Summer thunderstorms are stronger and more frequent in urban areas

Summer storms are typically more frequent, more intense, and more concentrated in cities than in rural areas, according to new detailed observations of eight cities and their surrounding areas. The findings could change how urban planners prepare for flooding in their cities, especially as urban areas expand and climate change alters global weather patterns.

The new study finds that more thunderstorms form over urban areas and their boundaries than in surrounding areas, and that large cities intensify rainfall more than small towns. The study was published in The Future of the Earth.

“Cities are expected to become more populated and larger in the coming decades,” said Herminia Torelló-Sentelles, an atmospheric scientist at the University of Lausanne and lead author of the study. “Being able to quantify urban flood risk is important for urban planning and the design of urban drainage systems.”

The effect of rain has been reported in studies of isolated cities, but the new research sought to identify trends and differences across multiple cities. The differences in urban rainfall patterns underscore the need to continue studying storm activity in cities around the world, Torelló-Sentelles said.

Storming the cities

Some storms produce rain that falls evenly, like a sprinkler, while others dump rain in concentrated bursts, like a fire hose. The new study finds that cities can turn storms into fire hoses, dumping bursts of rain on small urban areas instead of spreading the rain over a larger area. These concentrated bursts of rain can exacerbate flood risks if urban infrastructure can’t handle the deluge.

Most cities generate more fire hose storms than rural areas. Cities also generate more storms than their surrounding areas, and large cities generate stronger storms than small towns.

“It’s not just the intensity of rainfall that matters when studying flood risk. You also have to consider how it’s distributed in space,” Torelló-Sentelles said. “If very large amounts of rain fall on a very small area, it can cause the drainage system in an urban area to collapse.”

Several factors could be behind the formation and intensification of urban storms, Torelló-Sentelles said. Cities are typically warmer than their cool, humid, and vegetated surroundings, which could cause air to be drawn into cities and lifted. This warm, lifted air then condenses into rain clouds over urban centers.

Storms often form when air rises over mountain ranges, producing rain clouds on the mountaintops. Like mini-mountain ranges, urban landscapes can create favorable environments for air masses to rise and create storms.

“You can think of a city as an obstacle,” Torelló-Sentelles explains. “When a storm is heading towards it, the air can rise above and around it.”

Aerosol pollution suspended in the atmosphere above cities can also increase or decrease precipitation.

The researchers used seven years of high-resolution weather data from eight cities in Europe and the United States (Milan, Italy; Berlin, Germany; London and Birmingham, United Kingdom; Phoenix, Arizona; Charlotte, North Carolina; Atlanta, Georgia; and Indianapolis, Indiana) to track the formation and intensity of summer storms in and around the cities. The cities varied in size, climate, and urban form, but all are in relatively flat regions and far from large bodies of water—factors that could influence local rainfall patterns.

The researchers tracked the formation and evolution of storms outside and above cities and their boundaries, identifying the average direction, average intensity, maximum intensity and area of ​​each storm.

The researchers found that storms formed more over cities and their boundaries than in surrounding rural areas. Storms were generally more intense over city centers or on the outskirts of cities, such as in Berlin and Birmingham. Large cities experienced greater precipitation intensification than smaller cities: in smaller cities, precipitation intensified by 0.9% to 3.4%, while it increased by 5.2% to 11% in larger cities compared to outlying areas. Some cities also experienced much greater precipitation intensification at specific times of the day.

Precipitation has also become more concentrated in urban areas, up to 15% higher. Concentrated precipitation can put more strain on urban water management systems than evenly distributed precipitation.

Severe storms increase risk of urban flooding

Growing urban areas could generate and amplify more storms than their surroundings, even as climate change continues to intensify storms worldwide. The combined impact of urban growth and climate change could strain urban stormwater management systems and lead to more frequent and severe flooding.

The researchers observed consistent trends across all cities, but each city changed its precipitation patterns in unique ways. For example, while most cities experienced storms with heavier precipitation than the surrounding areas, Berlin and Charlotte saw more scattered precipitation. In Atlanta, storms intensified most during the day, while in Birmingham, they only intensified at night. And unlike the other six cities studied, Berlin and Phoenix did not experience more storms over the city than in the surrounding areas.

These findings highlight the need for individual urban planning strategies and studies, Torelló-Sentelles said. As the climate changes and the world urbanizes, cities will need to develop their own adaptation and mitigation strategies.

“We need to study a wider variety of cities so that we can generalize the results and determine which urban characteristics have the greatest effects on the potential for changing precipitation in cities,” she said. “The mechanisms that drive urban precipitation are quite complex, and we still need to study these processes in more depth.”