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Stanley A. Changnon Jr.

Abstract

The distribution of hail days during 1961–80 in the northern Great Plains-Midwest was evaluated on a temporal and spatial basis to help interpret crop-hail losses. Comparisons with earlier (1901–60) hail day data revealed the seven-state study area contained eight permanent areas of high and low incidences found in any 5-year or longer period. The high hail incidence areas were related either to major topographic features or to areas of frequent frontal occurrences. Certain other areas of high or low hail incidence appeared at random locales, lasted 5 to 20 years, and disappeared. The annual and July incidences of hail increased sporadically but steadily from 1901 to 1980 in the Dakotas, Nebraska and Minnesota, reaching a peak during 1961–80. This has led to relatively more crop damage in recent years. In Montana, eastern Iowa, and Illinois, hail has decreased to a low in 1961–80. During the 1961–80 period, hail maximized in 1961–65, being 30% more frequent than in any subsequent 5-year period.

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Stanley A. Changnon Jr.

Abstract

Convective raincells, as delineated by surface rainfall data from large and dense raingage networks in Illinois, were analyzed to define various characteristics. Raincells are the surface expression of convective entitles, and as such are important in furthering our understanding of convective storms, for designing mesoscale studies, and for evaluating numerical cloud models. The raincells exhibited great variability in most characteristics, but were typically 16 km long, 6 km wide, and had a mean rainfall of 2.5 mm. Their orientations varied widely but the most frequent was WSW–ENE. There was a second but infrequent class of large raincells that often exceed 43 km in length and can be up to 40 km wide.

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Stanley A. Changnon Jr.

Abstract

Unique detailed hail and rain data from dense networks operated in central Illinois during 1967 and 1968 have been used to define and study in detail hailstreaks and their associated rainfall. A hailstreak is an area of continuous hail with temporal coherence and is considered an entity of hail generated within a thunderstorm. The average hailstreak represents a fast-moving, short-lived, and relatively small phenomenon. Eighty per cent of all hailstreaks had areas <16 mi2 and hail impact energy values <0.1 ft-lb ft−2, but areas extremes were 0.9–788 mi2 and energy extremes sampled were 0.0001–12.6 ft-lb ft−2. A hail-producing system in a 1600 mi2 area normally produced five hailstreaks with an average separation distance of 15 mi. Cold fronts and unstable air mass conditions led in the production of hall systems and hailstreaks, respectively. Hailstreaks produced by different synoptic weather conditions differed considerably. Point durations of hailstreaks averaged 3 min, 43% occurring in the 1500–1900 CDT period. Eighty-eight per cent of all hailstones had diameters ≤¼ inch and only 1% were ≥1 inch. A 1-ft2 area in a hailstreak normally experienced 24 hailstones. Hailstreaks occurred in all locations and stages of age of their associated rain cells, but their preferred location was along the major axis and in the mature stage. Normally, a rain cell produced only one hailstreak, although 20% produced four or more. However, 52% of the rain cells in hail-producing systems did not produce hail. The average rainfall in a hailstreak was 0.19 inch, with an average point rainfall rate of 0.63 inch hr1. Rainfall produced throughout the network areas during hail-producing systems accounted for 38% of the 1967 network warm season total and 57% of the 1968 total.

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Stanley A. Changnon Jr.

Abstract

Temporal and spatial relationships between thunderstorms (events) and flashes were investigated using data for 1983–85 for 25 first-order stations (10 in the West and 15 along the East Coast). Thunder events were compared with flashes within three ranges: 5 km, 10 km, and 20 km, around each station. Cluster analysis revealed six geographic regions: Florida, Southeast (South Carolina, Georgia), Mid-Atlantic (Virginia, Maryland Pennsylvania), Northeast (New York and New England), Rocky Mountains, and an intermontane area.

