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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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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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Marcia K. Politovich
and
Gabor Vali

Abstract

The relatively simple orographic clouds forming in winter over Elk Mountain, Wyoming provided useful opportunities for field studies of cloud formation and of ice crystal development. In this paper, the observations of cloud droplet populations spanning a range of five consecutive years are summarized.

Date are presented which describe the climatology of the cloud droplet spectra. Selected cases are described in detail to illuminate the process at work and to allow comparisons with theoretical predictions.

Droplet concentrations are mostly around 300 cm−3 in accordance with the weak updrafts of the clouds and with the mid-continental, unpolluted cloud condensation nucleus concentrations prevailing in the region. In general, the data are in agreement with one-dimensional microphysical model calculations.

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William A. Cooper
and
Gabor Vali

Abstract

Ice crystal development in relatively simple layer clouds was studied using airborne instrumentation. The patterns in the development of ice in those clouds suggest that the ice originates in association with the initial condensation process, near the upwind edge of the cloud. Since continued ice production does not occur beyond that region, the ice development can be attributed to nucleation. There is no evidence for secondary ice generation. Either condensation-freezing or contact nucleation could account for the observed nucleation process, but special properties are required for the nuclei in either case. Ice crystal concentrations show a clear temperature trend, as expected for a nucleation process.

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J. B. Doolittle
and
Gabor Vali

Abstract

Heterogeneous freezing nucleation in electric fields was studied on samples of water containing organic nuclei or silver iodide. Electric fields of 6000 V cm−1 (dc) were applied over sets of supercooled drops supported on a silicone varnish coated surface during different time-temperature sequences. In no case was a significant difference in the nucleation rates observed due to application of the field. It is concluded from these experiments that electric fields of up to 6000 V cm−1 have no intrinsic effect on the heterogeneous freezing process and that the probability of enhanced ice nucleation in the atmosphere due to natural electric fields is quite remote.

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R. C. Schnell
and
Gabor Vali

Abstract

Decayed plant leaf litters from North America, Europe and Asia have been found to contain copious numbers of ice nuclei, some active at −4°C. The abundance of nuclei in a litter was noted to vary according to the climate of the plant's origin; litters from tropical, A-type climates (according to the Köppen classification) contain fewer ice nuclei (103 g−1 active at −10°C) than litters from mid-latitude, C-type climates (105 g−1 active at −10°C) which in turn contain fewer nuclei than litters from high-latitude, D-type climates (109 g−1 at −10°C). The rate of release of freezing nuclei to the atmosphere from in situ litters from D-type climates was determined experimentally: the flux of nuclei active at −12°C was found to be 101−103 cm−2 day−1 during daylight hours.

Active ice nuclei also have been found in seawaters rich in phytoplankton; seawaters devoid of plankton are poor sources of ice nuclei. Some of these nuclei are active at temperatures around −4°C and concentrations reach up to 107−103 nuclei at −10°C per gram of plankton.

Using numerous measurements from around the globe, atmospheric ice nucleus concentrations, and also freezing nucleus concentrations in rainfall, were shown to exhibit a climatic dependence similar to that of biogenic nuclei sources at the surface. This correlation suggests that large proportions of atmospheric ice nuclei are possibly of biogenic origin.

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