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Taneil Uttal, Robert M. Rauber, and Lewis O. Grant

Abstract

The phase distribution of the water mass of a cold orographic cloud into vapor, liquid, and ice is calculated from measurements made from an instrumented aircraft. The vapor values are calculated from thermodynamic measurements, and the liquid is measured directly with a Johnson-Williams hot-wire device. Ice mass is calculated from particle size spectra obtained with a two-dimensional optical array cloud probe (2-D probe) and a knowledge of crystal habit based on decelerator measurements and cloud temperatures. Maximum vapor mass in the cloud is 2.0 g m−3, which is comparable with maximum ice mass in the cloud of 1.5 G m−3. Maximum liquid mass is approximately one order of magnitude lower at 0.15 g m−3 and appears to be a small remainder between the vapor and the ice as they compete for the major portion of the cloud water mass. In the cloud upwind of the mountain, liquid + vapor + ice is nearly constant, suggesting that precipitation does not deplete the water mass at the levels studied by the aircraft. Maxima in both ice and liquid mass appear just over the windward crest of the mountain, indicating a strong orographic effect on condensation of vapor to liquid and growth of ice by vapor diffusion and riming. The distribution of crystal habits also suggests a significant downdraft exists just downwind of the mountain.

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Hermann E. Gerber, Paul A. Allee, Ulprich Katz, Charles I. Davis, and Lewis O. Grant

Abstract

The Goetz Aerosol Spectrometer, generally considered to possess only a fair ability in resolving size distributions of polydispersed aerosols, operates properly following a modification to the geometry of the entrance to the instrument's deposition channels. Its accuracy is demonstrated with an electron microscopic evaluation of a collecting surface deposit of a thermally produced polydispersed AgI aerosol with particle sizes ranging from 60 to 1000Å In diameter.

Thus calibrated, the instrument was utilized to investigate the activity of the same aerosol as freezing nuclei. The AgI particles on the hydrophobic chrome-plated collecting foil were nucleated by sorption at water saturation for temperatures of −15 and −20C. The results appear to reflect the influence of the Kelvin effect since the activity decreased at a faster rate than predicted by the “surface area rare” and since it showed a sharp cutoff corresponding to Fletcher's theoretical size temperature predictions for ideal sublimation nuclei.

Also, field measurements were conducted on 12,000-ft Chalk Mountain (Climax, Colo.) for the purpose of measuring the sizes of active AgI-NaI nuclei emanating from acetone ground generators located at least 6 mi upwind. The size distribution of the nuclei on seeding days proved similar to what might he expected from this generator type. On non-seeding days, the number of active nuclei decreased sharply while the peak of the size distributions shifted to larger sizes.

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