A potential emergent constraint on cloud liquid water path adjustments to aerosol&ndash;cloud interactions

MÃ¼lmenstÃ¤dt, Johannes; Ma, Po-Lun

doi:10.5194/egusphere-2026-748

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https://doi.org/10.5194/egusphere-2026-748

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13 Feb 2026

| 13 Feb 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

A potential emergent constraint on cloud liquid water path adjustments to aerosol–cloud interactions

Johannes MÃ¼lmenstÃ¤dt and Po-Lun Ma

Abstract. Emergent constraints are relationships between an observable in the present-day climate (such as cloud state variables) and an unobservable response (such as adjustments to radiative forcing) of the climate system to a perturbation. Here we present a candidate emergent constraint arising from the relationship across members of a perturbed parameter ensemble (PPE) between the observable present-day cloud droplet number–liquid water path (ð‘_ð‘‘–ð“›) correlation and the unobservable liquid water path adjustment to anthropogenic aerosol–cloud forcing (RA_ð“›). Emergent constraint candidates require scrutiny to distinguish them from, for example, spurious correlations. The candidate presented here meets several criteria delineated by Klein and Hall (2015): high correlation coefficient, plausible underlying physical mechanism, and emergence from a PPE that perturbs the physical parameters relevant to both the observable and the climate response. Constraining the observable ð‘_ð‘‘–ð“› regression slope to present-day satellite estimates yields a constrained estimate for the ratio of present-day to preindustrial cloud liquid water path ð“›_PD/ð“›_PI= 0.976 ± 0.009 (PPE regression slope uncertainty only) with a regression coefficient of 0.92. The constrained ð“›_PD/ð“›_PI implies a robustly positive RA_ð“›; this disagrees in sign with all other general circulation model (GCM) estimates, but agrees with non-GCM lines of evidence. However, the constrained estimate requires extrapolating the emergent-constraint relationship past the minimum ð“›_PD/ð“›_PI produced by any of the PPE members.

Received: 07 Feb 2026 – Discussion started: 13 Feb 2026

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RC1:
'Comment on egusphere-2026-748', Anonymous Referee #1, 09 Mar 2026 reply
This manuscript proposes and examines an emergent constraint between the radiative adjustment associated with the liquid water path (LWP) adjustment and the empirical LWP-drop number concentration (LWP-Nd) relationship. The latter is measurable in the present day while the former is the unknown target variable of great relevance to climate change. LWP-Nd relationships have been studied using multiple data sources including satellite remote sensing, climate model output and large eddy simulation (LES). An obvious concern is whether LWP-Nd relationships represent causality or simply correlation.
The manuscript is appropriately brief and well-written. There is a lot of information packed into the list of simulations that were performed and it is hard to figure out what they all mean. On the other hand, perhaps it doesn’t matter much, for reasons I will discuss below:
Major concerns:
The methodology is so briefly mentioned. The set-up seems to be a modern era simulation that does not allow for significant circulation changes (fixed SST, sea ice, nudging to MERRA-2 winds) so this isn’t a PD vs PI study. I appreciate the need to constrain the system so that clouds are not changing as they might in a true PD vs PI study. But this then begs the question as to why the authors stick with such coarse resolution/such poorly resolved clouds and cloud processes? How long were these simulations run for?

The one-off PPE set up is troubling because we know that it is very difficult to assess the influence of processes when a limited number of settings are changed. The interactive influence of different parameter settings can provide unexpected responses. It means that drawing conclusions about the importance of a given process from the difference between the experiments in Table 1 is highly uncertain. Also, the number of parameters in this model is huge and I suspect that different combinations of parameter settings could give similar results, leading (possibly) to different conclusions.

Following from 1), have the authors looked at an equivalent set of E3SM simulations at finer resolution/presumably better resolved cloud processes? For that matter, why not take a large set of LES simulations where processes are (mostly) resolved https://rmets.onlinelibrary.wiley.com/doi/10.1002/gdj3.70049. These were developed in-house and should be easily accessible. If results point in similar directions, one would have more confidence in the current results. Absent this, the current simulations are extremely difficult to parse.

m_l is dismissed based on Fig. S1. Why, other than it helps to limit the analysis to m_h? The two branches of LWP-Nd supposedly represent physical processes and if the model physics doesn’t show a clear emergent constraint, shouldn’t we question the representation of the warm phase physical processes that you have shown to be dominant in the emergent constraint with respect to m_h? The autoconversion and accretion processes are, after all, central to m_l.

Following from 1)-4), my concern is that the current manuscript presents another set of climate modeling results of limited use to the community because they are not sufficiently well-rooted in real-world clouds and aerosol-cloud interactions. My advice would be to pursue the ‘structural’ rather than the ‘parameteric’.

Other:
If my understanding of the model set-up is correct–i.e., this is not an historical PD vs PI set of simulations, doesn’t some of the text need to be more carefully written? E.g., the CMIP6 models are historical PD vs PI runs. In that case there are other reasons why a model might exhibit a negative LWP adjustment in PD while at the same time LWP in PD is higher than that in PI.

Coloring the points by class will be more effective than the change in symbol.

Long sentence starting on line 232 is hard to follow.

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Citation: https://doi.org/10.5194/egusphere-2026-748-RC1

Johannes MÃ¼lmenstÃ¤dt and Po-Lun Ma

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https://doi.org/10.5194/egusphere-2026-748-supplement

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Short summary

Aerosols emitted by human activities change the planetary energy budget by, among other mechanisms, changing how much liquid water is contained in clouds and thus their reflectance. Different lines of evidence (observations and models) disagree on the sign of the liquid water change even though they agree on the sign of the correlation between aerosols and liquid water in present-day internal variability. We show one way that observations can constrain models to resolve this disagreement.


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