Numerical Study of the Effect of CCN on the Size Distribution of Cloud Droplets

Part I. Cloud Droplets in the Stage of Condensation Growth

Abstract

The effect of the size distribution and chemical composition of cloud condensation nuclei (CCN) on the size distribution of cloud droplets was studied by a numerical model of an adiabatically ascending air-parcel. Specially the formation of a broad droplet size distribution was discussed.
Results of computation show that the supersaturation realized in the air-parcel is reduced, a broader size distribution of cloud droplets is produced. If the number concentration of CCN is higher in one air-parcel than that in the other air-parcel, the broader size distribution of droplets is produced in the former air-parcel. Both the addition of anthropogenic CCN in the range of Aitken size and the slower ascent of the air-parcel also produce the broader size distribution of droplets. The growth rate of a droplet formed on a small nucleus is sensitive to the change in supersaturation because of small molarity of the solution droplet, while a droplet formed on a large nucleus grows insensitively to the change in supersaturation because of large molarity. Therefore the droplet size distribution in the air-parcel of lower supersaturation is broader than that in the air-parcel of higher supersaturation. The size distribution of cloud droplets formed on nuclei in which water-insoluble matter is internally mixed is narrower than that of cloud droplets on non-mixed nuclei, if the size distribution of mixed nuclei is the same as that of non-mixed nuclei. But the size distribution of droplets formed on mixed nuclei is hardly different from that associated with non-mixed nuclei, if the mass distribution of soluble matter of mixed nuclei is the same as that of non-mixed nuclei.