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Hermann E. Gerber

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Hermann E. Gerber

Abstract

The ability of AgI particles to nucleate ice by the freezing mechanism was measured with the Goetz aerosol centrifuge as a function of particle size, temperature and time. When the cooled centrifuge deposit was exposed to a puff of supersaturated nitrogen, each particle was immersed in a supercooled droplet which, if it contained an active AgI particle, froze and formed a visible ice crystal. The fraction of active particles was found by comparing the number of ice crystals with the size distribution of all the particles on the deposit.

The mean nucleation rate per particle depended on particle size for a given temperature, which suggested that the surface of the particles was smooth with respect to the size of the critical ice embryos. The measured rates were about a factor of 1014 ± 16 smaller than predicted by Fletcher's theory. Activity spectra, calculated from the measured rates by numerically integrating the activities of all the particles in log-normal size distributions, showed the importance of the nucleation time lag at temperatures warmer than about −12°C or when the AgI generator produces very small particles.

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Hermann E. Gerber and Edward E. Hindman

The amount of light absorbed by aerosol particles has not been determined with certainty because errors in measurement techniques have been difficult to quantify. To improve this situation, a workshop was conducted to establish experimentally the errors for the various techniques. The workshop was held between 28 July and 8 August 1980 at the Cloud Simulation and Aerosol Laboratory at Colorado State University. Preliminary results show that, for the same well-characterized aerosol particles, substantial differences exist between results from the various techniques. These differences can explain a fraction of the variations reported for the light absorption properties of similar types of atmospheric aerosol particles.

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Robert W. Fenn, Hermann E. Gerber, and Dieter Wasshausen

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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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Yong-Feng Ma, Szymon P. Malinowski, Katarzyna Karpińska, Hermann E. Gerber, and Wojciech Kumala

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The authors have analyzed the scaling behavior of marine boundary layer (MBL) clouds using high-resolution temperature (T) and liquid water content (LWC) fluctuations from aircraft measurements collected over the Pacific Ocean during the Physics of Stratocumulus Top (POST) research campaign in summer of 2008. As an extension of the past studies for scale-invariant properties of MBL clouds, the authors studied the variability of scaling exponents with height. The results showed that both LWC and T have two distinct scaling regimes: the first one displays scale invariance over a range from about 1–5 m to at least 7 km, and the second one goes from about 0.1–1 to 1–5 m. For the large-scale regime (r > 1–5 m), turbulence in MBL clouds is multifractal, while scale break and scaling exponents vary with height, most significantly in the cloud-top region. For example, LWC spectral exponent β increases from 1.42 at cloud base to 1.58 at cloud top, while scale break decreases from ~5 m at cloud base to 0.8 m at cloud top. The bifractal parameters (H 1, C 1) for LWC increase from (0.14, 0.02) at cloud base to (0.33, 0.1) at cloud top while maintaining a statistically significant linear relationship C 1 ≈ 0.4H 1 − 0.04 in MBL clouds. From near surface to cloud top, (H 1, C 1) for T also increase with height, but above cloud top H 1 increases and C 1 decreases with height. The results suggest the existence of three turbulence regimes: near the surface, in the middle of the boundary layer, and in the cloud-top region, which need to be distinguished.

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