Tsuneya Takahashi

The Growth of Atmospheric Ice Crystals: A Summary of Findings in Vertical Supercooled Cloud Tunnel Studies

Measurements of ice crystal growth under free fall in a generation of vertical supercooled cloud tunnels and some static cloud chambers as well as the related theoretical works are summarized. Growth parameters, that is, mass (m), dimensions, apparent density, and fall velocity ( w), show extrema at about 258, 2108, and 2158C where crystals are predominantly column-needle, isometric, and plate-stellardendrite, respectively. Crystal shape enhances with time (t) at about 258 and 2158C, whereas at 2108C the effect is minimal and crystals show strongest tendency to grow into graupel due to the fall velocity maximum discovered early in the series of present studies. At this temperature, switch-over of growth mode toward graupel occurs more quickly as liquid water content (WL) increases. Under a fixed cloud condition, the shape-enhanced crystals hardly grow into graupel and vice versa. The diffusional growth theory, with Maxwellian surface condition and without ventilation, describes well the behaviors of intermediate size crystals for which m } t 3/2 } (2w) 3/2 } (2z) 3/4; z, being the fall distance, is identified. Small crystals grow more slowly due to accommodation coefficient effects and larger ones grow faster due to enhanced ventilation and riming. To include these effects, a generalized growth theory is formulated. A simple theory is developed for graupel/hail growth where m } (WLt) 6 } w6 } (2z) 3, ra being the air density. 23 ra Based on these relationships, the dominance of diffusional growth mechanism for precipitation development in shallow, convectively weak, winter clouds and that of graupel/hail-type riming growth in deep, convectively strong, summer clouds is explained.

Ice Crystal Replication with Common Plastic Solutions

Abstract Use of common plastics, i.e., polystyrene, Plexiglas (polymethyl methacrylate) and Lexan (polycarbonate), was investigated for ice crystal replication. The results suggest that all common plastics tested are usable for ice crystal replication with proper combinations of solvents. These solutions seldom get murky due to dissolved water as opposed to the Formvar solution. The combinations of Plexiglas and ethylene dichloride or trichloroethylene, and of polystyrene and 1,1,1-trichloroethane or carbon tetrachloride are advantageous over other solutions tested. The vapor method using the plastic and solvent combinations tested is found, in general, not suitable for replicating detailed surface structures of minute ice crystals.

Influence of the growth mechanism of snow particles on their chemical composition

Abstract Case studies of chemical analysis of solid precipitation were made at sites on the outskirts and central part of Sapporo City, Japan to investigate the difference in precipitation chemistry owing to the mechanism by which snow particles grow, i.e. by vapor deposition or by riming. Even in vapor depositional growth, the samples were found to be acidic. NO 3 − and nss-Cl − contributed to the acidification. In the samples of riming growth, the nss-SO 4 2− concentration was considerably high. Snow samples from the site in the central city area were less acidic due to the higher concentration of nss-Ca 2+ in the early period of snowfalls.

Influence of Liquid Water Content and Temperature on the Form and Growth of Branched Planar Snow Crystals in a Cloud

AbstractHow liquid water content (LWC) and temperature affect the growth of branched planar snow crystals is poorly understood. To address this issue, a vertical supercooled cloud tunnel was used to grow 167 individual snow crystals for 10 min under nearly constant conditions. The LWC varied within 0.07–0.76 g m−3 and average temperature varied from −12.4° to −16.3°C, with the latter varying by at most 0.2°C per run.The crystal habits are divided mainly by temperature, warm to cold, into nine regions: sector above −12.5°C; then broad branch to −13.0°C; and then stellar, dendrite, and fern to −14.5°C. Then the pattern reverses, with dendrite to −14.8°C, stellar to −15.7°C, broad branch to −16.1°C, and finally sector. From −13.3° to −13.8°C, stellar changes to dendrite with LWC increase as well as with temperature decrease. From −13.8° to −14.5°C, dendrites coexist with ferns below an LWC of 0.25 g m−3, but only ferns exist at higher LWC. At other temperatures, a higher LWC does not produce greater side-bra...

A Vertical Wind Tunnel for Snow Process Studies

Abstract A vertical wind tunnel using an artificially generated supercooled cloud was constructed to study snowfall processes. It is 18 m high and operates to a temperature as low as −25°C. Ultrasonic atomizers supply the supercooled water droplets, and ice crystals are generated by an automated adiabatic expansion of compressed air. The system operation is preprogrammed. Snowfall is sustained for about 20 min after injection of the seed ice crystals. Ice crystals of various forms and sizes including dendritic crystals up to 2 mm in diameter, and snowflakes more than 4 mm in diameter were successfully grown in the tunnel.