more justification. It reads a bit as if there was an available data collection put together but not in a consistent manner. For example, there are differences within both used reanalyses data sets which are not negligible, especially in the UTLS, see e.g. the special issue of S-RIP (https://acp.copernicus.org/articles/special_issue829.html) or the S-RIP report (https://www.sparc-climate.org/activities/reanalysis-intercomparison/). I do not fully understand why the trajectories have not been calculated with ERA5, there are many open source trajectory models available which allow for such calculations. Or otherwise, why the NCEP data has not been used instead of ERA5 in the SOM analysis. More so, the definition of the UTLS is rather arbitrary. On page 4, line 140 ff, the UTLS height range is defined on a mean tropopause altitude taken from another study. From this the height range of the UTLS is defined to be within 6 and 16 km. (By the way, the UTLS definition for the extratropics with +/- 5 km around the local tropopause is to my knowledge defined in Gettelman et al., 2011). Just below this statement, Figure 1 (c panels) show the seasonal cycle of the ozone distribution at the four sonde stations. Taking a value around 100 – 120 ppbv ozone to mark the tropopause, it becomes clear that at all four stations the tropopause is above the UTLS definition. This leads to a strong systematic bias in the analysis. This definition of the UTLS could have been made much more tailored to this analysis. The tropopause altitudes could have been derived from the ERA5 data, either using the lapse rate or dynamical (potential vorticity based) tropopause definition. The ozonesonde data could have been used to define an ozonepause (see Bethan et al. (1996)) or, since the ozonesonde data usually comes with a temperature measurement, the lapse rate tropopause. The data to do such an analysis is available but not used to its full potential and I do not fully understand why. Defining a tropopause would also give the possibility to do this analysis in a tropopause relative framework. For instance the flow could be analyzed relative to the tropopause (three layers: one below, one at, one above the tropopause) and all ozone profiles could have been analyzed in such a coordinate system. This would significantly reduce the ozone variability and would strengthen the connection between ozone distribution and dynamics. I also wondered about the trajectories. First of all, why have the trajectories been started at 8 and 14 km? Why not at the altitudes of the SOM analysis (three layers) or covering all the altitudes in the UTLS (at least between 6 and 16 km)? Then I also wondered why the trajectories have only been calculated for three days backward in time. This does not cover the full synoptic time scale and no justification is given for why the authors have chosen a duration of three days.
In summary, I would like to see how well the reanalysis used here fit together and that the results do not depend on the various choices made for this paper. - Transport diagnostics and stratosphere troposphere exchange
My third major point is again related to methods and could have been part of the second point. However, this point left me thinking so much that I thought it might better to be an individual point. In section 3.3 transport indicators are defined to study the O3 variability and in particular stratosphere troposphere exchange (STE). The MTI indicator is based on the trajectory calculations to quantify meridional transport, but I think it could have been realized much easier. A mean trajectory with standard deviation could have been calculated for each flow pattern or a mean meridional displacement. However, the issue that I see here is that there is definitely no direct connection to STE with the MTI. The meridional displacement mostly shows the position of the Rossby wave relative to the observational point. The other transport metric is introduced due to the bad vertical transport representation in the trajectory calculation. However, instead satellite data is used which has a very limited vertical resolution in the UTLS. So I question if this is really a better choice. Since this indicator is used to evaluate STE, I wonder why the trajectories have not been used to assess STE (e.g., based on potential vorticity). If the resolution is the issue, then this would call for better data to calculate the trajectories (e.g., ERA5 which has the same vertical resolution as ECMWF forecast, roughly 300 m in the UTLS) which is much finer than what is available for the NCEP reanalysis data. In summary, I wonder why the trajectories have not been used more directly and more extensively to analyze transport pathways and where the benefit of the transport indicators is. If, however, the goal was to establish these transport indicators as novel diagnostics then I would wish for a more detailed introduction and discussion of these parameters.
Minor comments (in order of appearance)
- L2, L62-66: To my understanding the main convective outflow is around 360 K for tropical convection while the tropopause is located further upward between 370 and 380 K. In the subtropics the mentioned process might be of relevance and the references point also more to STE in subtropics (or even mid-latitudes).
