Effect of Cloud Condensation Nuclei on the Optical Properties of a Layer Cloud: Numerical Simulation with a Cloud-Microphysical Model

The influence of aerosols on the atmospheric radiative budget, as seen in the effect of cloud condensation nuclei (CCN) on the optical properties of clouds, for example, is one of the key issues in understanding and predicting global climate change. To study quantitatively the effect of CCN on the optical properties of low-level layer clouds, we developed a cloud microphysical adiabatic parcel model. The model relates the size distribution of cloud droplets both to the updraft velocity in a cloud and to the size distribution and constituents of CCN. The growth of cloud droplets by condensation is calculated, with emphasis on avoiding numerical diffusion of the droplet spectrum. Near the cloud base, the activation of CCN and the subsequent growth of cloud droplets are calculated in a Lagrangian framework to accurately model the effect of CCN on the droplet's growth by condensation. In the middle and upper parts of the cloud, the cloud droplet size distribution is partitioned into fixed bins of cloud droplet radii in an Eulerian framework to estimate the growth by coalescence. In the middle and upper parts of the cloud, the growth by condensation is calculated using an advection algorithm, and the growth by coalescence is calculated using a flux method. Using simulated vertical profiles of droplet size distribution, the radiative properties (reflectance, transmittance and absorptance) of the cloud (for short-wavelength radiation) are computed by solving the radiative transfer equation using a discrete ordinate method without parameterization. The absorptance of a cloud is controlled primarily by the integrated liquid water content (ILWC). Reflectance and transmittance depend on both the ILWC and the concentration (or effective radius) of cloud droplets. Our results show that the concentration of cloud droplets is not uniquely determined by the ILWC, but depends on both the updraft velocity and the size distribution and constituent of CCN. Although the concentration of cloud droplets increases as the concentration