A study shows that built-up cities receive more rain than rural suburbs

Cities often receive more rain and experience extreme rainfall more often compared to surrounding areas, a new study shows, a phenomenon that exacerbates other risk factors for urban flooding.

Of the more than 1,000 cities worldwide that researchers considered, 63 percent received more annual rainfall on average than outlying rural areas, according to an analysis of two decades of satellite data published Monday in Proceedings of the National Academy of Sciences. In some cases, the difference was more than half a foot per year. Many cities also saw more frequent and dramatic extreme rainfall.

Urban areas are already vulnerable to flooding due to factors including impervious surfaces such as roads and parking lots and inadequate drainage systems. In addition, climate change is fueling heavier rainfall events – and the new study suggests that cities are directly in the firing line.

Urbanization shapes the weather in several ways, said Dev Niyogi, a professor of earth and planetary sciences at the University of Texas at Austin and one of the study’s authors. First, cities tend to be warmer than rural areas, a phenomenon called the urban heat island effect. When warm air rises, it creates updrafts that can lead to increased cloud formation and precipitation.

The uneven urban landscape of tall buildings and hard infrastructure can slow local airflow or prolong rain. And the air above cities has a higher concentration of aerosols, meaning there are more tiny particles in the atmosphere that collect water around them in raindrops and form fuel clouds.

“Cities can storm on steroids,” Niyogi said. “That’s how you add risk for (an) increased flood.”

The effects of what Niyogi and his coauthors call “wet urban islands” can be dramatic. The urban area of ​​Ho Chi Minh City, Vietnam, saw an average of 274 millimeters (about 11 inches) of extra annual rainfall compared to its rural fringes. Guangzhou-Shenzhen, China, received an additional 186 millimeters and Houston an additional 124 millimeters.

The authors defined cities based on satellite observations of land cover and then established three concentric zones around each, assuming that the outermost zone “lies outside the dominant range of city influence” on precipitation, they write.

A handful of urban areas, including Kyoto and (surprisingly) Seattle, were found to be “dry islands” because they received less rain than their surrounding areas. Geography appeared to play a role in some cases, such as cities located in valleys receiving less rainfall than nearby hills. The researchers also found that rainfall was higher upstream of cities, relative to surrounding areas, and lower in rural areas upstream.

Scientists have long known about urban heat islands. There is a growing awareness of the other implications of urbanization for local climates, and the new research is an “important step forward” in understanding the effect on rainfall, said William Solecki, a professor of geography and environmental sciences at Hunter College, who did not was involved in the study. .

Previous research has explored how various urban environments, from Atlanta to Beijing, shape precipitation. The new study suggests that the effect, like rising heat in cities, is a global phenomenon, Niyogi said.

The intensity of urban wet islands increased over time, the authors found, with the average difference in precipitation for those areas nearly doubling between 2001 and 2020. Larger cities were more likely to be wet islands, a sign that urban development is fueling the anomalies .

Even as cities grow, understanding the conditions they will face in the future can help harden them against flooding. Critical infrastructure can be built on the upper floors of buildings, Solecki said. He also emphasized efforts to prioritize permeable surfaces and build water retention areas, such as sunken basketball courts that fill with floodwater and slowly release it.

“We are on a dynamic trajectory” of climate change, Solecki said. “Any information will be very valuable to help designers and planners.”

Photo: Kuala Lumpur, Malaysia; photo credit: Ian Teh/Bloomberg

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