Supercooled Cloud Tunnel Studies on the Growth of Branched Planar Snow Crystals below Water Saturation

Snow crystals grow by vapor diffusion and play an important role in the formation of precipitation and related atmospheric processes. Of these, the branched planar crystals are the most popular and have the greatest variety of habit. Under what conditions do the branched snow crystals grow in the atmosphere? To address this issue, we carried out experiments with the finer-scale temperature resolution of 0.1°C using a vertical supercooled cloud tunnel that can simulate the process by which a snow crystal continuously grows while falling in the atmosphere (Takahashi 2014). It was shown that the crystal habits are divided mainly by temperature: sector above -12.5°C, then broad-branch, then stellar, dendrite, and fern; then the pattern reverses, with dendrite, stellar, broad-branch, and finally sector below -16.1°C. Between -13.3° and -14.5°C, the side-branch density increases with LWC. The cloud droplets contribute not only to the development of sidebranches but also to the increase in crystal thickness. The crystal diameter and mass show a maximum at -15.0°C, at which a stellar crystal, but no ferns, grows.

Although water vapor densities around natural snow crystals are between ice saturation and a slight excess over water saturation, the above-mentioned experiments were run only at water saturation. Here, we made experiments at saturation ratios less than one with respect to water. The experiments were carried out for growth time of 10 min under isothermal conditions from -12.5 to -16.5°C. Crystals having fern type, dendrite type, stellar type, and broad-branch type branches were grown even below water saturation, contrary to the results obtained thorough the experiments in static chambers (Kobayashi, 1961; Rottner and Vali 1974). As a snow crystal was suspended by applying the upward wind velocity that was adjusted to the terminal fall velocity of the crystal in our tunnel, the growth of these branched planar snow crystals may be ascribed to ventilation that brings fresh environment to the crystal surfaces. The crystal habits were temperature dependent, with habit regime about the fern regime near -14.2°C, which results are in a manner similar to those at water saturation described above. The crystal habits were also dependent on supersaturation ratio: for example, broad branch, stellar, dendrite, and fern forms occurred at -14.1°C as the ratio increased. The crystal diameter and mass and increased with increasing the ratio: the ferns were grown over ice supersaturation of 9%.