Characteristics Of Ice Crystals Research Articles

Kinetics of freezing of an oil-in-water emulsion by pressure release. Impact on ice crystals and oil droplets

Pressure release freezing (PRF) of an oil-in-water emulsion is studied. The characteristics of ice crystals as a function of freezing process is studied.

Three‐dimensional observations of ice crystal characteristics in polar ice sheets

In ice core studies, mean crystal size is generally measured on a section image of an ice specimen. However, an ice crystal with a complex shape may appear as several cells on a section image; consequently, the mean crystal size may be underestimated by overcounting the crystals on the image. We examined this phenomenon, which we called duplication effect, through three‐dimensional observation of crystal shapes. In terms of results, we found that the frequency in which duplicate cells appear on a section image is linear with respect to the frequency of which randomly drawn lines on the image pass a cell more than once. On the basis of this empirical relationship, the error on the mean crystal size caused by the duplication effect can be revised from the section image itself. As to its application, we examined crystal sizes in some Antarctic ice specimens and found that the extent of change of crystal size steadily increased with depth up to 100 m and then fluctuated between 6% and 10% in the deeper parts.

Transfer of polarized infrared radiation in optically anisotropic media: application to horizontally oriented ice crystals

We have developed a theory for the computation of the polarization of infrared radiation in optically anisotropic media, with specific application to horizontally oriented ice crystals that frequently occur in cirrus clouds. Both emission and scattering contributions are accounted for in the basic formulation concerning the transfer of thermal radiation in anisotropic media. The symmetry relations of the phase matrix elements for horizontally oriented ice crystals, which are required in the infrared polarization formulations, are presented for the first time to our knowledge. Phase matrix elements for horizontally oriented hexagonal ice crystals are computed by a geometric ray-tracing technique. Radiance and linear-polarization patterns at a 10-μm wavelength that are emergent from cirrus clouds that contain plates and columns oriented in two-dimensional space are presented and discussed in physical terms. Downward polarization emergent from the cloud base is negative, while upward polarization emergent from the cloud top has a positive maximum value near the limb directions. These polarization configurations differ distinctly from the configurations of polarization emergent from ice clouds that contain randomly oriented ice crystals in three-dimensional space. Given these results, it appears feasible to infer the orientation characteristics of ice crystals in cirrus clouds with the use of infrared polarization measurements either above or below the cloud.

A numerical study of thunderstorm electrification: Model development and case study

We have developed a numerical model for examining the thunderstorm electrification process in which we assume the electrification is entirely due to noninductive charge transfer between colliding ice crystals and hail. Since this ice‐hail charge mechanism is very dependent on particle sizes and distributions, we use an explicit microphysical framework. To maintain simplicity, the electrification model is kinematic; thus the temperature and velocity fields are input into the electrification model. These fields can be either calculated by a background model or retrieved from observations. For this study, we have used the cloud model of Taylor (1989) to generate the temperature and velocity fields to examine the July 19, 1981, CCOPE thundercloud. Using these fields, the electrification model produced time‐dependent ice particle concentrations, radar reflectivities, charge and vertical electric field distributions in good general agreement with those observed. The model produced a maximum electric field strength of 1.27 kV cm−1, which is on the order of that needed for lightning initiation, and this maximum occurred very close to the time of the observed discharge (as inferred by the sailplane measurements). Thus the ice‐hail charge mechanism appears to have played an important role in the electrical development of the July 19 cloud. The details of the electrification depended on the liquid water content and the glaciation processes, and particularly on the ice crystal characteristics. Rapid growth of the crystals to riming sizes (> 400 μ) yielded the most efficient charging. The electrification was also sensitive to the ice‐ice sticking efficiency but not to the characteristics of the large riming ice.

Radar observations of polarization characteristics and lightning‐induced realignment of atmospheric ice crystals

The reflectivity and polarization characteristics of ice crystals as measured with a 1.8‐cm radar are reported. Included are measurements on stratiform ice crystal clouds, showing cancelation ratios of 16 to 20 dB and reflectivities of 7 to 28 dBZ. The degree of preferred orientation was approximately 40 to 50%. Some vertical soundings in snow appear to show changes in the snow crystal form with height. Observations of the rapid changes in polarization characteristics in the upper levels of thunderstorms show some cases for which the depolarizing particles are not the same as those which are responsible for the radar backscatter.

The growth and nucleation of ice in a batch Couette flow crystallizer

An experimental investigation into the crystallization characteristics of ice crystals from brine solution was carried out using a batch Couette flow crystallizer. Particle size distributions were determined by a photographic technique. The nucleation rate was correlated by the power law model: B 0 = 26.2( P/ V) 0.52 (δ C *) 1.8 μ 0.2 2 Qualitative growth characteristics were also inferred and found to be consistent with a diffusion controlling mechanism. The results obtained here suggest that early time data be emphasized in future batch crystallization experiments to more accurately determine the nucleation rate model.