the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 Sep 2023
Gravity waves generated by the Hunga Tonga-Hunga Ha‘apai volcanic eruption and their global propagation in the mesosphere/lower thermosphere observed by meteor radars and modeled with the High-Altitude General Mechanistic Circulation Model
Sharon L. Vadas
Erich Becker
Alan Liu
Alexander Kozlovsky
Diego Janches
Zishun Qiao
Witali Krochin
Guochun Shi
Jie Zeng
Peter Brown
Denis Vida
Neil Hindley
Christoph Jacobi
Damian Murphy
Ricardo Buriti
Vania Andrioli
Paulo Batista
John Marino
Scott Palo
Denise Thorsen
Masaki Tsutsumi
Njål Gulbrandsen
Satonori Nozawa
Mark Lester
Kathrin Baumgarten
Johan Kero
Evgenia Belova
Nicholas Mitchell
Abstract. The Hunga Tonga-Hunga Ha‘apai volcano erupted on 15th January 2022, launching Lamb waves and gravity waves into the atmosphere. In this study, we present results using 13 globally distributed meteor radars and identify the volcanic-caused gravity waves in the mesospheric/lower thermospheric winds. Leveraging the High-Altitude Mechanistic General Circulation Model (HIAMCM), we compare the global propagation of these gravity waves. We observed an eastward propagating gravity wave packet with an observed phase speed of 240 ± 5 5.7 m/s and a westward propagating gravity wave with an observed phase speed of 166.5 ± 6.4 m/s. We identified these waves in the HIAMCM and obtained very good agreement of the observed phase speeds of 239.5 ± 4.3 m/s and 162.2 ± 6.1 m/s for the eastward and the westward waves, respectively. Considering that HIAMCM perturbations in the mesosphere/lower thermosphere were the result of the secondary waves generated by the dissipation of the primary gravity waves from the volcanic eruption affirms the importance of higher-order wave generation. Furthermore, based on meteor radar observations of the gravity wave propagation around the globe, we estimate the eruption time to be within 6 minutes of the nominal value of 15th January 2022 04:15 UTC and localized the volcanic eruption to be within 78 km relative to the World Geodetic System 84 coordinates of the volcano confirming our estimates to be realistic.
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Gunter Stober et al.
Status: open (extended)
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RC1: 'Comment on egusphere-2023-1714', Anonymous Referee #2, 23 Oct 2023
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This study investigates the secondary gravity wave (GW) response to Hunga Tonga-Hunga Ha’apai (HTHH) volcanic eruption, which took place on the 15th of January 2022. The work combines an extensive network of meteor radar wind measurements with simulations performed by the MESORAC/HIAMCM modeling system, to identify the global characteristics of the secondary GW response. The results add to the rapidly expanding body of literature surrounding the whole-atmosphere impacts of the HTHH event, for which secondary GWs play an important role in a range of atmospheric and ionospheric processes.
While the paper presents interesting and relevant results, the manuscript can be improved by including a more detailed description of a number of analysis and discussion points. Since many figures can also be improved, a considerable revision is in place. Some general and specific comments are given below.
General comments
Experimental setup
A more detailed (but concise) description of the modeling setup (as described in Vadas et al. 2023a) should be included, which includes a discussion of the impact of dissipation and resolution on the analysis presented in the current work.
By the nature of the experimental design, only the effects of secondary gravity waves can be investigated using the HIAMCM. In the discussion (lines 277-278), it is mentioned that the HIAMCM zonal and meridional winds exhibit variable phase relations between both wind components and a much less coherent phase front than expected for a Lamb wave. However, there is no mention of Lamb waves being present in the analysis or HIAMCM results. Can the authors elaborate on how the results support this discussion point?
A discussion should be included on the fact that the HIAMCM model generally underestimates GW amplitudes by ~50% and up to 150% (Vadas et al., 2023a), which would then include a comparison between the fitted observed and simulated GW packet amplitudes. Currently, it is difficult to make out whether or not the HIAMCM model adequately captures the wave forms needed to explain the observed HTHH wave-response. The concluding statement (lines 312-315) that the secondary GWs can explain the majority of the analyzed radar wind GW perturbations therefore does not carry much weight, given also that other possible sources of wind perturbations are not discussed in detail.
Polar vortex and Figure 2
The statement that HIACMC shows that the polar vortex disturbs the westward traveling GWs (lines 120-124) should be discussed in more detail. At first glance, a discussion of the impact of the polar vortex on the HTHH GWs is not given in the cited literature (e.g. Vadas et al., 2023b). For example, what are the mechanisms behind the polar vortex altering GW propagation, and what was the state of the polar vortex during the HTHH event? Fig. 2 does not seem to show any clearly discernible polar vortex structure, as alluded to in the text.
On lines 175-176 the impact of the polar vortex on the observed wind variations is further noted as being caused by GWs generated by the vortex. As a reader, it is impossible to verify such a statement, given also the other possible sources of wind variability mentioned in the text. While it is well known that the polar vortex can generate GWs, how and to what extent that applies to the current analysis should be discussed in more detail (including a discussion on the current state of the vortex). It is further noted that orographic GWs add noise to the European sector winds. But can orographic GWs not just as well add noise to the South American sector winds?
