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  • Author or Editor: W. R. Barchet x
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W. R. Barchet

Abstract

On the basis of a general formulation of the Gibbs free energy change for heterogeneously nucleating systems, the difference in the free energy change for the formation of a spherical cap embryo and a sessile drop-shaped embryo is evaluated. This difference represents the error introduced by assuming the embryo is a spherical cap in the presence of a gravitational field. Power series expressions for the volume and surface areas of the sessile drop, based on the work of Bashforth and Adams, are used to arrive at the free energy change. Ratios allowing a comparison of the error for different systems are presented along with numerical values for a hypothetical, unit system. This hypothetical system is compared to water and to mercury with the result that the maximum relative error is 10−6. Based on this value, it is concluded that the spherical cap model is an adequate and useful approximation of the shape of an embryo in heterogeneous nucleation under the action of gravity.

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W. R. Barchet
and
R. S. McKenzie

Abstract

There are requirements for a laboratory source of pure silver iodide aerosol with long-term output stability. Aerosol generation by a power-coated hot wire is unsatisfactory. A method of filament preparation providing a uniform, tightly bound layer of silver iodide on a nichrome heating filament is presented. Size, condensation activity, and ice nucleation properties of silver iodide particles produced by a plated filament meet laboratory needs. Long-term stability of aerosol output is an important added benefit.

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K. M. Weickmann
and
W. R. Barchet

Abstract

A numerical model is constructed to simulate the evolution of pre-ice-nucleation conditions within a settling cloud chamber. Drop nucleation, growth, evaporation, sedimentation, and vertical and radial heat and moisture diffusion are included in the model. Measured vertical temperature profiles and a prescribed condensation nucleus distribution serve as inputs to the model. Time-height cross sections of saturation ratio, liquid water content and drop concentration, and time evolution of drop size spectra at various locations are presented. The results show that maximum supersaturation is attained in a shallow layer just above the maximum curvature of the temperature profile, i.e., near the top of the brass cylinder. Subsaturation with respect to water but supersaturation with respect to ice exists in the cloud-filled iso-thermal portion of the chamber. Although the ice phase is not included in the model, the results suggest that the dominance of a particular mode of ice nucleation may be a function of the initial condensation nucleus spectrum.

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Cecil S. Lo
,
Wm R. Barchet
, and
David W. Martin

Abstract

The model of Sikdar, for inferring upward transport of mass in a cumulonimbus cloud from expansion of the anvil in a satellite picture sequence, is refined and tested. In the present mass transport model, the anvil is configured as a stack of slabs, and the thickness of each slab is inferred from cloud radiance, by means of multiple scattering results. In addition, mass transport from the base of the anvil to the base of the cumulonimbus cloud is required to decrease according to the formula of Austin and Houze, in which the entrainment is inversely proportional to total cloud depth. With a sounding of ambient temperature and water vapor mixing ratio, the model yields profiles of upward mass transport and production of condensate.

The model is applied to seven cumulonimbus clouds in the eastern Atlantic Occean on 18 member 1974, one of the last days of the GARP Atlantic Tropical Experiment. Transports of the present model at the base of the anvil average 27 &times 1010 g s-1. This is 2.5 times larger than transports calculated by Sikdar’s model for the same clouds, but only slightly larger than earlier results for other clouds. Transports at the haw of the cumulonimbus clouds average 9 × 1010 g s−1. They tend to be like earlier results from aircraft, radar and soundings. Updraft speeds are 2–7 m s−1 at the base of the anvils, 0.5–1 m s−1 at the base of the clouds. The mean total condensate production is 6.6 ×1012 g h−1 per cloud. Comparisons with satellite and radar rainrates give average rain efficiencies of 33% (radar) to 75% (satellite). Apparently, the stacked slab model tends to slightly overestimate upward transport of mass at the base of the anvil; however, in the present case this is compensated toward the base of the cumulonimbus by too large an entrainment rate.

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