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Narayan R. Gokhale

Abstract

The dependence of the median freezing temperature of supercooled water drops on rate of cooling is studied over a wide range of cooling rates. A polished metal surface, coated with a water repelling film of liquid paraffin is used to support the water drops being frozen. Deeper supercooling is observed with higher cooling rates in the range 0.3 to 5.0 deg C min−1. However, for cooling rates greater than 5.0 C min−1 a reverse trend is indicated and the drops freeze at warmer temperatures.

The importance of tins experimental result is discussed in the case of thunderstorm clouds with strong updrafts. A similar effect for sublimation nuclei tested in cold chambers has been noted by Schulz (1947) and by Smith and Heffernan (1954).

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Narayan R. Gokhale and Orest Lewinter

Abstract

Water drops of 2 mm diameter were supercooled on a glass slide, sprayed with AgI particles, and photographed at low magnification with a 16-mm movie camera at a speed of 64 frames per second to determine the nature of freezing by contact nucleation. In each case; nucleation was initiated at the point of particle contact and continued over the entire surface of the drop. The interior of the drop then froze toward the center at a much slower rate. The surface structures and crystallization rates by contact nucleation compare favorably with those found by other workers where water was frozen in containers.

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Narayan R. Gokhale and K. M. Rao

Abstract

No abstract available.

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John D. Spengler and Narayan R. Gokhale

Abstract

Recent attempts to understand the development of intense precipitation has led several investigators to speculate about the role of drop impactions. Working with a large vertical wind tunnel, investigations were carried out on the interactions of millimeter size drops. On the basis of these studies it is concluded that drop impactions produce a rapid increase in the number of precipitation size drops (average four or five per collision) while limiting the growth of larger drops. For drops ≳4 mm in diameter there exists a spectrum of smaller drops whose relative kinetic energy of impaction exceeds the critical value of 15 ergs, thus preventing permanent coalescence. However, impactions seldom completely destroy the larger drops, but they do remove some mass. Drop impactions reveal a self-regulating mechanism in nature that enables collisions to influence both the initial growth and the determination of final size for large drops.

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Narayan R. Gokhale and John D. Spengler

Abstract

The freezing of freely suspended, supercooled water drops by contact nucleation has been studied. Water drops were balanced in an updraft of a large vertical wind tunnel and allowed to supercool to the ambient temperature. Ice crystals introduced into the updraft were, as expected, the most effective nucleants in freezing the drops at ambient temperatures colder than and up to 0C. The results of this experiment using silver iodide and clay as contact nucleants closely agree with earlier work performed with a constant rate of cooling apparatus. The effective temperature for 100% nucleation efficiency in the case of AgI particles was −4 to −5C and for silicate particles −7 to −10C.

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Narayan R. Gokhale and Earl G. Droessler
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John D. Spengler and Narayan R. Gokhale

Abstract

The design and operation of a large vertical wind tunnel (1.8 m diameter updraft) are described, together with the associated special photographic equipment and techniques required for studies of the interactions and freezing of freely suspended water drops.

During the first two winters of operation several new and important observations have been made while freezing freely suspended, large supercooled water drops. The terminal velocity of the frozen pellet was found to be very different than that of the liquid drop. If individual drops freeze at −6C and colder they often exhibit a marked decrease (up to 4 m sec−1) in terminal velocity. Coalescence of a frozen pellet and a liquid drop produces an elongated ice pellet (8–15 mm horizontal axis) with a terminal velocity of 9 m sec−1. When an ice pellet becomes unstable and spins about a horizontal axis, it can obtain a rapid horizontal velocity. Two ice pellets frozen together display the same erratic tumbling. These observations indicate that some ice pellets have greatly increased distances and residence times to grow in the supercooled region of a cloud.

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Narayan R. Gokhale and James Goold Jr.

Abstract

Droplet freezing by surface nucleation has been observed during the course of studies of ice nucleation in our laboratories. A constant rate of cooling apparatus and a specially constructed cold chamber were used for this study. Silver iodide particles sprinkled on supercooled, millimeter-size water drops are effective in freezing the drops at −5C. Particles of naturally occurring silicates were found to be effective in the range −7 to −10C. Thus, the ice nucleating ability of such dry particles is much greater by the surface contact mechanism than when particles are embedded in the drops.

The importance of the surface nucleation mechanism in forming solid hydrometeors in cumulus clouds is assessed.

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