A new study from the University of Lausanne highlights how cities modify precipitation patterns, with urban areas experiencing more intense and localized rainfall compared to surrounding regions. This phenomenon could heighten the risk of flooding in the future, particularly as urban populations expand and global warming continues to intensify.
Summer storms are generally more frequent, intense and concentrated over cities than over rural areas, according to new study published in Earth's Future, and conducted by scientists from the Faculty of Geosciences and Environment at the University of Lausanne (UNIL).
Scientists used 7 years of high-resolution weather radar data to analyze rainfall intensity and storm formation across eight urban areas within Europe and the United States: Milan, Berlin, London, Birmingham, Phoenix, Charlotte, Atlanta, and Indianapolis. The cities were selected due to their location on relatively flat land and their distance from large bodies of water, two factors that could affect rainfall patterns, and thus interfere with the analysis.
When cities shape storms
The new study finds that more storms form over urban areas than in surrounding areas, and that larger cities intensify rainfall more than smaller cities.
Another key finding is that, while precipitation is generally evenly distributed and dispersed in rural areas, much like a sprinkler, in cities it often falls in concentrated bursts over small areas, much like a fire hose, significantly raising the risk of flooding. “If you have a very large amount of rainfall falling over a very small area, that can collapse the drainage system in an urban environment,” explains Herminia Torelló-Sentelles, researcher at UNIL's Faculty of Geosciences and Environment, and first author of the study. “Moreover, the threat of flooding could worsen in the future, with the expected growth of cities and global warming trends, which continue to intensify storms around the world.”
Several previous studies have shown that cities impact rainfall through three different mechanisms: cities are generally warmer than their cool, moist and vegetation-dense surroundings, which can cause air to be drawn toward the cities and uplifted. That warm, uplifted air then condenses into rain clouds over urban centers. The presence of tall buildings in cities can also result in this uplift of air. On the other hand, urban pollution, consisting of suspended aerosols, can also modify the rain formation process.
Urban planning and cooling: ways of dealing with the phenomenon
The next step is to conduct analyses on a larger scale, encompassing a greater number of cities. “Thanks to our research, we have been able to identify some trends, but each city has modified precipitation patterns in a unique way. It is therefore necessary to deepen our knowledge, to enable planners, in the future, to develop individual urban planning strategies,” says Torelló-Sentelles. In the meantime, the researcher suggests a number of avenues to explore, extrapolated from the published study. These include improving urban drainage systems, lowering urban temperatures, and increasing surface permeability. “
Source: H. Torelló-Sentelles, F. Marra, M. Koukoula, G. Villarini, N. Peleg, Intensification and Changing Spatial Extent of Heavy Rainfall in Urban Areas, Earth’s Future, 2024