| 27 Jun 2025
Aerosol-Cloud Interactions in Marine Low-Clouds in a Warmer Climate
Abstract. We explore the impact of aerosol perturbation on the stratocumulus-to-cumulus transition (SCT) in a warmer climate in the North-East Pacific region using a Lagrangian large-eddy simulation model coupled to a two-moment, bin-emulating bulk microphysics scheme. We explore two SCT cases with different free-tropospheric (FT) humidities – moist FT and dry FT. For each case, we consider two Shared Socioeconomic Pathways (SSPs), SSP3-7.0 and SSP1-2.6, from the most recent Coupled Model Intercomparison Project (CMIP6) to determine the extent of warming and changes in aerosol concentration at the end-of-the-century. We find that the cloud radiative effect (CRE) in non-precipitating stratocumulus clouds is more susceptible to climate change than to aerosol. However, after the breakup of the cloud deck, the impact of aerosol tends to dominate. Furthermore, in these low-clouds, aerosol-cloud interactions (Twomey effect and liquid water path adjustments) are to leading order immune to climate change, unless aerosol-induced cloud fraction adjustment is significant. We extend the analysis to marine cloud brightening and show that its efficacy decreases with warming because of the reduction in cloud fraction. We also explore the impact of climate change and aerosol perturbation on SCT. In the moist FT case, climate change advances the onset of cumulus activity and cloud breakup. However, in the dry FT case, climate change does not affect the onset of cumulus activity but delays cloud breakup. In both cases, aerosol injection delays cloud breakup via precipitation suppression but does not affect cumulus onset unless it is coupled to rain formation.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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The study investigates how climate change and aerosol perturbations influence the stratocumulus-to-cumulus transition over the North-East Pacific. Using Lagrangian large-eddy simulations along idealized trajectories, the authors examine two distinct free-tropospheric humidity conditions, moist and dry, which aims to control the transition mechanism (precipitation driven vs entrainment driven). They analyze two climate scenarios (SSP1-2.6 and SSP3-7.0) from CMIP6 to capture a range of warming and aerosol changes. Results show that in non-precipitating stratocumulus, the CRE is more sensitive to climate change than to aerosol perturbation. However, after the cloud deck breaks up, aerosol effects, mainly through precipitation suppression, become more influential. The study also finds that aerosol–cloud interactions such as the Twomey effect and LWP adjustments are mostly robust to climate change unless cloud fraction is significantly altered.
The topic addressed in the manuscript is important and interesting, with relatively few publications on the subject to date. I therefore think it is valuable for this manuscript to be published. Given the complexity of the problem, which involves considering both changes in meteorology and aerosol effects, I think the authors have done a nice analysis. I do have a few points that I believe should be clarified before publication:
1. The domain size of the simulations is 48 km. Given that the SCT involves boundary layer deepening and the development of wide convective cells, I am concerned that a 48 km domain may be too small to adequately capture the full extent of boundary layer circulations. If the characteristic cell size exceeds the domain size, can the simulation reliably resolve the SCT dynamics?
2. The simulations are based on the composite trajectories from Sandu and Stevens (2011). However, if aerosol–cloud interactions are nonlinear with respect to environmental conditions, the use of a composite trajectory may not reliably capture the true impact of climate change on cloud properties. Could the authors comment on the extent to which this approach might mask important variability or lead to biased conclusions?
3. To create two types of SCT breakup (entrainment-driven and precipitation-driven), the authors reduce the humidity to 27% of the reference value, which leads to an entrainment-driven transition. However, by manipulating the humidity in this way, aren’t the authors effectively prescribing specific climate change scenarios characterized by a drier free troposphere?
4. I am unclear about the need to lower the free-tropospheric humidity in order to trigger an entrainment-driven breakup. As I understand it, the trajectories from Sandu and Stevens (2011) are intended to represent typical SCT cases that include entrainment-driven breakup. If that is the case, wouldn’t increasing the humidity be the approach needed to shift the system toward a precipitation-driven breakup instead? Clarification on this point would be helpful.
5. The paragraph starting at line 157 and the following one are quite difficult to follow. More generally, the results are highly descriptive throughout the manuscript. In my view, the level of detail is overly comprehensive, which makes it challenging to follow. It may be helpful allow the figures to also speak for themselves.
6. I think that a significant portion of the current discussion section would be more appropriately placed in the results section. Several parts read more like continued presentation of results rather than higher-level synthesis.
Specific comments:
1. Figure 4: why are the dashed lines in subplot 1?
2. Line 219-224. This part is somewhat confusing, as it begins with a focus on the entrainment-driven transition (as indicated by the subsection title), but then shifts to emphasizing the role of precipitation.
3. Figure 7: You could add “Moist” and “Dry” labels above the left and right columns.
4. Equation 4: Could you clarify the role of this equation in your analysis? It’s not clear how it is used in the manuscript.
5. Line 348: “meteorological variables or cloud controlling factors” – Aren’t meteorological variables essentially the same as cloud-controlling factors? If not, could you clarify the distinction? If they are the same, using “or” may be misleading.
6. Line 451: You mention here “ the albedo contributions associated with precipitation-suppression are non-linearly coupled to fc changes and cloud albedo” (this is also stated earlier in the manuscript). It is unclear to me what exactly is meant by “non-linearly coupled” in this context. Could you clarify the nature of this coupling?
7. Isn’t the mathematical form in line 451 is redundant? Mathematically it reduces to dCRE. Perhaps a comma is missing after fc.