Dendritic Ice Crystals Research Articles

Precipitation from Stratocumulus Clouds Affected by Fallstreaks and Artificial Seeding

Cloud and precipitation processes in a stratocumulus cloud layer (∼1 km thick) were investigated by means of airborne, radar and ground observations for three situations: 1) the stratocumulus alone, 2) fallstreaks from altocumulus falling into the stratocumulus, 3) regions of stratocumulus not appreciably affected by fallstreaks but strongly affected by artificially seeded dry ice. In case 1) the cloud was composed primarily of supercooled droplets. In cast 2) dendritic ice crystals in the fallstreaks increased their mass by riming as they passed through the stratocumulus; derived precipitation rates for this case were ∼0.02–0.08 mm h−1. In addition, it appeared that the dendrites provided a source for high concentrations of needle crystals in the stratocumulus; these crystals were estimated to give a precipitation rate of ∼0.01–0.03 mm h−1. In case 3) high concentrations of needle crystals were produced by the dry ice seeding and it was deduced that these also produced precipitation rates ∼0.01–0.03 mm h−1. Some implications of the results for areal artificial seeding experiments are discussed.

Hail Growth Mechanisms in a Colorado Storm: Part II: Hail Formation Processes

Hail growth mechanisms in a Colorado multicellular hailstorm were investigated through use of dual-wavelength radar data, three-dimensional wind field data, in situ aircraft measurements and trajectory calculations. Hail production consisted of a transient, pulsating component and a weak, quasi-steady component. Peak hail production resulted when graupel particles and hailstones which grew within the updraft region of a feeder cell were advected in the wind field and fell into the main updraft region where they continued to grow. A minimum in hail production resulted when hail growth was confined to the main updraft region. Recirculation of particles within the main updraft had minor importance in the hail production mechanism. Aggregates of dendritic ice crystals probably comprised most of the hail embyros. These aggregates grew along the forward portion of the storm in a region of precipitation debris associated with decaying cumulus congestus clouds. They were introduced into developing feeder cells and were rapidly converted into graupel and small hail. Water drops, which resulted from the melting of aggregates, comprised some of the embryos of graupel particles which were growing within the main updraft region.

Anisotropic segregation of (K +) by dendritic ice crystals

The effective distribution coefficient K eff was measured spectrophotometrically for K + in melted ice which grew unidirectionally from water solution (0.01–5 wt%) of KOH. For the initial concentration C 0 larger than 0.1 wt%, ice was dendritic, of milky appearance, and the value of K eff was about 0.10 for a crystal with the c-axis in the direction of growth ( C v grain), while it was about 0.15 if the c-axis was perpendicular to the direction of growth ( C h grain). This anisotropy in the impurity entrapment disappeared for C 0 smaller than 0.1%. Then, the ice crystals were transparent and K eff was about 10 −3. For dendritic ice the coefficient K eff decreases with an increase in the concentration C 0. During the freezing a nucleation and preferred growth of parasite crystals took place on the interface of both C v and C h grains.

Temperature and contamination dependent freeze-etch images of frozen water and glycerol solutions

Patterns of crystallization and the appearance of rapidly frozen water and glycerol solutions have been examined in freeze-etch specimens prepared at temperatures from −100 to −190°C. The surface of freshly cleaved, rapidly frozen pure water specimens is characterized by conchoidal and other types of fracture marks. Sublimation of water vapor from such surfaces at −100°C produces etch pits and asperites. The size of asperites seems to depend on the amount of water vapor recondensation. This observation supports the idea that they may be related to whiskers seen by others at higher temperatures. Ice crystal boundaries are apparent in etched and unetched specimens. Rapidly frozen 20% glycerol solutions contain highly branched dendritic ice crystals (many micrometers in diameter) surrounded by what appears to be a glycerol hypoeutectic phase. In freshly cleaved, unetched 20% glycerol specimens nearly all the fracture marks are limited to the surface of the ice phase. Very few of these marks seem to propagate into the surrounding glycerol hypoeutectic phase although the same sequence of cleavage steps can frequently be followed on neighboring ice crystal dendrite branches. Lowering of the processing temperature from −100 to −190°C produces cleavage marks that become increasingly finer and more numerous. Some of these bear a relationship to the underlying crystal lattice. Etching of rapidly frozen 20% glycerol solutions at −100°C leads to a lowering of the surface of the ice phase with respect to the glycerol phase and to the exposure of etch-resistant material within the ice crystal dendrite branches. The disposition of this etch-resistant material of unknown origin is in the form of irregular honeycomb-like structures. The appearance of frozen glycerol solutions freeze-etched at temperatures between −110 and −190°C can be considerably altered by the condensation of water vapor onto the specimen surface. The condensation patterns are highly substrate specific and temperature dependent. Small specimen chips located on the edge of the liquid nitrogen cooled microtome knife seem to be the major source of water vapor contamination in the Balzers freeze-etch apparatus operated under normal conditions.

Structures Formed by Cryoprotective Agents Used in Freezc-Etching

The freeze etching technique is potentially useful for examining dilute solutions or suspensions of macromolecular materials. Quick freezing of aqueous solutions in Freon or propane at or near liquid nitrogen temperature produces relatively large ice crystals and these crystals may damage the structures to be examined. Cryoprotective agents may reduce damage to the specimem, hut their use often results in the formation of a different set of specimem artifacts.In a study of the structure of polyethylene oxide gels glycerol and sucrose were used as cryoprotective agents. The experiments reported here show some of the structures which can appear when these cryoprotective agents are used.Figure 1 shows a fractured surface of a frozen 25% aqueous solution of sucrose. The branches of dendritic ice crystals surrounded hy ice-sucrose eutectic can be seen. When this fractured surface is etched the ice in the dendrites sublimes giving the type of structure shown in Figure 2. The ice-sucrose eutectic etches much more slowly. It is the smooth continuous structural constituent surrounding the branches of the dendrites.

角板状雪結晶の基底面表面積の測定結果について

Examinaticn of basal plane surface areas of sectorlike, stellar and dendritic ice crystals found in natural clouds was made. Empirical relationships describing basal plane surface area and diameter have been developed. The results suggest that the surface area of the basal face for certain crystals is nearly constant, and thus independent of the crystal size.

Dislocations in Ice

The dislocation structures of dendritic ice crystals have been directly observed by x-ray diffraction topography. Growth occurs without the intervention of dislocations and very low dislocation dencities may initially prevail. Straining expands dislocations having 11[unknown]20 direction Burgers vectors from sources at grown-in inclusions. Slip of these dislocations in the basal plane (0001) and on pyramidal planes, probably (1 101), with frequent occurrence of primatic punching, is observed. Dislocation reactions of the type (a(0)/ 3) [11 20] + (a(0)/ 3) [1 210] = (a(0)/ 3) [2 110] and a strong preference for pure screw orientation are characteristic. These dislocations generally account for the known anisotropy of plastic flow in ice.