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Griffith M. Morgan Jr. and Neil G. Towery

Abstract

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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Griffith M. Morgan Jr. and Neil G. Towery

Abstract

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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NEIL G. TOWERY and STANLEY A. CHANGNON JR.

Abstract

Data from 103 hail echoes on 24 days in 1967 and 50 no-hail echoes from the same days were analyzed to describe hailstorm characteristics and to provide information useful in operational detection and forecasting of hail-producing echoes. Echo characteristics investigated included locations of echo formation and dissipation, echo reflectivities, echo-top heights, echo duration, direction of motion, speed, time of occurrence, and associated synoptic weather conditions. A single hail-echo model could not be derived because of the extreme variability found in all characteristics. However, distinctive echo models could be developed for the three predominant hail-producing synoptic weather conditions, cold fronts, stationary fronts, and low-pressure centers. The frontal hailstorms were faster moving, longer lived, and had taller echoes than those with low-pressure systems. Hail production after echo inception varied from an average of 32 min for low conditions to 59 min for cold frontal echoes. The average hail-echo top exhibited a 5,000-ft growth in the 15-min period prior to the average time of hail, suggesting that a major updraft surge was the prime producer of hail. The no-hail echoes occurring on hail days had characteristics of speed, direction of motion, reflectivity, and location that were very similar to the hail-producing echoes. The only distinct consistent difference between the hail and no-hail echoes in all synoptic situations was that the hail-echo tops averaged between 2,000 and 4,000 ft higher throughout their entire durations.

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Neil G. Towery and Griffith M. Morgan Jr.

Aerial photographs of crop-hail damage areas have revealed a smaller-scale feature of damage patterns than has previously been reported. These features, called “hailstripes,” are long, narrow (15–30 m width) stripes of alternating light and heavy damage. These are believed to be caused by a combination of strong wind and “hailstrands,” long string-like substructures of the hailshaft.

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Peter H. Hildebrand, Neil Towery, and Michael R. Snell

Abstract

Techniques of measuring area-mean convective rainfall over small areas (<2000 km2) are investigated using data from several gage-radar rainfall measurement studies. These data suggest that for low gage densities (≲1 gage per 250–300 km2) and for some climates (e.g., Illinois) that gage-radar area-mean convective rainfall measurements may be more accurate than gage-only measurements. The same result was not supported by data from another climate, suggesting that changes in raincell size and rain evaporation rate may affect the results. Further studies are needed to verify these suggestions. The data indicate that radar adds little to gage measurement of mean areal convective rainfall for gage densities of ≳1 gage per 100–200 km2.

Use of a space-variable gage adjustment of radar data is seen to be preferable to use of an averaged single adjustment factor, except in cases involving extrapolations of the space-variable adjustment factor. For such cases, e.g., over large lakes or outside of gage networks, use of the averaged single adjustment factor may be preferable.

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Neil G. Towery, Griffith M. Morgan Jr., and Stanley A. Changnon Jr.

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Stanley A. Changnon, Griffith M. Morgan Jr., Gary L. Achtemeier, Neil G. Towery, and Ronald C. Grosh

Abstract

A description is given of a broad program to Design and Experiment to Suppress Hail (DESH) in Illinois. This program draws on results acquired during 17 years of extensive hail research in Illinois. There are two principal tasks to DESH: the determination of the desirability and the feasibility of hail suppression experimentation in Illinois and the Midwest. Socio-economic studies have led to an affirmative conclusion on the desirability issues. The feasibility decision appears affirmative and rests on certain key results. Airborne cloud base seeding in the humid midwestern environment is possible but will be more difficult and expensive than in less humid areas. Radar will be needed for short-term forecasting, aircraft operations, identification of potential hailstorms, and in the evaluation of seeding effectiveness. Weather forecasting by objective techniques will be valuable in both operations and evaluation, and adequate objective techniques have been largely developed. The overall shape of the proposed experiment is now clear. It will consist of an impact monitoring effort, which will make assessments of societal, environmental and economic impacts and communicate with the public; an operational effort to execute the experiment according to the final detailed design; and an evaluation effort combining a variety of surface, synoptic and radar data to assess the efficacy of the chosen seeding technique.

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Neil G. Towery, Stanley A. Changnon Jr., and Griffith M. Morgan Jr.

The characteristics of commonly used surface hail instruments are reviewed and instruments evaluated. The instruments evaluated have been classified into two major categories: integrating and recording. The integrating sensors are relatively inexpensive and provide certain useful hail data. The recording sensors are much more expensive but provide more useful data including time of hail. The review includes the principles of operation, types of data obtained, operational advantages and disadvantages, and approximate cost of each type of instrument.

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