the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 16 Apr 2025
Radiative impact of increased middle atmospheric water vapour in the aftermath of the Hunga 2022 volcanic eruption at two locations in the Northern Hemisphere
Abstract. Increases in middle atmosphere water vapour as a result of the 2022 Hunga volcanic eruption have now been detected almost globally, with above-average mixing ratios predicted to persist until around 2032. Changes in the middle atmosphere water vapour volume mixing ratio impact chemical reactions in this section of the atmosphere, can result in more favorable conditions for polar stratospheric and mesospheric cloud formation, and have a significant radiative effect on the middle atmosphere and below. For this reason, precise radiative transfer calculations are important to make accurate and precise assessments of changes to both long-wave and short-wave fluxes, and how this may impact the heating rates at different heights in the atmosphere. In this study, water vapour profiles from two microwave radiometers deployed at two different latitudes in Europe are used to analyse changes in water vapour in the aftermath of the Hunga volcano, and a line-by-line radiative transfer model is used to analyse the thermal impact of this increase over Bern, Switzerland, and Ny-Ålesund, Svalbard.
Competing interests: One of the co-authors is a member of the editorial board of Annales Geophysicae.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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RC1: 'Comment on egusphere-2025-1396', Anonymous Referee #1, 19 May 2025
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Review of the manuscript
„Radiative impact of increased middle atmospheric water vapour in the aftermath of the Hunga 2022 volcanic eruption at two locations in the Northern Hemisphere“ (egusphere-2025-1396)
by Bell et al.
This study is based on MW H2O profile measurements at two NH sites (Zimmerwald, Switzerland and Ny Alesund) over the period from 2016 until the end of 2024. The Hunga H2O plume can be detected above Zimmerwald already in summer 2022, but only in mid-2023 at Ny-Alesund. Radiative transfer simulations with the ARTS code are used to determine the effects of the H2O enhancements on the net radiative fluxes, both in the shortwave (solar) and longwave (thermal) spectral regions. The resulting flux changes are small, but they are still important. The topic of the manuscript fits well within the scope of the journal, is well written and in my opinion the manuscript can eventually be published. There are a number of major and minor points that should be addressed, before the paper should be accepted in my opinion.
I see the following major points:
- Important information is missing: retrieval altitude range (i.e., range sensitive to H2O), vertical resolution of the profiles, degrees of freedom. It would also be good to show sample averaging kernels of the H2O retrieval.
- Retrieval errors are not discussed at all. How do the H2O retrieval errors translate to flux errors and errors in the heating rates. Right now, the reader does not know, whether the heating rates are larger than the corresponding errors.
- There are significant biases and discontinuities in the MIAWARA-C time series that should be addressed and understood before the dataset is used for scientific analyses.
- The net radiative fluxes should be defined early in the paper (downward – upward (my recommendation) or the other way around). This is not done, but you rather speak of upward and downward fluxes (which can in principle be negative and positive), which makes it more difficult to follow the descriptions. See also the detailed comments below.
It may well be that I didn’t understand some of the points mentioned below. If I’m wrong or if I misunderstood something, please let me know.
Detailed comments:
Line 13: “Matoza et al. (2022)”
Wrong cite command used.
Line 22: “The water vapour in the middle atmosphere is relatively stable because of the few sinks in this part of the atmosphere.”
I suggest mentioning the sinks here.
Line 51: “(Basha et al., 2023)”
Wrong cite command.
Line 85: “A balancing calibration scheme using a reference view that optimizes noise and linearity (Forkman et al., 2003).”
Sentence is incomplete.
Line 90: “Buehler et al. (2018)”
Again, the wrong cite command.
Lines 90 and 104: How are the covariance matrices chosen, both the a priori and the measurement covariance matrix? How many degrees of freedom are there? What is the vertical resolution of the profiles and the altitude range sensitive to H2O. Please provide more information on the retrievals.
Line 108: “the key advantage that is offered when compared to ground-based instrumentation is the horizontal coverage of measurements.”
And the typically much better vertical resolution, right?
Line 132: “Much has been written about the evolution of the initial water vapour plume (Schoeberl et al., 2023a; Nedoluha et al., 2024).”
