Observing the role of wind-driven processes in the evolution of warm marine cloud properties

Abstract. The cloud droplet effective radius is a key variable when evaluating the interactions between aerosols and clouds. The activation of fine-sized sea salt from the ocean results in the formation of more but smaller cloud droplets (reducing the effective radius) in marine stratocumulus. Coarse sea spray aerosols are generated for high surface wind speeds and act as giant cloud condensation nuclei, which activate to form larger droplets. This increases the effective radius and initiates precipitation. These high wind speeds also lead to enhanced moisture fluxes from the ocean surface. Although the opposing impacts of wind-driven fine and coarse marine sea spray aerosols have been documented, their observations have been limited to instantaneous satellite images. In this work, a novel framework is introduced that uses short-timescale observations of the temporal evolution of clouds to identify, isolate, and extract the process fingerprints of marine sea-salt and surface fluxes on stratocumulus cloud properties. This method shows that changes in droplet size previously attributed to aerosol are actually due to increases in evaporation from the ocean surface due to high surface wind speeds. However, when this is accounted for, a clear impact of giant cloud condensation nuclei is observed, reducing cloud droplet number concentrations by initiating precipitation in polluted clouds. By isolating the causal aerosol impact on clouds from confounding factors, this method provides a pathway to improved constraints on the human forcing of the climate, whilst also demonstrating how marine aerosols limit the effectiveness of anthropogenic aerosol perturbations.