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

摘要

Cloud optical properties are determined not only by the number density n(d) and mean radius (r) over bar 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)/(r) over bar. 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%.