资源环境科技发展态势分析平台

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%.

Plain Language Summary Clouds cover a large fraction of the Earth and play an important role in determining Earth's climate. Their optical properties, such as how much sunlight they reflect back to space, are determined in part by the number of aerosol particles in the atmosphere. In addition to the mean cloud droplet size, the range of droplet sizes influences cloud optical properties, and that influence is called the dispersion effect. A positive dispersion effect means that an increase in cloud droplet number leads to a more reflective (brighter) cloud. We have carried out experiments in a laboratory cloud chamber to observe how the average size and the range of droplet sizes changes as aerosol concentration is varied. The laboratory chamber creates a turbulent environment. The results show that the the dispersion effect is positive, but small in magnitude. Cloud droplet activation, condensation growth in turbulence, and sedimentation are enough to reproduce stratocumulus observations.