In this study, the authors use a climate model to simulate the large Australian wildfires of December 2019 and January 2020. They build on the study of Heinold et al. (2022), which looked at injection height and aerosol properties, to focus instead on rapid adjustments in the stratosphere and troposphere. They find that these adjustments are substantial and modulated by underlying clouds. Dynamical adjustments in the stratosphere are driven by different responses in the south and north branch of stratospheric circulation. In the troposphere, adjustments impact the water budget and may ultimately impact precipitation.

The paper is well written, and the description of the mechanisms of the response is generally convincing. Some aspects of the simulations and the discussion could however be clarified, as commented below. The revisions to address those comments should not require additional analyses, so should be minor revisions.

Main comments:

• The aerosol simulations are not described. It is difficult to determine whether adjustments are local or not (e.g., line 227) when not knowing where aerosols are located in the simulations. A figure showing the extinction and absorption aerosol optical depth, or related variables, would be useful to follow the reasoning. In addition, it would be good to state the absorbing properties of BC and OC aerosols in the version of ECHAM6.3-HAM2.3 used in this study early on, rather than waiting for the discussion (lines 412-414).
• The use of a combination of nudged and free-running simulations to isolate rapid adjustments should be presented more clearly in Section 2.2. My expectation was that dynamical adjustments are suppressed in nudged simulations, but not thermodynamical (temperature-driven) adjustments because temperature is not nudged. The comparison to free-running simulations therefore isolates dynamical adjustments. But the reasoning presented on lines 218-220 and 226-228 does not follow my expectations. Is it because dispersion is also different in the two sets of simulations? Overall, what does it say when both nudged and free-running simulations show similar responses?
• The simulated changes in stratospheric temperature and cirrus cloud cover are probably large enough to be observable. Are there observational studies that would support these findings?

Other comments:

Line 41: The first instance of “radiative forcing” is ambiguous. Do you mean effective radiative forcing, or the radiative effect of the adjustments? From context I would say the latter, but that is not clear.

Line 53: Is pyrocumulonimbus formation always happening when wildfire aerosols are injected high in the atmosphere?

Line 76: I do not understand the use of “for” in this sentence. Isn’t it the other way around, that radiative coupling between troposphere and stratosphere allows the chain of effect to happen?

Lines 135-136 and 241-242: I am not sure why the FIRE experiments need to be rescaled and averaged. Aren’t the ensemble simulations enough to deal with statistical significance? And then check from the scaled FIRE experiments whether perturbations are indeed linear functions of the aerosol injection amounts?

Lines 157-160: This is an unusual ensemble initialization technique. Does that risk making the ensemble too narrow by applying only a small perturbation?

Lines 269-270: I understand how the mere presence of clouds modulate the SW forcing, but how does that work in terms of LW adjustments?

Lines 275-276: Just to confirm, that heating is directly due to absorption by the injected aerosols?

Lines 279-281: And the heating decay is driven by the decrease in aerosol mass (or absorption)?

Line 340: What range of pressures do you call the “upper troposphere” here?

Lines 357-358: I do not understand what that sentence is saying, and what it implies for the results that were just presented.

Line 371: I would be good to specify which variability modes are referred to here.

Technical comments

Line 51: in several kilometers -> several kilometers

Line 59: smoke -> of smoke

Line 129: impact -> the impact on

Line 132: analysis -> analyse

The manuscript by Senf et al. investigates the radiative forcing and stratospheric circulation response to the extreme 2019-2020 Australian wildfires in nudged and free-running climate model simulations. The topic is of interest to ACP readership and the paper is clearly written. My main criticism is that the current manuscript lacks a comparison of aerosol optical properties between the different simulations and the observations. Without such a comparison, it is difficult for the reader to make an opinion on the realism of the different scenarios and to place the estimated forcings in the context of the literature. There are other limitations inherent in the study, and although most of them are briefly alluded to in the paper, the authors should consider expanding those points in the discussion.

For these reasons, I recommend that the paper be reconsidered after major revisions. My concerns are detailed below.

Major comments

1) It would be helpful to include a description of the evolution of the aerosol field in the different simulations and a comparison with the (already published) observations to assess how realistic the different runs are. CALIOP, SAGE and OMPS-LP are all suitable instruments and were used to characterize the Australian wildfire plume. This validation is necessary since the representation of the smoke plume in the large-scale model misses important processes which strongly affect dispersion, such as the confinement of the plume within vortices.

2) An important amount of water vapor was injected together with the wildfire aerosols. A significant reduction in stratospheric ozone was also reported (e.g., Yu et al., 2021). As both water vapor and ozone are radiatively active gases in the stratosphere, they will impact the circulation response and stratospheric adjustment. Lines 432-434 of the manuscript, it is conceded that those effects are not well-represented in the simulations. However, I disagree with the authors when they write that such perturbations ‘‘mainly occur in the second half of 2020’’ (lines 432-433). A number of papers (for instance, Kablick et al., 2020 ; Khaykin et al., 2020 ; Schwartz et al., 2020) documented that perturbations in composition were already present a few days after the injection. The authors could at least comment further on the impact that neglecting those perturbations may have on their results.

