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Stratiform clouds in collapsed boundary layers are relatively rare and their cloud water content see

Figure: The black lines of the left-hand side figure show the relative distribution of all stratocumulus decks worldwide for two different organisational states: open and closed cells [Courtesy: Fig. 10 from Muhlbauer et al (2014)]. Overlaid colour bars denote the depth ranges sampled during field experiments. The numbers are placed on the figure according to the depth and year of publication of each process-model study investigating aerosol-cloud interactions within marine stratiform clouds (numbered 1-20). Possner et al. (2020) find that 75% of process-model studies (grey bar) are undertaken in a very narrow range of boundary layer depths. The right-hand side figure demonstrates that the adjustment in cloud water content due to changes in aerosol (measured here by the magnitude of s_lwp; a lower value corresponds to a stronger decrease in cloud water content with increasing aerosol) may be considerably smaller in these shallow boundary layers.

Stratocumulus cloud decks cover vast regions of the ocean surface around the world and influence Earth's energy balance by reflecting incoming sunlight which would otherwise be absorbed by the dark ocean surface and heat up surface waters. It remains one of the key challenges in climate science to quantify to which extent the ability of these clouds to reflect sunlight, i.e. their albedo, is affected by changes in man-made particulate emissions, i.e. anthropogenic aerosol. These emissions could stem from ships, or be advected out over the ocean from nearby continents.

Overall, cloud reflectivity is likely increased (e.g. Bellouin et al (2020)) by anthropogenic aerosol. However, the exact amount of this increase remains uncertain due to different partially compensating processes. Most of the uncertainty is generated by deviating estimates of the change in cloud water content triggered by a change in anthropogenic aerosol. In recent years a body of literature has grown that demonstrates that this change is likely relatively small and contributes a moderate negative offset to the overall increase in reflectivity. However, estimates still deviate substantially between different time-scales and frameworks of analyses.

Possner et al (2020) find that a lot of these estimates of cloud water content adjustments from pollution tracks, process modelling and in situ observations predominantly sample relatively shallow stratocumulus decks. Meanwhile 70% of all cloud decks reside in boundary layer depths currently underrepresented in many datasets. So can estimates be extrapolated to all depth-regimes? Or do we need to account for potential deviations in deeper boundary layers?

Possner et al (2020) find within a ten-year remote sensing record, that the strength in cloud adjustment due to changes in aerosol (approximated here by the change in cloud droplet number concentration) roughly triples in strength as the boundary layer deepens from 500 m near the coast to 1200 m. Most stratocumulus decks over the oceans reside at depths of 1200m and deeper.

Thus, in order to provide a robust global constraint on the strength of cloud water content adjustments representative for all stratocumulus decks around the globe, we need to increase our confidence in cloud water adjustment estimates in deep boundary layers, which are currently underrepresented within the scientific literature. Meanwhile, numerical estimates of cloud water content adjustments in deep boundary layers remain highly uncertain.

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