Impact of a strong volcanic eruption on the summer middle atmosphere in UA-ICON simulations
Abstract. Explosive volcanic eruptions emitting large amounts of sulfur can alter the temperature of the lower stratosphere and change the circulation of the middle atmosphere. The dynamical response of the stratosphere to strong volcanic eruptions has been the subject of numerous studies. The impact of volcanic eruptions on the mesosphere is less well understood because of a lack of large eruptions in the satellite era and only sparse observations before that period. Nevertheless, some measurements indicated an increase in mesospheric mid-latitude temperatures after the 1991 Pinatubo eruption. The aim of this study is to uncover potential dynamical mechanisms that may lead to such a mesospheric temperature response. We use the upper-atmospheric icosahedral non-hydrostatic (UA-ICON) model to simulate the atmospheric response to an idealized strong volcanic injection of 20 Tg S into the stratosphere (about twice as much as the eminent 1991 Pinatubo eruption). The simulation shows a significant warming of the polar summer mesospause of up to 15–21~K in the first November after the eruption. We argue that this is mainly due to intrahemispheric dynamical coupling in the summer hemisphere and potentially enhanced by interhemispheric coupling with the winter stratosphere. This study will focus on the first austral summer after the eruption, because mesospheric temperature anomalies are especially relevant for the properties of noctilucent clouds whose season peaks around January in the southern hemisphere.
Sandra Wallis et al.
Sandra Wallis et al.
Sandra Wallis et al.
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Review of “Impact of a strong volcanic eruption on the summer middle atmosphere in UA-ICON simulations” by Wallis et al.
The manuscript is focused on understanding the response of the summer mesosphere to a strong volcanic eruption. These effects are investigated using the UA-ICON model, with the volcanic effects included by simulating the influence of an injection of 20 Tg S into the stratosphere. The simulations indicate that a large response (~15-20 K) occurs in the summer mesosphere several months after the simulated eruption. Two sets of ensemble simulations with different gravity wave forcing are used to diagnose the mechanism by which the volcanic eruption influences the summer mesosphere, with a particular focus on inter- versus intra-hemisphere coupling. The manuscript provides insight into how the mesosphere responds to volcanic eruptions, and would be suitable for publication. However, I believe that there are a number of aspects that would first need to be addressed prior to publication. These are provided in the specific comments below.
1. The manuscript would benefit from additional description of how the volcanic eruption is simulated in the model. Although a description is provided in Section 2.2, the reviewer found it difficult to understand exactly how the effects of the volcanic eruption are included. My interpretation from the text is that this is done by specifying a modification of the aerosols in the model, which then influence the stratosphere heating. It is recommended that the authors revise the description of the simulation setup in order to make the description of how the volcanic eruption is included in the model clear to the reader. It would also be beneficial to explicitly state the timing of the simulated eruption, which can only be inferred from the text and figures currently.
2. There are clear differences in the results for the two experiments with different gravity wave parameters. However, it is unclear how to interpret these results. My understanding is that the results in experiment 1 use the default gravity wave parameters, which were presumably tuned to obtain accurate model climatology, but that using modified gravity wave parameters provides responses to the volcanic eruption that are more consistent with expectations, especially in the response of the Northern Hemisphere polar vortex. The results would thus partly seem in conflict. That is, the tuned gravity wave parameters would give a better climatology but potentially worse volcanic eruption response, while the modified parameters give worse climatology but better response to the eruption. Is this correct? It is recommended that the authors include some additional discussion with regards to how to interpret the results with the two different specifications of the gravity wave parameters.
3. The UA-ICON model does not include interactive chemistry. This represents a possible limitation to the simulations. For example, the effects of the volcanic eruption on ozone are not included. This limitation is not discussed at all in terms of how to interpret the results. Additional discussion of the potential limitations of the study due to neglecting the chemical effects should be included.
4. The interpretation of the results in terms of the effects of a large volcanic eruption on the mesosphere are unclear. Should the effects in terms of the summer mesosphere cooling be considered only qualitatively? That is, the results of the study show the potential mechanisms that would lead to the summer mesosphere cooling, but the magnitude of the cooling is uncertain.
1. Line 50: “below as as” should be “below as”
2. Line 66: “the dynamic core” should be “the dynamical core”
3. Line 86: The authors should clarify that the two reference experiments are also ensemble simulations.
4. Lines 111-112: The authors should consider moving this text to the beginning of Section 3 so that it is immediately clear to the reader why the results in Figure 3 are focused on November-February.