The processing of aerosol by droplet collision‐coalescence is analyzed in three‐dimensional simulations of drizzling stratocumulus using a Lagrangian cloud model for the representation of aerosol and cloud microphysics. Collision‐coalescence processing is shown to create a characteristic bimodality in the aerosol size distribution. We show that the large‐scale dynamics of the stratocumulus‐topped boundary layer are key to understanding the amount of time available for collision‐coalescence processing. The large‐scale dynamics enable aerosol particles to repeat a cycle of droplet condensation, collision‐coalescence, and evaporation, which causes a steady increase in aerosol size. This process is continued until the aerosol grows so large that droplet growth is substantially accelerated and multiple collisions occur within one cycle, forming precipitation‐sized droplets that are lost to the surface, including the aerosol. An analytical relationship is derived that captures the fundamental shape of the processed aerosol size distribution.
N2 - Plain Language Summary: Clouds consist of cloud droplets, and cloud droplets grow from aerosol particles, which are tiny particles suspended in the atmosphere. But clouds also modify aerosol particles. This study shows that the merging of cloud droplets, a process related to the production of rain, can lead to larger aerosol particles, causing characteristic changes in the aerosol size distribution that are revealed in this study. These changes are important because larger aerosol particles will create cloud droplets more easily, with commensurate effects on the development of clouds. N2 - Key Points: The effect of droplet collision‐coalescence on the aerosol size distribution is analyzed in three‐dimensional simulations. Collision‐coalescence processing introduces a characteristic bimodality in the aerosol size distribution. The large‐scale stratocumulus dynamics are key to the development of a stable population of processed aerosol particles. UR - http://resolver.sub.uni-goettingen.de/purl?gldocs-11858/10921 ER -