Coarse-mode sea spray aerosols enhance warm rain formation in in marine low clouds

Abstract. Precipitation in low clouds affects cloud responses to aerosol perturbations, thereby impacting aerosol radiative forcing of climate, a significant but indeterminate contribution to uncertainty in future warming. The onset of drizzle formation in marine low clouds may be influenced by the presence of supermicron sea salt aerosols. These particles can act as giant CCN (GCCN) that benefit from rapid condensational growth to a size that initiates collision-coalescence. Aircraft-based in-situ observations of cloud microphysical properties and aerosol size distributions from the Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE) are analyzed from January–June 2022. Observations from a cloud aerosol spectrometer and a cloud droplet probe in clear air are used to quantify GCCN size distributions. Modest correlations are found between GCCN and near surface-level wind speeds when averaged together into wind speed bins, a result consistent with other studies. Observed GCCN distributions and cloud thicknesses are used to drive simulations within a 1-dimensional kinematic super droplet model, exploring the conditions under which the observed GCCN distributions induce a first-order impact on precipitation rate. The model demonstrates that even relatively small GCCN salt masses (~1 μg m^-3) can accelerate the onset of drizzle in lightly precipitating marine low clouds. Similarly, a statistical analysis of in-cloud data shows the ratio of rainwater to total condensed water is higher when GCCN concentration/mass is above average. Together, the results indicate a significant influence of GCCN on precipitation formation in marine low clouds.