Clean and Smoky Aerosol Competition Response of Cloud Supersaturation in Tropical Atlantic Marine Cloud Observations

The brightening of warm clouds by increasing droplet concentrations to aerosol loading decreases cloud supersaturation in response to more competition for water vapor, a feedback that is not well understood. The sensitivity of marine low cloud activation and competition to aerosol conditions were quantified using clean and smoky observations at Ascension Island in the tropical South Atlantic during the Layered Aerosol Smoke Interactions with Cloud (LASIC) campaign. 38% of the clean and smoky 2-hour conditions were identified as daytime, overcast, single-layer warm cloud scenes with liquid water path between 20-300 g m-2. Based on synoptic regime clustering, the cloud micro- and macrophysical properties were similar between regional meteorological conditions, which allowed for the isolation of aerosol effects on clouds. Accumulation-mode aerosol number concentrations from measured size distributions were shown to have similar increases in cloud droplet number from surface and satellite retrievals, demonstrating the importance of below-cloud aerosol observations to resolve aerosol-cloud interactions in clean and smoky conditions. Decomposing aerosol-related changes in cloud albedo and optical depth showed the calculated competition response of supersaturation accounts for dampening the activation of aerosol into cloud droplets, explaining the diminished Twomey effect at high aerosol concentrations observed for smoky conditions at LASIC. This result was consistent for independent supersaturation retrievals by cloud scene-based droplet number and cloud condensation nuclei and parcel-based size-resolving Lagrangian methods. These results provide an essential process-specific observational evaluation of aerosol feedbacks on supersaturation that are represented in climate model simulations of indirect radiative forcing.