Środowiskowe Seminarium Fizyki Atmosfery

A significant fraction of the precipitation that falls on Earth is formed by the collision-coalescence of cloud droplets, yet the rate of this process during the rain initiation stage is poorly understood. Recent studies have significantly advanced our understanding of the effects of in-cloud air turbulence on collisions between cloud droplets. In this talk, we review results from our on-going, systematic effort to quantify various effects of air turbulence on the rate of collision-coalescence of cloud droplets. A rigorous computational approach is being developed to treat the motion and interactions of a large number of cloud droplets in a turbulent suspension, in order to measure the increased settling velocity, relative motion, spatial clustering, collision efficiencies of the droplets. The resulting collection kernel of sedimenting, finite-inertia cloud droplets in cloud turbulence will be discussed and compared with the gravitational collection kernel and other published results. A preliminary impact study using the most realistic collection kernel shows that moderate levels of turbulence, similar to those found in cumulus clouds, can produce significant acceleration of the coalescence process leading to rain. We also find that the coalescence process can be separated objectively into three stages and that separation provides useful quantification for rain initiaiton.