|Second review of Frey et al., ‘Cloud albedo changes in response to anthropogenic sulphate and non-sulfate aerosol forcings in CMIP5 models|
The authors have made improvements to the manuscript since the last review. In particular, the methodology is clearer, and they have moved away from using only AOD as an aerosol metric. The article should be published, following a few more minor revisions.
Page 1, line 10: ‘The magnitude of cloud albedo changes in response to the aerosol changes are most closely related to changes in cloud droplet number and liquid water path in the models.’ This seems obvious to me – my understanding is that these are the only related variables in models. In most climate models reff is a function of CDNC and LWP, optical depth is a function of reff and LWP, and albedo is a function of optical depth. It would be much more interesting to know if one of reff or LWP is more closely related to the change in albedo.
Page 4, L30: This repeats the sentence at line 27, but is phrased much better. I recommend moving this to line 27, in place of the original L27 sentence.
Page 5, L19: I suspect there is a degree of model tuning behind this statement. Model AODs are much more uniform than the aerosol loadings, where even sulfate can have a factor of 4 spread in the global mean (e.g. Wilcox et al., 2015). Since you focus on the indirect effect, I think it is worth highlighting this difference, as you are likely to see effects of it in your results. Of course, AOD is easier to observe than aerosol loads, so it is harder to know which model is closer to the truth in this case, but an awareness of model diversity in aerosol load is useful for interpretation of the modeled cloud changes.
Page 7, L27: The CDN variability is primarily [typo in manuscript] related to anthropogenic sulfate, but this is not to be expected from the model parameterization of CDNC. CDNC in the models is a function of the mass/number concentration of hydrophilic aerosols, which is, in all the cases where it is specified in literature for the models used in this study, a linear sum of the relevant species, with no preferential weighting given to sulfate. CDN variability is primarily related to sulfate because sulfate accounts for most of the mass (as shown earlier in the manuscript, and in previous studies). The only model I can find where this finding might be a true result of the parameterization is HadGEM2-ES, where the number concentration of hydrophilic aerosols is a function only of sulfate and sea salt (Bellouin et al., 2007; Wilcox et al., 2015).
Figure 3: Should the color bar be CDNC, not albedo?
Figure 3/Page 8, L2: I think it would be interesting to see the results for LWP. We know from the parameterization in the models that increased CDN and LWP will increase albedo. It would be interesting to see which has the greatest effect in each model.
Figure 4: Should the color bar be AOD, not CDNC? Are Figures 3 and 4 paired with the wrong captions?
Figure 5: Is there a reason for using green twice? More distinct colors may be helpful.
Page 8, L16: Why do you think this is?
Page 9, L14: Why do you think this is? LWP?
Page 13, L21: CDNC typically depends on other species, as well as sulfate. I think you’re seeing the effect of having a larger mass concentration of sulfate aerosols.