Significance of Size‐Dependent Hygroscopicity Parameterization for Aerosol‐Cloud Interactions

Abstract

Aerosol‐cloud interactions (aerosol indirect effects) are the most significant source of uncertainties in the effective radiative forcing (ERF) due to aerosols which can act as cloud condensation nuclei (CCN). Uncertainties in the indirect effect can result from systematic uncertainties in aerosol water uptake (hygroscopicity). Here, we implemented a new hygroscopicity parameterization derived from the Hybrid Activity Model (HAM) in the Community Atmosphere Model (CAM). Hygroscopicity parameterized using the HAM framework is a function of particle size and combines the effects of adsorption and aqueous solubility on water uptake. We simplified the HAM hygroscopicity as a power law function of particle size to implement in CAM. We also performed simulations with aerosol hygroscopicity parameterized from the ideal Köhler theory framework. We investigated the variability in the radiative properties of aerosols and clouds resulting from different representations of aerosol hygroscopicity and evaluated the sensitivity to different modal properties. Low‐cloud CCN concentrations from the HAM configuration were ∼28% smaller and ∼16% larger than the default model at and supersaturations, respectively. The global mean low‐cloud drop number concentrations in the HAM configuration was ∼19% larger than that in default model. The HAM configuration also produced weaker cooling and stronger warming compared to the default model. Overall, HAM hygroscopicity configuration of CAM produced a ∼6.7% reduced cooling ACI effect compared to the default model. The results in this work suggest that accounting for complex aerosol chemistry with the HAM hygroscopicity parameterization may be important for estimating physical and radiative properties of aerosols and clouds.