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Robert W. Scott and Bernice Ackerman

Abstract

Sudden changes in surface meteorological parameters were observed to propagate across a densely-instrumented network in central Illinois during a summer night in 1979. The changes were due to the outflow from an eastward moving, organized storm system passing well north of the network. Although no precipitation was observed within 45 km of the area (i.e., its passage across the network was “dry"), the change from ambient to outflow air was seen in other surface weather indicators nearly 100 km south of the point at which the outflow is estimated to have been initiated and more than 3 h after the generating storm had dissipated.

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Robert R. Czys and Robert W. Scott

Abstract

A simple objective procedure used exploratively to forecast the occurrence, height, and coalescence activity of summertime convective clouds in Illinois during the cloud-seeding trials of the 1989 Precipitation Augmentation for Crops Experiment is described. The method used the temperature of the convective condensation level (T CCL) and potential buoyancy (PB) at 500 mb, easily determined from morning National Weather Service sounding data, to forecast afternoon convection. Maximum echo top heights were found to group according to T CCL and PB. The physical basis of T CCL and PB to implicitly represent a period of time for coalescence to produce supercooled drizzle and raindrops is discussed. The technique performed well at forecasting the occurrence and height of afternoon convective clouds. Aircraft measurements of supercooled raindrop concentrations showed that a discriminator function, dependent only on T CCL and PB, gave a good indication of the presence or absence of supercooled drizzle and raindrops in the updrafts of clouds at the − 10°C seeding level. Median concentrations of supercooled drizzle and raindrops (N D>300) in updraft regions at the − 10°C level were found to be best approximated by a third-order polynomial dependent on T CCL and PR, presenting a possible physical link between cloud-scale environment and in-cloud conditions.

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Robert M. Rauber and Robert W. Scott

Abstract

Numerous cold air funnels developed in close proximity over central Illinois on 23 May 1988 as the core of an upper-level cutoff low pressure center passed over the region. Five separate funnels were observed by one of the authors from a single location over a period of 33 min. Images of these funnels captured from video are presented, one showing two funnels extending toward one another from the same cloud and two others illustrating stages in the life cycle of one of the funnels. Surface and upper air analyses, radar data, and two soundings taken at the time of the outbreak are presented to illustrate the environment in which these funnels formed. The family of cold air funnels occurred along a weak stationary front. The fact that so many funnels occurred along the same line is suggestive of a vortex sheet breaking down due to the development of horizontal shearing instability. If this is the case, then the cold air funnels observed on 23 May are similar dynamically to nonsupercell tornado families that develop along cold fronts and outflows.

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Patrick C. Kennedy, Nancy E. Westcott, and Robert W. Scott

Abstract

No abstract.

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Stanley A. Changnon, Robin T. Shealy, and Robert W. Scott

Abstract

Analysis of precipitation events in the St. Louis aera, based on pre-event low-level wind flow, was pursued to ascertain the presence of urban effects on fall, winter, and spring precipitation. Data from a circular, dense, raingage network were used to define quadrant (NW, NE, SE, SW) average precipitation. Winds before each event (443 events in 1971–75) were used to define the urban plume and identify which quadrant was “downwind” of the city. Results for fall revealed a 17% increase in precipitation downwind of St. Louis and a 13% increase in events with their peak rainfall occurring downwind, both outcomes were statistically significant at the 1% level. The downwind enhancement was greatest when pre-event winds were from the SE, and when average precipitation in the quadrant with the maximum value was either light (<5.1 mm) or quite heavy (>17.9 mm). The fall results agree well with earlier findings for summer rainfall that revealed a 25% increase due to enhancement in isolated airmass showers and during heavier, well-organized convective system Winter precipitation indicated little precipitation change downwind of St. Louis. However, when SW pre-event winds existed (a flow often associated with conviction), there was a statistically significant downwind increase in winter precipitation; but when pre-event winds were from SE or NW (flows frequently associated with stratiform precipitation), downwind decreases occurred. The number of spring precipitation conditions that maximized downwind of St. Louis was significantly greater than expected by chance particularly in light (<5.1 mm) events, but the total spring rainfall downwind increased only 4%. There was no suggestion of decreased precipitation in spring or fall. The urban influences to enhance precipitation appeared to be related to precipitation conditions with convective processes, and urban influences in more stratiform precipitation situations were negligible.