Could the authors discuss the source of wind perturbations above Europe and Scandinavia in Figure 2? These are already present at the 12:00 snapshot (Fig. 2a), and grow rapidly before the GWs reach the sites (16:00 snapshot, Fig. 2b), with the ‘noise’ quickly becoming comparable in magnitude (or even greater than) the GW signal itself. It seems hard to argue that this is due to the polar vortex, since both the ‘Tonga run’ and ‘base run’ simulations specify the same vortex structure, and the noise is limited to the European sector.
Figures 3, 4 & 5These figures should have labels added to the axis, and the font size should be enlarged to make the legend and labels more readable. Due to the small panel size, variations in the observed winds are difficult to distinguish, giving the figures an overall noisy impression. The y-axis scale on the observation can probably be changed to limits [-40, 40] without loss of information.
Throughout the text it is argued that increases in wind variability can be identified by looking at these figures (e.g., lines 176-178 and line 239), but this is not so straightforward. Perhaps increases or decreases in wind variability at certain time periods can instead be quantified, for example using a sliding window approach of the calculated variance. Currently, it is difficult to judge if a supposed increase in variability (e.g. before the arrival of the westward HTHH GWs at the MEN radar) is just that, or if the increased in variability is due to the presence of some (possibly unaccounted for) waveform.
As the left-hand and right-hand panels of Fig.3,4,5 show fundamentally different things, it would be beneficial if the fitted waveforms and amplitudes are overlain (and possibly zoomed in on) for both the observatory and simulated winds. In addition, showing the observed and modeled wind fluctuations above and below each other (rather than side-by-side), would make the difference in arrival time easier to distinguish.
As mentioned earlier, HIAMCM simulated GWs are smaller by ~50% than observed due to the choice of turbulent diffusion coefficient D0 (Vadas et al. (2023a)). The impact of D0 on simulated GW amplitudes should therefore also be discussed within the context of Fig.3,4,5, including how it impacts the choice of y-axis scaling for the right-hand plots, and the amplitude of the fitted waveforms.
Q2DW
The impact of the Q2DW on the GW propagation lacks detail. It is not clear from the text or earlier cited work what the amplitude of the Q2DW is and how it compares to the concurrent tidal and low-frequency GW amplitudes. Discussing the mechanisms of the Q2DW impacts is important, as one of the concluding statements of this work is that the observations reveal that the Q2DW played an important role in the GW propagation.
L263 - Stober et al. (2023) are cited in support of the statement that the GW packet is doppler shifted due to a strong Q2DW, thus explaining the difference in arrival time between the observations and HIAMCM for McM and DAV. It is unclear how the cited work supports the current statement; could this be elaborated further?
L205-208 That the GW packet faced a strong headwind towards the south due to the presence of the Q2DW would surely depend on the phase of the Q2DW in the region of propagation? While on lines 204-205 it is clarified that the Q2DW meridional wind component showed clear northward winds over the South American continent, the Antarctic stations (and the associated propagation paths) are at considerably different longitudes (as shown in Fig. 1). Given the zonal wavenumber 3 structure of the Q2DW, it is not obvious what the phase of the Q2DW was (i.e., headwind or tailwind) facing the Antarctic stations, and it may well have been a tailwind.
While the Q2DW was said to be present, its magnitude and relative importance versus the semidiurnal and diurnal tidal mode amplitudes is not discussed. That the superposition of these wave components leads to high wind speeds, does not necessarily imply that Q2DW winds are especially large (even though this may very well be the case), or that the semidiurnal and diurnal tides do not have a similarly large impact on GW propagation. The presence of other (large amplitude) tidal waves along the different great circle propagation paths (and their representation in the HIAMCM model), should therefore be discussed in more detail.
Other comments
L17 - The wording would be more scientific if ‘huge’ is removed.
L22 – Total Electron Content (TEC) abbreviation should be defined.
L43 – The way the sentence starting with “In this study,...” is worded, suggests that TEC perturbations are also discussed in the current work, which they are not.
L47 - "strongest gravity waves launched by the HTHH". Maybe this is a matter of semantics, but can secondary GWs really be considered as being launched by the HTHH? Perhaps ‘resulting from’ would be more appropriate.
L62 – If the 2 km resolution with a 5-kilometer vertical averaging window centered around the respective altitude is identical to the technique described in Stober et al. 2023, this article should be cited here. If the methodology has been substantially modified since Stober (2023), more information about the analysis technique should be given instead.
Fig. 2 - The quality would be improved if the axis labels would not overlap with the plot.
L77 - “Could” to “can”.
L78 – The dashed red line not only connects (or lies close to the) ALO and HTHH, but also KUN, MEN, and CAR, as noted in the caption of Fig. 1. The caption and text statements should be the same.
