Lake- and sea-breezes have been extensively studied using observational, theoretical, numerical modeling, and laboratory techniques. Large population centers in coastal regions, such as Chicago, make predicting and understanding of these circulations particularly important due to their impacts on dispersion of pollutants, heat wave relief, and energy use. While recent numerical model simulations have suggested that sea- or lake-breezes should move more slowly through urban areas than in the surrounding suburbs due to the Urban Heat Island (UHI) circulation, there have been few fine-scale observations of the spatial and temporal variations in lake-breeze movement to evaluate these results. The goal of this research is to utilize high-resolution WSR-88D observations to evaluate the effect of the UHI on lake-breeze frontal movement through Chicago and nearby suburban areas. This was accomplished by identifying and analyzing a total of 44 lake-breezes which occurred during the April – September 2005 period.
During 2005 the monthly frequency of lake-breezes near Chicago gradually increased from 5 in April to 12 in August, despite a maximum in air-lake temperature differences in April. The August peak appears to be related to a decrease in the wind speed later in the warm season. A great deal of temporal and spatial variability of inland motion of the lake-breeze front (LBF) was noted. For example, while many LBF remained close to the shore, one LBF progressed more than 130 km inland.
The hourly position of the radar fine line was used to determine inland movement of the LBF along several shore-perpendicular cross-sections throughout the Chicago, IL, area. This allowed for detailed analyses of the relationship between Chicago’s UHI on lake-breeze frontal movement. Interestingly, stronger nighttime UHI preceding lake-breezes were found to decrease the speed of LBF movement as it progressed from downtown Chicago to its southwest suburbs.The observed daytime UHI, which was substantially weaker than it was at night, did not have a significant effect on lake-breeze frontal movement through Chicago. Accordingly, it is possible that the UHI lasts much longer than the surface signature indicates.
|
