Can general circulation models (GCMs) represent cloud liquid water path adjustments to aerosol–cloud interactions?

Journal Article · 10 December 2024 · Atmospheric Chemistry and Physics (Online)

DOI: https://doi.org/10.5194/acp-24-13633-2024 · OSTI ID:2480967

Mülmenstädt, Johannes  ^[1]; Ackerman, Andrew S.  ^[2]; Fridlind, Ann M.  ^[2]; Huang, Meng  ^[1]; Ma, Po-Lun  ^[1]; Mahfouz, Naser G.  ^[1]; Bauer, Susanne E.  ^[2]; Burrows, Susannah M.  ^[1]; Christensen, Matthew W.  ^[1]; Dipu, Sudhakar  ^[3]; Gettelman, Andrew  ^[1]; Leung, L. Ruby  ^[1]; Tornow, Florian  ^[4]; Quaas, Johannes  ^[3]; Varble, Adam C.  ^[1]; Wang, Hailong  ^[1]; Zhang, Kai  ^[1]; Zheng, Youtong ^[5]

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1. Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
2. NASA Goddard Inst. for Space Studies (GISS), New York, NY (United States)
3. Univ. of Leipzig (Germany)
4. Columbia Univ., New York, NY (United States); NASA Goddard Inst. for Space Studies (GISS), New York, NY (United States)
5. Princeton Univ., NJ (United States); Univ. of Houston, TX (United States)

General circulation models (GCMs), unlike other lines of evidence, indicate that anthropogenic aerosols cause a global-mean increase in cloud liquid water path ($$\mathscr{L}$$) and thus a negative adjustment to radiative forcing of the climate by aerosol–cloud interactions. In part 1 of this series of papers, we showed that this is true even in models that reproduce the negative correlation observed in present-day internal variability in $$\mathscr{L}$$ and cloud droplet number concentration (N_d). 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 of 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.

• Research Organization: Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF); USDOE Office of Science (SC), Biological and Environmental Research (BER); USDOE Office of Science (SC), Biological and Environmental Research (BER). Earth & Environmental Systems Science (EESS)
• Grant/Contract Number: AC02-05CH11231; AC05-76RL01830
• OSTI ID: 2480967
• Report Number(s): PNNL-SA--197852
• Journal Information: Atmospheric Chemistry and Physics (Online), Journal Name: Atmospheric Chemistry and Physics (Online) Journal Issue: 23 Vol. 24; ISSN 1680-7324
• Publisher: Copernicus Publications, EGUCopyright Statement
• Country of Publication: United States
• Language: English