Evidence for Ice Particle Multiplication from In-Situ Measurements

Environment Canada has made in-situ measurements in cold stratiform cloud layers for many years. Evidence has accumulated showing that ice multiplication probably occurs quite often in clouds containing both liquid water and ice, and those containing only ice. Ice particle concentrations appear to be much higher than can be explained by conventional ice nucleus measurements. Ice particle concentration number frequency distributions appear to be the same over a wide temperature range. Ice particle size spectra consistently show larger numbers of particles at smaller sizes, suggesting that small particles are being produced continuously. An interesting case study during profiles made within a freezing rain layer over an airport showed the freezing rain converting to ice near the surface at temperatures close to -5oC with the sudden immergence of a high concentration of needles/columns that could not have been formed through the Hallett-Mossop mechanism. There are measurement issues associated with ice particles shattering off probe parts and these artifacts being counted. However, the bulk of the evidence points to the fact that ice multiplication must be occurring in the atmosphere over a wide temperature range. Some suggestions for ice multiplication mechanisms will be discussed and evaluated based on the measurements.