Maximum Supersaturation in the Marine Boundary Layer Clouds Over the North Atlantic

Abstract

The maximum supersaturation (S_x) in clouds is a key parameter affecting the cloud's microphysical and radiative properties. We investigate the S_x of the marine boundary layer clouds by combining airborne and surface observations in the Eastern North Atlantic. The cloud droplet number concentration (N_c) in the least diluted cloud cores agrees well with the number concentration of particles larger than the Hoppel Minimum (HM) (N_>HM) below clouds, indicating that the HM represents the average size threshold above which particles are activated to form cloud droplets. The S_x values derived from surface observations vary from 0.10% to 0.50% from June 2017 to June 2018, with a clear seasonal variation exhibiting higher values during winter. Most of the S_x variance (∼60%) can be explained by the cloud condensation nuclei (CCN) concentration and updraft velocity (w), with the CCN concentration playing a more important role than w in explaining the variation of S_x. The influence of CCN concentration on S_x leads to a buffered response of N_c to aerosol perturbations. The response of N_c to low aerosol concentration during winter is further buffered by the high w. The global Community Earth System Model (CESM) simulated S_x values in the Azores have a positive bias compared to measured S_x, likely due to overestimated w and underestimated CCN concentration. The CESM simulated S_x exhibits higher values further north over the North Atlantic Ocean, which is attributed to stronger w. The suppression of S_x by aerosol is also evident in regions with high CCN concentrations.