An Approximate Criterion for Morphological Transformations in Small Vapor Grown Ice Crystals

Observations and measurements show that crystals remain relatively compact at low ice supersaturations, but become increasingly hollowed and complex as the ice supersaturation rises. Prior measurements at temperatures >-25 degrees C indicate that the transition from compact, solid ice to morphologically complex crystals occurs when the excess vapor density exceeds a threshold value of about 0.05 g m(-3). A comparable threshold is not available at low temperatures. A temperature-dependent criterion for the excess vapor density threshold (Delta rho(thr)) that defines morphological transformations to complex ice is derived from laboratory measurements of vapor grown ice at temperatures below -40 degrees C. This criterion depends on the difference between the equilibrium vapor density of liquid (rho(el)) and ice (rho(ei)) multiplied by a measurement-determined constant, Delta rho(thr)similar or equal to 0:27(rho(el )- rho(ei)). The new criterion is consistent with prior laboratory measurements, theoretical estimates, and it reproduces the classical result of about 0.05 g m(-3 )above 225 degrees C. Since Delta rho(thr )defines the excess vapor density above which crystals transition to a morphologically complex (lower density) growth mode, we can estimate the critical supersaturation (s(crit)) for step nucleation during vapor growth. The derived values of s(crit )are consistent with previous measurements at temperatures above -20 degrees C. No direct measurements of s(crit )are available for temperatures below -40 degrees C; however, our derived values suggest some measurement-based estimates may be too high while estimates from molecular dynamics simulations may be too low.