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Robert G. Knollenberg

Abstract

A comparison study is made of liquid water content measurements using conventional devices with those integrated from optically measured drop size distributions. In clouds of low to medium droplet size (<30 μ diameter) the liquid water content from a conventional hot wire device was found to be in excellent agreement with that determined with an optical array particle size spectrometer. In populations containing more mature cloud droplets the integrated liquid water content from the spectrometer data is consistently higher. This is attributed to underestimates of larger droplet masses by the hot wire device. In low populations the spectrometer is shown to provide adequate liquid water content information while the hot wire device is limited by its signal-to-noise ratio.

Liquid water content comparisons with the paper tape device show less signature correlation and questionable agreement in absolute value.

The lack of high resolution in drop size measurements is shown to be of less importance in liquid water content determination than an adequate sample number.

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Robert G. Knollenberg

Abstract

Urea was observed to have extraordinary ice nucleating properties in laboratory and natural clouds. Ice nucleation was induced in laboratory clouds at temperatures as warm as +6C. The nucleation mechanism involves the high endothermic heat of solution and high solubility of urea. Three field releases of urea in supercooled stratus resulted in marked visual transformation of the cloud and substantial snow showers. It is concluded that urea is an effective seeding reagent for supercooled clouds. In these experiments urea appeared to be equally effective as dry ice in causing shower formation.

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Robert G. Knollenberg

Abstract

Laboratory experiments were conducted to determine the local cooling resulting from the dissolution of soluble ice nuclei which have endothermic heats of solution. The local cooling was determined from ice nucleation activity measurements. The bulk of the measurements involved an expansion technique which produced a supersaturation to activate the ice nuclei. The local cooling under supersaturated conditions was found to be slightly less than the cooling one would obtain from bulk adiabatic dissolution in forming a saturated solution.

The local cooling is greatly reduced when condensation is entirely due to particle hygroscopicity.

Maximum local cooling results if nuclei collide with cloud droplets.

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Robert G. Knollenberg

Abstract

A local cooling ice nucleation model is proposed to explain the high ice nucleation activity of soluble compounds which have endothermic heats of solution. Partial dissolution of the nuclei particle is required to produce local cooling and high activity. The amount of local cooling is shown to be governed by the heat and mass transfer processes in the boundary layer immediately adjacent to a dissolving ice nucleus. Variations in the heat transfer processes can be predicted from changes in experimental conditions. The effects of various experimental conditions upon local cooling and ice nucleation activity are the general result.

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Robert G. Knollenberg

Abstract

No abstract available.

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Robert G. Knollenberg

Abstract

An electro-optical technique is described for the measurement of the size distributions of cloud and precipitation particles. The technique utilizes a linear array of photodetectors as a size measuring grid in a typical shadowgraph-type imaging system. Several instruments have been designed and are described in various stages of testing and development. A thorough description of one such instrument and preliminary field data are given.

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Robert G. Knollenberg

Abstract

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Keith T. Griffith
,
Stephen K. Cox
, and
Robert G. Knollenberg

Abstract

Longwave emissivities and the vertical profile of cooling rates of tropical cirrus clouds are determined using broadband hemispheric irradiance data. Additionally, a broadband mass absorption coefficient is defined and used to relate emissivity to water content. The data used were collected by the National Center for Atmospheric Research (NCAR) Sabreliner during the GARP Atlantic Tropical Experiment (GATE) in the summer of 1974.

Three case studies are analyzed showing that these tropical cirrus clouds approached an emissivity of 1.0 within a vertical distance of 1.0 km. Broadband mass absorption coefficients ranging from 0.076 to 0.096 m2 g−1 are derived. A comparison of these results with other work suggests that tropical cirrus cloud emissivities may be significantly larger than heretofore believed.

Ice water content of the clouds was deduced from data collected by a one-dimensional particle spectrometer. Analyses of the ice water content and the observed particle size distributions are presented.

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