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William A. Cooper

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William A. Cooper

Abstract

Selected concentrations of ice crystal concentrations attributable to nucleation are compiled and summarized. The variability in the observations is discussed, and some conclusions related to natural precipitation formation and to seedability are discussed.

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William A. Cooper

Abstract

A theoretical framework is developed that permits estimation of the effects of fluctuating supersaturation on the development of cloud droplet size spectra. The studies focus on the role of turbulent fluctuations in vertical wind and in the microphysical environments in which droplets grow, and represent the effects of droplets mixing together that have encountered different trajectories through the cloud. It is contended that the effects can be analyzed in terms of two contributions to the variance in supersaturation history, one dependent on the average microphysical environment (specifically, integral radius) of the near environment in which a droplet grows, and the other dependent on the correlation between the integral radius and the updraft along the droplet trajectory. Variations in the possible trajectories that all end at a given point (and so form the droplet spectrum there) are used to estimate the possible widths of droplet spectra, and methods of testing these predictions using experimental data are also proposed. Possible broadening effects due to fluctuations in the updraft at cloud base are also analyzed. It is suggested that simple turbulent motions in a stochastically varying cloud may provide significant broadening of the cloud droplet spectrum if those motions are accompanied by a variable microphysical structure produced by dry-air entrainment.

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William A. Cooper

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William A. Cooper

Abstract

A contact nucleation mechanism is suggested, by which ice embryos formed on a nucleus in vapor are able to nucleate supercooled water on contact. Using conventional nucleation theory, the activity of a contact nucleus is calculated as a function of the contact angle of ice on the nucleus in vapor, and as a function of nucleus size. It is predicted that the threshold supercooling required for deposition nucleation should be about 2.3 times as great as that required for contact nucleation (for nuclei >0.1 μm radius), and this prediction is found to be in reasonable agreement with the limited experimental evidence. A relation between the deposition and contact nucleus concentrations in air samples is predicted.

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William A. Cooper

Abstract

A PMS Forward Scattering Spectrometer Probe (FSSP) may fail to detect a droplet that enters the illuminated volume in coincidence with another droplet, or such a coincident pair may be assigned an erroneous size. This effect is shown to distort the droplet size spectra measured by the FSSP, and the effects can be quite important in clouds having droplet concentrations exceeding 500 cm−3. The most common coincidence error is one that arises when a droplet within the sample volume of the FSSP is rejected or sized incorrectly because of the coincident passage of another droplet outside that sample volume. Droplet spectra are measured to be too broad, and to contain too many large but too few small droplets, as a result of this effect. Some implications for past and future studies using this probe are discussed.

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William A. Cooper
and
R. Paul Lawson

Abstract

The general characteristics of the clouds that were included in the HIPLEX-1 experiment are reviewed, and the results for the response variables are interpreted in light of other measurements from the instrumented aircraft. In most seeded clouds, the HIPLEX-1 experimental hypothesis corresponded with the observed precipitation development for only the first ∼8 min after seeding. The failure to obtain a stronger statistical result is attributed to the inherent inefficiency of the small cumulus congestus selected as experimental units. This inefficiency was only partly due to low ice concentrations; a more significant cause of the low precipitation efficiency was the limited lifetime and low liquid water content of these clouds. Some calculations which indicate that these clouds could not support a rapid enough accretional growth process to lead to precipitation after seeding are discussed. Other reasons for the successes and failures of the experiment are discussed.

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William A. Cooper
and
John D. Marwitz

Abstract

The potential for snowfall augmentation in the San Juan Mountains of southwestern Colorado is considered. We show that the seeding criteria and delivery method used in the Colorado River Basin Pilot Project were not suited to the storm structure and characteristics described in the preceding two papers. New criteria are suggested and compared to the available statistical results. It is suggested that opportunities for precipitation enhancement by seeding occur in the latter part of the storm sequence, are associated with the release of convective instability, and can be identified by the presence of a zone of horizontal convergence upwind of the mountain range.

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Marcia K. Politovich
and
William A. Cooper

Abstract

The quasi-steady supersaturation in cumulus clouds from the Cooperative Convective Precipitation Experiment of 1981 are calculated from measurements of vertical wind, cloud droplet size, temperature and pressure. Mean values and spectral characteristics of the supersaturation are presented. The supersaturation values in 147 cloud regions averaged near 0%, with standard deviations in the (10-m average) supersaturation values that ranged from 0.1% in unmixed regions to about 0.4% in regions about 80% diluted by entrainment. The Eulerian variance spectra for vertical wind, integral radius and supersaturation were determined, and a technique is described for estimating the Lagrangian spectra. The Lagrangian characteristic times for supersaturation were estimated to be about 50 s for mixed regions but substantially larger for some regions with strong updrafts. It is argued that the observed variability in supersaturation is comparable to that needed to account for typical dispersions in droplet size spectra.

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William A. Cooper
and
Darrel Baumgardner

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No abstract available

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