As city populations increase worldwide, meteorologists are raising concerns about the potential for urban areas to experience heavier rainfall and flooding risks in the future.

Decades of research has demonstrated that climate change can increase the intensity of precipitation events because a warmer atmosphere can hold more water. Research has also shown that major cities can create their own microclimates, influencing the local atmosphere in ways that can both magnify or suppress precipitation. 

For scientists, a pressing question is how the combination of warming temperatures and growing urbanization will affect the risk of extreme precipitation in major cities. This is an increasingly consequential issue for society, since more than two-thirds of the world’s population is projected to live in urban areas by 2050, up from about 55% today.

A pair of papers co-authored by scientist Fei Chen at the National Center for Atmospheric Research (NCAR) and published over the last year indicates that urban inhabitants may indeed be in harm’s way for more extreme hourly precipitation. Moreover, cities in the midlatitudes appear to be at especially high risk.

Applying high-resolution computer modeling techniques, scientists simulated hour-by-hour rainfall in a warmer world for two of the world’s megacities: Singapore in the tropics and Tokyo in the midlatitudes. The research indicated that, overall, extreme rainfall will become more intense and frequent in both places, although the urban effect will be blunted in Singapore. 

The authors warned that heavier precipitation will pose major challenges for urban infrastructure designed for more traditional rainfall patterns.

“These studies have significant implications for urban design,” said Chen, who co-authored the papers. “Urban planners who designed projects over the last 150 years or so did not anticipate such huge amounts of rain pouring down over a short period of time.”

Both studies were led by Quang-Van Doan of the University of Tsukuba in Japan in collaboration with an international team of scientists. Funding came from NASA, the U.S. Department of Agriculture, and the U.S. National Science Foundation, which is NCAR’s sponsor.

Paved surfaces, tall buildings

Meteorologists have long known that cities and precipitation have a complex relationship.

As far back as the 1960s, scientists attributed a decades-long trend of increased precipitation in a northwest Indiana town to the proximity of nearby industrial activity in Chicago. Since then, studies have shown significant increases in summertime precipitation in certain U.S. cities, including New York and Houston, as well as intensified precipitation in or around major urban areas worldwide from Beijing to Mexico City. In other cases, however, urbanization appeared to lead to a warmer, drier climate.

The reasons for urban impacts on precipitation have to do with the effects of dense development on the local atmosphere. Miles of paved surfaces and wasted heat in cities warm up the air, creating urban heat islands that, along with tall buildings, alter local winds, sometimes drawing in moist air from nearby bodies of water or accentuating vertical air movements that increase precipitation. In other cases, however, the hot air can become stagnant and lead to more drying, especially because the pavement, unlike many natural surfaces, does not contain water that can evaporate and moisten the atmosphere.

To better understand how these processes will influence precipitation in a warming climate, Chen and his colleagues focused on Singapore as an example of a large, tropical city, and Tokyo as an example of a large, midlatitude city. They ran a set of specialized, high-resolution computer simulations that used the NCAR-based Weather Research and Forecasting (WRF) model to compare current-day precipitation with precipitation toward the end of the century. 

The results showed that future warming will enhance both the frequency and intensity of extreme hourly precipitation, especially if society emits a high amount of greenhouse gases. In the case of Singapore, for example, very heavy precipitation of more than 30 millimeters (1.2 inches) per hour would become about 50% more frequent toward the end of the century, assuming high amounts of greenhouse gas emissions. Light and moderate precipitation, in contrast, would not change as much. 

Climate change will have a far more pronounced effect on Tokyo. There, the simulations showed that low-intensity precipitation would decrease, while very heavy precipitation (more than 30 millimeters, or 1.2 inches, per hour) would triple with high greenhouse gas emissions. Even if society were to emit only moderate amounts of greenhouse gases, heavy precipitation would increase significantly.

“This new normal can be expressed as ‘extreme events get more extreme,’” the authors wrote in the paper about Singapore.

The research team also looked into the extent to which the cityscapes themselves affected future precipitation. To do so, they created another set of computer simulations of future warming, replacing the urban landscape with a hypothetical land cover of cropland in the same location. 

In Singapore, future precipitation became more extreme over the cropland than over the city. This indicated that the urban surfaces of Singapore had a somewhat restraining effect on rainfall in a warmer climate. The reason, the simulations indicated, is that the increased heating of the atmosphere caused by extensive urban development tended to cause low-level stratification of the atmosphere, suppressing the types of vigorous updrafts and downdrafts that can generate heavy rain and thunderstorms.

In the simulations of Tokyo, the situation was reversed: precipitation became more extreme over urban surfaces than over the hypothetical cropland. Unlike Singapore, the heating and resulting stratification merely delayed the onset of major storms but did not prevent them entirely, Chen and his co-authors concluded. Once the storms did form, they became even more intense.
 

Chen and his colleagues plan to learn more about urban precipitation by expanding their research to Lagos and other coastal cities. These will include Houston, which has suffered several devastating floods in recent years, most notably when Hurricane Harvey in 2017 inundated the city with more than 50 inches of rain over four days. The scientists will also look at inland cities, such as Austin.

“Urban environments have very complex effects on the atmosphere,” Chen said. “We need to study a number of cities in different geographical and climatic regions so we can prepare for a future in which hourly precipitation will change and potentially affect billions of city residents.”

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