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- Author or Editor: Charles A. Knight x
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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.
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.
Abstract
No abstract available.
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No abstract available.
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.
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.
Abstract
Detailed radar echo structures and histories of two storms are presented. Both advanced into cloudless skies and had prominent, bounded weak echo regions. The storms had comparable size and intensity, and their environments provided similar amounts of shear and potential instability. One was an organized, multicellular storm, and had detailed photographic coverage from an aircraft. The combination of visual and radar data suggests the possibility of seeding of turrets in the “flanking line” by ice particles falling from the anvil. The other storm was a supercell. It had a rather steady echo configuration with a radar echo vault for about 40 min, and produced an exceptionally heavy hailswath, with hail up to 10 cm deep. However, the heavy hailfall at the ground started before vault formation and ended well before vault dissipation. The hailfall relates best to the onset of the strong updraft that (presumably) produced the vault, but does not relate to the mere fact of the existence of the bounded weak echo region.
The radar reflectivity structure and evolution of these two storms provide an interesting contrast. They are discussed in terms of the distinction between multicellular and supercell storms, and the concepts of storm and cell motion.
Abstract
Detailed radar echo structures and histories of two storms are presented. Both advanced into cloudless skies and had prominent, bounded weak echo regions. The storms had comparable size and intensity, and their environments provided similar amounts of shear and potential instability. One was an organized, multicellular storm, and had detailed photographic coverage from an aircraft. The combination of visual and radar data suggests the possibility of seeding of turrets in the “flanking line” by ice particles falling from the anvil. The other storm was a supercell. It had a rather steady echo configuration with a radar echo vault for about 40 min, and produced an exceptionally heavy hailswath, with hail up to 10 cm deep. However, the heavy hailfall at the ground started before vault formation and ended well before vault dissipation. The hailfall relates best to the onset of the strong updraft that (presumably) produced the vault, but does not relate to the mere fact of the existence of the bounded weak echo region.
The radar reflectivity structure and evolution of these two storms provide an interesting contrast. They are discussed in terms of the distinction between multicellular and supercell storms, and the concepts of storm and cell motion.
Abstract
No abstract available.
Abstract
No abstract available.
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.
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.
Abstract
The forms of snow crystals and flakes in various stages of melting can be observed with substantial fidelity after collecting the particles in subfreezing hexane. These forms are largely determined by minimum surface area considerations, which can lead to the breakup of some forms. The collection method might be useful in detailed studies of the radar bright band.
Abstract
The forms of snow crystals and flakes in various stages of melting can be observed with substantial fidelity after collecting the particles in subfreezing hexane. These forms are largely determined by minimum surface area considerations, which can lead to the breakup of some forms. The collection method might be useful in detailed studies of the radar bright band.
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.
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.
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.
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.
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.
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.
