Numerical Simulation of the Impact of Giant Cloud Nuclei on Cloud Droplet Activation

The effect of giant cloud condensation nuclei (GCCN) on the cloud droplet activation at different initial concentrations, median cloud condensation nuclei (CCN) radii, and updrafts and cloud base temperatures is investigated using a bin model that considers the nucleation of CCN and GCCN, condensation, coalescence, and collection of water droplets. A numerical simulation shows that the total number concentration of active cloud droplets simply increases with increasing GCCN if the water vapor supply is sufficiently strong; in contrast, when the updraft and water vapor supply are weak, it decreases with higher GCCN number concentration. Further, a shift in the media diameters of the CCN distribution to larger sizes or a decrease in the cloud base temperature both increase the relative impact of GCCN on the activation of cloud droplets. A comparison of cloud droplet spectra 300 m above the cloud base in clear and polluted conditions shows that the initial CCN concentration has little effect on GCCN activation, which would produce large cloud droplets and broaden the size spectra. However, at a large updraft, the addition of GCCN has little impact on the number concentration of droplets, but activation of a large number of cloud droplets at high peak supersaturation exhausts the water vapor and decreases the growth rate of water droplets. Thus, added GCCN can first be activated to form big droplets and facilitate the collection process.