Abstract
Abstract. The liquid water path (LWP) adjustment due to aerosol–cloud interactions in marine stratocumulus remains a considerable source of uncertainty for climate sensitivity estimates. An unequivocal attribution of LWP adjustments to changes in aerosol concentration from climatology remains difficult due to the considerable covariance between meteorological conditions alongside changes in aerosol concentrations. We utilise the susceptibility framework to quantify the potential change in LWP adjustment with boundary layer (BL) depth in subtropical marine stratocumulus. We show that the LWP susceptibility, i.e. the relative change in LWP scaled by the relative change in cloud droplet number concentration, in marine BLs triples in magnitude from −0.1 to −0.31 as the BL deepens from 300 to 1200 m and deeper. We further find deep BLs to be underrepresented in pollution tracks, process modelling, and in situ studies of aerosol–cloud interactions in marine stratocumulus. Susceptibility estimates based on these approaches are skewed towards shallow BLs of moderate LWP susceptibility. Therefore, extrapolating LWP susceptibility estimates from shallow BLs to the entire cloud climatology may underestimate the true LWP adjustment within subtropical stratocumulus and thus overestimate the effective aerosol radiative forcing in this region. Meanwhile, LWP susceptibility estimates in deep BLs remain poorly constrained. While susceptibility estimates in shallow BLs are found to be consistent with process modelling studies, they overestimate pollution track estimates.
Highlights
The aerosol radiative forcing due to changes in cloud reflectivity of low-level marine clouds remains one of the largest sources of physical uncertainty in climate sensitivity estimates
Isolating the liquid water path (LWP) adjustment due to changes in Nd from potentially co-varying meteorological factors has remained a significant hindrance to quantifying the radiative forcing of aerosol–cloud interactions
It is a likely cause of diverging estimates from low-cloud climatology, process-scale models, and pollution track estimates
Summary
The aerosol radiative forcing due to changes in cloud reflectivity of low-level marine clouds remains one of the largest sources of physical uncertainty in climate sensitivity estimates. Adjustments in cloud fraction (CF) by changes in aerosol concentration may increase the overall albedo of the cloud scene (Gryspeerdt et al, 2016; Andersen et al, 2017; Possner et al, 2018) These effects cannot be addressed within the framework of this study due to the insufficient accuracy in CF retrievals under polluted conditions (e.g. Twohy et al, 2009).
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