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Scott W. Powell, Robert A. Houze, Jr., and Stella R. Brodzik

Abstract

An algorithm used to classify precipitation echoes by rain type without interpolating radar data to a constant height is detailed. The method uses reflectivity data without clutter along the lowest available scan angle so that the classifications yield a more accurate representation of the rain type observed at the surface. The algorithm is based on that of Steiner et al. but is executed within a polar coordinate system. An additional procedure allows for more small, isolated, and/or weak echo objects to be appropriately identified as convective. Echoes in the immediate vicinity of convective cores are included in a new transition category, which consists mostly of echoes for which a convective or stratiform determination cannot be confidently made. The new algorithm more effectively identifies shallow convection embedded within large stratiform regions, correctly identifies isolated shallow and weak convection as such, and more often appropriately identifies periods during which no stratiform precipitation is present.

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Robert W. Scott, Edward C. Krug, and Stephen L. Burch

Abstract

Long-term data collection of volumetric soil moisture under sod has been conducted in Illinois for more than 25 years. Numerous applied and modeling studies have been undertaken with these data, often relating results to regional conditions under a variety of surface covers. However, the actual level of representation of these data to nearby areas with different surface covers is unknown. In 2006/07, the Soil Moisture under Sod Experiment was conducted at Bondville, Illinois, to increase understanding of soil moisture variability across a very small area of seemingly uniform surface and near-surface conditions. Ten locations were chosen at random within a 5.9-ha sodded field for twice weekly neutron probe soil moisture observations over a period of more than 13 months. Measurements were taken at the surface and at 20-cm intervals down to 2 m, precisely matching the historic Illinois depth observations. A detailed surface terrain analysis was conducted to consider effects on soil moisture attributable to surface slope or ponding potential at each monitoring location across the very low relief surface. The near-surface water table level at the field location was monitored. At the end of observations, soil property heterogeneity (i.e., soil porosity, bulk density, and soil color) was determined by digging trenches and extracting soil cores immediately adjacent to each monitoring site at all observation levels within the predominantly loess soil.

Results indicate a strong temporal consistency in intrasite trends of volumetric soil moisture at all depths throughout the experiment. However, intersite spatial variability increased with depth, indicated by an average standard deviation of all temporal observations of 2.26% in the top 30 cm of soil and 5.19% in the 170–200-cm layer. Differences between the average field soil moisture at all primary randomly selected sites and the historic Bondville site were 2.39% and 6.51%, respectively. In addition, an apparent strong relationship was observed between soil moisture in deeper layers and surface terrain slope, and to a lesser extent with soil porosity and bulk density.

The question of representativeness of soil moisture under sod to adjacent surface covers was not answered with this work, but the large differences measured across this seemingly uniform field suggest that proper use of the historic Illinois dataset by future research related to adjacent areas may need greater attention. Most of Illinois is under an agricultural cover, not sod. Adequate data monitoring of surface terrain slope, soil profiles, and water table climatology under various major surface covers within a region may be necessary prior to the installation of new soil moisture monitoring networks and before useful assumptions concerning spatial representation can be made that attribute individual soil moisture datasets to adjacent areas. These results highlight the importance of a strict globally unified protocol for soil moisture network design and data collection in support of quality in situ global soil moisture assessment, a primary goal of the International Soil Moisture Working Group of the Global Energy and Water Cycle Experiment.

