The Role of Aerosol Particles in the Formation of Raindrops and Hailstones in Severe Thunderstorms

Abstract

Variable concentrations of different sized water-insoluble particles were found in bulk rainwater and single raindrops (liquid phase) and in hailstones (solid phase) collected from severe thunderstorms in the Colorado-Nebraska region. It was possible to draw the following conclusions from studies of aerosol particles transferred into the liquid phase of the storm: 1) aerosol particles constitute an intrinsic part of a convective cloud in a severe thunderstorm; 2) the size distribution of cloud droplets is extended to larger sizes by thepresence of giant aerosol particles (> 75 n diameter); 3) giant aerosol particles begin to accrete cloud droplets as soon as they enter the cloud; and 4) the size distribution of raindrops falling at the leading edge of a severe storm depends on the concentration of giant aerosol particles ingested by the storm. The presence of different sized particles in the solid phase led to the following conclusions: 1) transparenthailstones are formed from the frozen Equid phase of a cloud (cloud droplets and raindrops) around giantaerosol particles, some of the giant aerosol particles acting as ice-forming nuclei at temperatures as warmas - 6C; 2) milky hailstones originate in the presence of ice crystals which have grown through the liquid-vapor-solid phase transition; 3) the majority of ice-forming nuclei in milky hailstones are particles between6 and 12 y. diameter, this size range being the result of particle separation by size in an ascending cloud; and4) detailed analysis of particulate matter present in different forms of ice in mixed hailstones provides information on the environmental conditions during their formation and growth. A numerical model shows possible conditions for the formation of raindrops around giant aerosol particles. The models calculations also predict strong separation of different sized aerosol particles ingested by the storm. The latter was verified in samples from a storm in which rainfall contained no 50-75 μ diameterparticles, while hailstones contained up to 2300 particles in the same size range, per gram of ice.