Seasonal updraft speeds change cloud droplet number concentrations in low-level clouds over the western North Atlantic

To determine the impact of dynamic and aerosol processes on marine low clouds, we examine the seasonal impact of updraft speed w and cloud condensation nuclei concentration at 0.43 % supersaturation (N-CCN0.43%) on the cloud droplet number concentration (N-C) of low-level clouds over the western North Atlantic Ocean. Aerosol and cloud properties were measured with instruments on board the NASA LaRC Falcon HU-25 during the ACTIVATE (Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment) mission in summer (August) and winter (February-March) 2020. The data are grouped into different N-CCN0.43% loadings, and the density functions of N-C and w near the cloud bases are compared. For low updrafts (w < 1.3 m s(-1)), N-C in winter is mainly limited by the updraft speed and in summer additionally by aerosols. At larger updrafts (w > 3 m s(-1)), N-C is impacted by the aerosol population, while at clean marine conditions cloud nucleation is aerosol-limited, and for high N-CCN0.43% it is influenced by aerosols and updraft. The aerosol size distribution in winter shows a bimodal distribution in clean marine environments, which transforms to a unimodal distribution in high N-CCN0.43% due to chemical and physical aerosol processes, whereas unimodal distributions prevail in summer, with a significant difference in their aerosol concentration and composition. The increase of N-CCN0.43% is accompanied with an increase of organic aerosol and sulfate compounds in both seasons. We demonstrate that N-C can be explained by cloud condensation nuclei activation through upwards processed air masses with varying fractions of activated aerosols. The activation highly depends on w and thus supersaturation between the different seasons, while the aerosol size distribution additionally affects N-C within a season. Our results quantify the seasonal influence of w and N-CCN0.43% on N-C and can be used to improve the representation of low marine clouds in models.