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Floyd A. Huff

An unusually severe microstorm system developed on 11 August 1993 in Champaign in east-central Illinois where continuous weather records have been maintained for 104 years. Total rainfall in the 1993 storm exceeded the previous 103-year maximum by nearly 50%. Analysis provides strong evidence that even a 100-year record of point rainfall may be misleading in estimating the frequency of extreme rainfall events for that point and the immediate vicinity. Mean frequency relations based on several stations in a region of homogeneous precipitation climate are more reliable predictors, as illustrated in the text. The problem of natural time and space variability in the distribution of extreme rain events is shown from analysis of the 104-year record.

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DOUGLAS M. A. JONES and FLOYD A. HUFF

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Floyd A. Huff and Stanley A. Changnon Jr.

Abstract

Two datasets were used to investigate the potential presence of urban-related precipitation anomalies in the fall, winter and spring seasons St. Louis, Missouri, and to ascertain under what conditions anomalies occurred- if indeed they did occur. The 1971–75 METROMEX dense raingage network data were used along with 1941–80 data from NWS stations in the area. Spatial and temporal analyses of seasonal precipitation showed the reality of urban-related influences northeast of St. Louis in all seasons, plus urban-related increases to the east and southeast in winter and fall. The maximum alterations in precipitation were northeast of St. Louis with increases of 14% in spring, 5% in winter, and 7% in fall when averaged over the 40-yr sampling period. Topographic effects that increased rainfall, particularly in the winter and fall, were quite evident in the hill and bluff areas southwest and southeast of St. Louis. Studies of snowstorms during 1971–75 revealed 5% to 10% less snowfall over the city than over adjacent rural areas.

Only 10% to 15% of the rain events related to areas of urban increases were altered in each season, and in most cases, they occurred with well-organized precipitation systems having convection. This agrees with the METROMEX summer findings. Good agreement between the precipitation patterns of METROMEX and climate network stations suggest that future studies of urban influences on winter and transition season precipitation can be based on the less dense climatic network of NWS.

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Stanley A. Changnon and Floyd A. Huff

During a 50-yr history of diverse scientific endeavors, the Illinois State Water Survey's weather group has constantly shifted to meet new challenges as the nation's only sizable atmospheric research and services group housed in a state agency. The hundreds of projects encompass basic and applied research; data collection, evaluation, and storage; field projects; instrument design, development, and testing; and a services program in response to the needs of the public and specialized users of weather and climate information. Areas of major achievements include development of numerous meteorological instruments and data systems including the CHILL radar; participation in several national field projects, including the Metropolitan Meteorological Experiment (METROMEX); and pioneering research in radar meteorology, weather impacts, planned weather modification, inadvertent urban weather and climate change, applied climatology, and hydrometeorology.

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Stanley A. Changnon Jr. and Floyd A. Huff

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Studies during the Metropolitan Meteorological Experiment (METROMEX) sought to define influences of St. Louis on the summer atmosphere that led to alterations in rainfall. These studies defined how city influences caused an afternoon maximum of rainfall cast of the city. Rain data indicated a second rain maximum northeast of the city during the 2000–2400 CDT period. Study of this nocturnal maximum revealed a 58% localized rain increase, relative to the mean rainfall in the 5200 km2 network. The anomaly was present in all summers from 1971–1975. The northeast rain maximum is preceded by a local increase beginning 2 h earlier and 30 km west over the urban-industrial area. Most northeast anomaly-related storms were found to move either from the southwest (from over the urban area) or from the west-northwest (from a major industrial area), and to produce heavy rainfall rates; 19 storms moved from St. Louis between 2100–2400 and these produced 69% of the rainfall in the maximum rainfall area. The afternoon and nocturnal maximums both occurred when the entire area was receiving relatively heavy rainfall indicating that urban influences are most effective during relatively heavy rainfall conditions. All of the nocturnal anomaly rainfall occurred with well-organized convective systems. The individual convective raincells which led to heavy rainfall in the anomaly typically began over the urban industrial area and ended in the anomaly area. The raincell areas, volumes, and intensifies were much greater than rural raincells. Collectively, the results strongly suggest that the nocturnal anomaly is a result of urban influence that affect a few of the heavier rain events.

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Paul T. Schickedanz and Floyd A. Huff

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Storm rainfall data from dense raingage networks in Illinois were employed in a study to determine the length of time required to obtain significance for various increases in storm rainfall due to weather modification efforts. The primary purpose was to evaluate the effect of stratifying the storm data on the detection of seeding effects for a given design using highly accurate measurements of the rainfall parameters. It was also desired to evaluate the efficiency of various rainfall parameters and the efficiency of various statistical designs in detecting various increases. Results indicate that the length of experimentation necessary for detection of seeding effects varies according to weather type, precipitation type, rainfall parameter, and statistical design employed. Results also indicate that as the seeding-induced increase becomes large, the choice of stratification, rainfall parameter, and statistical design becomes less important. An evaluation procedure is recommended which incorporates desirable features from several of the designs, stratifications and rainfall parameters considered in this study. Although it is difficult to verify, a 20% increase in precipitation can be detected in a five-year experiment provided proper choices are made of weather types, statistical designs, data stratifications and rainfall parameters.

