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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.
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.
Abstract
The highest recorded temperature in Illinois, 117°F (47.2°C) occurred on 14 July 1954 in East St. Louis. This occurred in the midst of a widespread, long-lasting heat wave covering significant parts of 11 states: from eastern Colorado through Kansas, Oklahoma, part of Texas, Missouri, and Arkansas, southern Illinois, and extending to western Tennessee, Alabama, Georgia, and parts of the Carolinas. According to historical climate data, this event ranked as one of the top five extended periods of heat in these states since 1895. No such prolonged heat wave has occurred in the Midwest since 1954. It stands to reason that since prolonged widespread heat waves have occurred in the last 100 years, there is a distinct possibility that they will occur again, and reviewing past impacts could help us plan for future events.
This research examines the impacts of the heat felt in the Illinois, Missouri, and Kansas region, as well as the responses to the extreme temperatures. Impacts on human health and well-being, water resources, utilities, agriculture, and commerce are described, as well as responses by individuals, communities, and governmental bodies. The extreme heat resulted in many deaths and much discomfort. Sizeable infrastructure repair costs from buckled streets and warped railroad ties were accrued in 1954. Energy and water resources were significantly strained. However, the most costly governmental interventions were those related to the agricultural community. Recent activities in heat wave and drought preparedness that may help alleviate impacts of future heat waves are discussed.
Abstract
The highest recorded temperature in Illinois, 117°F (47.2°C) occurred on 14 July 1954 in East St. Louis. This occurred in the midst of a widespread, long-lasting heat wave covering significant parts of 11 states: from eastern Colorado through Kansas, Oklahoma, part of Texas, Missouri, and Arkansas, southern Illinois, and extending to western Tennessee, Alabama, Georgia, and parts of the Carolinas. According to historical climate data, this event ranked as one of the top five extended periods of heat in these states since 1895. No such prolonged heat wave has occurred in the Midwest since 1954. It stands to reason that since prolonged widespread heat waves have occurred in the last 100 years, there is a distinct possibility that they will occur again, and reviewing past impacts could help us plan for future events.
This research examines the impacts of the heat felt in the Illinois, Missouri, and Kansas region, as well as the responses to the extreme temperatures. Impacts on human health and well-being, water resources, utilities, agriculture, and commerce are described, as well as responses by individuals, communities, and governmental bodies. The extreme heat resulted in many deaths and much discomfort. Sizeable infrastructure repair costs from buckled streets and warped railroad ties were accrued in 1954. Energy and water resources were significantly strained. However, the most costly governmental interventions were those related to the agricultural community. Recent activities in heat wave and drought preparedness that may help alleviate impacts of future heat waves are discussed.
Abstract
To better understand dense fog events in the midwestern United States, a fog climatology was developed that examines the surface weather conditions at dense fog onset and during dense fog events, in relationship to fog duration. Surface airways hourly observations for the period 1948–96 were examined, focusing primarily on Peoria, Illinois, during the cold season (October–March). Temperature, winds, and visibility at dense fog onset did not prove to be useful in differentiating between short- (1–2 h) and long- (>5 h) duration dense fog events. However, it was found that once dense fog forms, it is more likely to persist if the horizontal visibility is 200 m (1/8 mi) or less and the ceiling height lowers to 30 m (100 ft) or less. Further, dense fog events at Peoria tend to last longer if they are widespread, that is, when many other midwestern surface airways hourly stations also report dense fog. When dense fog develops early in the night to the hours just after midnight, it is more likely to persist than when it develops later in the night or during the day. This was found to be the case for many other midwestern stations as well. Fog events forming earlier in the night may last longer because of the absence of solar insolation upon the fog layer during the night. As longer-duration fogs often become more opaque and more widespread than short-duration events, more time may be required to dissipate fog once the sun has risen. Dense fog onset time and the physical dimensions of the fog events appear to be the best predictors of fog duration considering all types of fog in the Midwest.
Abstract
To better understand dense fog events in the midwestern United States, a fog climatology was developed that examines the surface weather conditions at dense fog onset and during dense fog events, in relationship to fog duration. Surface airways hourly observations for the period 1948–96 were examined, focusing primarily on Peoria, Illinois, during the cold season (October–March). Temperature, winds, and visibility at dense fog onset did not prove to be useful in differentiating between short- (1–2 h) and long- (>5 h) duration dense fog events. However, it was found that once dense fog forms, it is more likely to persist if the horizontal visibility is 200 m (1/8 mi) or less and the ceiling height lowers to 30 m (100 ft) or less. Further, dense fog events at Peoria tend to last longer if they are widespread, that is, when many other midwestern surface airways hourly stations also report dense fog. When dense fog develops early in the night to the hours just after midnight, it is more likely to persist than when it develops later in the night or during the day. This was found to be the case for many other midwestern stations as well. Fog events forming earlier in the night may last longer because of the absence of solar insolation upon the fog layer during the night. As longer-duration fogs often become more opaque and more widespread than short-duration events, more time may be required to dissipate fog once the sun has risen. Dense fog onset time and the physical dimensions of the fog events appear to be the best predictors of fog duration considering all types of fog in the Midwest.
