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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

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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

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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Gabor Vali
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Terry Deshler
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Gabor Vali

Abstract

Atmospheric concentrations of contact-freezing nuclei were measured using a technique primarily sensitive to submicron aerosol particles. Diffusion and phoretic forces were relied on for the capture of nuclei by supercooled drops of distilled water exposed to the sample air. Nucleus concentrations were deduced from the rate at which the drops were observed to freeze, interpreting that rate on the basis of a theoretical prediction of aerosol capture rate for different assumed sizes of the nuclei. Measurements at Laramie, Wyoming, yielded average concentrations of contact-freezing nuclei of 1.7 L−1 at −15°C and 3.1 L−1 at −18°C for an assumed radius of 0.01 μm for the nucleating particles.

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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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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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Rick Damiani
and
Gabor Vali

Abstract

Intense vortical circulations, often organized in counterrotating vortex pairs, were detected in midcontinental cumulus congestus over southeast Wyoming in July 2003. The sampled clouds developed in dry environments and at cold temperatures, and were a few kilometers in depth and width. Observations were obtained with the Wyoming Cloud Radar from aboard the Wyoming King Air research aircraft. Dual-Doppler analyses of the data yielded high-resolution (30–45 m) depictions of the horizontal components of air motions across vigorously growing clouds. The vortices found in the horizontal cross sections are interpreted as components of the toroidal circulations in thermals when those are tilted because of the effect of ambient cross flow. This configuration also leads to a partial stabilization of the vertical trajectory of the updraft, by opposing the drag by the ambient wind. Additionally, dry air intrusions were seen to accompany these features when the vortices developed near the cloud outer boundaries; recirculation of hydrometeors occurred when the vortices were adjacent to in-cloud downdrafts. These features are also evident in the radar reflectivity patterns. In general, gradients of velocities and vorticity values in horizontal planes are comparable to those found in vertical planes.

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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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