PNNL

The Science

In urban areas, the temperature is often a few degrees higher than surrounding areas; this is referred to as the urban heat island effect. Urban areas also produce more particles (aerosols) that enter the atmosphere and affect regional rainfall through their interactions with clouds and radiation.

Using high-resolution modeling, in situ measurements, and satellite observations, researchers at the U.S. Department of Energy's Pacific Northwest National Laboratory led a study showing that urbanization can induce intense rainfall over cities from convective storms during the Indian summer monsoon. While the urban heat island effect induces more cloud formation over cities, urban aerosols modify properties of cloud droplets and act as a catalyst to further intensify rainfall over and downwind of the cities.

The Impact

This study provides a process-level understanding of how urban aerosols can prompt invigoration of city-scale rainfall. It also underlines the importance of including aerosol-cloud processes, in addition to accounting for urban land use, in models to better understand and predict extreme rainfall and flash flood events over cities.

The findings have important implications for understanding extreme rainfall events around major cities and for water resource infrastructure planning.

Reference: C. Sarangi, S.N. Tripathi, Y. Qian, S. Kumar, L.R. Leung, "Aerosol and Urban Land Use Effect on Rainfall Around Cities in Indo-Gangetic Basin from Observations and Cloud Resolving Model Simulations." Journal of Geophysical Research: Atmospheres 123, 3645-3667 (2018). [https://doi.org/10.1002/2017JD028004]