Periods of multiple flashes not within thunder events and within 10 km of a point (most realistic for audibility), revealed that 10% to 20% (depending upon region) of all thunderstorms were missed. Also, 13% (Rockies) to 44% (Mid-Atlantic) of all thunderstorms have recorded durations too short (missed flashes before their reported start), and the average underestimated durations were from 55% (Northeast Mid-Atlantic) to 26% (Rockies). Flashes isolated in time and space, due to locational errors, represent 1% of all flashes in the east and 3% to 5% in the west where the data are poorer. Errors in flush data appear minimal but the errors in thunder events are sizable.

Thunder events and flash frequencies related well based on major features in their average areal patterns and their between-year changes at stations. Correlations of fishes with events varied; their annual point frequencies had coefficients of +0.83 (east) and +0.67 (west). Durations of events and Bash frequencies were poorly correlated with skewed distributions (often large flash frequencies in a few storms). The percent of all recorded flashes (within 10 km) in thunder events varied from 28% to 44% at western stations and from 13% to 20% at eastern stations. Thunder events with ≥ 1 flash varied widely, from 71 % of all events at Washington, D.C. to 30% at Boston.

Major east-west differences existed in the frequency of thunder events with flashes, reflecting poorer audibility of thunder in the west. Part of the difference is due to flash recording problems in the west, leaving flash frequencies that are underestimates of the true values. latitudinal distributions were marked with north-to-south increases in thunder events and their durations, frequency of flashes, and number of flashes not in events. More missed flashes in the south suggested that atmospheric conditions in northerly U.S. latitudes enhance audibility. With a 20-km sampling radius, between 6 (Northeast) and 23 (Southeast) thunder events are not recorded yearly, but these averages drop to 1 (Northeast) and 4 (Southeast) based on flashes within 5 km. The data on thunderstorms is generally poor from two perspectives: 1) the recorded data miss sizable numbers of storm events, and 2) when recorded, 30% to 50% often underestimate durations based on nearby lightning activity.

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Stanley A. Changnon Jr.

Collection of precipitation in a raingage located in Lake Michigan near Chicago over an 11-yr period has permitted a study of the average monthly, seasonal and annual precipitation received off the southwestern shore of the lake. These values are compared with average precipitation occurring in the Chicago urban area and with amounts from other nearby rural and shore stations. From these comparisons, the effects of lake and urban influences on the precipitation pattern are evaluated.

The results of this study compare favorably with results from similar lake-precipitation studies performed by the Corps of Engineers. The precipitation data from this station in the lake indicate that precipitation over southern Lake Michigan may be considerably less than previously estimated from data of stations along the shore. The effect of precipitation on lake levels may be better evaluated now that additional information on lake precipitation is available.

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Stanley A. Changnon Jr.

Abstract

Collection of hail data that will provide meaningful measures of the results of hail suppression projects varies according to five factors, including the geographical-climatic site, the statistical design, and the goal of the project. Eight possible techniques of collecting hail data are evaluated with respect to their use in different areas, availability of historical data, and with different project designs. Each technique provides data that have distinct limitations. However, the two data collection techniques rated best for projects having a continuous seeding (on all hail days) design are networks of passive hailpads and raingages, and crop-hail damage records. The best technique for use in projects utilizing a single-storm seeding design or a random daily seeding (single area) design is a network of recording hailgages.

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Stanley A. Changnon Jr.

Abstract

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Stanley A. Changnon Jr.

Abstract

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Stanley A. Changnon Jr.

Abstract

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Stanley A. Changnon Jr.

Abstract

Four forms of hail data in Illinois were analyzed to obtain indirect measures of the areal and seasonal variations in hail intensity. These data were also examined to ascertain which hail characteristics correlated best with crop damage.

The frequency of intense hail in the crop season (May–October) was found to increase with time, reaching a maximum in September. Insurance statistics indicated that corn damage from hail was usually greater in July than in the later months because corn was more susceptible to damage in July. Observations from a mesoscale network in central Illinois indicated that hailstone sizes and number (volume of ice) and durations of hailstorms related moderately well with crop-hail damage, but that strong surface winds were more closely related. Significant areal variations of hail intensity were found, with some portions of Illinois experiencing intense hail six times more often than other areas.

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