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Griffith Morgan
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GRIFFITH M. MORGAN JR.

Abstract

Calculation of the trajectories of precipitation particles growing in a simplified thunderstorm updraft model shows some effects that have a bearing on understanding the mechanism of water storage in thunderstorms, the growth of large hailstones, and the techniques to be used in hail prevention.

The updraft model is characterized by a region of inflow overlain by one of outflow, with the vertical component of air velocity increasing along inflowing streamlines and decreasing along outflowing streamlines. For simplicity, streamlines are assumed to be arcs of circles in the vertical plane, and the velocity distribution in this plane is equivalent to solid rotation. Air density is assumed constant everywhere, and particles are assumed to move horizontally at the same speed as the air and vertically at a speed equal to the vertical component of air velocity minus their terminal velocity. In such a flow regime, the motion of particles is along families of concentric circles centered on the intersection between the locus of points of zero horizontal air velocity and the vertical air speed isotach corresponding to the terminal fall speed of the particle (called the “balance point” of the particle). Particles of different sizes move along different families of circular trajectories, and consideration of the way in which these families intersect illustrates the large dispersion in the directions of motion of the different components of the precipitation size spectrum at each point in the cloud. Growth of particles is specified as a linear increase of fall speed with time, sawtooth fashion for liquid drops (to simulate the spontaneous breakup responsible for the Langmuir chain reaction), and continuous for hail particles.

The results of trajectory calculations show: (1) that in the region of the balance points of large raindrops (diameter = 3–5 mm) the drop trajectories are of an indefinitely (timewise) recirculating character, indicating a tendency to store water in that region (identified with the “accumulation zone” introduced by Russian hail researchers) ; (2) that in the higher speed regions of the updraft, drop trajectories lead to ejection of the drop from the storm and little or no storage of water (identified with the “echo-free vault” introduced by English cloud dynamicists) ; (3) that hail embryos, in the form of frozen large raindrops, start their growth in the region of liquid water storage and move along looping trajectories that carry them across the up draft to the region of highest velocities at a rate such that they achieve fall speeds (size) about equal to the maximum updraft speed; and (4) that introducing cloud seeding material into the high speed updraft core should be ineffective for hail prevention due to rapid ejection from the storm. Seeding material should be introduced directly into the accumulation zone (Russian method) or at points below the cloud where the airflow will carry it through the accumulation zone.

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Griffith M. Morgan Jr.

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The Italian Po Valley hail problem is examined and a description is presented of its economic, dimensions, its climatology, and some of the meteorological factors which influence it. A brief history of hail prevention in Italy is presented, covering ancient beginnings and tracing the development of the use of explosives employed by farmers at the present time in the Po Valley.

Italy has the worst hail problem in the world, the estimated average low being $1333 per square mile W annum, nationwide, with an average loss reaching $7106 per square mile on a smaller scale in the Po Valley of North Italy.

The great crop loss to hail is not due to excessively frequent hall. Point and areal hail-day frequencies are lower than many other hail areas in the world. The great loss value is due to a combination of 1) high crop value, 2) high hall frequency during the growth season, 3) storms that are large in areal extent, 4) frequent large hail, 5) long hailfall durations, and 6) large numbers of hailstones per square foot. The meteorological cause is found in the unique terrain configuration of the Po Valley and the cyclonic development which it causes during the passage of synoptic systems.

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Griffith Morgan Jr.

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A modern Becquerel phosphososcope has been constructed for detecting zinc sulfide tracer in melted snow samples. The device utilizes the components of the real-time sensor described by Nickola et al., adapted to a rotating shutter unit. Commercially available fluidized zinc sulfide powder has been found adequate for storm work when ultraviolet radiation is almost eliminated by cloud cover. The dependence of the light output of the pigment on temperature was found to be negligible at room temperature.

An absolute calibration of light output vs. mass concentration of pigment has not been performed; results are expressed as voltage developed by the photomultiplier sensor. Field trials have shown that the instrument is very useful for detecting differences in pigment concentration from time-to-time and place-to-place.

The background signal from a test tube of distilled water is 5 mV. Samples of clean snow and stream water produce a signal of 5–8mV. When the tracer is dispersed, the signals found in samples of the snow-pack vary from near background to several hundred millivolts at a distance of 8 mi from the dispersing site, to as high as 70 V in the immediate vicinity of the dispenser.

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Griffith M. Morgan Jr.

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Griffith Morgan and Franco Prodi

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Griffith M. Morgan Jr.

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Griffith M. Morgan Jr. and Paul A. Allee

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Potential ice nuclei, particles which become ice nuclei when properly treated, are produced in automotive exhaust. They are activated by iodine vapor and are believed to be lead particles originating from the tetraethyl lead mixed with the fuel. The sensitivity of nucleation measurements provides an excellent method of counting submicron lead particles and monitoring the automotive component of air pollution. Measurements reported here show that the production of potential ice nuclei by a gasoline engine is at least 2 × 107 per gram Pb at −10C, 1 × 1010 per gram Pb at −15C and 1 × 1012 per gram Pb at −20C.

Some simple calculations are presented, showing that large enough numbers of potential ice nuclei are produced by the routine burning of gasoline to be useful in cloud and weather modification research. In particular, a DC-6 aircraft burning 6 gal min−1 of gasoline should produce at least 4 × 1011 nuclei sec−1 at −20C.

It is demonstrated that ethylene diiodide can be added to the gasoline supply of an automobile and that the automobile will then act as an inexpensive source of large numbers of ice nuclei.

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Griffith M. Morgan Jr. and Neil G. Towery

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Studies of small-scale variability of hailfall parameters are being pursued using fine-scale networks of passive hail sensors of various designs. These studies have revealed the great variability which exists in objective hail parameters over very short distances. The objective of the fine-scale measurements is to eventually produce a statistical hailstreak model with which to assess the uncertainty produced by making areal hail estimates with coarse networks as part of hail prevention experiments. An example of such an assessment for a single hailstreak crop-loss pattern illustrates the problem and demonstrates that, for the particular damage pattern used, a square grid with 1 mi spacing would estimate the areal damage within 25% accuracy 80% of the time.

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Griffith M. Morgan Jr. and Neil G. Towery

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Surface winds which accompany the fall of hail have a profound effect on crop damage. Quantitative estimates of the horizontal and total flux of hailstone kinetic energy can be made with simple theoretical considerations using data obtained from a hailcube. A hailcube is a box with aluminum foil wrapped styrofoam pads on four sides and the top. The analytical procedure for obtaining the energy estimates from cubes is described. Results show that the total kinetic energy, which includes the effect of the wind speed, can be up to five times greater than the vertical kinetic energy—the energy imparted without the effect of the wind.

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