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Floyd A. Huff and James C. Neill

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An investigation was made of variations in corn and soybean yields resulting from natural fluctuations in weather conditions between years in a five-state area in the Midwest. Analyses were performed for crop districts within each state and for various combinations of the five states when crop yields are evaluated over periods of 1-5 years. Results were presented in the form of temporal-spatial probability distributions, in which the distributions were based on deviations from “expected” yield after adjustment for technology advancements during the period of record (1931-75). In general, it was found that weather-related deviations in corn yield were greater than in soybeans, a decrease in temporal variability occurs from west to east, negative deviations tend to be greater than positive deviations, but that the five-state area seldom experiences large deviations from expected yield and the occasional large deviations do not usually persist long.

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James R. Angel and Floyd A. Huff

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The rainstorm on 17–18 July 1996 in northern Illinois produced three rainfall records. The 43.0-cm total storm rainfall at Aurora was the greatest point rainfall recorded for storm durations of 24 hours or less in this century in Illinois and most surrounding states. The 27.9-cm storm rainfall recorded in the southwestern part of the Chicago metropolitan area was the heaviest 24-h amount ever recorded in that city. The July 1996 storm also produced the heaviest 24-h mean rainfall recorded in Illinois over areas of 5200 and 13 000 km2 immediately surrounding the storm center.

An area of approximately 12 000 km2 experienced 24-h point rainfall amounts that exceeded those expected to occur, on the average, once in 10 years. Similarly, the 25-, 50-, and 100-yr frequency values were exceeded over areas of 6730, 4920, and 3500 km2, respectively.

One concern resulting from a major rainfall event such as this storm is its impact on the rainfall frequency analysis. This new information may result in changes in the estimated rainfall amounts at selected return periods, which are used to design water-handling structures. Based on previous research, the Aurora rainfall appears to exceed the 1000-yr return period. However, fitting a statistical distribution to the annual maximum time series and using regional averages minimized the effect of this storm on rainfall frequency estimates.

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Stanley A. Changnon Jr., Floyd A. Huff, and Richard G. Semonin

METROMEX, a field project designed and now in progress at St. Louis, involves 4 research groups planning and working cooperatively to study inadvertent weather modification by urban-industrial effects, and, in particular, man-made changes of precipitation. Urban areas affect most forms of weather and some, such as winds, temperature, and visibility, are obvious and their changes are easily measured. Inadvertent precipitation changes are harder to measure, and except for the well-documented La Porte anomaly, urban-related rain changes have had only limited study. Examination of historical data at St. Louis has revealed summer increases in the immediate downwind area of: 1) rainfall (10–17%); 2) moderate rain days (11–23%); 3) heavy rainstorms (80%); 4) thunderstorms (21%); and 5) hailstorms (30%). METROMEX field measurements in the summer of 1971 involved 220 raingages and hailpads, 3 radar sets, 70 rainwater collectors, 14 pibal stations, 4 meteorological aircraft, unique atmospheric tracers, and a wide variety of standard and unusual meteorological equipment. These measurement tools were used to provide information on 1) the processes of cloud and precipitation formation, 2) the chemistry of aerosols and rainwater, 3) the urban heat budget, 4) the 3-D patterns of precipitation elements, and 5) the airflow and cloud development for numerical models.

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Stanley A. Changnon, Floyd A. Huff, and Chin-Fei Hsu

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The statistical relationships between monthly precipitation (P) and shallow groundwater levels (GW) in 20 wells scattered across Illinois with data for 1960–84 were defined using autoregressive integrated moving average (ARIMA) modeling. A lag of 1 month between P to GW was the strongest temporal relationship found across Illinois, followed by no (0) lag in the northern two-thirds of Illinois where mollisols predominate, and a lag of 2 months in the alfisols of southern Illinois. Spatial comparison of the 20 P-GW correlations with several physical conditions (aquifer types, soils, and physiography) revealed that the parent soil materials of outwash alluvium, glacial till, thick loess (≥2.1 m), and thin loess (>2.1) best defined regional relationships for drought assessment.

Equations developed from ARTMA using 1960–79 data for each region were used to estimate GW levels during the 1980–81 drought, and estimates averaged between 25 to 45 cm of actual levels. These estimates are considered adequate to allow a useful assessment of drought onset, severity, and termination in other parts of the state. The techniques and equations should be transferrable to regions of comparable soils and climate.

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