Review of Particle–Turbulence Interactions and Consequences for Cloud Physics

Abstract

This paper summarizes recent progress on particle?turbulence interaction and discusses the possible implications for cloud physics. The subject is of considerable current interest as reflected in a special session on droplet spectral broadening at the 1998 Conference on Cloud Physics. Laboratory and numerical work, done mostly in mechanical engineering, showed that the velocity and the spatial distribution of particles may be modified significantly in a turbulent flow field. A review of this work is presented and the important nondimensional parameters describing particle?turbulence interaction is discussed. For example, the spatial distribution of particles with a Stokes number approaching one in three?dimensional isotropic turbulent flows can deviate substantially from randomness. The pertinent scales for cloud droplets in clouds are then reviewed and the resultant parameter space and that explored in the mechanical engineering work is compared. The results of this analysis indicate that the effect of preferential concentration during diffusional growth cannot explain adequately the observed droplet spectral broadening in adiabatic cloud cores. The reason is attributable to the relatively small Stokes number of the cloud droplets under consideration and the relatively short duration associated with strong variations in the concentration of droplets. However, the few results available so far on collisions of cloud droplets in a turbulent flow did suggest that small?scale turbulence may impact significantly on collisions between droplets.