Generation of Ice Nuclei in the Surface Outflow of Thunderstorms in Northeast Colorado

Abstract

Measurements of ice-forming nuclei and other components of the atmospheric aerosol (cloud condensation nuclei, Aitken nuclei, giant particles) were made at two ground stations in the operational area of a Colorado field experiment on convection storms. Large increases in the concentration of ice nuclei, sometimes by a factor of 100 and typically tenfold, have been found to coincide with the arrival of cold thunderstorm outflows at the observing sites, confirming some previous reports. A study of a single case shows that the enhanced ice nucleus activity noted at the surface is also detectable aloft. An aircraft equipped with an ice nucelus counter detected ice nucleus concentrations of up to ten times background during a chance encounter with a zone of strong turbulent mixing between the inflow and outflow of a large hailstorm. This suggests that ice nuclei observed in the outflow may be introduced by turbulent mixing into the inflow air and their possible role in precipitation formation processes of storm should be subject to further study.