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Gabor Vali

Abstract

Freezing experiments using large numbers of small drops are frequently used for the study of both homogeneous and heterogeneous nucleation of water and of other substances. For heterogeneous nucleation, the spread in the observed freezing temperatures of drops has been shown to arise from the presence of nuclei of different activities in the sample. In the past no quantitative assessment of the nucleus content could be given independent of the drop sizes used. It is shown in this paper that from the observed freezing temperatures of the drops one can derive both a differential and a cumulative nucleus spectrum. The differential spectrum represents the concentrations of nuclei which are active at specific temperatures and the cumulative spectrum represents the concentrations of nuclei active at all temperatures warmer than the selected temperature. The accuracies of the derived spectra were examined by Monte Carlo simulation and are shown to be such that the concentrations are reliable to within factors of 2–4. The dependence of the average freezing temperature on drop volume is shown in general to be determined by the shape of the nucleus spectrum but is approximately exponential for many spectra.

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Gabor Vali

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Gabor Vali
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Gabor Vali

Abstract

The concentrations of freezing nuclei in precipitation from different storms have been measured and the variations of nucleus content with space, time, precipitation type and intensity have been examined. It was found that nucleus concentrations are higher in showery rain and in hail than in continuous-type rain. Peaks have been detected in several nucleus spectra and there is some recurrence of such peaks throughout the storms. This type of information may help to improve descriptions of precipitation processes which involve the ice phase.

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Gabor Vali

Abstract

The heterogeneous nucleation of ice from supercooled water is influenced by the nature of the foreign nuclei that serve as the sites for ice embryo formation, and by the stochastic nature of the process of embryo growth to critical size. The relative roles of these two factors have been the subject of some debate, especially as they influence the way nucleation of ice is modeled in clouds. “Freezing rate” is defined as the time-dependent rate at which a population of macroscopically identical samples (e.g., drops in a volume of air) freeze due to the nuclei contained in them. Freezing rate is the combined result of nucleus content and of time dependence. The time-dependent freezing rate model (TDFR) is consistent with available empirical evidence. For droplets cooled at rates of the order of −1°C per min, the nucleus content, or nucleus spectrum, predicts the freezing rate with reasonable accuracy. For samples exposed to a fixed temperature, the time dependence of the freezing rate becomes important, but the probability of freezing is not the same for each individual of the sample population. Stochastic models are not supported by the results. Application of the TDFR model and use of measured freezing nucleus data for precipitation provide a basis for the description of ice formation via immersion-freezing nucleation in cloud models. Limitations to full development of these models arise from inadequate knowledge about the freezing nucleus content of cloud water as a function of cloud evolution.

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J. Zikmunda and Gabor Vali

Abstract

Fall patterns of timed snow crystals between 0.02 and 0.5 cm in size were studied using a stereoscopic camera system with stroboscopic illumination. Relationships between terminal fall velocities and crystal dimensions were determined for graupel, rimed columnar crystals, rimed plate crystals, rimed capped columns, rimed broken branches, and aggregates of rimed crystals. For each crystal form the characteristic fall pattern is discussed and Reynolds numbers, drag coefficients and Best numbers are presented. Measured densities for graupel and for densely rimed columns are also reported. It is concluded that oscillatory and rotatory motions are quite common for rimed crystals although the mean fall attitude is mostly the same as for unrimed crystals of similar habit. Fall velocities of rimed crystals are on the average twice those of unrimed crystals.

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Binod Pokharel and Gabor Vali

Abstract

Measured 94-GHz reflectivity in midlevel, stratiform ice clouds was compared with reflectivity calculated from size distributions determined with a particle imaging probe. The radar and the particle probe were carried on the same aircraft, the Wyoming King Air, ensuring close spatial correspondence between the two measurements. Good overall agreement was found within the range from −18 to +16 dBZ, but there is an important degree of scatter in the results. Two different assumptions about particle density led to calculated values that bracket the observations. The agreement found for reflectivity supports the use of the data for establishing relationships between the measured reflectivity and ice water content and between precipitation rate and reflectivity. The resulting equation for ice water content (IWC vs Z) agrees with the results of Liu and Illingworth within a factor of 2 over the range of overlap between the two datasets. The equation here reported for precipitation rate (PR vs Z) has a shallower slope in the power-law relationship than that reported by Matrosov as a consequence of sampling particles of greater densities. Because the radar and the particle probe were collocated on the same platform, errors arising from differences in sampling locations and volumes were minimized. Therefore it is concluded that the roughly factor-of-10 spread in IWC and in PR for given Z is, primarily, a result of variations in ice crystal shape and density. Retrievals of IWC and PR from cloud radar data can be expected to have that level of uncertainty.

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Donai Rottner and Gabor Vali

Abstract

Artificial snow crystals were grown in a diffusion chamber Of parallel-plate design which afforded accurate control of the crystal growth conditions. Observations were made at small excesses of vapor density over ice equilibrium (Δρ = 0.015 − 0.169 gm m−3) within the temperature range −8 to −24C. Crystal type was found to be primarily determined by the magnitude of the vapor density excess, in apparent contrast with the situation at supersaturations with respect to water where growth habit is a function of temperature. Transitions between crystal types were noted to be gradual rather than abrupt.

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Mengistu Wolde and Gabor Vali

Abstract

Based on observations made with an airborne 95-GHz polarimetric cloud radar and in situ microphysical probes, the dependence of Z DR and linear depolarization ratio (LDR) values on ice crystal type and radar beam orientation was examined. Distinct ranges of Z DR and LDR values at various radar beam orientations were identified for simple planar and columnar crystals and for melting particles. The results also show that, based on Z DR and LDR values for different beam orientations, dendritic crystals can be distinguished from simpler hexagonal and branched crystals. Polarimetric signatures are almost exclusively associated with unrimed or slightly rimed crystals, therefore the presence of such signatures can help to identify cloud regions where such crystals dominate. The data generally agrees with previously reported results, though some differences are also noted. The observed Z DR and LDR values for simple crystal types are in reasonable agreement with theoretical predictions.

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Mengistu Wolde and Gabor Vali

Abstract

Data are presented, from a large collection of observations in wintertime clouds in Wyoming, which show that the fraction of cloud volumes from which significant radar polarimetric information can be obtained is small. For example, when averaged over all available samples, signals exceeding the chosen limits of 3 dB for Z DR and −18 dB for linear depolarization ratio were found in just a few percent of the observations for radar beam incidence angles of less than 45°. In general, the polarimetric signatures are interpreted as indicators of the prevalence of pristine and lightly rimed crystals, as opposed to more densely rimed crystals, graupel, or aggregates. However, specific cases are presented to illustrate exceptions to this interpretation.

The polarimetric signatures provide information regarding ice crystal types from larger cloud volumes than can be observed with in situ probes, and thus may aid in understanding the evolution and possible origin of hydrometeors in the clouds. They may also help to refine assumptions made in the modeling of radiative transfer through clouds.

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