the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Can GCMs represent cloud adjustments to aerosol–cloud interactions?
Abstract. General circulation models (GCMs), unlike other lines of evidence, indicate that anthropogenic aerosols cause a global-mean increase in cloud liquid water path (đť“›), and thus a negative adjustment to radiative forcing of the climate by aerosol–cloud interactions. In part 1 of this manuscript series, we showed that this is true even in models that reproduce the negative correlation observed in present-day internal variability of đť“› and cloud droplet number concentration (Nd). We studied several possible confounding mechanisms that could explain the noncausal cloud–aerosol correlations in GCMs and that possibly contaminate observational estimates of radiative adjustments. Here, we perform single-column and full-atmosphere GCM experiments to investigate the causal model-physics mechanisms underlying the model radiative adjustment estimate. We find that both aerosol–cloud interaction mechanisms thought to be operating in real clouds – precipitation suppression and entrainment evaporation enhancement – are active in GCMs and behave qualitatively in agreement with physical process understanding. However, the modeled entrainment enhancement has a negligible global-mean effect. This raises the question whether the GCM estimate is incorrect due to parametric or base-state representation errors, or whether the process understanding gleaned from a limited set of canonical cloud cases is insufficiently representative of the diversity of clouds in the real climate. Regardless, even at limited resolution, the GCM physics appears able to parameterize the small-scale microphysics–turbulence interplay responsible for the entrainment enhancement mechanism. We suggest ways to resolve tension between current and future (storm-resolving) global modeling systems and other lines of evidence in synthesis climate projections.
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Status: open (until 09 May 2024)
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RC1: 'Comment on egusphere-2024-778', Anonymous Referee #1, 21 Apr 2024
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Mülmenstädt et al. study how changes in cloud-condensation nuclei (CCN) aerosols lead to adjustments in the liquid water path (LWP) of warm liquid clouds. The two prevailing hypotheses are that more CCN lead to stronger entrainment drying and precipitation suppression, which act to decrease and increase the LWP, respectively. The authors perform global-climate model (GCM) and single-column model experiments and analyze boundary layer heat and moisture budgets to identify causal relationships related to the proposed mechanisms. The results indicate that both proposed mechanisms are active in GCMs, but the enhanced-entrainment mechanism has a negligible effect on global-mean LWP. The authors conclude by interpreting these results and posing guiding questions for future research.
           I believe that this topic is highly relevant to the aerosol-cloud-climate community, the analysis is well done, and the paper is clearly written. I have very little to say regarding criticisms of the current manuscript, but I offer a few ideas for additional analysis and discussion that I think could improve the paper. If the authors deem that these additional pieces would be beyond the scope of the study, then I believe the paper would also be publishable without them. I therefore recommend minor revision.
General Comments
- The authors conclude with a substantial discussion section in which they pose six guiding questions for future research. This is helpful for the community to think about next steps. However, it would be even more helpful if the authors could explicitly connect the questions to concrete examples from their analysis. For example, one guiding question is “What complexity is required (to simulate the global LWP adjustment)?” The authors suggest looking for the minimal set of parameterizations that capture relevant process understanding. Can the authors perform single-column experiments with a range of complexity to give some guidance about what this minimal set of parameterizations might look like in practice? The authors also pose the question “how representative are susceptibilities in small ensembles of individual cases?” Can the authors identify any cases with the single-column model in which the LWP adjustment differs substantially from the canonical LES cases that are widely studied? Where do we need to look to find this differing behavior? If the authors can provide specific, concrete examples from their analysis to motivate the six questions in the discussion, then I think the discussion would be more useful to the community.
- If the enhanced-entrainment mechanism is in fact negligible for the global-mean LWP adjustment, as suggested by the results in the study, then what are the implications for the historical effective radiative forcing from aerosol-cloud interactions (ERFaci)? The enhanced-entrainment mechanism was used to justify a positive radiative adjustment from LWP changes in the Bellouin et al. (2020). If this positive radiative LWP adjustment is in fact negligible, then doesn’t that imply an even stronger negative ERFaci? There is already a tension between ERFaci estimates derived from process understanding and aerosol-cloud relationships (“bottom-up estimates”) and ERFaci estimates from global energy-budget constraints (“top-down estimates”), with the former predicting a stronger negative ERFaci. Do the current findings exacerbate this tension? How do we interpret this, and how do we more forward? Given that the paper concludes with a substantial forward-looking discussion, I was hoping that the authors would have discussed this topic.
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Specific Comments
- Line 44: I suggest changing “cloud the…” to “complicate the…” or “contradict the…” to avoid confusion because the noun form of “cloud” is used often in the preceding text.
- Line 122: “The idealizations active in the baseline experiment are:” A colon should not be used here because colons should follow complete sentences, not sentence fragments (apologies for my obsession with grammar)
- Line 134: “top-of-atmosphere flux” -> “top-of-atmosphere radiative flux”
- Line 368: consider changing “is small enough to require far longer model runs” to “is small enough to require far longer model runs to detect”
Citation: https://doi.org/10.5194/egusphere-2024-778-RC1 -
RC2: 'Comment on egusphere-2024-778', Anonymous Referee #2, 22 Apr 2024
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Please find my review attached below.
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