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David L. Mitchell, Renyi Zhang, and Richard L. Pitter

Abstract

The masses, dimensions, and habits of over 2800 natural ice particles precipitating from orographic winter storms in the central Sierra Nevada were obtained using photomicrographs. Ice particles that could be unambiguously classified were used to generate empirical expressions relating snow particle masses and dimensions. Many of the ice particle types had not been investigated previously. The influence of riming and aggregation on ice particle masses was examined. When possible, comparisons are made between these results and those of other experimental observations. By incorporating these mass-dimensional relationships into an expression for the ice mass content in a snowstorm, it was possible to estimate the mass fraction of the fresh snowpack resulting from accreted supercooled cloud water. The results from two storms analyzed suggest that about 30 to 40 percent of the deposited snow is composed of accreted cloud water during moderately rimed snowfall.

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Steven K. Chai, William G. Finnegan, and Richard L. Pitter

Abstract

In a 1984–85 winter cloud-seeding program at Lake Almanor, California, indium sesquioxide (In2O3) aerosol particle generators were collocated with silver iodide (AgI) aerosol particle generators as a source of inert tracer aerosol. The In2O3 aerosol served as an indicator of the amount of AgI aerosol scavenged. Based on the aerosol emission rates, if AgI aerosol was only captured by scavenging processes, and played no part in forming ice crystals and snowfall, the silver to indium ratio (Ag:In) in the analyzed snow would be 0.8.

Analysis of snow samples from the target area produced frequent Ag:In ratio values in excess of 1.1. In the snowfall at the closest sampling sites to the aerosol generator the high ratios of Ag:In cannot be explained by the contact-freezing ice formation mechanism. A mechanism with a much faster rate than possible by contact freezing is necessary to produce the high Ag:In ratios that were observed. Part of the AgI seeding aerosol functioned rapidly to produce ice crystals by a forced condensation-freezing mechanism immediately after generation, and those ice crystals contributed to the snowfall at those sites closest to the generator.

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Belay B. Demoz, Renyi Zhang, and Richard L. Pitter

Abstract

Systematic observations of the sizes, shapes, and degrees of riming of ice particles falling at a downwind station of a major mountain barrier are presented. The observational station was equipped to measure ice-particle masses from 1 µg to a few milligrams, and to measure ice-particle dimensions, habits, degrees of riming, and degrees of aggregation. The results are shown to be useful in learning where ice nucleation and growth take place in the cloud system.

The present study analyzed dissipating and developing winter orographic storm systems, which are representative of more than 60% of the storms observed over the study region. It suggests that most of the needles and columns observed at the ground may be formed by secondary ice production. Heavy riming was associated with light precipitation, while high precipitation rates were correlated with a high number fraction of aggregate crystals. Aggregation was found to be important in the process of precipitation development and the aggregate mass was mostly contained in the dendritic crystal growth region.

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