Effects of CCN Regeneration on Cumulus Cloud Microphysics and Aerosol Distribution

Cloud–aerosol interactions are central to understanding the coupling between microphysical and dynamical cloud processes and precipitation. Our study focuses on shallow cumulus clouds, employing a high-resolution (10 m) large-eddy simulation using the System for Atmospheric Modeling (SAM) coupled with a Spectral Bin Microphysics (SBM) scheme. The model explicitly tracks aerosol evolution both in the air and within droplets, including activation, transport, growth through coalescence, and release back to the atmosphere via droplet evaporation, which is called the aerosol regeneration process.

Simulations of single clouds, under clean and polluted background conditions, show that droplet evaporation efficiently returns large CCN to the atmosphere, demonstrating that shallow convective clouds are an efficient source of these particles in the lower and middle atmosphere. Regeneration significantly modifies the aerosol size distribution and its vertical profile in the atmosphere, and also alters droplet number and size distributions, particularly in diluted cloud regions. Under clean conditions, including the regeneration process reduces surface precipitation by approximately 50%, highlighting a strong microphysical effect.

These findings underscore the importance of accurately representing aerosol regeneration in models to better quantify aerosol–cloud–precipitation interactions and their influence on the Earth’s radiation and water budgets.