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  • Author or Editor: Gabor Vali x
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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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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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Robert D. Kelly
and
Gabor Vali

Abstract

The University of Wyoming King Air (KA) research aircraft was used in controlled, in situ experiments to determine whether or not, and under what cloud and aircraft operating conditions, a twin-turboprop aircraft would itself produce ice crystals during passage through clouds containing supercooled liquid water. Such crystals are termed “Aircraft Produced Ice Particles” (APIPs). Computer-aided, air-relative navigation was used to pilot the KA back through the diffusion volumes of earlier flight segments. To protect against false-positive and false-negative conclusions, large concentrations of artificially nucleated ice crystals were used as tracers at one or two points along each flight segment. These tracers should have the same diffusion and sedimentation characteristics as the APIPs, and their detection should indicate that any APIPs, if present, would also be detected.

The results of 15 experiments in which the cloud volume affected by the aircraft was subsequently sampled suggest that the KA produces APIPs only in a limited range of cloud and operating conditions. The range of experimental conditions included temperatures from −3° to −25°C, liquid-water contents up to 0.5 g m−3 mean drop diameters 7–20 μm, maximum drop diameters up to 30 μm, true airspeeds 80–110 m s−1, engine speeds 1700–1900 rpm, and low to heavy airframe icing. For the single case in which APIPs were detected, the conditions were − 12°C temperature, 0.5 mg cm−3 liquid-water content, 20 μm mean diameter, 27-μm maximum diameter and heavy airframe icing. An experiment with very similar conditions, but with little or no airframe ice, generated no APIPs. The two most plausible APIP generation mechanisms consistent with these results are splinter production during airframe icing and/or enhanced ice nucleation rates due to adiabatic cooling in propeller tip or wing tip vortices.

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David C. Rogers
and
Gabor Vali

Abstract

Evidence is presented for a process of ice crystal generation in supercooled orographic clouds in contact with snow-covered mountain surfaces. Comparisons of the crystal concentrations at the surface with aircraft sampling indicate that the “anomalous” crystals originate at the interface of the cloud with the surfaces. Crystal concentrations at the surface, over the temperature range −5° to −23°C, were found to be roughly 100 times higher than in the main body of the clouds. Occasionally, the effects extends to altitudes as much as 1 km above the ground in the clouds studied, and indications are that even greater depths of clouds might be influenced over extended mountain ranges. The mechanism of ice crystal generation involved has not yet been firmly established; several possibilities are discussed in the paper. The phenomenon can be expected to have significant implications for the characteristics of low-altitude orographic clouds with respect to their propensity to produce precipitation; radiative, chemical and electric properties; and their suitability for cloud seeding.

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Paul J. Huffman
and
Gabor Vali

Abstract

The depletion of water vapor during humidification of filter samples, which introduces complications in the determination of ice nucleus concentrations, is considered in detail. A complete solution of the problem appears beyond reach because of the dependence of the depletion effect on the activation spectrum of nuclei; these spectra cannot be determined independently.

Measurements showed that depletion by activated nuclei is more important than by hygroscopic particles. A simple “area-of-effect” model was found to fit the observations relatively well. A generalized correction factor was derived with which the true concentrations of nuclei can be found from measured concentrations.

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James R. Hicks
and
Gabor Vali

Abstract

The inducement of cloud glaciation by cooling with evaporating droplets of liquefied propane was investigated in laboratory experiments and in field tests. The efficiency of ice crystal production was shown to be near 1012 crystals per gram of propane for temperatures colder than −2C; 1010 crystals were produced at supercoolings of few tenths of a degree. Observed variations of the efficiency with changes in cloud liquid water content and in wind velocity indicate that the rate of vapor supply to the cooled plume is the limiting factor in ice crystal production. Ice crystal habits and growth rates were also examined and inferences could be drawn regarding the nucleation mechanisms of the ice crystals in the cooled zone. The results indicate that liquefied propane is an effective, easy-to-use, and safe nucleating agent. The experiments helped delineate the factors which are to be considered in designing practical applications of the technique.

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David C. Rogers
,
Darrel Baumgardner
, and
Gabor Vali

Abstract

A ground-based technique is described for determining the liquid water content of supercooled clouds orfog by measuring the mass rate of rime accumulation on a small rotating wire. Development of the techniqueis described, examples of the data are presented, and comparisons are made with two conventional methodsof liquid water measurement. The comparisons were quite favorable for the winter orographic clouds studied.

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Gabor Vali
,
L. R. Koenig
, and
T. C. Yoksas

Abstract

No abstract available.

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Gabor Vali
,
L. Randall Koenig
, and
Thomas C. Yoksas

Abstract

Field investigations for the Precipitation Enhancement Project (PEP) were undertaken during the winter months of 1979–1981 in the upper Duero River Basin of Spain. The purpose of these studies was to examine what potential might exist for the enhancement of precipitation from different cloud types via ice-nucleus seeding of clouds. This paper describes procedures for estimating that potential. Specific regions within natural clouds were qualified as potentially seedable on the basis of observations by instrumented aircraft of persistent zones of supercooled water content. The observed “regions of potential” are described, and precipitation increases that might result from seeding the regions are estimated using two relatively simple models. Summed over all cloud types, and expressed as averages over the 100-km radius project area, increases of 10% and 23% are estimated with the two models for the days of seeding. For the February–May season as a whole, the increases are 0.75% and 1.8% of the normal precipitation (160 mm) for the season. The major contributions to these increases come from cumulus mediocris and cumulus congestus, and from shallow stratiform and clouds.

The low values obtained for the estimated increases indicate that a major augmentation of total seasonal precipitation in the Duero Basin is not likely to result from seeding the “regions of potential”. Consequently, within a 5-yr period envisaged for the project, it would be difficult to discern a seeding effect in terms of area-averaged precipitation. Useful increases in precipitation may possibly be produced by seeding the regions of potential, but demonstration of the seeding effects would have to be based on evaluations which are focused (in time and space) on the treated clouds or cloud regions.

Since no seeding was actually carried out during, or subsequent to these field studies, the validity of the criteria employed for defining the regions of potential, and the derived estimates, remain unverified. The criteria which define the regions of potential, and the methods of estimation developed in this study, can also be applied, in principle, to other situations, where precipitation enhancement is sought via seeding with ice nuclei.

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