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Charles A. Knight

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Charles A. Knight

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Charles A. Knight

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.

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Charles A. Knight

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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

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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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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

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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

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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

The Cooperative Convective Precipitation Experiment, CCOPE, was an outgrowth of the perceived need for more comprehensive data sets on convective clouds. It was planned and executed by a large group of participants, with the leadership of the Convective Storms Division of NCAR and the Office of Atmospheric Resources Research of the Bureau of Reclamation. The field program ran from 18 May through 7 August 1981, involving networks of eight radars—seven Doppler, two dual-wavelength—123 mesonet stations, seven upper-air sounding sites, and 14 research aircraft (of which as many as eight were flown in coordinated missions on single storms). The field program was an operational success, with a lot of convective activity within the densely instrumented area, permitting many relatively complete data sets to be obtained. The data are now becoming available, and the analysis effort is commencing.

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