Sensitivity of cloud microphysics to aerosol is highly associated with cloud water content: Implications for indirect radiative forcing

The sensitivity of cloud microphysics to aerosol loading, quantified by the aerosol-cloud interactions (ACI) index, plays a crucial role in calculating the radiative forcing due to ACI ( RF aci ). However, the dependence of the ACI index on liquid water content (LWC) LWC ) and its impact on RF aci are often overlooked. This study aims to investigate this dependence and evaluate its implications for RFaci, aci , based on ground in-situ aerosol-cloud observations on Mt. Lu in eastern China. The results demonstrate that the ACI index exhibits an initial increase, followed by a decline with increasing LWC. . In the unconstrained LWC scenario, the ACI index, calculated based on cloud droplet number concentration ( ACI n of 0.13), is found to be lower than the lower bound of ACIn n (0.17 to 0.35) obtained from the constrained LWC scenario. Neglecting this LWC-dependence leads to a significant underestimation of the mean RF aci by 53%. Furthermore, when calculating RF aci using the ACI index from droplet effective diameter (ACId), ACI d ), implying the neglect of the dispersion effect by assuming a fixed droplet spectrum width, it results in an overestimation of RF aci by 22% compared to using ACIn. n . These findings shed new light on the assessment of RF aci and help reconcile differences between observed and simulated RFaci. aci . Plain language summary: Aerosol-cloud interactions are the major source of uncertainty in understanding human- induced climate change. This study focused on the relationship between aerosol and clouds, specifically investigating how the sensitivity of cloud properties to aerosol loading associates with the amount of liquid water present in the clouds. Using ground in-situ observations of clouds and aerosols on Mt. Lu in eastern China, this study showed that the sensitivity of cloud properties to aerosol loading initially increases and then decreases as the cloud water content increases. Neglecting this dependence can lead to a significant underestimation of the impact of aerosol-cloud interactions on climate. These findings provide valuable insights for improving our understanding of how aerosols and clouds interact and contribute to climate change.