Distinct Impacts of Increased Aerosols on Cloud Droplet Number Concentration of Stratus/Stratocumulus and Cumulus

In situ aircraft measurements obtained during the RACORO field campaign are analyzed to study the aerosol effects on different cloud regimes. The results show that with increasing cloud condensation nuclei (CCN), cloud droplet number concentration (Nd) significantly increases in stratocumulus (Sc) while remains almost unchanged in cumulus (Cu). By using a new approach to strictly constrain the dynamics in Cu, we found that neither simultaneously changing cloud dynamics nor dilution of cloud water induced by entrainment‐mixing can explain the observed insensitivity of Nd. The different degree of reduction in cloud supersaturation caused by increasing aerosols might be responsible for the observed different aerosol indirect effect between Sc and Cu. Plain Language Summary Aerosol‐cloud interactions have the largest uncertainty in assessing the anthropogenic contribution to present and future climate change. There still exist large differences in aerosol indirect radiative forcing between the satellite‐based and model‐based studies. Moreover, recent satellite‐based studies suggested that an increase of aerosols may reduce the cloud droplet number concentration over land, which is contrary to the conventional aerosol indirect effect. This discrepancy might be subject to the inherent limitations of satellite retrieval. In this study, detailed in situ measurements are employed to investigate the impacts of aerosol on continental stratocumulus and cumulus, respectively. We found that with increasing cloud condensation nuclei, cloud droplet number concentration (Nd) increases for stratocumulus, but exhibits a negligible change for cumulus. The analysis shows that neither the simultaneously changing cloud dynamics nor the entrainment‐mixing can explain the observed insensitivity of Nd in cumulus. The difference between stratocumulus and cumulus is likely caused by the different degree of reduction in cloud supersaturation caused by increasing aerosols. This finding would improve the understanding on aerosol effects on different cloud regimes, and is useful to the communities for an accurate estimates of first indirect radiative forcing from both observation and model.