- P2, L 71: I'd be careful. TST has many sources such as radiation (e.g., Zierl and Wirth, 1997), gravity wave dynamics along with KHI occurrence (e.g., Kunkel et al., 2019, Whiteway et al., 2004, Pepler et al., 1998) or convective overshoots (e.g., Homeyer et al., 2014, 2015, Tang et al., 2011). WCBs transport air from low levels to upper levels but seem not be too relevant for direct TST (see also Madonna et al., 2014ab).
- P5, L166 and L 172: the analysis areas do not match. Which did you use?
- P5, L192: What is meant with standard error? Is it the standard deviation in time and space?
- P6, L196: What is meant with self-organized circulation patterns? I tend to think of convective related circulations when associating self-organized with circulation.
- P11, L340: Is this potentially related to the different abundances of O3 in the UT and LS? What about some relative quantities like relative standard deviation? More so, looking at Figure 4, what is the point? Is there a real statistical difference between the O3 abundances per node? Has a statistical test been used to determine whether the medians differ statistically from each other?
- Figure 5: An analysis of potential vorticity similar to the O3 analysis could potentially reveal how the ozone distribution is linked to the dynamics here. Also this figure in a tropopause relative coordinate system would provide more insight in the differences of O3 between the various nodes. A lot of the the differences that seem to be apparent here might be due different tropopause heights.
- P 11, L 358: This would imply a significant transport of tropospheric air into the LS during WCB. However, other studies suggest that this transport pathway does not often occur (Madonna et al., 2014). See comment before.
- P13, L 413: Why anomaly? I would rather argue, this is a displacement, the jet is either shifted north or southward and thus is the sloped region of the tropopause.
- P13, L416: A tropospheric intrusion means a high tropopause at higher latitudes, initially, that does not affect the ozone in the LS but only the ozone at a given altitude. A proper tropopause definition would help here.
- P14, L 429-434: This is synoptic scale meteorology at play. Upstream of the trough axis, there is usually more upward transport in the troposphere (often associated with WCB) while downstream of the axis, downward motions prevail (often associated with a dry intrusion). The usage of upwelling and downwelling in this context should be avoided. Upwelling/downwelling occurs is more often used to characterize the residual circulation in the stratosphere with tropical upwelling and extratropical downwelling. This occurs on much slower time scales and has nothing to do with the vertical motions associated with Rossby wave dynamics in the extratropical troposphere.
- P15, L 451: In my eyes, this can not be generalized as such. ASMA shedding events do not necessarily interact with WCB nor do they necessarily subsequently occur.
- P15, L455: Depends on how the lower stratosphere is defined. This looks much more like a vertical displacement due to a high tropopause altitude.
- P15, L467: What is a climatological trajectory?
- P15, L467/468: MTI gives no information on the altitude.
- P17, L 511: Which statistical test has been used to test for significance?
Technical (in order of appearance)
- P2, L 53: climatologically instead of climatically
- P2, L 56: for STE there is a review from Stohl et al. (2003)
- P7, L252: troughs instead of roughs
- P16, L506: do you mean numerical instead of numerous
References:
S Bethan, G Vaughan, SJ Reid, A comparison of ozone and thermal tropopause heights and the impact of tropopause definition on quantifying the ozone content of the troposphere, Quarterly Journal of the Royal Meteorological Society, 1996
Gettelman, A., Hoor, P., Pan, L. L., Randel, W. J., Hegglin, M. I., and Birner, T. (2011), The extratropical upper troposphere and lower stratosphere, Rev. Geophys., 49, RG3003, doi:10.1029/2011RG000355.
Homeyer, C. R.: Numerical simulations of extratropical tropopause penetrating convection: Sensitivities to grid resolution, J. Geophys. Res., 120, 7174–7188, https://doi.org/10.1002/2015JD023356, 2015.
Homeyer, C. R., Pan, L. L., Dorsi, S. W., Avallone, L. M., Weinheimer, A. J., O’Brien, A. S., DiGangi, J. P., Zondlo, M. A., Ryerson, T. B., Diskin, G. S., and Campos, T. L.: Convective transport of water vapor into the lower stratosphere observed during double tropopause events, J. Geophys. Res.-Atmos., 119, 10941–10958, https://doi.org/10.1002/2014JD021485, 2014.
