A one-dimensional model with detailed cloud microphysics and size segregated aerosol particles and hydrometers is used to investigate the possible reinforcement or inhibition effects of different concentrations of cloud condensation nuclei (CCN) on precipitation formation in low-level, warm stratiform marine and continental clouds. In- and below-cloud scavenging of aerosol particles have also been investigated.

Precipitation is found to be inhibited due to newly activated droplets when a precipitating marine cloud moves over polluted regions with higher CCN concentrations. Precipitation can also be initiated in a non-precipitating cloud having a high concentration of small droplets by seeding giant CCN into the cloud. The precipitation intensity in warm, stratiform clouds with and without background giant CCN differs little, implying a very limited effect of background giant CCN on precipitation formation. This limited effect includes a slightly slower precipitation formation, which is found to be caused by lower supersaturation, in clouds containing background giant CCN compared to clouds without giant CCN. The delay of precipitation development is longer for continental clouds with high CCN concentration than for marine clouds with low CCN concentration. Sensitivity tests show that with the increase of the giant CCN concentration an earlier precipitation can be initiated. These results agree with recent observations and explain controversial findings among earlier studies of the role of giant CCN on precipitation formation. It is concluded that the role of giant CCN on precipitation formation in warm, stratiform clouds is minor and depends on their concentrations, especially those of the largest CCN.

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 few hours. Total in-cloud scavenging removes aerosols more than 70% 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-hour period, weak precipitation having less than 0.1 mm hr-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 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.