3) The treatment of the aerosols in the model should be presented in more detail in Sect. 2, in particular their interaction with chemistry and their radiative properties etc.

4) The precipitation response (Sect 3.3) does not seem very robust to me. Figure 10 and 11 show that, among the different simulations, it is not a monotonic function of the injected amount of black carbon aerosols. Is it really significant ? Furthermore, the mechanisms behind this response are not clearly explained. I would recommend either providing more information (and a mechanism) to support this hypothesis or shortening that point.

Other comments

l4: ‘as high as the stratosphere’: Could you be more specific and provide an altitude range ?

L 13: ‘in the Southern hemisphere..’ :’averaged over the Southern hemisphere..’

l 18-19: consider being more quantitative here

l 73 : Actually none of the cited papers explain the dynamical mechanisms behind the formation and maintenance of smoke vortices. They just describe the phenomenon in reanalysis or model simulations. A more insightful paper in that respect may be Lestrelin et al (ACP, 2021), which could be cited, although it does not provide a fully satisfactory explanation from a dynamical point of view either.

L 101-104: Could you include more detail (part of the table) ?

L 121-125: Please recall the level of injection (the reader should go to Heinold et al. only for the details) and add a reference for the aerosol mass here. Also, the end of the sentence seems to be missing a verb.

L 133: I don’t understand this sentence. Why can one not compare the different experiments when the response is not linear ? Would all the experiments be useful if the response were linear (would 2 not be enough)?

L 143-144: I am skeptical that the nudging of wind only does not affect the energy budget, since wind is directly related to temperature through geostrophic/thermal wind balance as mentioned later in the paper. The Zhang et al reference might not be sufficient here, since those authors did not consider a large stratospheric aerosol injection but rather the background aerosol state.

L 157: ‘order 10^-6’ : ‘10^-4 %’ would make clearer that it is the relative variation which is meant

l 174: I would remove ‘very popular’

Figure 1: MATM could be defined in the main text as well as in the caption.

Line 276-277: ‘‘The nudged simulations seem to reach higher heating maxima for smaller fire strengths’’: do you have any idea why ?

L 280 : This longer decay time in the free running simulation seems at odds with Fig. 1 (which suggest a longer lasting SW perturbation for the nudged simulation) and the expectation that the plume is more diluted in free-running simulations. Again, it would helpful to have a comparison of the aerosol field between the two sets of runs.

line 340: what is the lifetime of the black carbon aerosols in the upper troposphere in the model ? Do you confirm that they are refilled by sedimentation from above ?

Line 365: See major comment 4

Line 402: You might consider citing De Laat et al (2012) regarding self-lofting. It is one of the first papers to mention this effect.

Figure 10: Please reproduce the legend here, so that the reader does not have to go to a different figure to interpret this one

l 395: data : model ?

l 457-460: this sentence might rather belong in the introduction

l 481: remove ‘as’ ?

References

de Laat, A. T. J., Zweers, D. C. S., Boers, R., & Tuinder, O. N. E. (2012). A solar escalator: Observational evidence of the self-lifting of smoke and aerosols by absorption of solar radiation in the February 2009 Australian Black Saturday plume. Journal of Geophysical Research, 117, D04204. https://doi.org/10.1029/2011jd017016

Kablick, G., Allen, D. R., Fromm, M., and Nedoluha, G.: Australian PyroCb Smoke Generates Synoptic-Scale Stratospheric Anticyclones, Geophys. Res. Lett., https://doi.org/10.1029/2020GL088101, 2020.

Khaykin, S., Legras, B., Bucci, S., Sellitto, P., Isaksen, L., Tencé, F., Bekki, S., Bourassa, A., Rieger, L., Zawada, D., Jumelet, J., and Godin-Beekmann, S.: The 2019/20 Australian wildfires generated a persistent smoke-charged vortex rising up to 35 km altitude, Commun Earth Environ, 1, 22, https://doi.org/10.1038/s43247-020-00022-5, 2020.

Lestrelin, H., Legras, B., Podglajen, A., and Salihoglu, M.: Smoke-charged vortices in the stratosphere generated by wildfires and their behaviour in both hemispheres: comparing Australia 2020 to Canada 2017, Atmos. Chem. Phys., 21, 7113–7134, https://doi.org/10.5194/acp-21-7113-2021, 2021

Schwartz, M. J., Santee, M. L., Pumphrey, H. C., Manney, G. L., Lambert, A., Livesey, N. J., et al. (2020). Australian new year's pyroCb impact on stratospheric composition. Geophysical Research Letters, 47, e2020GL090831. https://doi.org/10.1029/2020GL090831

Yu, P., Davis, S. M., Toon, O. B., Portmann, R. W., Bardeen, C. G., Barnes, J. E., et al. (2021). Persistent stratospheric warming due to 2019–2020 Australian wildfire smoke. Geophysical Research Letters, 48, e2021GL092609. https://doi.org/10.1029/2021GL092609