Kunkel, D., Hoor, P., Kaluza, T., Ungermann, J., Kluschat, B., Giez, A., Lachnitt, H.-C., Kaufmann, M., and Riese, M.: Evidence of small-scale quasi-isentropic mixing in ridges of extratropical baroclinic waves, Atmos. Chem. Phys., 19, 12607–12630, https://doi.org/10.5194/acp-19-12607-2019, 2019.
Madonna, E., H. Wernli, H. Joos, and O. Martius, 2014: Warm Conveyor Belts in the ERA-Interim Dataset (1979–2010). Part I: Climatology and Potential Vorticity Evolution. J. Climate, 27, 3–26, https://doi.org/10.1175/JCLI-D-12-00720.1.
Madonna, E., S. Limbach, C. Aebi, H. Joos, H. Wernli, and O. Martius, 2014: On the Co-Occurrence of Warm Conveyor Belt Outflows and PV Streamers. J. Atmos. Sci., 71, 3668–3673, https://doi.org/10.1175/JAS-D-14-0119.1.
Pepler, S. J., Vaughan, G., and Hooper, D. A.: Detection of turbulence around jet streams using a VHF radar, Q. J. Roy. Meteor. Soc., 124, 447–462, https://doi.org/10.1002/qj.49712454605, 1998.
Stohl, A., et al., Stratosphere-troposphere exchange: A review, and what we have learned from STACCATO, J. Geophys. Res., 108(D12), 8516, doi:10.1029/2002JD002490, 2003.
Tang, Q., Prather, M. J., and Hsu, J.: Stratosphere-troposphere exchange ozone flux related to deep convection, Geophys. Res. Lett., 38, 1–5, https://doi.org/10.1029/2010GL046039, 2011.
Whiteway, J. A., Klaassen, G. P., Bradshaw, N. G., and Hacker, J.: Transition to turbulence in shear above the tropopause, Geophys. Res. Lett., 31, 2–5, https://doi.org/10.1029/2003GL018509, 2004.
Zierl, B. and Wirth, V.: The influence of radiation on tropopause behavior and stratosphere-troposphere exchange in an upper tropospheric anticyclone, J. Geophys. Res., 102, 23883, https://doi.org/10.1029/97JD01667, 1997.
Citation: https://doi.org/10.5194/egusphere-2023-1393-RC1 - Analysis of flow patterns and major goals of this study
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RC2: 'Comment on egusphere-2023-1393', Anonymous Referee #1, 13 Dec 2023
Review of “Impact of upper-level circulation on upper troposphere and lower stratosphere ozone distribution over Northeast Asia” by Liao et al
This study primarily evaluates the prevailing synoptic patterns over Northeast Asia and their relationship to ozone anomalies in a layer that spans both troposphere and stratosphere. Using output from ERA5 reanalysis and self-organizing maps, the leading 12 upper-level circulation patterns are identified at pressure levels of 500, 250, and 100 hPa. Ozone data from satellite and ozonesonde observations are evaluated in the context of these 12 nodes based on the self-organizing map results. The ozone results largely mirror the prevailing differences in synoptic mode and appear to be largely related to variability in the altitude of the tropopause. While these synoptic patterns seem robustly evaluated, the narrative and analysis metrics used to interpret and assess the role of stratosphere-troposphere exchange for the observed co-variability in ozone is weakly justified. To this end, I provide detailed summaries of my impressions and suggested revisions below. A few suggested technical edits follow.
