Soft Matter and Complex Systems Seminar

Precipitation in warm (ice-free) clouds can only be initiated after the droplet-size distribution evolves to include a spread of sizes and fall speeds. This talk presents cloud parcel stochastic simulations to study the evolution of the cloud droplet-size distribution under the influence of cloud turbulence, entrainment, and activation of entrained cloud condensation nuclei (CCN). Entrainment events, turbulent mixing inside the parcel, and the resulting stochastic droplet activation and growth by condensation are simulated using a Monte Carlo scheme. In consistency with previous large-eddy simulations (LES), our idealized parcel model reproduces broadening of the droplet size distribution towards smaller sizes due to permanent insertion of environmental CCN. Turbulence plays a key role in stochastically activating the entrained CCN and broadening the spectrum also towards larger sizes.