Supercooled Cloud Tunnel Studies on the Growth Conditions of Branched Planar Snow Crystals between -12°C and -17°C

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. How does liquid-water content and temperature affect the growth of branched snow crystals? To address this issue, we carried out experiments with the finer-scale temperature resolution of 0.1„aC using a vertical supercooled cloud tunnel in which a single snow crystal could be freely suspended and continuously grown in front of the observer (Takahashi and Fukuta 1988). A snow crystal grew for 10 or 20 min under isothermal and water-saturated conditions from -12.4 to -16.3¢XC with constant LWC values below 0.75 g m-3. The crystal habits are divided mainly by temperature: sector above -12.5¢XC, then broad-branch to -13.0¢XC, then stellar, dendrite, and fern to -14.5¢XC; then the pattern reverses, with dendrite to -14.8¢XC, stellar to -15.7¢XC, broad-branch to -16.1¢XC, and finally sector. From -13.2 to -13.8¢XC, stellar changes to dendrite with LWC increase as well as temperature decrease. From -13.8 to -14.5¢XC, dendrites coexist with ferns below a LWC of 0.25 g m-3, but only ferns exist at higher LWC. At other temperatures, a higher LWC does not produce greater sidebranch development. The crystal diameter and mass show a maximum at -15.0¢XC, at which a stellar crystal, but no ferns, grows. The apparent crystal density, however, show a local maximum at -14.1¢XC, indicating the growth of side branches or a fern. On each side, local minima occur at -13.2 and -15.3¢XC, both where stellars grow. The crystal mass increases with an increase of liquid water content, although the crystal diameter is little dependent of the content. The higher mass growth rate is most likely due to the larger local gradients of vapor density and temperature where the droplets closely approach the crystal. At the temperatures where sidebranches are not developed with LWC increase, and consequently the apparent crystal density is almost constant, the crystal mass increase ascribes from an increase in crystal thickness. Thus, the cloud droplets contribute not only to the development of sidebranches but also to the increase in crystal thickness.