Dear Authors

Thank you for providing the revised manuscript “Aerosol indirect effects on temperature-precipitation scaling” in response to the request for revisions.
Previous marks given by the reviewers were not sufficient to reach the standard required for publication in ACP. Having re-assessed the reviewers’ comments and conducted my own assessment, I have now concluded that a number of outstanding issues need to be addressed before reaching the required standard. Most importantly, it is insufficient to frame the complexity of potential aerosol effects on convection modulated via microphysics as “indirect effects” in the simplistic Albrecht framework. There exists a whole body of research and literature on these effects that cannot simply be ignored.
Please consider each of the items below in your response.

Kind regards,

Philip Stier
Co-Editor
Atmospheric Chemistry and Physics


 
Response P1:
“The first sentence of the abstract has been modified in order to be explicit: «Convective precipitation are known to be negatively affected by aerosol indirect effects through reduced precipitable water and convective instability, as stated in the previous literature.» “

The verdict on aerosol effects on convective precipitation through modulation of cloud microphysics is still open and very likely to be regime dependent. There exists no clear consensus in the literature so this statement will need to be either more general and inclusive or omitted.

Response P2:
“The following sentence has been added in the ‘simulation experiment’ section: « It is however important to keep in mind that the ranges that will be found in this study should be interpreted as an upper bound of aerosol indirect effects. »”

Given the significant structural uncertainties involved it is not appropriate to conclude on upper or lower bounds of the total effect from a single modelling study. Even if structural errors were negligible there exist a large number of parametric uncertainties that have not been sampled. The only dimension you sample is the perturbation strength so this need so be clear. However, given the extreme range of values chosen, these perturbations are likely to act as on/off switch for some processes, such as autoconversion. The implications should be discussed.

Response P4:
“This entire study is performed within WRF. That’s OK, but I find little discussion of any possible limitations of that particular model. How broadly applicable do the authors think their results are? Are crucial elements still missing, even for WRF at such high resolution? (There is some discussion in the conclusions, but I would encourage expanding a bit on it.)
We expanded a bit the conclusion in order to reveal the main limitations of our model setting. A deeper discussion is made on the previous companion paper (Da Silva et al., 2018). “

Extensive work has been performed to understand uncertainties in cloud (resolving) modelling that should be acknowledged.

Response P9:
“Precipitation extremes are supposed to wring out all of the moisture from an ascending parcel and are therefore expected to scale with the CC law. However many departures from the CC-scaling”

Statements as “supposed to” needs to be backed up by reference. “Wring out” does not correspond to a physical process. Language needs to refer to physical processes.

Response P10:
“The CC scaling is less expected for mean precipitation which are more constrained by an energetic budget than extreme precipitation (Allen and Ingram, 2002; Held and Soden, 2006; Muller et al., 2011; Muller, 2013). “

Please discuss the scales on which these constraints hold.

Response P11 / Introduction:
You discuss “indirect effects” on precipitation in the framework of Abrecht etc. However, your results include the influence of convection for which a wide range of hypothesis exist regarding aerosol effects on precipitation due to potential interactions with the cloud dynamics, including the convective invigoration hypothesis. This needs to be reflected in the introduction and discussed later on as appropriate.

Response P13:
You need to discuss which processes you may resolve in convection permitting simulation that you do not capture in parameterised convection. Presumably you now capture aerosol effects on convection? This then needs to be linked to the mechanism referred to in the last comment..

Response P13:
“at first vertical grid level” -> at lowest vertical grid level (the ordering in models is arbitrary)

Response P14:
Figure 2 and caption: it is unclear if results are all from same simulations/resolutions or not? How do they differ?

Response P14:
Define precipitation efficiency at first occurrence.

Response P16:
Figure 3: consider adding visual legend

Response P17:
“It confirms the weaker occurrence of clouds at high temperatures in our simulations, which results”

This sentence is confusing and needs to be clarified. What is “weaker occurrence”?

Response P17
COD is not well defined here. Are we seeing a shift in cloud regimes or a change in cloud radiative properties within regimes? The shape is not obvious.

Response P17:
“We believe that the inhibition of convective precipitation is mainly due to the processes described in Da Silva et al. (2018), i.e. a stabilisation of the atmosphere and a reduction of precipitable water in the polluted simulations.”

