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William A. Cooper

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William A. Cooper

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William A. Cooper

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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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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

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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

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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 and Darrel Baumgardner

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William A. Cooper and Diana Rogers

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Potential-flow calculations of the airflow around two research aircraft are used to estimate the effect of flow distortion on measured fluxes of sensible heat and water vapor. From the calculated flow patterns, flow-distortion coefficients are determined and used to characterize biases and contamination terms in the measured fluxes. These calculations provide the basis for estimating the magnitude of the errors and in some cases for correcting the measurements. The errors in measured fluxes for typical mid-day planetary boundary layers are usually less than 5%, if the normal choices for sensor locations are used, but could be much larger for other possible locations. The estimated errors for realistic measurement conditions are smaller than statistical uncertainties in the flux estimates for those same flight segments, when the flight legs are about 10 min in duration.

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William A. Cooper and Gabor Vali

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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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William A. Cooper and Marcia K. Politovich

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