Natural Ice-Forming Nuclei in Severe Convective Storms

Abstract

There are two populations of aerosol particles in severe storms: normal background aerosol and aerosolized soil particles. Concentration of the latter, which depends on local wind speed and soil conditions, may be orders of magnitude higher than that of the former. Condensation nuclei are derived principally from the first source. Concentration of ice-forming nuclei, which derive from the soil particles, increases during storms up to 100 times the pre-storm (background) value. This concentration increase is less than that of the aerosol population, indicating that only a fraction of soil particles exhibit ice-nucleating properties. The fraction of soil particles active as ice-forming nuclei in a given particle size range increases with particle size; however, the concentration of ice-forming nuclei in air is counteracted by a decrease in the concentration of aerosol particles with size. Supercooled water drops are nucleated by hydrosol soil particles at temperatures as high as ?5.3C. The quantity of water vapor released during the freezing of supercooled water drops was determined theoretically and experimentally. This value depends primarily on the size of water drops and, to a lesser degree, on the temperature of supercooling. The released water vapor, equal to 0.03 to 3.5 mg per 1?5 mm diameter drops, produces high supersaturation with respect to water at the temperature of the environment in a volume of 300 to 105 cm3, respectively. The water vapor recondenses on cloud droplets and aerosol particles acting as condensation nuclei at higher supersaturation. Some of the aerosol particles acting as ice-forming nuclei will form ice crystals in the water vapor recondensation zone, and these particles will propagate the ice phase within an updraft. Giant aerosol particles, after becoming hydrosol particles, are the most effective freezing nuclei derived from the soil and should be responsible for the appearance of ice at the lowest altitude (warmest temperature). The freezing temperature spectrum of different hydrosols made of various ices separated from natural hailstones revealed that the warmest freezing temperature was not necessarily associated with hailstone embryos. This indicates that many hailstone embryos form at higher altitudes (lower temperature zones) rather than forming at a freezing level corresponding to the temperature at which the warmest ice-forming nuclei are active.