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  • Author or Editor: Nancy E. Westcott x
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Nancy E. Westcott

Abstract

Cloud-to-ground lightning flash data collected by the National Lightning Detection Network were analysed in and around 16 central U.S. cities for the period 1989–92. Lightning data are well suited to study storm activity in and around large urban areas since their continuity and coverage in space and time is superior to historical, spatially limited records of thunderstorm activity. Frequency of cloud-to-ground lightning flashes (of negative and positive polarity) in the area immediately upwind, within, and immediately downwind of the cities were compared. An enhancement of lightning frequency on the order of 40%–85% was found over and downwind of many of these cities.

A number of possible urban-related causal factors were examined including effects of increased urban concentrations of cloud condensation nuclei, urban population and size, and the presence of distinct topographic features in and around the cities. Various factors, physical and anthropogenic, appeared to interact in diverse ways to account for changes in lightning flash frequency. The enhancement of lightning activity was largest during the afternoon hours when the urban–rural temperature differences are usually smallest, but when the atmosphere is generally the most unstable and when there is often a maximum in convective activity. The spatial distribution of the first 50 lightning flashes from each storm suggested that the urban area did not initiate new lightning storms. Thus, the overall results suggested that existing thunderstorms were the most strongly affected.

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Nancy E. Westcott
and
David A. R. Kristovich

Abstract

This study focuses on dense fog cases that develop in association with low clouds and sometimes precipitation. A climatology of weather conditions associated with dense fog at Peoria, Illinois, for October–March 1970–94 indicated that fog forming in the presence of low clouds is common, in 57% of all events. For events associated with low pressure systems, low clouds precede dense fog in 84% of cases. Therefore, continental fogs often do not form under the clear-sky conditions that have received the most attention in the literature. Surface cooling is usually observed prior to fog on clear nights. With low cloud bases, warming or no change in temperature is frequent. Thus, fog often forms under conditions that are not well understood, increasing the difficulty of forecasting fog. The possible mechanisms for fog development under low cloud-base conditions were explored for an event when dense fog covered much of Illinois on 7 November 2006. Weather Surveillance Radar-1988 Doppler (WSR-88D) and rawinsonde observations indicated that evaporating precipitation aloft was important in moistening the lower atmosphere. Dense fog occurred about 6 h following light precipitation at the surface. The surface was nearly saturated following precipitation, but relative cooling was needed to overcome weak warm air advection and supersaturate the lower atmosphere. Surface (2 m) temperatures were near or slightly cooler than ground temperatures in most of the region, suggesting surface sensible heat fluxes were not important in this relative cooling. Sounding data indicated drying of the atmosphere above 800 hPa. Infrared satellite imagery indicated deep clouds associated with a low pressure system moved east of Illinois by early morning, leaving only low clouds. It is hypothesized that radiational cooling of the low cloud layer was instrumental in promoting the early morning dense fog.

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Nancy E. Westcott
,
Steven E. Hollinger
, and
Kenneth E. Kunkel

Abstract

This study evaluated the suitability of rain estimates based on the National Weather Service (NWS) Weather Surveillance Radar-1988 Doppler (WSR-88D) network to estimate yield response to rainfall on a county scale and to provide real-time information related to crop stress resulting from deficient or excessive precipitation throughout the summer. The relationship between normalized corn yield and rainfall was examined for nine states in the central United States for 1997–99 and 2001–02. Monthly rainfall estimates were computed employing multisensor precipitation estimate (MPE) data from the National Centers for Environmental Prediction and quality-controlled (QC_Coop) and real-time (RT_Coop) NWS cooperative gauge data. In-season MPE rain estimates were found to be of comparable quality to the postseason QC_Coop estimates for predicting county corn yields. Both MPE and QC_Coop estimates were better related to corn yield than were RT_Coop estimates, presumably because of the lower density of RT_Coop gauges. Large corn yields typically resulted when May rain was less than 125 mm and July rain was greater than 50 mm. Low yields often occurred when July rainfall was less than 100 mm. For moderate July rains (50–100 mm), positive and negative normalized yields resulted. Parameterization of heat stress (number of July days > 32.2°C) improved the correlation between rainfall and normalized corn yield, particularly for years with the poorest yield-vs-rain relationship (1998 and 1999). For the combined analysis years, the multiple regression correlation coefficient was 0.56, incorporating May and July rainfall and July heat stress and explaining 31% of the variance of normalized corn yield. Results show that MPE rainfall estimates provide timely yield projections within the growing season.

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

Abstract

Findings are reported from an analysis of AgI seeding effects on individual cumulus congestus clouds in the 1989 Illinois Exploratory Cloud Seeding Experiment. The experiment was designed around a dynamic seeding hypothesis. Randomized treatments of individual clouds were based on “floating” experimental units, initially cantered on the first treated cloud. The analysis was based on 12 experimental units having a total of 67 treated echo core—32 treated with sand and 35 with AgI. Prior to any analysis for seeding effects, a check of many of the physical conditions at the time of treatment that would govern future cloud growth showed a bias for the sand-treated clouds to be expected to ultimately grow larger than the AgI-treated clouds. Thus, even though randomization produced numerical balance, direct comparison between the posttreatment behavior of the entire sample of sand- and AgI-treated echoes could not be expected to provide a true impression of possible seeding effects.

In an attempt to overcome the bias, an empirically defined seedability index composed of criteria consistent with the Illinois dynamic seeding hypothesis was developed and applied as a filter to reduce the sample bias, and thereby reveal possible seeding effects. Results of two representative applications of the seedability index are reported: one for a subgroup of clouds with higher index values, and the other for a subgroup with lower index values. The primary impression from the ability index analysis was that AgI treatment did not have a pronounced initial effect on the behavior of individual echo cores, and that if seeding had any effect at all it may have been negative on maximum cloud-top height. This finding was not consistent with that expected from the Illinois dynamic seeding hypothesis.

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Stanley A. Changnon
,
K. Ruben Gabriel
,
Nancy E. Westcott
, and
Robert R. Czys

Abstract

Radar-indicated rainfall characteristics from six experimental units randomly selected for AgI treatment were compared with those from six experimental units treated with sand during the summer of 1989 in an exploratory analysis. No differences were found between AgI and sand cell frequencies before and after treatment. The areal extent of rain in all AgI-treated units grew during treatment, whereas four of the sand units decreased, but the differences were not significant. Rainfall amounts were determined for the units and for the extended areas around them. AgI units had higher median rainfall values by the end of the treatment and thereafter for 90 min than did the sand units, whereas extended area rainfall was less in AgI than sand cases. The unit rainfalls, adjusted to account for the unit-extended area relationships, showed a systematic difference with AgI rain higher at the end of treatment (and statistically significant) and for 90 min thereafter. Comparisons based on 20 relevant meteorological variables showed that seeding appeared to produce an effect on days with high net buoyancy and low initial echo heights. Much of the AgI-sand difference was due to two AgI-treated units with heavier rain conditions across the area. Both were cold-frontal cases, and in general, the results resembled those found at Chicago and St. Louis, where urban influences act to increase summer rain during some frontal cases when moderate to heavy rains occur. Pretreatment conditions on the two AgI cases, found notably different from the sand cases, were compared with conditions on the other AgI cases, showing that the echoes were younger on the two potentially effected days. This study found some weak evidence of augmented rainfall due to seeding but the sample size is too small to draw definitive conclusions of an effect. As in many other seeding experiments, results suggest that if a seeding effect occurred, it was present during only certain atmospheric conditions (some frontal situations producing heavy rains) and only with certain clouds (rapidly growing congestus treated in the early stages of development).

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