Observations of Urban Heat Island Influence on Lake-Breeze Frontal Movement

Abstract

redictions of lake and sea breezes are particularly important in large coastal population centers because of the circulations? influence on heat-wave relief, energy use, precipitation, and dispersion of pollutants. While recent numerical modeling studies have suggested that sea or lake breezes should move more slowly through urban areas than in the surrounding suburbs because of urban heat island (UHI) circulations, there have been few quantitative observational studies to evaluate these results. This study utilizes high-resolution Weather Surveillance Radar-1988 Doppler (WSR-88D) observations to determine the effect of the UHI on lake-breeze frontal movement through Chicago, Illinois, and nearby suburban areas. A total of 44 lake-breeze cases from the April?September 2005 period were examined. The inland movement of the lake-breeze front (LBF) was calculated by tracking ?fine lines? of radar reflectivity along several cross sections perpendicular to the Lake Michigan shoreline. The average inland propagation speed of the LBF was 5.0 km h?1; there was substantial spatial and temporal variability in LBF propagation, however. Chicago?s UHI magnitude on lake-breeze days exhibited an average nighttime maximum urban?rural temperature difference near 4.5°C and an afternoon minimum near 0°C. The observed daytime UHI magnitude did not have a significant relationship with lake-breeze frontal movement through Chicago. However, the maximum magnitude of the nighttime UHI preceding lake-breeze development was found to be strongly related to a decrease in speed of LBF movement through Chicago?s southwest (inland) suburbs. This relationship is consistent with previous studies of the diurnal evolution of UHI circulations and may represent a useful method for predicting lake-breeze inland movement.