On the Processes Determining the Slope of Cloud-Water Adjustments in Non-Precipitating Stratocumulus

Cloud-water adjustments are a part of aerosol-cloud interactions, affecting the ability of clouds to reflect shortwave radiation by processes altering the vertically integrated cloud water content L in response to changes in the droplet concentration N. In this study, we utilize a simple entrainment parameterization for mixed-layer models to determine entrainment-mediated cloud-water adjustments in non-precipitating stratocumulus. At lower N, L decreases due to an increase in entrainment in response to an increase in N suppressing the stabilizing effect of evaporating precipitation (virga) on boundary layer dynamics. At higher N, the cessation of cloud-droplet sedimentation sustains more liquid water at the cloud top, and hence stronger preconditioning of free-tropospheric air, which increases entrainment with N. Overall, cloud-water adjustments are found to weaken distinctly from dln(L)/dln(N) = -0.48 at N = 100 cm^-3 to -0.03 at N = 1000 cm^-3, indicating that a single value to describe cloud-water adjustments in non-precipitating clouds is insufficient. Based on these results, we speculate that cloud-water adjustments at lower N are associated with slow changes in boundary layer dynamics, while a faster response is associated with the preconditioning of free-tropospheric air at higher N.