On the six-cornered snowflake: Recent advances in our understanding ofsmall ice particle evolution and growth

A number of recent observations suggest that traditional ice particle habit diagrams, showing the observed aspect ratios of compact ice crystals as functions of growth conditions, are not sufficient to explain important features of small ice crystals in fully glaciated stratiform cloud conditions. When data sets taken over the last 50 years are sorted by growth rate and experimental arrangement, a large amount of scatter is found in the observed aspect ratio and the production of crystals with irregular and complex shapes is not discussed. In this talk, results from recent laboratory experiments investigating the first stages of evolution of small (100 to 200 um) ice crystals are presented. We find that at slow growth rates particle morphology is not uniquely determined by ambient temperature and humidity as is commonly assumed; other parameters, such as the mode of ice initiation, appear to have important effects on particle shape. A model for understanding the microphysics of this behavior will also be discussed.