PNNL

Abstract

Collisional growth of cloud droplets is an essential yet uncertain process for drizzle and precipitation formation. To improve quantitative understanding of this key component of cloud-aerosol-turbulence interactions, observational studies of collision-coalescence in a controlled laboratory environment are needed. In an existing convection-cloud chamber, collisional growth is limited by low liquid water content and short droplet residence times. In this work, we use numerical simulations to explore various configurations of a convection-cloud chamber that may increase collision-coalescence. We employ a large-eddy simulation (LES) model with a size-resolved (bin) cloud microphysics scheme to explore how cloud properties, including the intensity of collision-coalescence, are affected by the chamber size and aspect ratio, surface roughness, side-wall wetness, walls' temperature arrangement, and aerosol injection rate. Runs without condensation and evaporation within the domain are firstly performed to explore the turbulence dynamics and wall fluxes in convection chambers. The LES wall fluxes are further used to improve the Scalar Flux Model for predicting continuous results of supersaturation. Results of LES with full cloud microphysics reveal that effects of collision-coalescence are greatly enhanced by employing a taller chamber with saturated side walls, non-uniform side-wall temperature (two side walls as warm as the bottom and the two others as cold as the top), and rough surfaces. For the conditions explored, although lowering aerosol injection rate, and thereby reducing droplet number concentration, broadens droplet size distribution, which favors collision-coalescence, the reduced droplet number concentration decreases the frequency of collisions and their effect on the droplet spectrum.