Review of particle-turbulence interactions and consequences for cloud physics

This paper summarizes recent progress on particle-turbulence interaction an d 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 vel ocity and the spatial distribution of particles may be modified significant ly in a turbulent flow field. A review of this work is presented and the im portant nondimensional parameters describing particle-turbulence interactio n is discussed. For example, the spatial distribution of particles with a S tokes 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 o f this analysis indicate that the effect of preferential concentration duri ng diffusional growth cannot explain adequately the observed droplet spectr al broadening in adiabatic cloud cores. The reason is attributable to the relatively small Stokes number of the clo ud droplets under consideration and the relatively short duration associate d with strong variations in the concentration of droplets. However, the few results available so far on collisions of cloud droplets in a turbulent fl ow did suggest that small-scale turbulence may impact significantly on coll isions between droplets.