Wednesday, 5 June 2002
Laboratory Simulation of Hydrometeor Structure Resulting from Crystal Growth from the Vapor on Previously Frozen Supercooled Droplets
Supercooled drops at temperatures between - 10 C and -20 C were suspended on thin vertical glass filaments and frozen, followed by ice growth under conditions of controlled temperature and supersaturation in both a dynamic diffusion and a static diffusion chamber. Supercooled drops were first produced by condensation or by melting of previously grown crystals and were frozen quickly by introducing a seed crystal or gradually by contact with the chilled glass filament. In the dynamic diffusion chamber, the frozen drops were subjected to an airflow under ice supersaturated conditions to simulate terminal fall velocity which led to ice growth on the upstream side of the drops. In the static diffusion chamber, growth at a higher ice supersaturation than that of the dynamic chamber for the same temperature approximately produces the same conditions but with symmetric growth about the frozen droplet. The frozen droplets were grown until well defined secondary crystals extended from their surfaces, typically plates, plate-like polycrystals or dendrites. A tendency for frozen drops to remain spherical without producing extended secondary crystals is observed for periods of time which increase with increasing initial droplet size. These results are compared with in situ observations of frozen drops which have experienced subsequent crystal growth. The density structure of these particles at different stages of this process yields a hard inner core and soft outer shell with varying density, as has been measured in the atmosphere by real time processing following capture by the DRI cloudscope.