Compounding effects of Lake and urbanization on summer precipitation in the Greater Chicago area

Abstract

This study explores the impacts of Lake Michigan and Chicago's urbanization on precipitation patterns over the Greater Chicago Area using long-term observational gridded dataset and Weather Research and Forecasting (WRF) model ensemble simulations focused on an early summer rain event. Observational analysis reveals that the Chicago core urban area consistently experience more precipitation than the adjacent southern Lake Michigan region throughout the year, particularly before 2015. This disparity has narrowed since 2016 due to a more rapid increase in heavy precipitation over the lake compared to the urban area. Specifically, lake precipitation has risen by 18.6 mm per year, compared to11.5 mm per year over the urban area in warm-season (April–October). Additionally, the number of days with precipitation exceeding 5 mm has been rising at a rate of 0.82 days per year over the lake and 0.42 days per year over urban areas. Ensemble modeling experiments reveal that both urbanization and lake effects enhance precipitation over the urban area, primarily through convergence with sufficient moisture, driven by interactions between land and lake breezes, a warm surface layer, and a low lifted condensation level. In contrast, these factors suppress precipitation over the lake. The suppression results from Lake Michigan's stable environment, characterized by cooler surface temperatures, limited evaporation in early summer, and a high-pressure anomaly over the lake driven by urban heating, which creates upward motion over urban areas and downward motion over the lake, further influencing precipitation patterns.