Adiabatic Evolution of Cloud Droplet Spectral Width: A New Look at an Old Problem

Spectral width of the cloud droplet spectrum is important for radiative properties and drizzle/rain development in warm ice‐free clouds. We use an adiabatic rising parcel model to study activation and diffusional growth of cloud droplets, focusing on the spectral width evolution, and contrasting clean and polluted environments. A comprehensive droplet growth equation is used that includes kinetic, solute, and surface tension effects. We show that those effects have an appreciable impact on the spectral width evolution above the cloud base. Without those effects, the droplet area standard deviation should not change once activation is completed. In contrast, simulation results show that the area standard deviation does increase with height, especially for weak updrafts and polluted environments. Implications of those results for cloud modeling, especially applying conventional bin microphysics, are discussed. Plain Language Summary Shape of the droplet spectrum is an important parameter affecting cloud radiative properties as well as drizzle and rain development in warm ice‐free clouds. We consider formation and growth of cloud droplets in an air parcel rising across the subcloud layer, reaching the cloud base, and continuing its rise carrying cloud droplets growing by condensation of water vapor. We focus on the evolution of the spectrum shape. We show that the simplified droplet growth equation often applied in cloud simulations is not accurate enough to faithfully predict shape of the droplet spectrum aloft, especially in polluted weak‐updraft shallow clouds such as a continental stratocumulus. Key Points Evolutions of cloud droplet spectral width in an adiabatic parcel rising through cloud base differ between clean and polluted environments Smaller droplet sizes make the solute and surface tension effects more influential for spectral width evolutions in polluted clouds Conventional bin microphysics are not suitable for studying weak‐updraft shallow polluted clouds such as continental stratocumulus