Maximum Supersaturation in Marine Boundary Layer Clouds over the North Atlantic

Description

Aerosols strongly influence Earth’s radiation budget and climate by modifying the properties and lifetime of clouds. Currently, the effect of aerosols on clouds (i.e., aerosol indirect effects) remains one of the largest uncertainties in climate simulations. The maximum supersaturation (SS_max) inside clouds affects the population of aerosol particles that can form cloud droplets, and thereby cloud microphysical and radiative properties. However, at present, there have been few experimental studies of the seasonal variation of SS_max inside ambient clouds, despite its importance to cloud formation and the aerosol indirect effect. Here, we investigate SS_max in marine boundary layer clouds by combining airborne measurements and surface observations during the Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign. The number concentrations of particles larger than the Hoppel Minimum (N_>HM) below clouds are positive correlative with cloud droplet number concentration (N_c) measured in-situ during flights under both well-mixed and de-coupled boundary layer conditions, suggesting that the Hoppel Minimum (HM) represents the average threshold size at which particles are activated to form droplets in the clouds. Based on this result, we derived SS_max by combining surface measurements of aerosol size distribution and size-resolved CCN activated fraction from June 2017 to June 2018. SS_max in the ENA boundary layer clouds varied from 0.1% to 0.5%, showing a clear winter high and summer low. SS_max is negatively correlated with cloud condensation nuclei number concentration (N_CCN) (R=-0.63), indicating the suppression of SS_max by increased condensation sink of water vapor at high N_CCN. We also investigate the relationships between SS_max and meteorology conditions (e.g., lower tropospheric stability, inversion layer height, and inversion strength). The derived SS_max values are used to evaluate simulations by the Community Earth System Model (CESM). As the CESM simulated SS_max shows good correlations with the derived value, the CESM simulation is then used to examine the temporal-spatial variations of SS_max over the North Atlantic Ocean.