The effects of entrainment and mixing on the droplet size distributions in cumuli

This study addresses a central problem in cloud microphysics: the broadening of the cloud droplet spectrum in cumulus clouds, which contributes to the initiation of warm rain.

The evolution of cloud droplet spectra is calculated using the EMPM (Explicit Mixing Parcel Model). This model explicitly represents spatial variability due to entrainment and turbulent mixing down to the smallest turbulent scales in a 1D domain. Several thousand individual droplets evolve by condensation or evaporation according to their local environment. The focus of this study is on a simple mixing scenario: a single entrainment event followed by isobaric mixing, in a 20-m domain.

The impact of the mixing process on the droplet size distribution (DSD) depends on the time scales for mixing and evaporation of droplets, as well as on the volume fraction of the entrained air. Broadening of the DSD is favored by relatively long mixing and short evaporation time scales. Increasing the volume fraction of entrained air also has a major broadening effect on the DSD.

Our results suggest that it is possible to parameterize the impact of entrainment and mixing on the DSD in this simple scenario as a function of the two time scales and the volume fraction of the entrained air.