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

Abstract. Climate models continue to exhibit strong sensitivity to the representation of aerosol effects on cloud reflectance and cloud amount. This paper evaluates a proposed method to constrain modeled cloud liquid water path (LWP) adjustments in response to changes in aerosol concentration N_a using observations of precipitation susceptibility. Recent climate modeling has suggested a linear relationship between relative LWP responses to relative changes in N_a, i.e., dln LWP / dln N_a, and the precipitation frequency susceptibility S_pop, which is defined as the relative change in the probability of precipitation for a relative change in N_a. Using large-eddy simulations (LES) of marine stratocumulus and trade wind cumulus clouds, we show that these two cloud regimes exhibit qualitatively different relationships between λ and S_pop; in stratocumulus clouds, λ increases with S_pop, while in trade wind cumulus, λ decreases with S_pop. The LES-derived relationship for marine stratocumulus is qualitatively similar but quantitatively different than that derived from climate model simulations of oceanic clouds aggregated over much larger spatial scales. We explore possible reasons for variability in these relationships, including the selected precipitation threshold and the various definitions of precipitation susceptibility that are currently in use. Because aerosol–cloud–precipitation interactions are inherently small-scale processes, we recommend that when deriving the relationship between λ and S_pop, careful attention be given to the cloud regime, the scale, and the extent of aggregation of the model output or the observed data.