Major Comments
- The foundation of the composition analysis direction in the paper is an assumption that one can take the average tropopause altitude over the study region to represent a distinction between upper troposphere and lower stratosphere as a basis for transport analysis and interpretation. Unfortunately, this assumption is immediately invalidated by the fact that approximately half of the domain (the southern portion) is typically characterized by tropical tropopause heights uniformly near 16 km and the other half (the northern portion) is typically characterized by midlatitude tropopause heights nearer the stated 11 km level. The transition between these two modes in tropopause altitude is referred to as the tropopause break and there are both vertical and mostly lateral stratosphere-troposphere exchange processes that take place in this region. Ultimately, this makes a constant-altitude analysis like that included in the present study intrinsically flawed for assessing the role of stratosphere-troposphere exchange to the UTLS ozone budget (or composition of other gases) unless the altitude of the tropopause is considered. Furthermore, none of the pressure levels considered lie fully within the stratosphere and one likely lies fully within the middle-to-upper troposphere (500 hPa). The 100 hPa surface, in particular, lies at the approximate level of the tropical tropopause, which is demonstrated well in Figure 7. The select altitude layers in Figure 4 also do not reliably fall within (or are suitable characterizations of) the upper troposphere or lower stratosphere. Ultimately, this means that all the composition analysis is dominated by variability in the height of the tropopause that follows latitudinal meandering of the subtropical jet and tropopause break. The ozonesonde stations used straddle this tropopause break, as evidenced by the annual variability in ozone composition shown in Figure 1. As such, assessing the role of cross-tropopause transport in accomplishing the observed composition will be unreliable unless the analysis is completed in relative altitude to a reference tropopause. Revisions to address this will be required for much of the paper.
- Using NCEP reanalysis winds to drive the trajectory calculations introduces unnecessary inconsistencies with the remainder of the analysis built upon ERA5. The trajectory analyses should instead be driven by ERA5 winds. Moreover, the transport indicators used could be made more resilient to some of the issues above if a diagnosed potential vorticity change along the trajectory is used to increase confidence in stratosphere-troposphere exchange. Several prior studies have done such with great success.
Specific Comments
Line 17: “simulating the stratosphere” should be “simulating stratosphere”
Section 2.2: need to specify the vertical grid resolution of this dataset
Section 2.3: similarly here, what vertical resolution data do you use? Tropopause definition?
Line 187: these are levels, not layers. See above major comment #1 about the inappropriate labeling/interpretation of these levels representing upper troposphere and/or lower stratosphere.
Line 232: “tracer-to-tracer” should be “tracer-tracer”
Line 245: Add “The” to the start of this sentence.
Figure 2: Since this is showing anomalies, it would be more appropriate to use a diverging color scale with distinct hues for positive and negative values (e.g., blue-red).
Line 276: should add citations to support this here.
Citation: https://doi.org/10.5194/egusphere-2023-1393-RC2 - The foundation of the composition analysis direction in the paper is an assumption that one can take the average tropopause altitude over the study region to represent a distinction between upper troposphere and lower stratosphere as a basis for transport analysis and interpretation. Unfortunately, this assumption is immediately invalidated by the fact that approximately half of the domain (the southern portion) is typically characterized by tropical tropopause heights uniformly near 16 km and the other half (the northern portion) is typically characterized by midlatitude tropopause heights nearer the stated 11 km level. The transition between these two modes in tropopause altitude is referred to as the tropopause break and there are both vertical and mostly lateral stratosphere-troposphere exchange processes that take place in this region. Ultimately, this makes a constant-altitude analysis like that included in the present study intrinsically flawed for assessing the role of stratosphere-troposphere exchange to the UTLS ozone budget (or composition of other gases) unless the altitude of the tropopause is considered. Furthermore, none of the pressure levels considered lie fully within the stratosphere and one likely lies fully within the middle-to-upper troposphere (500 hPa). The 100 hPa surface, in particular, lies at the approximate level of the tropical tropopause, which is demonstrated well in Figure 7. The select altitude layers in Figure 4 also do not reliably fall within (or are suitable characterizations of) the upper troposphere or lower stratosphere. Ultimately, this means that all the composition analysis is dominated by variability in the height of the tropopause that follows latitudinal meandering of the subtropical jet and tropopause break. The ozonesonde stations used straddle this tropopause break, as evidenced by the annual variability in ozone composition shown in Figure 1. As such, assessing the role of cross-tropopause transport in accomplishing the observed composition will be unreliable unless the analysis is completed in relative altitude to a reference tropopause. Revisions to address this will be required for much of the paper.
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