Dimensional Growth Rates of Ice Crystals at Cirrus Cloud Temperature in a Diffusion Chamber

The mechanisms controlling ice crystal habit formation at low temperature (T<-40C) are relatively unknown. A new thermal-gradient diffusion chamber was developed to capture high-resolution images of ice crystals growing from a substrate. Time-series of dimensional growth rates of columnar ice crystals at cirrus-like temperatures (-66.9 to -46.1C) and moderate to high supersaturation (27.7 to 80.4%), were extracted from these images. Preliminary results show that growth rates in both primary dimensions (a and c) decrease over about the first hour of each experiment, but they asymptotically approach constant values. The growth rate of the a-dimension appears to be most sensitive to the environmental conditions, declining with decreasing temperature and increasing supersaturation. The growth rate of c is independent of temperature and slightly correlated with supersaturation. Together, these trends produce aspect ratios that approach constant values that are negatively correlated with temperature. The ratio of the asymptotic growth rate values is tightly correlated with the aspect ratio, which supports the predictions of crystal growth theory assuming that steps nucleate near crystal edges (Frank, 1982; Nelson and Baker, 1996). In contrast, predictions from capacitance theory or those based on Chen and Lamb (1994) are not consistent with the measurements.