Observed precipitation enhancement and suppression downwind of a major U.S. city

While urban enhancement of precipitation during summer has been heavily studied, there has been little research on urban effects on precipitation during other seasons. Therefore, this study assesses urban effects on precipitation throughout the year in the Atlanta, Georgia USA region during 2014–2023. Precipitation totals and frequencies of precipitation days and heavy-precipitation days from a network of 50 gauges were aggregated for 12 overlapping three-month periods. The two main methods used were the calculations of correlations between precipitation and upwind imperviousness and differences between groups of downwind gauges and control gauges. Urban enhancement and suppression of precipitation totals and heavy-precipitation days occurred east/northeast of Atlanta, primarily in Gwinnett County. Enhancement (suppression) was revealed by significant positive (negative) correlations between precipitation and upwind imperviousness and significantly larger (smaller) precipitation values at downwind gauges compared to control gauges. Precipitation totals and heavy-precipitation days were enhanced by 13 % and 19 %, respectively, during the warm season, and those two variables were suppressed by 5 % and 11 %, respectively, during winter/spring. Precipitation days were enhanced throughout the year, with the largest enhancement (7 %) occurring during summer. The urban heat island and urban aerosols are the likely causes of the urban effects on precipitation.