Abstract:

The Twomey effect brightens clouds by increasing aerosol concentrations, which activates more droplets and decreases cloud supersaturation in response to more competition for water vapor. To quantify this competition response, we used marine low cloud observations in clean and polluted conditions from two different sites, namely Ascension Island during the Layered Aerosol Smoke Interactions with Cloud (LASIC) campaign and La Jolla during the Eastern Pacific Cloud Aerosol Precipitation Experiment (EPCAPE). The LASIC observations show increases in droplet number for increased accumulation-mode particles from surface-based and satellite cloud retrievals, demonstrating the importance of below-cloud aerosol measurements for retrieving aerosol-cloud interactions in clean and smoky aerosol conditions. By noting the dependence of the Hoppel minimum diameter on the accumulation-mode number concentration, we were able to quantify the dampening of the Twomey effect by aerosols competing for water vapor. Interestingly the higher-concentration aerosol regime in coastal California compared to the tropical South Atlantic meant that the aerosol signature of this feedback effect was masked, precluding a dependence of the Hoppel minimum diameter on the accumulation mode. Other regions including the North Atlantic and Arctic provide further region-specific evidence of aerosol-cloud interactions. For LASIC and other cleaner aerosol conditions, aerosol-related changes in cloud albedo and optical depth show the calculated competition response accounts for dampening the activation response, explaining the diminished Twomey effect at high aerosol concentrations. Translating aerosol effects to local radiative forcing with clean conditions as a proxy for pre-industrial and smoky conditions for present-day showed that the competition response reduces cooling from the Twomey radiative forcing by 12-35%, providing an essential process-specific constraint for improving the representation of aerosol competition in climate model simulation of indirect aerosol forcing.