“General Comments”

This work represents an impressively complete description of a highly simplified GCM with the capability to simulate two specific impacts of stratospheric-volcanic injections: stratospheric warming at the altitude of the plume and surface cooling. The authors do a good job contextualizing this model in the hierarchy of complexity of climate models and present a justification for this model’s level of complexity. Generally, this article represents excellent scientific work that is significant, of high quality, and reproducible. I suggest the authors make minor revisions in two areas: (1) making a case that this model is useful for attribution and (2) specific references to the Pinatubo literature before publications. Finally, the technical presentation of the article is excellent—I have no technical corrections.

“Specific Comments”

The authors do not show that this first-order treatment of transport, temperatures, radiation, and aerosol processes create a trustworthy climate-attribution environment. One criticism, for example, might stem from a lack of a quasi-biennial oscillation in the model. This deficiency both eliminates one way in which volcanic eruptions impact circulation and one mode of dynamical variability that impacts the transport of volcanic aerosols and their precursors.

A related issue stems from the assertion that, on line 62, “the goal is not to accurately replicate any particular historical eruption”. This assertion seems at odds with the rest of the paper which is dominated by an example of tuning parameters in order replicate the specific (and unique) eruption of Pinatubo in 1991. This highly idealized setup requires tuning these parameters; the authors should clarify how this scheme could be used in a general fashion that isn’t based on tuning parameters to a specific eruption. One suggestion for future work might be to tune the parameters to some kind of average of many eruptions.

Finally, specific to the Pinatubo eruption, the authors should expand on parameter choices. Observations (especially early on) of Pinatubo are uncertain. That being said, a plume center of mass at 14km for Pinatubo is extremely low. It is implied that this is due to unrealistic plume rise observed in this system—could the authors expand on that? In a similar vein, the 30-day e-folding time used for the Pinatubo SO2 is considered fairly uncertain—faster e-folding times (23±5 days or 25±5 days depending on choice of dataset) have been proposed (Guo et al., 2004, https://doi.org/10.1029/2003GC000654).

"Technical Corrections"

The study is excellently presented. No noticed technical errors.

General comments:

This paper describes a new parameterization used to simulate volcanic stratospheric aerosol processes in an idealized Held-Suarez-Williamson atmospheric general circulation model. The aerosol transport is based on advected tracers, producing a time evolving 3D aerosol field. The study introduces a physically motivated scheme to couple the radiative effects of the aerosols to the model dynamics, producing an idealized but self-consistent coupling between the aerosol radiation and the model dynamics. The scheme thus provides a novel method of simulating stratospheric aerosol, which is highly computationally efficient compared to comprehensive prognostic aerosol models while also avoiding the decoupling between dynamics and aerosol properties inherent in prescribed aerosol forcing techniques. 

The description of the aerosol scheme is extremely well done. The motivation is well described, the physical basis of the parametrizations detailed with enviable clarity, and the results from an ensemble set of simulations of a Pinatubo-like sulfur injection expertly presented.

My only more general comment relates to the lack of any mention of the Brewer-Dobson circulation (BDC), which is the main process which controls the transport and spread of stratospheric aerosol. Around line 496, some discussion of the tropospheric large scale circulation is present, which may be relevant to the simulations given the rather low injection height used in the “Pinatubo-like” simulations, but the general utility of this model set-up will depend in part on the fidelity of the BDC: in terms of general features like isolation of the tropical pipe, large-scale mixing in the extratropical stratosphere, and polar subsidence. If an assessment of the stratospheric meridional circulation in the HSW model configuration is given in other studies, it would be useful to summarize some of that work in the introduction. If not, maybe the authors can provide some guidance based on their own results. 

Specific comments:

L29: there are of course many different stratospheric aerosol models used by groups around the world. Therefore it could be here widen the scope of references on such model beyond one single model. A possibility might be to include a reference to a study which includes multi-model comparison (e.g., Clyne et al., 2021) and/or a model-focused review paper (e.g., Timmreck, 2012).

L150: Reference here to the HSW “forcing set” seems like a different definition of “forcing” as the term is applied to volcanic aerosol forcing. 

L222: “timestep n”?

L289: I guess it should be “either absorption or scattering” or “absorption and scattering”, the latter being more generally accurate.  

L300: Probably useful to specify you refer specifically to the SW AOD here. There is also a LW AOD even if it isn’t directly considered. 

L328: It would be good to be explicit about ignoring heating from near-IR solar radiation which was shown by Stenchikov et al. (1998) to be a contributing factor to the total aerosol heating. 

L446: missing “we” in sentence

Fig. 7 caption describes time axis as “time since eruption” which appears inaccurate.

Fig. 8 caption: the black marker should be said to denote the time and latitude of injection.

L558: here and/or in introduction, it would be useful to make reference to the work of DallaSanta et al., (2019) who used a model hierarchy to investigate the dynamical impacts of volcanic forcing, using a much simpler prescribed aerosol forcing.  

L566: A stratovolcano is a particular type of volcano, not one that produces injections of sulfur to the stratosphere. 

L588: a little copy editing required

References:

Clyne, M., Lamarque, J. F., Mills, M. J., Khodri, M., Ball, W., Bekki, S., Dhomse, S. S., Lebas, N., Mann, G., Marshall, L., Niemeier, U., Poulain, V., Robock, A., Rozanov, E., Schmidt, A., Stenke, A., Sukhodolov, T., Timmreck, C., Toohey, M., Tummon, F., Zanchettin, D., Zhu, Y., and Toon, O. B.: Model physics and chemistry causing intermodel disagreement within the VolMIP-Tambora Interactive Stratospheric Aerosol ensemble, Atmos. Chem. Phys., 21, 3317–3343, https://doi.org/10.5194/ACP-21-3317-2021, 2021.

Dallasanta, K., Gerber, E. P., and Toohey, M.: The circulation response to volcanic eruptions: The key roles of stratospheric warming and eddy interactions, J. Clim., 32, https://doi.org/10.1175/JCLI-D-18-0099.1, 2019.

Stenchikov, G. L., Kirchner, I., Robock, A., Graf, H.-F., Antuña, J. C., Grainger, R. G., Lambert, A., and Thomason, L.: Radiative forcing from the 1991 Mount Pinatubo volcanic eruption, J. Geophys. Res., 103, 13837–13857, https://doi.org/10.1029/98JD00693, 1998.

Timmreck, C.: Modeling the climatic effects of large explosive volcanic eruptions, Wiley Interdiscip. Rev. Clim. Chang., 3, 545–564, https://doi.org/10.1002/wcc.192, 2012.