Columnar Ice Dimensional Growth Rates at Temperatures below −40°C: Measurements in a Novel Thermal Gradient Diffusion Chamber

The mechanisms controlling ice crystal growth rates at lowtemperature ( T < −40°C) are relatively unknown. A new thermal-gradient diffusion chamber was developed to capture high-resolution images of ice crystals growing from a substrate with minimal vapor competition or shadowing. Time series of dimensional growth rates of columnar ice crystals at cirrus-like temperatures (−67 to −46°C) and moderate to high supersaturation (28 to 80 %) were determined from these images. Results show that growth rates of both primary facet dimensions ( a and c ) decrease over about the first hour of each experiment, but asymptotically approach constant values. The a -dimension growth rate is well correlated with the environmental conditions, declining with decreasing temperature and increasing supersaturation. In contrast, c -dimension growth rates from individual experiments are not correlated with 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 rates ( dc/da ) is tightly correlated with the aspect ratio ( ø = c/a ), which supports the predictions of crystal growth theory assuming that steps nucleate near facet edges. In contrast, predictions from capacitance theory are not consistent with the measurements.