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the Creative Commons Attribution 4.0 License.
| 02 Jun 2025
Adiabatic versus diabatic transport contributions to the ozone budget in the northern hemispheric upper troposphere and lower stratosphere
Abstract. Ozone in the extratropical lowermost stratosphere (LMS) is important for the local radiative balance and contributes to the tropospheric and near-surface ozone burden via stratosphere-troposphere exchange. Here, we aim to deepen our understanding of the transport contributions to LMS ozone in the Northern Hemisphere by studying the ozone budget in isentropic coordinates, which allows for a clean distinction of adiabatic and diabatic transport contributions. This is done by analyzing 20 years of ERA5 reanalysis output on model levels and a free-running simulation using the EMAC chemistry-climate model. Our analysis confirms that the ozone tendencies in the extratropical LMS at high latitudes are dominated by diabatic mean flow advection (associated with downwelling within the Brewer-Dobson circulation) and quasi-horizontal adiabatic eddy mixing due to planetary- and medium-scale Rossby waves. These transport contributions are somewhat weaker during summer compared to winter, although seasonality is found to be weaker in the LMS compared to higher altitudes. Horizontal mean flow advection is found to be relevant near the tropopause and just above the subtropical jet core. Notably, vertical (i. e., diabatic) eddy ozone transport is found to be important near the tropopause. While the adiabatic eddy ozone fluxes in the LMS are consistent with diffusive, down-gradient eddy transport, the vertical eddy ozone transport also features up-gradient regions, which by itself would act to reinforce the background ozone gradients near the tropopause. Closer analysis reveals that this is due to long-wave radiative damping of planetary waves, which acts to dampen the down-gradient horizontal eddy transport.
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RC1: 'Comment on egusphere-2025-2195', Kris Wargan, 28 Jun 2025
Review of “Adiabatic versus diabatic transport contributions to the ozone budget in the northern hemispheric upper troposphere and lower stratosphere” by F. Harzer et al.
This paper uses ERA-5 and EMAC output to provide a detailed analysis of the contributions to the lowermost stratosphere (LMS) zonal mean ozone transport from advection and eddy mixing. In addition to the known dominant role of diabatic advection and isentropic mixing, the authors demonstrate that horizontal advection and vertical mixing play a non-negligible role near the tropopause. Furthermore, they separate the vertical eddy fluxes into radiative and non-radiative components and identify signatures of up-gradient mixing near the subtropical jet stream. I believe those are novel results that will be of interest to ACP readers. The paper is very well written. The findings are clearly supported by figures, including those in the supplementary material. I have only a few very minor comments and suggestions.
Minor comments
Please, define all symbols used in equations 1–3.
L34. “Quasi-observed” is an interesting term that I don’t think I’ve seen before. I take it to mean that reanalysis circulation, while not directly observed, is tightly constrained by data. I’m not sure if it will be clear to readers less familiar with data assimilation. I like the term, but perhaps one sentence of explanation would be useful.
L56. MLS is assimilated starting 2004 but this analysis uses the 2000–2019 period. Are the results from 2000–2004 as reliable as in the later period? I would think that assimilation of MLS makes a big difference especially in the UTLS.
L58. How long are these forecasts? Presumably very short (several hours?).
LL58–62. More about this. Aren’t the temperature tendencies constrained by assimilation to some extent? Long wave cooling depends strongly on temperature, which is assimilated.
L67-68. Does it mean that the spinup was 2000–2001 or 1998–1999?
Figure 1 caption. Please, specify which dotted line marks which tropopause. I’m guessing the blue dotted line is the 2-PVU one.
LL96–100. All symbols used in Equation 1 should be defined in the text.
Figure 3. Would it make sense to include analysis tendencies in panels a and b? Would the sum of all the terms (and the analysis tendency) reproduce the “total ozone tendency” shown in gray? It looks to me like in EMAC total transport (and horizontal eddy transport) is balanced by chemistry near the STJ. I wonder if analysis tendencies play an analogous role in ERA-5 to produce a “total tendency” profile very similar to that in EMAC. I also wonder if the black line in Fig. 3a would look more like 3c if the analysis tendency was included in it. The differences between panels a and b between 20N and 40N are substantial (even the sign of the net transport tendency is different) and require more explanation than what’s provided in the text.
Figure 3. The gray shading should be described in the caption, not just in the main text. Also, is it possible to call this something other than ”total ozone tendency”, which suggests vertically integrated ozone?