“Believe” is not appropriate in results section. Such statements need to be backed up by analysis / facts.

Response P18:
“To analyse the reduction of convective precipitation at low temperatures we consider that precipitation can be approximately described by the following equation:”

The origin of this equation is not entirely obvious. Needs to be backed up by primary references.

“This description is mostly valid for convective precipitation which result from a parcel that raises from the surface.”

Same here: such statements need to be referenced.

Response P18:
You discuss maximal vertical velocity in parameterised convection. Where does this come from? Not clear from the model description. Is this prognostic (unlikely)? How is it diagnosed?

Response P21:
“In this set of simulations with explicit convection, changes in aerosol concentrations may have an impact on precipitation efficiency through a change in autoconversion rate (second indirect effect). “

As discussed above, this is too simplistic for potential effects of aerosols on convective precipitation and ignores a wide body of research and literature.

Response P27 / conclusions:
“Figure 14 summarizes the various involved processes and their qualitative contribution (size of the arrows).”

It summarises the investigated processes but does not discuss other relevant processes (see above).

Response P29 / conclusions:
“These results should be interpreted as an upper bound of the aerosol climatological indirect effect on convective precipitation, since extremely and high aerosol concentrations were used in this study. A more realistic estimation of the aerosol indirect effect on convective precipitation could be carried out with the use of online-coupled models in which aerosol concentrations are evaluated with precise emission and transport schemes. Although taken into account in our simulations with explicit convection, our study suggests that the second aerosol indirect effect may not affect convective precipitation efficiency in a significant waycompared to the stabilisation effect. It is however likely that the second indirect effect plays a role in stabilising the atmosphere and hence in reducing convective precipitation, a result that remains to be established.”

These statements are too simplistic and will need to be modified based on the points raised above.

Dear Authors

Thank you for providing your revisions to the comments raised in the last iteration. While many of the comments have been addressed, there are still some outstanding issues that will need to be resolved before publication. Please address each of them in your response.

Best regards,
Philip Stier

Issues (page numbers from author response):

1) Response page 2: "On the flip side, extreme values of aerosol concentrations reach the bounds of permitted values in the microphysical scheme, suggesting that for these ranges of concentrations, the microphysical parameterizations may lead to more uncertainties."

This still (wrongly) suggests that the model could represent microphysics perfectly within the bounds of permitted values. Unfortunately, we do not yet have the sufficient confidence in mixed-phase cloud microphysics schemes to conclude this.

2) The discussion (and possibly understanding) of the energetic constrain appear incomplete. On page 9 it is suggested that "The study of Hardwick et al. (2010) suggests that this constraint also acts at a specific location over a long time record." This does not make sense as divergence of dry static energy can certainly balance any changes in latent heating on local scales (c.f. Muller and O'Gorman, 2011). Please make sure to reflect on this fact and update the manuscript accordingly.

3) Model uncertainties cannot be ignored. On page 10 you suggest that "A few
studies fairly represented the overall indirect effects of aerosol by realising high resolution simulations over few months... . The study of Da Silva et al. (2018) (your own work... is one them". The wording fair vs unfair is subjective and it is clear that we are not yet aware of a perfect model - but know many uncertainties. Please update the wording accordingly based on physical arguments.

4) On page 17 you refer to "changes of vertical velocities between
... may be explained by the convective invigoration effect,". However, as acknowledged in the updated manuscript, there exist multiple potential invigoration mechanisms so there is not a single "invigoration effect".

5) Conclusions. You conclude on page 30 that "Despite these limitations, our configuration highlights the importance of the cloud albedo effect on convective precipitation". Presumably you mean due the surface dimming and subsequent reduction of surface fluxes? This should be clear and the physical mechanism explained. Would this then not hold for precipitation in general? The current wording seems to suggest a direct link between cloud albedo and convective precipitation, which is confusing.

6) The revised section contain a number of grammatical errors that should be fixed before publication.

Dear Authors,

Thank you very much for addressing the final comments raised during the last revision. I am pleased to inform you that I have now accepted the manuscript for publication in ACP.

Best regards
Philip Stier