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Stanley A. Changnon

Abstract

Two large, dense recording rain gauge networks of identical size and gauge density, the Central Illinois Network (CIN) and the Southern Illinois Network (SIN), operating over the same 10-yr period, offered unique data to assess causes for the climatological differences in their precipitation and to identify results relevant to hydrologic design and operations. Long-term averages show SIN has 32% more precipitation in the colder half year and has slightly higher heavy rainfall frequency values, but both networks experience the same average number of days with precipitation and thunderstorms, with nearly identical precipitation amounts in the warmer half year. The 10-yr data sample (1958–67) had near-normal precipitation conditions, but likely is representative not of long-term conditions but only a 10-yr period. Storms, discrete periods of precipitation in a network, had durations in SIN that were 2.5 h longer than in CIN in the colder half year. The primary direction of precipitation movement was from the northwest for CIN and from the west-southwest for SIN, revealing important differences related to the predominance of stationary-frontal rain conditions in the south as compared with cold-frontal conditions being primary in the central area. Both networks measured the same annual frequency of storms producing measurable precipitation, but SIN had 23% more storms producing average network amounts in excess of 1.27 cm. The SIN network had 2 times the number of storms with 2-yr frequency values in winter–spring. Precipitation from these additional storms in the south represented 46% of the total annual difference between the two networks. In summary, many cold-season rain-producing conditions in SIN tended to last longer, have stronger convection, and produce heavier, more-widespread rainfalls than in CIN. Further, many cold-season rains were tied to the higher frequency of stationary-frontal conditions in the south. Warm-season network rainfall amounts were similar, but many rainfall characteristics differed. The southern area exhibited more rainfall variability and more airmass-type storms, whereas the central area had longer summer storms. Data from these dense rain gauge networks were essential in ascertaining the causes for the regional hydroclimatic differences. Furthermore, several results reveal the magnitude within these networks of point–area and point–point differences likely to be sampled in a 10-yr period. The number of 2-yr events at some gauges was 4 times greater than at others.

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Stanley A. Changnon

Abstract

Operation in Illinois of eight dense rain gauge networks of varying sizes, with each operating for 5–22 yr, provided data used to assess the temporal frequency of very heavy rain events on small- to moderate-sized areas. Initial testing reveals that incidences of heavy rainstorms with amounts equating to 25-yr values for 6–12 h, and those equating to 100-yr values for 1 day, increase rapidly as area increases and that values for the average storm incidence for 5-yr periods fit a logarithmic distribution. Results indicate an area of 300 km2 will experience one 25-yr 6–12-h storm in an average 5-yr period, whereas a 10 000 km2 area would experience 10 such storms in an average 5-yr period. Assessment of incidences of 100-yr storms or greater with a 1-day duration also found that the eight network values fit a logarithmic distribution of the area and storm frequency. In a 5-yr period, one such severe 100-yr rainstorm will occur in an area of 1500 km2, and three such storms are average for a 10 000 km2 area. The results serve as a meaningful basis for more extensive research, and the values derived have relevance to storm transposition as part of hydrologic design and for storm water management for urban and rural basins. The results should be applicable for estimating storm occurrences in areas with similar climate throughout the central United States.

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Stanley A. Changnon

Abstract

Rain-yield findings were integrated with the average incidence of rain days and areas distribution of rain in a potential rain-modification area in Illinois to simulate regional aspects of a cloud-seeding project over a 13 000 km2 area. Potential seeding opportunities are limited because clouds cannot be effectively seeded at night, and 46% of all rain events occur at night. Further, 32% of all remaining rain events occur with severe weather warnings when Illinois law does not allow seeding. Hence, the number of candidate rain periods for modification is drastically reduced from a regional average of 31 days to only 11 days. Yield increases from the best treatment, based on all years’ performance (25% increases in rain on all days with moderate rainfalls, 2.5 mm–2.53 cm) are further reduced regionally because on 52% of the moderate rain events, 50% of the simulated project area receives less than the minimum moderate rain level, 2.5 mm, and thus has no appreciable yield gains. These various factors combine to reduce yield gains from 20% to 43% of the yield responses found in the 1987–91 field trials. The effects of the resulting crop-yield changes over the simulated project area, as calculated for varying rain-modification capabilities applied over a series of years, ranged from an average annual increase of $3.4 million to an average decrease of $2.6 million per year. The estimated annual cost of a quality cloud- seeding project over the area is $1 million; hence, regional benefits would be marginal, ±3% of the total farm income. They could be much larger if summer rainfall forecasts were sufficiently accurate to allow selection of the rain treatment best suited to the actual summer conditions, including no seeding in those summers like 1989 when natural rainfall met crop water needs. If one had advance knowledge that an Illinois summer was to be extremely hot and dry like that in 1988, could have a well-organized seeding project ready on 1 June, and had a technology that could produce a 40% increase in all summer rains, the estimated crop-yield benefit in the simulated project area would have been $25 million.

The 1987–91 field trials sampled only 30% of the growing conditions that occur in Illinois, and thus, the rain-modification results are only estimates of the possible outcomes from added rainfall. Nevertheless, they reveal clear needs for research relating to weather modification in the humid climate of the Corn Bell. First, more field trials are needed to define crop yield-rain relations in other types of growing seasons. Second, methods for seeding clouds at night must be developed if agriculturally useful increases are to occur. Since the value of choosing to modify rain in a given summer depends on the availability of an accurate forecast of summer rainfall, increased attention should be given to seasonal forecasting research.

