|FRONTIER Newsletter No.11 July.2000|
|The Effect of Cloud Condensation Nuclei on the Optical Properties of Clouds|
|NaomiKuba(Hydrological Cycle Research Program)|
|The influence of aerosol particles on the atmospheric radiative budget is a key issue in predicting and understanding the mechanism of global climate change. This is especially so for the effect of cloud condensation nuclei (CCN) on the optical properties of clouds (the indirect effect). In addition, the resultant size distribution of cloud droplets is a very important factor that affects the precipitation efficiency of clouds, and the chemical composition (or acidity) of rain. We have developed a cloud-microphysical model which can estimate the formation of droplets on CCN, condensation and coalescence growth of droplets. Special attention is paid to the avoidance of numerical diffusion. We study the sensitivity of the size distribution of cloud droplets to the size distribution and constituents of CCN and the ascending velocity of air-current.
Figure 1 shows the relationship between the optical thickness and liquid water path (LWP) of a cloud. Our results are plotted as circles for 4 kinds of LWP. The solid line is the approximate relationship derived by Stephens (1978) with assumption that microphysical properties of a cloud are roughly decided by LWP. This figure shows that the optical properties can be changed due to the difference in CCN or in ascending velocity of air-current even if the LWP of the cloud is fixed. The relationship between the optical properties and the number concentration of cloud droplets near the cloud base in 78 cases of 200m cloud depth (LWP is 44 g / m2) is shown in Fig.2. This shows that the optical properties of clouds mostly depend on the number concentration of cloud droplets if LWP is fixed.
Figure 3 shows the relationship between the number concentration of cloud droplets near the cloud base, and that of the CCN. We calculated the number concentration of cloud droplets, by changing the number concentration of all sizes of CCN, while keeping the overall shape of the size distribution unchanged (see Fig. 3a). In contrast to the results of Twomey (1959), the number concentration of cloud droplets was found to approach a unique value that depended on the updraft velocity. Fig. 3b indicates the effect of aerosol pollutants: here, the number concentration of cloud droplets was calculated by adding only Aitken-sized (less than 0.1micro meter) particles to the maritime CCN. In contrast to the results shown in Fig. 3a, the number concentration of cloud droplets monotonically increases with an increase in the CCN concentration. This result means that there is a marked effect of aerosol pollutants on the optical properties of clouds in the maritime air mass, especially when the updraft velocity is large. This result supports the idea that the indirect radiative effect of aerosol pollutants is negative. However, it also suggests that the cooling effect is not large when the number concentration of background CCN is sufficiently abundant.