On the relationship between cloud water responses to changes in aerosol and the probability of precipitation

Climate models continue to exhibit a strong sensitivity to the representation of aerosol effects on cloud reflectance and amount. This work focuses on efforts to constrain modeled cloud liquid water path (LWP) adjustments to changes in aerosol number concentration (N_a) through observations of precipitation susceptibility. Recent climate modeling efforts have suggested a linear relationship between the relative LWP responses to relative changes in N_a (i.e., dlnLWP/dlnN_a) and the precipitation susceptibility (S_pop), which is defined as the relative change in the probability of precipitation for a relative change in N_a. Measurements of S_pop from satellite-based sensors together with this model-based relationship provide a constraint on dlnLWP/dlnN_a. A thorough review of the literature and a suite of large eddy simulations are analyzed to demonstrate that the climate model-based relationship is not unique. Moreover, the climate-model based relationship may sometimes even have an opposite slope to that predicted by finer-scale simulations. We therefore propose more careful examination of the scale- and regime-dependence of this relationship.