On the processes determining the slope of cloud water adjustments in weakly and non-precipitating stratocumulus

Abstract. Cloud water adjustments are a part of aerosol–cloud interactions, affecting the ability of clouds to reflect solar shortwave radiation through 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 used in mixed-layer models to determine entrainment-mediated cloud water adjustments in weakly and 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 decrease in cloud droplet sedimentation sustains more liquid water at the cloud top, and hence stronger preconditioning of free-tropospheric air, which increases entrainment with N. Using this idealized framework that neglects interactive surface fluxes, changes in boundary layer depth, and the diurnal cycle of solar radiation, we are able to show that cloud water adjustments weaken distinctly from dln⁡(L)/dln⁡(N)=-0.48 at N=100cm-3 to −0.03 at N=1000cm-3, indicating that a single value to describe cloud water adjustments in weakly and 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.