L213. This is very interesting. Up-gradient eddy transport seemingly contradicts the statement made in lines 201–202 about eddy transport (horizontal, in that case) being generally down-gradient. Perhaps it would be worth mentioning right here that this is explained later in the discussion of Figure 9, because eddy transport is diffusion-like and uphill diffusion is counterintuitive.
Equation 2. Again, most symbols are undefined.
LL244-248. I think this comparison should be explicitly shown in a figure to make the argument more quantitative.
Technical corrections
L31. “our knowledge on…” “our knowledge of…”
L252: “rather plays a minor role” “plays a rather minor role”
Citation: https://doi.org/10.5194/egusphere-2025-2195-RC1 -
RC2: 'Comment on egusphere-2025-2195', Anonymous Referee #2, 05 Aug 2025
Review of “Adiabatic versus diabatic transport contributions to the ozone budget in the northern hemispheric upper troposphere and lower stratosphere” (F. Harzer et al.)
This is a nice and interesting paper that improves our understanding of the contributions of different transport processes (in particular advection and mixing) to the total ozone budget in the extratropical UTLS region. The authors show very clearly how vertical advection and horizontal eddy transport are the most important contributors, although they demonstrate that horizontal advection and vertical mixing also play an important role along the extratropical tropopause. In addition, they analyse radiative and non-radiative effects on the vertical eddy ozone flux, finding that the upward eddy ozone fluxes near the tropopause and the STJ core are mainly due to radiative dumpling within planetary-scale Rossby waves.
Overall, I consider the paper to be very well-written, with several very nice and novel results well supported by the figures. I consider it to be of interest to ACP readers and I recommend it for publication after addressing a few minor comments and suggestions which I list below.
- Line 24: Perhaps it would be a good idea to include here the recent paper by Benito-Barca et al. 2025 (https://doi.org/10.1029/2024JD042412), which confirms the importance of transport processes in ozone trends in the most recent CCMs (CCMI-2).
- Line 67: This is a bit confusing… the spin-up years are 1998-99 or 2000-2001? Please clarify.
- Line 68: Just out of curiosity, why do you use ERA5 to prescribe SSTs and SICs instead of observations (e.g. HadISST or ERSST)?
- Line 85: There is something here that I have not fully understood. In line 48 it says that ERA5 has been interpolated to match the resolution of the model. However, now here it says that the model has been interpolated to the same levels as ERA5, which “led to slight vertical oversampling of the EMAC data”. Why isn't ERA5 interpolated directly to the same levels as the model, instead of interpolating the model to ERA5 levels? I wonder if the vertical oversampling of the model could cause problems in the region near the tropopause. I think this point needs to be better clarified, as understanding the vertical resolution of each dataset is important for interpreting the results in the UTLS region.
- Caption Figure 1: The thermal tropopause is indicated by the blue dotted line and the 2 PVU isolines by the black one, right? Please indicate in this caption.
- Line 99: It is not clear to me what you mean by ‘overbars (with asterisks)’. What overbars without asterisks indicate? Also, there are symbols that are not defined. Please define all symbols in the equation.
- Figures 2b and 2f: I am curious about the negative ozone tendency on the equatorial flank of the subtropical jet in EMAC, which does not appear in ERA5. Do you have any idea why this might be happening?
- Line 124: please define STJ (here is the first mention).
- Line 137: Here, in addition to the differences between EMAC and ERA5, I think it is important to mention that the challenge of this region can also be seen in the difference between the ozone tendency obtained directly from the actual ozone distribution and the one obtained through the sum of the transport and chemical terms. I get the impression that equation (1) closes the total ozone budget reasonably well north of 40ºN but has problems in the subtropics.
- Lines 176-178: my impression is that for large-scale waves (WV 1-3) there is also a stronger transport during winter and spring.
- Line 236/ Equation 2: For those who are unfamiliar with these equations, it would be convenient to define all the symbols.
- Caption of Figure 9: Is the black contour interval in Figure 9b in hPa or Pa units? Is it possible that there are anomalies of more than 100 hPa? It sounds strange to me...
- Line 248 (and also 285): I think this result is interesting enough to include a figure to see it explicitly (at least as supplementary material).
Citation: https://doi.org/10.5194/egusphere-2025-2195-RC2 -
AC1: 'Comment on egusphere-2025-2195', Frederik Harzer, 03 Sep 2025
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2025/egusphere-2025-2195/egusphere-2025-2195-AC1-supplement.pdf
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