Recognition of Inter‐Cloud Versus Intra‐Cloud Controls on Droplet Dispersion With Applications to Microphysics Parameterization

Abstract

The amount of aerosol is a key parameter modulating cloud microphysical and macrophysical properties and their subsequent long-term impact on climate. Broadly speaking, aerosol has two primary effects on cloud microphysics of warm-phase clouds. The first is the cloud albedo effect (Twomey, 1974(Twomey, , 1977: given a fixed amount of liquid water content, an increase in aerosol concentration ( a E N ) leads to smaller droplets but higher total surface area, which reflects more sunlight. The second is the lifetime effect (Albrecht, 1989): as smaller droplets are slower to precipitate, more liquid water remains as part of the cloud and spend more time reflecting sunlight. These two mechanisms can change the average albedo of the Earth and thus affect the climate.A lesser-discussed effect is the proposed "dispersion effect" (Liu & Daum, 2002). Dispersion, or more specifically, relative dispersion ( E  is defined as the ratio of standard deviation and mean droplet size, and it is one way to quantify the width of the distribution, i.e., smaller E  for a narrower distribution, and vice versa. The dispersion effect refers to the idea that cloud droplet distributions formed in higher aerosol concentration environments will have different values of relative dispersion from those formed in low aerosol concentration, which in turn impacts the albedo of the clouds.