The growth of atmospheric ice crystals: A summary of findings in vertical supercooled cloud tunnel studies

Abstract

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 -5°, -10°, and -15°C where crystals are predominantly column-needle, isometric, and plate-stellar-dendrite, respectively. Crystal shape enhances with time (t) at about -5°and -15°C, whereas at -10°C 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 (W(l)) 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 proportional to t(3/2) proportional to (-w)(3/2) proportional to (-z)(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 proportional to ρ(a)-3 (W(L)t)6 proportional to w6 proportional to (-z)3, ρ(a) being the air density. 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.