Nucleation effects on the habit of vapour grown ice crystals from −18 to −42°C

Ice crystals have been nucleated and grown by a new method where supersaturation is controlled by combining static diffusion chamber and expansion chamber techniques. Crystals were nucleated and grown on diameter glass filaments at pressures of 500 to 300 hPa, typical of clouds with temperatures from −18 to −42°C, respectively. Crystals were also nucleated on particles of finely powdered silver iodide and kaolinite adhering to glass filaments, in order to investigate effects on habit due to different nucleating materials. Critical supersaturations for nucleation were measured for each case and were in good agreement with previous nucleation studies. Crystals were grown at ice supersaturations which ranged from less than 1% to just above water saturation. For ice growth below a critical supersaturation, nucleation was achieved by applying a brief adiabatic expansion to the static diffusion chamber with a filament in place and the chamber evacuated to the target pressure for the temperature of interest. Nucleation occurred at a critical supersaturation, followed by growth at a lower value. A review of past laboratory experiments and recent in situ observations reveals good agreement between laboratory and field observations when the effects of nucleating methods and growth times are taken into account. Contrary to studies which report the habit of ice as ‘prism’-like or columnar below approximately −22°C, the habit is found to be plate-like for temperatures ranging from −18°C to −40°C at ice supersaturations relevant to the atmosphere. Additionally, the plate-like-habit distribution is dominated by polycrystalline forms, which is consistent with aspects of past laboratory studies and recent observations obtained with improved aircraft instrumentation. The habits and habit distributions obtained from glass and kaolinite most closely resemble those observed in the atmosphere, while the results from silver iodide show large differences. Below −40°C, the habit becomes columnar except at low supersaturation, and is still dominated by polycrystals which appear as rosettes for ice supersaturations greater than about 20%. ‘Bullet rosettes’ were observed to nucleate and grow on clean glass threads below −40°C but not on kaolinite which predominantly grew single columns and irregular polycrystals with columnar components; silver iodide produced single columns with few polycrystals. This behaviour indicates that rosettes result from ice nuclei which share few or no crystallographic similarities with ice, and is consistent with that expected from homogeneous nucleation. Critical supersaturation measurements for nucleation were extended down to −70°C, and revealed that below −40°C ice crystals can nucleate on glass at ice supersaturations of 25%, and at values of approximately 15% on kaolinite or silver iodide. Copyright © 2002 Royal Meteorological Society