Numerical studies of aerosol scavenging by low-level, warm stratiform clouds and precipitation

Numerical studies have been performed to investigate aerosol scavenging by low-level, warm stratiform clouds and precipitation using a one-dimensional model with detailed cloud microphysics and size resolved aerosol particles and hydrometeors. Activation processes remove most aerosol mass within the cloud layer despite the very low supersaturation, since a large fraction of the aerosol mass is associated with large aerosols which can be quickly activated into cloud droplets. Impaction scavenging inside the cloud layer removes little aerosol mass; however, this process removes aerosols as high as 50% in number during a period of a few hours. Total in-cloud scavenging removes more than 70% of aerosols in number and more than 99% in mass. Below cloud scavenging is linked to aerosol concentration and size distribution, precipitation intensity and droplet spectra. During a 4-h period, weak precipitation having less than 0.1 mm h −1 intensity can remove 50–80% of the below-cloud aerosol in both number and mass. Scavenging coefficients for large particles vary significantly with precipitation rates and/or droplet mean radii while for small particles such variation is not apparent. As a result, bulk aerosol mass-scavenging coefficients depend strongly on precipitation intensity while bulk number scavenging coefficients have less dependence. A dependence of scavenging coefficients for all size particles on total droplet surface area is found to be possible and such dependence is stronger for smaller particles. With the same precipitation amount, precipitation with more small droplets can remove aerosols more effectively due to larger total droplet surface area. Size-resolved scavenging coefficients have to be used in order to correctly track both aerosol number and mass distributions. It is suggested that parameterizations for bulk or size-resolved scavenging coefficients should be a function of other precipitation properties as well as precipitation intensity.