Deconvolution of boundary layer depth and aerosol constraints on cloud water path in subtropical stratocumulus decks

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.

The Impact of Resolving Subkilometer Processes on Aerosol‐Cloud Interactions of Low‐Level Clouds in Global Model Simulations

“…However, when we separate our analysis into clouds that rain and do not rain, we find that other factors, namely, the baseline fraction of clouds that rain and the response of nonraining clouds, better explain the overall difference in LWP response to aerosols in UPCAM compared to SPCAM. This distinction of ACI in raining and nonraining clouds has been done in previous observational analyses (e.g., Possner et al, 2020; Toll et al, 2017), but here we show how an analogous distinction of ACI in raining versus nonraining clouds can be done even in global models, and proves helpful in understanding emergent ACI effects, provided we nudge the large‐scale conditions.…”

Assessing the potential efficacy of marine cloud brightening for cooling Earth using a simple heuristic model

“…Large eddy simulations of deep stratocumulus-topped MBLs indicate that albedo can be increased substantially by injected aerosol emissions even when a clear track is not discernible (Possner et al, 2018). In Durkee et al (2000) no ship tracks were detected in MBLs deeper than 800 m, but Possner et al (2020) show that over 80% of all stratocumulus-topped MBLs over the oceans are deeper than 800 m, where surface emissions can increase cloud albedo but tracks may not be easy to detect.…”

Assessing the potential efficacy of marine cloud brightening for cooling Earth using a simple heuristic model

“…Different assessments of the hygroscopicity parameter (κ; from Petters and Kreidenweis, 2007) of CCN in the MBL provide a significant diversity of values, from values as low as 0.45 (Wex et al, 2010) to ∼ 0.7 (Andreae and Rosenfeld 2008). Here, we use the mean marine value of 0.7 from the model study of Pringle et al (2010) for the unperturbed accumulation mode. The background coarse mode is lognormal, with GMD D 0,coarse = 615 nm and GSD S 0,coarse = 1.8 taken from summertime measurements at Graciosa island in the Azores (Zheng et al, 2018).…”