Dispersion Aerosol Indirect Effect in Turbulent Clouds: Laboratory Measurements of Effective Radius

Cloud optical properties are determined not only by the number density n _d and mean radius $\overline{r}$ of cloud droplets but also by the shape of the droplet size distribution. The change in cloud optical depth with changing n _d , due to the change in distribution shape, is known as the dispersion effect. Droplet relative dispersion is defined as $d={\sigma }_r/\overline{r}$ . For the first time, a commonly used effective radius parameterization is tested in a controlled laboratory environment by creating a turbulent cloud. Stochastic condensation growth suggests d independent of n _d for a nonprecipitating cloud, hence nearly zero albedo susceptibility due to the dispersion effect. However, for size-dependent removal, such as in a laboratory cloud or highly clean atmospheric conditions, stochastic condensation produces a weak dispersion effect. The albedo susceptibility due to turbulence broadening has the same sign as the Twomey effect and augments it by order 10%.