The equivalent size of cloud condensation nuclei

The role of turbulence in the growth process of cloud droplets in the condensation phase is investigated by means of direct numerical simulations in two dimensions. Both curvature effect and solute effect are incorporated in the model for the evolution of cloud droplet radii. The latter effect is allowed because of the explicit description of cloud condensation nuclei (CCN) dynamics. Spectrum broadening appears as a spectacular effect of turbulence. In order to give a quantitative measure of the importance of turbulence in the growth process, we introduce the concept of equivalent CCN radius. The latter naturally emerges as a sort of parametrization of turbulence effects to be incorporated within a classical description of the condensation phase. In that context, it is the minimal CCN radius required to produce spontaneous condensation of cloud droplets. The equivalent CCN size behaves with the dissipation rate epsilon as epsilon(-1/3), thus becoming smaller and smaller as turbulence intensity increases.