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Patrick C. Kennedy, Nancy E. Westcott, and Robert W. Scott

Abstract

During the early evening hours of 19 May 1989, the CHILL 10-cm Doppler weather radar observed most of the lifetime of an unusually small tornadic thunderstorm. Throughout the event, the parent thunderstorm echo top remained below 6.7 km MSL The low-altitude echo diameter, as defined by the 25-dBZ contour, was only 15 km. Despite its small size, both visual and radar observations indicated that this storm contained many of the organizational features often noted in large, “classical” southern Great Plains supercells. The synoptic setting in which this storm occurred was atypical for supercell development in that both the thermodynamic instability and vertical wind shear magnitudes were limited. This documentation of a tornadic storm that developed in a nonthreatening environment mid that presented a small, seemingly inconsequential radar appearance demonstrates some of the challenges that will be faced by automated Doppler radar-based severe weather detection algorithms.

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Robert R. Czys, Robert W. Scott, K. C. Tang, Ronald W. Przybylinski, and Michael E. Sabones

Abstract

A nondimensional parameter is presented that can he used to help distinguish between conditions favorable for the occurrence of freezing rain and ice pellets. The parameter was derived from the well-established condition that most incidents of freezing rain and ice pellets are associated with a layer of above-freezing air elevated above a layer of below-freezing air adjacent to the earth's surface and the requirement that any cloud ice must completely melt for freezing rain, otherwise ice pellets would result. The parameter was obtained from the ratio of the time available for melting to the time required for complete melting. The parameter was tested on the mesoscale thermodynamic conditions that existed with the 1990 St. Valentine's Day ice storm that affected much of the Midwest and on a number of other episodes of freezing rain and ice pellets in the Midwest. Testing showed excellent spatial agreement between diagnosed and observed locations for freezing rain and ice pellets. An isonomogram is presented to allow the parameter to be easily used as a tool in determining winter precipitation type.

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Stanley A. Changnon, Robert R. Czys, Robert W. Scott, and Nancy E. Westcott

At the heart of the 40-year atmospheric research endeavors of the Illinois State Water Survey have been studies to understand precipitation processes in order to learn how precipitation is modified purposefully and accidentally, and to measure the physical and socioeconomic consequences of cloud and precipitation modification. Major field and laboratory activities of past years are briefly treated as a basis for describing the key findings of the past ten years. Recent studies of inadvertent and purposeful cloud and rain modification and their effects are emphasized, including a 1989 field project conducted in Illinois and key findings from an on-going exploratory experiment addressing cloud and rain modification. Results are encouraging for the use of dynamic seeding on summer cumuliform clouds of the Midwest.

Typical in-cloud results at −10°C reveal multiple updrafts that tend to be filled with large amounts of supercooled drizzle and raindrops. Natural ice production is vigorous, and initial concentrations are larger than expected from ice nuclei. However, natural ice production is not so vigorous as to preclude opportunities for seeding. Radar-based studies of such clouds reveal that their echo cores usually can be identified prior to desired seeding times, which is significant for the evaluation of their behavior. Cell characteristics show considerable variance under different types of meteorological conditions. Analysis of cell mergers reveals that under conditions of weak vertical shear, mid-level intercell flow at 4 km occurs as the reflectivity bridge between cells rapidly intensifies. The degree of intensification of single-echo cores after they merge is strongly related to the age and vigor of the cores before they join. Hence, cloud growth may be enhanced if seeding can encourage echo cores to merge at critical times. Forecasting research has developed a technique for objectively distinguishing between operational seeding and nonoperational days and for objectively predicting maximum cloud-top height and seeding suitability. An accuracy rate of up to 60% in predicting maximum echo-top height using four categories has been achieved and suggests its use as a covariate in future experimentation. Impact studies illustrate that sizable summer rain increases would be necessary to produce economically beneficial outcomes for Corn Belt agriculture. Increases of ≥25% in July rainfall across certain high-production crop districts of the Corn Belt would produce economic effects realized nationally.

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