A statistical analysis of the occurrence of polar stratospheric ice clouds based on MIPAS satellite observations and the ERA5 reanalysis

Zou, Ling; Spang, Reinhold; Griessbach, Sabine; Hoffmann, Lars; Khosrawi, Farahnaz; Müller, Rolf; Tritscher, Ines

doi:https://doi.org/10.5194/egusphere-2024-547

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https://doi.org/10.5194/egusphere-2024-547

Preprints

14 Mar 2024

| 14 Mar 2024

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

A statistical analysis of the occurrence of polar stratospheric ice clouds based on MIPAS satellite observations and the ERA5 reanalysis

Ling Zou, Reinhold Spang, Sabine Griessbach, Lars Hoffmann, Farahnaz Khosrawi, Rolf Müller, and Ines Tritscher

Abstract. Small-scale temperature fluctuations can play a crucial role in the occurrence of ice clouds. This study analyzes a decade of ice polar stratospheric clouds (PSCs) occurrence obtained from Michelson Interferometer for Passive Atmospheric Sounding (MIPAS/Envisat) measurements. The points with the smallest temperature difference (ΔT_{ice_min}) between the frost point temperature (T_ice) and the environmental temperature along the limb line of sight are proposed here to identify the location of ice PSC observations. In MIPAS observations, we find approximately 56 % of the Arctic and 28 % of the Antarctic ice PSCs are detected at temperatures above the local T_ice based on ERA5 data at ΔT_{ice_min}. Ice PSCs above T_ice are concentrated around mountain regions and their downwind directions. A backward trajectory analysis deduced from the ERA5 reanalysis is performed to investigate the temperature history of each ice PSC observation. Based on 24-hour backward trajectories, the cumulative fraction of ice PSCs above T_ice increases as the trajectory gets closer to the observation point. The most significant change of the fraction of ice PSCs above T_ice occurs within the 6h preceding the observations. There is an impact of previous temperature fluctuations on the interpretation of MIPAS ice PSC observations. At the observation point, the mean fractions of ice PSCs above T_ice taking into account temperature fluctuations along the backward trajectory are 33 % in the Arctic and 9 % in the Antarctic. The results provide quantitative assessments of the correlation between orographic waves with ice PSCs above T_ice based on the Lagrangian model by using MIPAS measurements and ERA5 reanalysis data. Additionally, the observational statistics presented can be utilized for comparison with chemistry-climate simulations.

Received: 23 Feb 2024 – Discussion started: 14 Mar 2024

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Ling Zou, Reinhold Spang, Sabine Griessbach, Lars Hoffmann, Farahnaz Khosrawi, Rolf Müller, and Ines Tritscher

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RC1:
'Comment on egusphere-2024-547', Anonymous Referee #1, 22 Apr 2024 reply
In this article, the authors propose a way to evaluate the fraction of ice Polar Stratospheric Clouds (PSC) affected by short-lived small-scale temperature fluctuations generated by gravity waves, using a combination of MIPAS PSC detections and reanalyses temperatures. They support their analysis by the use of backward trajectories to document the temperature history of air masses. Their results bring new quantified information on the relationship between ice PSC and gravity wave activity over both poles.

The article describes the methodology and results in a very clear way and is extremely readable. The proposed methodology is sound, the results are consistent with what is known about PSCs and gravity waves, while bringing new information that completes what we know in a useful manner. The authors clearly have a good handle on the data from MIPAS and reanalyses, which are very well described. References abound, the bibliography is complete and well selected. The figures are well designed and convey their message with clarity. I support the publication of this article in ACP, once the few remarks I have below have been addressed.

Major comment
My main comment is a suggestion to the authors to clarify their intent, make some of their assumptions explicit, and better relate the approach they've devised with the scientific questions that are raised. As currently written, it is a bit hard to understand why the authors are doing what they are doing.

For instance, the abstract begins with "Small-scale temperature fluctuations can play a crucial role in the occurrence of ice clouds". This may be true, but rather vague. What small-scale temperature fluctuations are we talking about? Are we talking about temperature fluctuations created by mountain/orographic waves? Are "ice clouds" meant to be ice PSC? This first sentence is the only one that suggests what questions the authors are interested in. The rest of the abstract describes the methodology and results, but fails in my opinion to relate them to the scientific questions raised by the first sentence. After the first sentence, the abstract goes on explaining that the study is analyzing PSC occurrences from MIPAS, using the smallest difference temperature etc. How are all these steps related to better understanding the crucial role the small-scale temperature fluctuations play on the occurrence of ice clouds? Please fill in the gaps.

