Impacts of Wind Speed Driven Coarse Mode Sea Spray Aerosol on Precipitation in Marine Boundary Layer Clouds over the Western Atlantic Ocean.

Precipitation in low clouds affects cloud adjustments to aerosol perturbations, thereby impacting aerosol radiative forcing of climate. Cloud adjustments are poorly represented in climate models. Drizzle formation in marine boundary layer (MBL) clouds depends on cloud-aerosol interactions within and beneath the cloud layer. Collision-coalescence is enhanced in clouds with low droplet concentrations and therefore large cloud droplets, and/or by the existence of a tail of large “collector” droplets formed on giant cloud condensation nuclei (GCCN) that can initiate collision-coalescence. Due to their chemical composition, large sea salt aerosols serve as GCCN and can grow rapidly in a saturated environment, where they can initiate collision-coalescence. Large cloud drops also have a higher collision efficiency and terminal velocity due to their large diameter and are more likely to undergo collision and coalescence. Evidence suggests that sea salt aerosol size distributions in the lower MBL are controlled by low-level wind speed across the ocean surface. An increase in wind speed leads to an increase in the concentration of GCCN, here defined as particles with dry diameters larger than 1 um. The overall objective of this study is to ascertain the extent to which observed GCCN concentrations in the MBL can modify precipitation in marine stratocumulus clouds. We use aircraft observations from the 2020-2023 Aerosol-Cloud-Meteorology Interactions over the Western Atlantic Experiment (ACTIVATE) to assess the dependence of coarse mode aerosol size on wind speed over the Western Atlantic by characterizing GCCN size distributions. Results explore the number and size distribution of GCCN and their dependence on wind speed using observations. Observed GCCN distributions and observed cloud thicknesses are used to drive simulations with an explicit microphysics parcel model to explore the range of conditions under which the observed range of GCCN is sufficient to induce a first-order impact on precipitation rates.