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James R. Angel and Scott A. Isard

Abstract

Case studies have shown that the Great Lakes can intensify and alter the speed of passing cyclones in winter by contributing latent and sensible heat to the storms. However, the influence of the Great Lakes on cyclones has not been systematically examined using an extensive dataset. In this research, a National Climate Data Center dataset for the period 1965–90 was used to examine the rate of movement and change in mean sea level pressure of 583 cyclones as they passed over the Great Lakes.

The Great Lakes had a strong effect on the passing cyclones during the ice-free/unstable season from September through November. As cyclones approached the lakes during this season, they accelerated. Once in the Great Lakes region, their rate of intensification increased (the change in pressure tendency at the center of the cyclone was negative). The acceleration into the region was less for cyclones during the ice-cover/unstable season, and rates of intensification for these cyclones did not change within the region. Cyclones that traversed the Great Lakes region during the stable season from May through July exhibited essentially the same behavior as those in the ice-free/unstable season.

The authors’ results for the unstable seasons (ice free and ice cover) are consistent with previous modeling case studies of the influence of the Great Lakes on passing cyclones. Because the lakes are generally cooler than the overriding air during spring and summer, a satisfactory explanation for the influence of the Great Lakes on cyclones during the stable season is not apparent.

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Scott A. Isard and William E. Easterling

Abstract

Palmer's z-index, calculated to reflect only the planting-emergence and anthesis-grainfill stages of the growing season, is related with detrended corn yields to produce a predictive model for Illinois corn production. The model is evaluated to see how well it can predict mean bushel per acre corn yields for large areas (state of Illinois). Results suggest the z-index, if calculated to emphasize moisture-sensitive periods in corn production, is a reliable predictor of yields, and, moreover, this predictive ability improves with more extreme moisture conditions.

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Steven E. Hollinger and Scott A. Isard

Abstract

Ten years of soil moisture measurements (biweekly from March through September and monthly during winter) within the top 1 m of soil at 17 grass-covered sites across Illinois are analyzed to provide a climatology of soil moisture for this important Midwest agricultural region. Sod moisture measurements were obtained with neutron probes that were calibrated for each site. Measurement errors are dependent upon the volumetric water content with errors less than 20 percent when soil moisture is above 10 percent of soil volume. Single point errors in moisture measurements from the top 1 m of soil range from 6 percent to 13 percent when volumetric soil moisture is 30 percent of soil volume. The average depletion in moisture between winter and summer over the 10-year period for the top 2 m of soil in Illinois was 72.3 mm. Three-quarters of this decrease occurred above 0.5 m and only 5 percent occurred between the 1.0-m and 2.0-m depths. The average moisture decrease between winter and summer during a wet year (1985) and a drought year (1988) in the top 2 m of soil was 64 percent and 204 percent of the average for the 10-year period, respectively. Seasonal means in soil moisture averaged for the state show the effects of different seasons and soil types on soil moisture. In the winter and spring a latitudinal gradient exists with the wetter soils in the southern part of the state. During summer and autumn there is a longitudinal gradient with the wetter soils in the eastern half of the state. The longitudinal gradient is closely associated with the depth of loess deposits. A north to south latitudinal gradient of soil moisture variability for the summer season is also evident in the 10 yr of records. A comparison of time series of soil moisture from sites with differing soil texture shows that a silty loam soil holds 2 to 3 times more water in the top 1 m than a loamy sand soil. Time series of soil moisture indicate that seasonal variations in water in the top 1 m at a grass-covered site was 1 to 2 times greater than at an adjacent nonvegetated site.

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James R. Angel and Scott A. Isard

Abstract

Cyclones are an important feature of the Great Lakes region that can have important impacts on shipping, lake temperature profiles, ice cover, and shoreline property damages. The objective of this research is to analyze the frequency and intensity of cyclones that traversed the Great Lakes region, the changes of these characteristics since 1900, the interrelationship of cyclone frequency and intensity, and their relationships to circulation patterns and regional temperature and precipitation.

Significant increases in the number of strong (≤992 mb) cyclones over the twentieth century were found for the annual, cold season, November, and December time periods. In contrast, the frequency of all cyclones in the annual and warm season time series and the central pressure of all cyclones in the annual, cold, and warm season time series displayed significant decreases from 1900 to 1939.

Relationships between cyclone frequency and intensity and between cyclone and anticyclone frequency and intensity suggest that there is a partial compensation within the region. As the number of cyclones increases, their intensity decreases. As the number of cyclones increases, so does the number of anticyclones. And, finally, as the cyclones become stronger, so do the anticyclones. Comparisons with the Pacific–North American teleconnection index indicate that lower (higher) cyclone frequency is associated with more zonal (meridional) flow. Comparisons of cyclone characteristics with temperature and precipitation in the Great Lakes region shows that cyclone frequency is inversely related to temperature and directly related to precipitation in most month and season categories. In contrast, the relationships between cyclone intensity and climate variables are inconsistent.

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Scott A. Isard, James R. Angel, and Geoffrey T. VanDyke

Abstract

The zones of origin for all cyclones that traversed the Great Lakes region from 1899 to 1996 are analyzed using a digital daily record of central pressure and location for individual cyclones. Plots of latitude of formation show that Great Lakes cyclones form (or reform) east of the Rocky Mountains at all latitudes between 25° and 65°N. In winter, about the same number of cyclones originate to the northwest as to the southwest of the Great Lakes region. In spring, the southwest zone is dominant. The number of summertime cyclones is greatly reduced, with the west zone of origin most active, while the fall plot displays a transition between the summer and winter distributions. The proportion of strong Great Lakes cyclones that originate in the southwest zone is greater than for all cyclones; however, the seasonal shifts in the latitudinal distributions of origin in the two datasets are similar.

An analysis of differences in frequencies by zone of origin for Great Lakes cyclones during months characterized by positive and negative Pacific–North American (PNA) index patterns reveals a statistically significant relationship between the midtropospheric flow pattern and cyclogenesis. The results indicate that the number of cyclones per month for the positive (PNA index > 0.5) and negative (PNA index < −0.5) categories are approximately equal and that the combined frequencies for positive and negative PNA pattern categories for the northwest, west, and southwest zones of origin are similar. The study supports the intuitive assertion that more Great Lakes cyclones originate from the northwest during months characterized by positive PNA index values than the negative pattern while more cyclones from the west and southwest are associated with the negative PNA index pattern than the positive one.

Approximately 20% of the cyclones that traversed the Great Lakes from 1899 to 1996 originated in the region. The most noteworthy and puzzling finding of the study is that cyclogenesis over the lakes as a proportion of cyclone presence in the region is highest in the summer months. This result corresponds with the finding that cyclones traversing the Great Lakes region in May–July accelerate as they approach the region and increase their rates of deepening over the lakes.

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Marcelo Chamecki, Nicholas S. Dufault, and Scott A. Isard

Abstract

Theoretical predictions for dispersion of heavy particles above an area source are used to formulate a new framework to interpret measurements of spore concentration above an infected field. Experimental measurements of mean spore concentration above an infected wheat field are used to validate theoretical predictions. The framework is then used to estimate total spore flux from the infected field and deposition patterns downwind. Results suggest that for the present case, consisting of a very low open canopy and friction velocity between 0.2 and 0.5 m s−1, the properties of the spore plume above the source field are mostly determined by the source strength (i.e., spore release rate) and are approximately independent of turbulence properties. Turbulence conditions have a strong effect on the distance downwind from the source traveled by spores, however, and are therefore critical in the spread of the disease. In addition, effects of spore clumping on dispersal are explored, illustrating the strong effect of clumping on reducing spore dispersal distance.

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