I was hoping the introduction would clarify these points, by describing how the methodological approach chosen by the authors is appropriate to answer the questions raised by that first sentence, but in my opinion it suffers from the same problem as the abstract. The first 3 paragraphs do a good job explaining why temperature perturbations generated by gravity waves affect the formation of PSC. The last paragraph dives straight in a summary of the selected methodology, but does not relate this methodology to the questions raised in the previous paragraphs, or explain what insights it will bring. I feel there is a missing paragraph before this one that should explain why MIPAS detecting a PSC, when ERA5 says the coldest synoptic temperatures are too warm to support PSC formation, is a sign of gravity wave activity. I have felt this as an implicit assumption throughout the whole text -- please make it explicit. I understand that the authors perhaps want to be careful and avoid stating it for some reason, but not making it explicit makes it hard to appreciate what the authors' work brings to the subject under study. In addition to the abstract and the introduction, the link between the authors' approach and gravity wave activity needs to be made clearer in section 4 and 5 also.

Minor comments
Title: the current title is very generic. Please at least refer to gravity waves in it.

L. 1: "a decade" please state which years we are talking about here (2002-2012 ?)

L. 18: "The surfaces of PSCs" I don't think we can say that clouds have surfaces. Maybe "the surface of PSC particles"

L. 46-47: This paragraph begins by explaining the primary focus of the study is to investigate the occurrence of ice PSC observed by MIPAS and characterized by temperatures above the ice existence threshold. Please explain why, when interested in the relationship between ice PSC and gravity waves (as the beginning of the section suggests), it would be a good idea to do that (see major comment).

L. 76-85: Here you explain your alternative to using the tangent point for identifying the location of a PSC that MIPAS has detected along its line of sight. Have you tried to investigate the spatial distribution of temperatures along the line of sight? Is it frequent for cold temperatures to be spatially spread along the line of sight far from the point identified as the coldest? Or can you have two locations along the line of sight with similar coldest temperatures? This would mean the PSC location is affected by strong uncertainties. In other words, how well defined spatially is the ΔTicemin?

L. 101, 103: References to Hoffmann et al. 2017b and a are missing parentheses. Also please fix the citations (a should be cited before b).

Section 2.2: Please explain why you use both ERA-Interim *and* ERA5. Spatial/temporal resolutions appear similar, so why isn't it possible to pick just one reanalysis dataset? If using two datasets is mandatory, could you talk about how potential disagreements in polar stratospheric temperatures from both datasets would affect your results?

Section 2.2: I was under the impression that Hoffmann et al 2017 (the one about Concordiasi) suggested that ERA-Interim suffers from a zonally increasing warm bias in the south pole. Is that affecting your results in any way? Is this bias corrected in ERA5?

L. 116: what does it mean for the Lagrangian transport to be significantly impacted? Is it better, worse, something else? Does this suggest that using ERA5 (instead of ERA-Interim) for locating and quantifying the ΔTicemin would lead to different results?

L. 119: Here you state that your aim is to "conduct a statistical analysis of ice PSCs where the temperature at the MIPAS observation is above the frost point temperature". Again, in my opinion you have not made clear enough why you might want to do that (see main comment). Please make explicit the link with gravity wave activity.

L. 135: After lines 76-85, this is the second time you explain your alternative to the tangent point for PSC detection. You explain it again on lines 240-242. Please try to limit these explanations.

L. 139: "highest occurrence frequency": over which time scales? 2% is not a lot.

L. 146: Here you describe that most ΔTicemin are found below the frost point. Reading this confused me at first, since on lines 119 you explain that your focus is on points that are above the frost point. I think you could limit reader confusion that making it clearer from the start that you expect ΔTicemin above the frost point to be rare, and that you take them as the sign of small-scale temperature perturbations from gravity waves, i.e. by clarifying why you are doing what you are doing (see main comment).

L. 148-149: The discussion about ice PSC particles nucleating at temperatures colder than Tice mirrors the one found on lines 123-125, but with fewer details. Please find a way to combine both discussions. I think the discussion should happen in Section 3.1, but with the details found in lines 123-125.