Abstract
This study examines the growth of radar echoes from the time of their initiation to several minutes after they have merged to ascertain what factors are important in determining the frequency of merging events, the manner in which echo cores join together, and the, effect of merging on subsequent echo core growth. Three-dimensional radar reflectivity data were examined for two convective periods, 25 July 1986 and 26 August 1986. It was found that echoes that merged were initially taller and slightly larger than those that dissipated without merging, suggesting they were more vigorous and thus more likely to grow and join with another echo. While meteorological conditions on the two days were quite different, 25% of the mergers occurred between young echoes, 65% occurred between a young echo and a parent storm, and 10% occurred between echo cores from two different systems. Previous case studies have indicated that merging is accomplished through differential storm motion or through the growth of a new echo core between adjacent cores. Echo cores in this study appeared to merge primarily through horizontal expansion. Two mechanisms were proposed to account for this expansion: moisture-laden oufflow air may have resulted in the presence of precipitation-sized drops at low altitudes in areas bridging the cores, and an intercell flow mechanism may have been at work at middle and upper levels. About 45%-70% of the cores were growing just after merger. Generally, the cores that grew were young, were growing prior to or at the time of merger, and thus were likely to continue growing.
Abstract
This study examines the growth of radar echoes from the time of their initiation to several minutes after they have merged to ascertain what factors are important in determining the frequency of merging events, the manner in which echo cores join together, and the, effect of merging on subsequent echo core growth. Three-dimensional radar reflectivity data were examined for two convective periods, 25 July 1986 and 26 August 1986. It was found that echoes that merged were initially taller and slightly larger than those that dissipated without merging, suggesting they were more vigorous and thus more likely to grow and join with another echo. While meteorological conditions on the two days were quite different, 25% of the mergers occurred between young echoes, 65% occurred between a young echo and a parent storm, and 10% occurred between echo cores from two different systems. Previous case studies have indicated that merging is accomplished through differential storm motion or through the growth of a new echo core between adjacent cores. Echo cores in this study appeared to merge primarily through horizontal expansion. Two mechanisms were proposed to account for this expansion: moisture-laden oufflow air may have resulted in the presence of precipitation-sized drops at low altitudes in areas bridging the cores, and an intercell flow mechanism may have been at work at middle and upper levels. About 45%-70% of the cores were growing just after merger. Generally, the cores that grew were young, were growing prior to or at the time of merger, and thus were likely to continue growing.
Abstract
A reflectivity and triple-Doppler radar study of the development of several cells and their successive union within a nonsevere thunderstorm is presented. Two characteristic separations were found between the newly formed cells and the parent thunderstorm, with the closer cells forming in response to the collapse of an active cell and the more distant cells forming in a previously existing storm-modified area characterized by mesoscale convergence and rain cooled air. The manner in which these cells evolved appeared to be partially related to differences in the environment in which they formed. As suggested by Peterson, the cells that formed closer to the main storm resembled the “weakly evolving” cells of Foote and Frank. The updraft of the “weakly evolving” cell analyzed here merged with the updraft in a cell in the main storm as one cell was decreasing in intensity and the other was increasing.
Later in the life cycle of the storm, two cells which initially formed further away from the main storm appeared more like classical “strongly evolving” cells. While the vertical air velocity analyses of these cells were incomplete, a trend towards the maintenance of a discrete cell updraft was noted. The ways in which the reflectivity cores of these two cells became merged with the main storm differed. In one case the development of a new cell between two existing cells produced the merger, in the second case differential cell motion played an important role. Additionally, periods of significant intercell flow at 4 km coincided with the times when the midlevel reflectivity bond linking the cell cores showed a rapid intensification. It is proposed that the intercell flow is a result of radial outflow observed at heights above the maximum updraft level in the actively growing echoes. The strengthening of the reflectivity bridge may have been the result of both particle transfer and environmental modification brought about by this radial outflow.
Abstract
A reflectivity and triple-Doppler radar study of the development of several cells and their successive union within a nonsevere thunderstorm is presented. Two characteristic separations were found between the newly formed cells and the parent thunderstorm, with the closer cells forming in response to the collapse of an active cell and the more distant cells forming in a previously existing storm-modified area characterized by mesoscale convergence and rain cooled air. The manner in which these cells evolved appeared to be partially related to differences in the environment in which they formed. As suggested by Peterson, the cells that formed closer to the main storm resembled the “weakly evolving” cells of Foote and Frank. The updraft of the “weakly evolving” cell analyzed here merged with the updraft in a cell in the main storm as one cell was decreasing in intensity and the other was increasing.
Later in the life cycle of the storm, two cells which initially formed further away from the main storm appeared more like classical “strongly evolving” cells. While the vertical air velocity analyses of these cells were incomplete, a trend towards the maintenance of a discrete cell updraft was noted. The ways in which the reflectivity cores of these two cells became merged with the main storm differed. In one case the development of a new cell between two existing cells produced the merger, in the second case differential cell motion played an important role. Additionally, periods of significant intercell flow at 4 km coincided with the times when the midlevel reflectivity bond linking the cell cores showed a rapid intensification. It is proposed that the intercell flow is a result of radial outflow observed at heights above the maximum updraft level in the actively growing echoes. The strengthening of the reflectivity bridge may have been the result of both particle transfer and environmental modification brought about by this radial outflow.
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.
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.
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.
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.
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.
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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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.
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.
