A new study highlights the increasing intensity and frequency of summer storms over urban areas, underscoring the need for better urban planning to mitigate flood risks. The research examines patterns across eight cities and their implications for the future.
Summer storms are becoming more frequent, intense and concentrated over cities compared to rural areas, according to a new study that has implications for urban flood management and future city planning.
The research team examined weather data from eight cities across Europe and the United States – Milan, Berlin, London, Birmingham, Phoenix, Charlotte, Atlanta and Indianapolis – and discovered a pattern of increased storm activity and rainfall intensity in urban areas. This phenomenon, documented in the interdisciplinary journal Earth’s Future, highlights the pressing need for cities to adapt their infrastructure to handle the deluge.
“Cities are expected to become more populated and increase in size in the coming decades,” lead author Herminia Torelló-Sentelles, an atmospheric scientist at the University of Lausanne, said in a news release. “Being able to quantify urban flood risk is important for urban planning and when designing urban drainage systems.”
Unlike rural areas, where rain can spread out more evenly, storms over cities often result in intense, localized bursts of rainfall. Such a pattern is akin to a fire hose, as opposed to a sprinkler, and can overwhelm urban drainage systems, increasing the risk of flooding.
“It’s not only intensity of rainfall that matters when you look at flood risk. It’s also how it’s distributed over space,” Torelló-Sentelles added. Concentrated bursts of rainfall can collapse urban drainage systems, leading to dangerous flooding events.
Several factors contribute to this urban storm creation and intensification, Torelló-Sentelles explained. Cities tend to be warmer than their more vegetative surroundings, drawing air towards them. This air, once uplifted, condenses into rainclouds over urban centers, resembling how mountain ranges can induce rainfall by lifting air masses.
“You can think of a city like an obstacle,” added Torelló-Sentelles. “When a storm is moving toward it, the air can be lifted over and around it.”
Aside from topographical influences, urban aerosol pollution also plays a role, affecting rainfall patterns and intensities.
Over a span of seven years, the researchers used high-resolution weather data to track storm formation and intensity. They found that urban areas, regardless of size or climate, generally experienced more intense summer storms than their rural surroundings. Larger cities, in particular, intensified rainfall by 5.2% to 11%, whereas smaller cities had a rain intensification rate of 0.9% to 3.4%.
While the study revealed consistent trends, it also showed unique rainfall modification patterns in different cities. For example, storms in Atlanta were more intense during the daytime, while Birmingham saw increased intensity overnight. Berlin and Phoenix did not generate as many storms as other cities, but their rainfall patterns still varied significantly from rural areas.
These variances underscore the need for tailored city planning strategies and further research. As the world urbanizes and climate change continues to influence weather patterns, individual cities must develop their own adaptation and mitigation strategies.
“We need to study a wider variety of cities so that we can generalize findings and determine which city characteristics have the largest effects on cities’ rainfall-modifying potential,” Torelló-Sentelles said. “The mechanisms driving urban rainfall are quite complex, and we still need to research these processes more.”
This study not only brings to light the increasing vulnerability of urban areas to intense storms but also provides critical insights that could help mitigate future risks through informed urban planning.