I suggest to have a look at the discussion in Niemeier et al., (2023), dealing with MLS observations and ICON simulations (https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023GL106482), including a nice description of the reasons for the evolution of the H2O plume.
Line 134: “Within a month, there was a significant northward movement of the plume across the equator”
But not much further (& it would be good to mention the latitude of the volcano)
Line 134: “which was attributed to infrared cooling effects associated with the high levels of water vapour”
The strong initial descend of the H2O plume was explained by the radiative cooling by H2O, but I'm not sure about the meridional transport? I may be wrong.
Line 135: “During the descending phase of the Quasi Biennial Oscillation (QBO)”
What does this mean exactly? The QBO always descends in a way. I suggest being more precise here.
Line 141: “With MLS data, the transport of the water vapour anomaly is visible to as far north as 80.“
Not before the beginning of 2023, right? Perhaps it would be good to mention briefly, when the H2O plume reached northern mid-latitudes.
Line 143: “(see Supplementary Material).“
There doesn’t seem to be a supplement?
Line 147: “Water vapour measurements above Ny-Alesund exhibit for the MIAWARA-C and ACE-FTS slightly lower values compared to MLS retrievals,”
Not for 2019 and 2020! What is going on in these years?
Figure 1: what about the different vertical resolutions of the measurements? Were the satellite profiles convolved with the AVK of the MW retrievals? Also: why don't you show profiles?
Figure 1, right panel: what is the reason for the MIAWARA-C high biases in 2019 and 2020. They are partly larger than the HTHH effect.
Line 150: “all ACE-FTS measurements were taken from within 5 latitude and 25 longitude of the observation sites”
What are the co-location criteria for MLS?
Line 154: “The observations from Zimmerwald, BE, show that already in summer of 2022, water vapour mixing ratios at 0.1 hPa are above preceding years”
Is 0.1 hPa correct, or is it 1 hPa as in Figure 1?
159 – 164: It would of course be very good to know what this bias is due to? And how it can be corrected for. The bias is larger than the HTHH H2O anomaly in the last year, so this is a major problem in my opinion. How can we trust the measurements in 2023 and 2024?
Figure 2, right panels: There seem to be jumps and biases in the MIAWARA-C H2O mixing ratios at 1 hPA, e.g., very large values in early 2024, and a discontinuity of about 2 ppm from end of December 2023 and beginning of 2024, a 1 ppm decrease around day 300, and the large values in 2019 or 2020. I think these signatures should be understood and corrected for. Also, no error bars are given and errors are not discussed at all.
Line 174: “can remain there for much longer, on the order of years to decades,”
Multiple decades are probably unrealistic, right?
Line 178: “(by approximately SI25 %)“
Please explain what SI 25% means.
Line 180: “It is orthodox in flux calculations for climate studies to calculate changes to downward fluxes at the surface”
Radiative forcing is typically determined at the tropopause level.
Same sentence: You should define here, how your net fluxes are defined: downward - minus upward or the other way around. You use the terms "downward" and "upward" fluxes (or anomalies), but they can be negative and positive, making it more difficult to follow the descriptions, e.g.: a negative upward flux is a downward flux. I suggest to define here, how the net fluxes are defined. And sticking to this definition throughout the paper, i.e., using the terms positive/negative fluxes rather than downward/upward fluxes.
Line 182: “and at the lower measurement response limit of the microwave radiometer observations at 10 hPa.”
This should have been mentioned before. What is the lowest pressure level measurable?
Line 187: “the longwave fluxes at the top of the troposphere could be more pivotal in determining global climate patterns than those at the surface.”
"could" or is it actually the case? The statement is quite weak and I'm not sure what its intention is?
Figure 3, caption: “hight” -> “height”
Line 222: “The temperature parameter decreases with each iteration,”
What is the “temperature parameter”?
Line 229. “The total flux is then found by integrating over all frequencies and 15 elevation angles between 0 (zenith) and 90 (horizon).”
I guess you also have to integrate azimuthally, i.e., the integration is actually done over solid angles, not only the elevation angle.
Line 249: “and the full ARTS method”
The “full ARTS method” is method 1 above, right? It would be good to introduce the term "full ARTS method" in the section above.
Line 255: “standard deviation of” -> “standard deviation is”?