L. 151: "T-Tice-3K" I'm guessing here you mean Tice-3K

L. 175: "Fig. 5 displays the fraction of ice PSCs above Tice". I did not understand this part. As I understand, MPTRAC calculates backward trajectories starting from the point where a PSC was located according to MIPAS observations + ΔTicemin from ERA5. MPTRAC provides the evolution of temperature and other parameters along the backtrajectory, but has no way to know when a PSC was present in the air mass that it is tracking, and does not provide that information. Are you assuming that the air mass where a PSC was detected at t=0 contains a PSC over the previous 24 h through the entire backtrajectory? Please clarify my misunderstanding. (also please fix related wording line 309)

L. 176: "(t) t=-24 (h)" -- do you mean t=-24h ?

L. 177-178: It appears strange to me to say the fraction decreases by going from t=0 to t=-6h, ie going backwards in time. It would appear more natural to say the fraction increases from t=-6h to t=0.

L. 180: "6h before the observation, temperatures of most ice PSC... are below Tice". If I read Fig. 5 correctly, it looks to me like temperatures of most ice PSC are below Tice at all times.

Section 3.4: If I understand correctly, your main hypothesis is that the influence of gravity waves on PSC can be inferred by ΔTicemin being positive, as it means that the reanalysis temperatures are failing to capture small-scale temperature variations due to GW. In this section, however, you look for short-scale temperature variations within the reanalysis as indicators for the influence of gravity waves. There seems to be a contradiction -- either GW influence is captured in reanalyses, or it isn't. Please clarify my misunderstanding. Are you expecting ERA5 (used by backtrajectories) to better capture the gravity wave influence on temperatures than ERA-Interim (used for ΔTicemin)? Why?

L. 209-210: "This observation strongly suggests a correlation with orographic waves with ice PSCs above Tice". This reads strange to me. We *know* that orographic waves trigger the formation of ice PSCs. Unless I'm mistaken, this is actually the (unstated) reason why you look for positive ΔTicemin -- because they are the sign of short-scaled temperature perturbations, that are not well captured by ERA-Interim, and are generated by gravity waves. It looks like you are trying here to avoid stating that we already know that orographic waves trigger PSC formation. Please be explicit about your assumptions.

L. 218-223: I am very confused by this paragraph. I understand your results find that the fraction of T > Tice related to mountain waves are higher in the Arctic than in the Antarctic (l. 220-223), no problem there. But, unless I'm mistaken, your main point is that the fraction of T > Tice related to mountain waves decreases as it gets closer to the observation point. This is what you open your paragraph with, and the main result from Table 1. And yet you make no attempt to explain this decrease. Why is this fraction decreasing as we get closer to observation point? I personally find this result perplexing.

L. 230: double parentheses

L. 226-238: This paragraph is the closest you get to state your assumption that PSCs observed at ΔTicemin > 0 are due to small-scale temperature fluctuations from gravity waves that brought temperatures below Tice in a way that is not captured by ERA-interim reanalyses. Still, you need to make this reasoning explicit. Also: if the main hypothesis is that ERA-Interim misses temperature fluctuations that generate PSCs, why should we trust the location of the ΔTicemin according to ERA-Interim? The PSC detected by MIPAS might be somewhere else along the line of sight, in a place more affected by gravity waves (which are not well captured by ERA-Interim). Please address this somehow.

L. 317: "with temperature fluctuations at the observation point" -- as I understand it, the observation point is fixed in time and space. How can there be temperature fluctuations are the observation point then?

L. 319: "... suggests a correlation with orographic waves with ice PSCs above Tice". That ice PSC are correlated with orographic waves is a given from the beginning -- you even explain the mechanism in the introduction. You could say that your results are a strong confirmation of these correlations.

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Citation: https://doi.org/10.5194/egusphere-2024-547-RC1

Ling Zou, Reinhold Spang, Sabine Griessbach, Lars Hoffmann, Farahnaz Khosrawi, Rolf Müller, and Ines Tritscher

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Short summary

This study quantified the correlation between orographic waves with ice PSCs above frost point (T_ice) based on the Lagrangian model by using MIPAS observations and ERA5 reanalysis. We found that ice PSCs above T_ice with temperature fluctuations along the backward trajectory are 33 % in the Arctic and 9 % in the Antarctic. This quantitative assessment enhances our understanding of ice PSCs, and the observational statistics can be utilized for comparison with chemistry-climate simulations.


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