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Stanley A. Changnon

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In-depth interviews with 27 executives in various agribusiness defined usage and needs for climate predictions. Predictions are acquired from various public and private sources but are seldom used in making major decision. Users exhibited little trust of climate predictions, relying heavily on recent weather conditions as the basis of prediction. Additions to predictions involving climatic information would better serve the needs of most of agribusiness. Improved predictive accuracies alone will not materially increase usage. A need exists to familiarize agribusiness leaders with the information currently available, and to realize benefits from this information; many agribusinesses will need to develop models and procedures that allow integration of future weather conditions (actual and predicted) with their corporate activities and economic conditions.

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Stanley A. Changnon

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A study of very detailed spatial and temporal data on damaging hail, associated cloud-to-ground (CG) lightning, and storm echoes in the Midwest was pursued to define their relationships and gain insight on their formation processes. Lightning activity was always closely associated with hailstreaks: the surface embodiment (4.5 km long by 1.3 km wide) of a single volume of hail generated aloft. Lightning seldom occurred where the hail fell and generally formed and moved forward in a 10–15-km2 area on either the left or right flank of the hailstreaks. These lightning centers, areas with CG flashes closely associated with hail, typically developed 9 min before hail at a point 5 km upstorm from first hail, suggesting that CG flashes began as the hailstones were developing aloft. The centers then grew in areal coverage and flash frequency until hail began and diminished shortly after hail ended, with a duration of 26 min. The hailstorm's severity was found to be well correlated to the rate of flashing during the hailfall. More than 75% of all lightning centers in the storms on the studied hail days were not associated with damaging hailfalls, indicating little chance to use lightning activity as a predictor of hail. Multicellular echoes in lines produced hailstreaks with severities twice those generated in single cell storms, and severities were much higher when hail-generating cores exceeded 65 dBZ. Merging of cells produced one-third of the hailstreaks but did not affect hail severity. Hailstreaks and associated lightning centers were generally found in storm cores (>45 dBZ) that developed well before the lightning and persisted well after the hail and associated lightning ended. These were relatively strong storms typical of the summer season in the central United States. The findings may not be representative of storms in other climatic zones. Lightning with hailfalls occurred throughout individual cores and adjacent steep reflectivity gradients, whereas hail tended to fall either along the left or right flank of the cores. Hailstorms occurring with cold fronts had more lightning activity and less complexity than did stationary frontal storms.

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Stanley A. Changnon

Abstract

Data from networks of lightning sensors operated during 1986–89 were employed to perform climatic assessments of cloud-to-ground (CG) flashes, and of the relationship between CG flashes and thunder events, as reported at 62 first-order stations. The average annual CG flash pattern resembles the nation's thunderstorm pattern, with lightning being most frequent at points in the southeast United States (>18 000 flashes in Florida) and least frequent at West Cost stations (< 100 flashes). Flashes in the intermontane area are relatively less than thunderstorms, and greater than 50% of all thunder events there are due to intracloud lightning. An anomalous high in CG frequencies exists from Virginia to New York. Cluster analysis identified ten regions of similar storm activity with several reflecting localized influence (Great Lakes, Florida, the Piedmont, and Arizona).

Thunder events provide poor estimates of CG lightning incidence and durations. Cloud-to-ground flash data reveal that 20% (far West) and 50% (southeast United States) of all thunder events are missed at weather stations; 30%–60% of all thunder events have durations too short; 10% (North and West), 40% (mountains), and 25% (Southeast) of all CG flashes within 20 km of weather stations occur at times that are not reported as thunderstorms. The average annual point duration of thunderstorm activity, when based on thunder data, is underestimated by 23% (West Coast) and 44% (East Coast). A real variations in CG flashes reveal that audibility of thunder is better in the cooler northern latitudes, poorer in the mountains, and good on the West Coast. Regional values allow estimation of the average thunderstorm frequency and durations of point storm activity. Peak single-storm CG flashes vary from 50 at West Coast stations to greater than 2000 flashes in the Northeast and Florida, being between 2 (West) and 15 (East) times the point averages. The results allow for assessment of the risks associated with CG lightning throughout the United States. Findings also reveal that the use of historical thunder data, as a surrogate for lightning activity, is improper, and thunder values need to be adjusted with the relationships presented. A future shift to making thunderstorm observations using lightning data, as opposed to the thunder-heard method of the past and present, will bring sizable difference in the incidence and duration of storm activity, with increases ranging from 10% to 50% depending on the area of the United States.

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Stanley A. Changnon Jr.

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No abstract available.

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Stanley A. Changnon Jr.

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Stanley A. Changnon Jr.

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Stanley A. Changnon

Abstract

Records of monthly sky cover, sunshine and temperature for 1901–77 in a 10-state midwestern area were analyzed on a temporal and spatial basis to discern long-term trends and indications of shifts potentially due to added cirrus generated by jet aircraft since about 1960. The skycover data show generally long-term increasing frequencies of cloudy days and decreases in clear days since 1901. Percent of possible sunshine also shows a decrease but to a lesser extent than clear day frequencies. Changes have been greatest since the 1930's. The greatest shifts to cloudier, less sunny conditions occurred since 1960 in an east-west zone across southern Iowa-northern Missouri, northern two-thirds of Illinois and Indiana, and extreme southern sections of Wisconsin and lower Michigan, the area where commercial jet traffic has been greatest. The long-term trends give evidence of natural climate changes, whereas the localized shifts to more cloudiness in the central area since 1960 suggest anomalous changes related to jet-induced cirrus. Months with moderated temperatures (below average maximum and above average minimum) have increased since 1960 in the central east-west zone and largely in summer and fall, the seasons with the major shifts to cloudiness.

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