Ice initiation in warm-base convective clouds: An assessment of microphysical mechanisms

Abstract Ice initiation in convective clouds at relatively high temperatures is examined by considering nucleus concentrations for various nucleation modes and by calculating nucleation rates of supercooled cloud droplets and small raindrops for the most efficient capture mechanisms. Concentrations of ice nuclei aloft for contact-freezing and condensation-freezing at −8°C were found from recent studies to be of order 1 m −3 for small particles (0.01–0.1 μm diameter) increasing to about 30 m −3 for giant particles (≥ 1 μm diameter). Reevaluation of Blanchard's experiments onfreezing of supercooled raindrops shows consistency with these atmospheric concentrations for capture by impaction of giant ice nuclei. Calculations for impaction show that ice initiation is critically dependent on the existence of drizzle drops with threshold values of about 100–200 μm in diameter. In addition, cloud droplets can be frozen by thermophoretic capture of small ice nuclei in mixing regions near cloud top. Initial ice, however, occurs at only about 0.01 m −3 , perhaps reaching several orders of magnitude larger given the natural variability in ice nucleus concentrations. Such nucleation mechanisms are too weak to explain glaciation in warm-base cumulus clouds, but the initiation of frozen drizzle by impaction of giant ice nuclei can serve as a trigger for secondary ice production by rime-splintering. Several mechanisms for enhancing ice nucleation were evaluated (evaporative cooling, thermophoretic shock, high supersaturation and electric charge), but were found to increase ice concentrations only moderately. Only ice nucleus concentrations, enhanced by many orders of magnitude (by about 10 5 ), seem to readily explain the rapid formation of significant ice concentrations (> 11 −1 ) in warm-base cumulus clouds in the absence of secondary production. One potential process at cloud top (or in downdrafts) is the creation of “evaporation ice nuclei” from a small fraction of the residue of cloud droplets. The active substances may be scavenged from organic trace materials, forming a hydrophobic shell during evaporation with hydrogen bonds and anchoring sites aided by soluble materials such as sulfate. Further research is needed on processes, such as evaporation ice nuclei, capable of producing high concentrations of ice nuclei at relatively high temperatures (≥ −10°C).

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