L92 – Sentence can be clarified by rewording to "...specify realistic large-scale meteorological fields through which the resolved GWs propagate (Becker et al., 2022)".
L100 - “cover” to “covering”
L124 - ‘This is consistent…’ should be reworded to clarify what exactly ‘this’ refers to. Currently it can be read as if the cited work also supports the result that the polar vortex disturbs GW propagation, which does not appear to be the case.
L144 - “theoretical arrival times” suggests the arrival times are based on theory, while they are derived from best-fit phase speed using all stations that detected this wave. Perhaps ‘derived’ would be more appropriate?
L145-150 In the discussion (L243 - 244), it is mentioned that the distance between the solid left and right vertical lines is 3 hours. Please add this information here already.
L152-154 Mentioning the power of the CMOR meteor radar relative to the other radars, suggests that the radar power may represent a considerable source of variability for the other radars. Considering also the impact of diurnal meteor count variations on wind variability alluded to on lines 163-165, the impact of the system power as well as diurnal meteor count variations on wind variability should be discussed in more detail.
L165 Perhaps ‘This provides…’ can be reworded to ‘Our results therefore support…’, as it currently reads as if the meteor count rate itself somehow impacts asymmetric azimuthal GW propagation.
L217 ‘The reasons for..’ could be changed to ‘Possible reasons for…’, since the discussion of the Q2DW impact on the DAV and McM stations is largely speculative. Furthermore, the discussion on lines 254-266 mention the impact of Southern Hemisphere MLT temperatures, which does not make sense chronologically. Merging the Q2DW and MLT temperature impacts on the DAV and McM stations into a single section would make sense.
L223 - MR should be defined.
L252-253 In Vadas and Azeem, the mesopause sound speed (at 98 km) was estimated to be 270 m/s and not 280 m/s. It would also be nice to see in the text for which conditions this number was obtained (March 25 at 100°W and 34°N).
L270 - It is unclear where the wave speed of 202.5 m/s comes from. This number has not been mentioned earlier in the text.
L279 - It is unclear what the visibility of a Lamb wave in the meteor radar wind data entails (Lamb waves are noted as being "hardly visible" - can evidence of this wave somehow be seen in Fig 3,4,5?). The sentence starting on line 279 also seems to imply that Lamb waves are effectively filtered out by the spatial and temporal averaging kernel employed in this work. If so, the statement that the observed wave period and horizontal wavelength observed in this work (lines 275-277) cannot be reconciled with that of a Lamb wave does not make sense, since by design, Lamb waves cannot be measured.
Fig. 7 - It would be good to label the ALO stations for the eastward and westward wave packets separately, and to add gridlines to the figure.
Many of the DOI’s in the reference list are formatted wrongly, having "https://doi.org" twice.
References
Vadas, S. L., & Azeem, I. (2021). Concentric secondary gravity waves in the thermosphere and ionosphere over the continental United States on March 25–26, 2015 from deep Convection. Journal of Geophysical Research: Space Physics, 126(2), e2020JA028275.
Stober, G., Liu, A., Kozlovsky, A., Qiao, Z., Krochin, W., Shi, G., ... & Mitchell, N. (2023, April). Identifying gravity waves launched by the Hunga Tonga–Hunga Ha′ apai volcanic eruption in mesosphere/lower-thermosphere winds derived from CONDOR and the Nordic Meteor Radar Cluster. In Annales geophysicae (Vol. 41, No. 1, pp. 197-208). Göttingen, Germany: Copernicus Publications.
Vadas, S. L., Becker, E., Figueiredo, C., Bossert, K., Harding, B. J., & Gasque, L. C. (2023a). Primary and secondary gravity waves and large‐scale wind changes generated by the Tonga volcanic eruption on 15 January 2022: Modeling and comparison with ICON‐MIGHTI winds. Journal of Geophysical Research: Space Physics, 128(2), e2022JA031138.
Vadas, S. L., Figueiredo, C., Becker, E., Huba, J. D., Themens, D. R., Hindley, N. P., ... & Bossert, K. (2023b). Traveling ionospheric disturbances induced by the secondary gravity waves from the Tonga eruption on 15 January 2022: Modeling with MESORAC-HIAMCM-SAMI3 and comparison with GPS/TEC and ionosonde data. Journal of Geophysical Research: Space Physics, 128(6), e2023JA031408.
Citation: https://doi.org/10.5194/egusphere-2023-1714-RC1 -
AC1: 'Reply on RC1', Gunter Stober, 01 Dec 2023
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We thank the reviewer for the comments and suggestions. We will include additional figures and expand the recommended discussion of the importance of the Q2DW. We remove all statements about the HIAMCM referring to the polar vortex, as this is not the main narrative of the paper. However, we include in the reply some additional data from MERRA2 to show the location of the polar vortex. Our detailed replies to each of the raised concerns are in the supplementary material. The final revision will also depend on the other reviewer.
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AC1: 'Reply on RC1', Gunter Stober, 01 Dec 2023
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Gunter Stober et al.
Gunter Stober et al.
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