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.

Automated summary

The spectral width of cloud droplet spectrum plays a crucial role in the radiative properties and drizzle/rain development in warm ice-free clouds. In this study, we use an adiabatic rising parcel model to investigate the activation and diffusional growth of cloud droplets, with a focus on the evolution of spectral width in both clean and polluted environments. Our results show that the effects of kinetics, solute, and surface tension have a significant impact on the spectral width evolution above the cloud base. These findings have important implications for cloud modeling, particularly in the application of conventional bin microphysics.