ELSEVIER Atmospheric Research 32 (1994) 45-53 ATMOSPHERIC RESEARCH Ice particle generation during evaporation Yayi Dong, Riza G. Oraltay, John Hallett Desert Research Institute, Reno, Nevada 89506-0220, USA (Received December 1, 1992; revised and accepted July 19, 1993) Abstract Simulated soft hail (graupel) particles are grown in a laboratory chamber and evapo- rated under controlled conditions of wind speed, temperature and undersaturation. Parti- cle breakup is observed directly and recorded by video, and also inferred from abrupt changes of charge. A particle with a diameter of a few mm may give rise to several hundred particles under some conditions. Breakup occurs by irregular evaporation of the rime branches, which thin to less than 10 #m prior to fracture. Break up is also observed during evaporation of individual faceted vapor grown crystals. All ice particles are strong; the fracture strength is estimated to be at least ten times the bulk fracture strength of ice. I. Introduction Evaporation of cloud and precipitation particles occurs as they fall into regions where the relative humidity is below saturation. Evaporation also takes place at the cloud top and to some extent at cloud edge (particularly down shear) where dry air is mixed into convective clouds. These panicles may break up during evaporation to produce secondary ice crystals, as observed in the present experi- ments. These experiments are designed to investigate the secondary ice crystal production during evaporation under various conditions. The results show that rime panicles break up at low relative humidities and imply that the mechanism could play a significant role in the ice economy of some clouds. Direct monitoring of the breakup processes made it possible to estimate the strength of very small ice crystals. It is found that small ice panicles are much stronger than bulk ice, a fact to be considered in any hypothesis of ice fragmen- tation in clouds. It was also found that ice charged negatively during evaporation, resulting in a consequential positive space charge. Sudden changing of particle charge resulted from panicle breakup and could be used to indicate these events. 0169-8095/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0169-8095 (93) E0050-9