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Charles A. Knight

Abstract

The application of two-dimensional, surface “phase” changes to explain activation and memory in heterogeneous ice nucleation is examined and found to contradict nucleation theory. At a temperature at which an ordered surface is stable, the unstable, disordered surface should be the better nucleator. The two-dimensional phase change theory is discussed from other points of view as well, with the conclusion that its validity is highly doubtful. Nevertheless, some of the evidence that led to its original proposal remains unexplained.

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Charles A. Knight

Abstract

A Lagrangian, trajectory-tracing scheme for modeling precipitation formation by the ice process is used for extensive sensitivity testing and is applied to a CCOPE (Cooperative Convective Precipitation Experiment) first-echo case. One major purpose is to try to gain a judgement of the degree of model simplification of the microphysical growth processes that is jusfiable in light of present uncertainties regarding both particle growth rates and cloud water content. Considerable microphysical simplification appears justified in that the time interval between nucleation and the onset of efficient accretional growth is far more important than any other factor in the growth equations.

The model reproduces semiquantitatively the observed, first precipitation formation in the modeled cloud; it does not show a need for any novel ice nucleation schemes or ice multiplication processes.

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Charles A. Knight

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Charles A. Knight

Abstract

The principle of formation of etch pits with crystal faces on ice crystals is explained as a natural consequence of evaporation (or any sort of dissolution) at concave surfaces of crystals. A new technique of ice etching using perforated metal foil is described. It is a useful way of determining grain orientations in hail stones.

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Charles A. Knight

Abstract

No abstract available.

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Charles A. Knight

Abstract

Results are summarized and illustrated from a long series of experiments on ice growth from the vapor, nearly all in a very small range of conditions: −5°C, slightly below liquid water saturation, with minimal environmental gradients and no imposed ventilation. The temperature was chosen because c-axis ice needles grow in a narrow temperature interval there, which coincides with the temperature at which the Hallett–Mossop ice multiplication process operates most effectively, and one may suspect that this coincidence is likely to be meaningful. The ice growth habit is poorly reproducible in these conditions, dictating many runs with little change. Growth as plates can persist for hours, and two distinct types of needle growth occur, called sheath needles and sharp needles. Both are distinct from thin columns in that they taper to a point, with no discernible basal face. Both deviate slightly from parallel to the c axis. Sharp needles have been reported before, but only as occurring with an applied high DC voltage. New crystal orientations nucleate occasionally at the tips of the sharp needles; this also has been seen before in the presence of strong electric fields. There appears to be an ice multiplication mechanism in these conditions that does not involve riming.

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Charles A. Knight

Abstract

It is argued that the mechanism of spongy hailstone growth, in certain conditions, is analogous to growth in conditions of constitutional supercooling. If so, this allows the application of the large body of information on, and theory of, crystallization in constitutional supercooling to be applied to spongy hailstone growth. Justification of this view is from fundamental principles, from observations of spongy icicles, and from the orientation fabrics of artificial spongy hail. Application to natural hail is restricted to cases in which the crystal size is fairly large.

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Charles A. Knight

Abstract

Examination of the early radar echo histories of several vigorous, cumulus clouds in northeast Colorado and northwest Kansas, with sensitive, dual-polarization radar, reveals the formation of millimeter-sized water drops at about the same time that the conventional, first precipitation echo (from ice) forms aloft. The early, positive Z DR values appear in the vicinity of the 0°C level (the radar data do not specify height accurately) and soon extend both above and below it. Positive Z DR is found within and to the upwind side of the updraft, separate from the conventional first precipitation echoes, which appear first at higher altitude, generally downwind of the updraft core, and have no significantly positive Z DR. Big, liquid drops were not expected this early in the formation of continental cumulus. The early presence of supercooled water drops larger than cloud droplets may be a significant factor in the glaciation of these clouds.

The kind of early radar coverage illustrated here would be a priceless adjunct to aircraft studies of precipitation formation in cumulus. Microphysical data from aircraft must be interpreted with numerical models in order to deduce (or verify) the processes, and such models require the kind of early data illustrated here, both for initialization and verification.

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Charles A. Knight

Abstract

A simple and computationally efficient method is described for estimating hydrometeor size distributions within a convective storm. The method requires air motion measurements (from Doppler radar in this case, but it could be used with a dynamic model), and specification of the cloud water field and the mechanism by which the hydrometeors originate. The cloud water field that corresponds to the wind field used is estimated by calculating condensation and depletion rates along air parcel trajectories. It is assumed that the storm is in steady state and that hydrometeors grow only by accretion.

The technique is applied to one of the storms documented in the Cooperative Convective Precipitation Experiment (CCOPE), assuming that hydrometeors originate by primary ice nucleation alone. The distribution of hydrometeor sizes that is obtained is very unrealistic, in such a way that one or more other sources must have dominated hydrometeor formation. Since the trajectory analysis indicated that the source had to be at temperatures above 0°C, it must have been either coalescence or some melting process. Alternatively, there could have been a strong transport of small hydrometeors on scales unresolved by the Doppler radar.

The analysis scheme is a useful tool for learning about precipitation mechanisms from held data, and it will be more useful if it can be extended to time-varying cases without becoming too unwieldy.

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Charles A. Knight

Abstract

No abstract available.

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