Line 256: “Despite this, the anomalies predicted by both methods, by comparing fluxes simulated from the climatology to fluxes calculated in the post-eruption period, a very good agreement is found. Throughout the period, the entire ARTS method showed more intense fluxes by a mean of 0.0017Wm-2,”
How is this possible? If the flux calculations for the two approaches differ by up to 0.5 W/m2, how can the differences wrt to the reference period be so small. This cannot be true. Or the description here is wrong or misleading? Or I am missing a point ...
Figure 4, caption: the caption does not describe both panels.
Also: are these daily averages? Or values for a specific local time range each day?
And these are fluxes / flux anomalies at the tropopause, right? Perhaps it would be good to mention this once more in the Figure caption.
Line 271: “The propagation of solar fluxes is …and thus IS ..”
Line 272: “but also highly seasonal dependent at high (arctic) and low antarctic latitudes.”
There is also a strong seasonal dependence at mid-latitudes.
Line 273: “A computation .. was also calculated”
Sounds a bit odd.
Line 276: “CO_4”??
Does this exist? Or do you mean methane?
Line 277: “Coddington et al. (2017)”
Wrong cite command.
Line 296: “.. the retrievals from Ny Alesund TAKE”
Same sentence: “take a lower a priori profile”
“Lower profile” can have different meanings.
Line 301: “The anomalies presented in figure 5 show a positive response (meaning less downwelling flux) for”
The flux has not been properly defined above and this should be done (see my earlier comment). As far as I know the standard definition is downward – upward, implying that a positive net flux leads to more downwelling radiation.
Line 312: “In contrast to the downward flux measurements”
Why "Downward flux measurements" ? You did not use flux measurements, did you.
Figure 5: These are not the fluxes, but the flux anomalies, right? Both the caption and the y-axis label state that fluxes are shown. This is not correct, as far as I can tell?
Line 319: “By contrast, at Ny-Alesund, the bulk of the water vapour increase extends to lower altitudes (and cooler stratospheric conditions), thereby inhibiting the net emission of energy to space.”
I am wondering at what altitude the H2O absorption bands really become optically thin?
Figure 6, top panel: The shortwave flux is (basically) zero, but the curves for the LW and the sum are not identical. Something must be wrong here.
Figure 6, top panel: I'm not sure I understand fully, why the SW flux anomaly is zero at the TOA, but not in the lower atmosphere. Perhaps you can explain this briefly.
Both panels of the Figure: again, the figures show flux anomalies, not fluxes themselves?
Line 327: “The mesosphere exhibits an atmospheric circulation from the summer pole to the winter pole, as air rises from the summer pole,”
The meridional circulation from summer to winter hemisphere is not a consequence of the upwelling above the summer pole and the downwelling at the winter pole, but caused by breaking gravity waves.
Determination of heating rates: it would be good to briefly explain, how the heating rates were calculated. I also suggest showing the central formula.
Figure 7, bottom panel: How do you deal with the high bias of the Ny-Alesund measurements during the two years before the eruption? They will directly affect the heating rate anomalies, i.e., the heating rate anomalies for Ny-Alesund will be systematically wrong and the results shown are not reliable. Or am I missing a point?
Figure 8, bottom panel: From this depiction it is not possible to tell what part of the signatures is related to HTHH H2O. What about interannual variability and the high bias in H2O at Ny-Alesund in 2019 and 2020?
Figures 7 and 8: The pressure range sensitive to H2O is of course important and should be mentioned.
Line 342: “Shortwave absorption by water vapour molecules is substantially ..”
So, the above and Fig. 7 only considers the LW effect. This must of course be mentioned in the paragraph above.
Line 351: “.. the combined effect .. is combined”
Citation: https://doi.org/10.5194/egusphere-2025-1396-RC1
Data sets
Retrieved Water Vapour Profiles from MIAWARA-C at the Ny-Ålesund Observatory, Svalbard Alsitair Bell and Axel Murk https://doi.org/10.60897/gk0h-3b75
Retrieved Water Vapour Profiles from MIAWARA at the Zimmerwald Observatory, Bern, Switzerland Alistair Bell and Axel Murk https://doi.org/10.60897/pdyc-8v84
Model code and software
pyarts-fluxes Manfred Brath https://github.com/atmtools/pyarts-fluxes
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