In a new study published in 'Proceedings of the National Academy of Sciences', researchers from The University of Texas at Austin examined rainfall patterns in 1,056 cities globally. They discovered that over 60% of these cities experience more rainfall than surrounding rural regions. In certain locations, the disparity is significant. For instance, Houston receives nearly 5 inches more rain annually compared to its rural neighbors.
This increased rainfall can have serious implications, particularly in terms of heightened flash flooding risks in densely populated urban environments.
While scientists have been aware of urban-rural rainfall differences for decades, this study is the first to examine the phenomenon on a global scale. Previous research focused on individual cities or specific weather events, according to Xinxin Sui, a doctoral student at the Cockrell School of Engineering and the study's lead author. For this study, Sui and her colleagues analyzed satellite and radar data from 2001 to 2020, focusing on daily precipitation anomalies in the 1,056 cities.
"In general, we found that over 60% of these global cities have more rainfall (than the surrounding countryside). Then we compared with different climate zones and found that if the local climate is hotter, if it's wetter, then it may have a larger rainfall anomaly compared to the cities in cooler and dryer places," Sui said.
Cities like Houston are not alone in experiencing larger-than-usual rainfall. Ho Chi Minh (Vietnam), Kuala Lumpur (Malaysia), Lagos (Nigeria), and the Miami-Fort Lauderdale-West Palm Beach area in Florida are also notable for significant rainfall anomalies.
Professor Dev Niyogi, a co-author from the Jackson School of Geosciences and Cockrell School of Engineering, likened the effect to squeezing a sponge, where water is concentrated and released more forcefully in specific areas. "The amount of water you have in the sponge is the same, but because now you have that dynamic sort of squeezing the atmosphere, you have more ability to take the water out from that location," Niyogi explained.
Not all cities experience more rainfall. Some urban areas, particularly those in valleys or lowlands, receive less precipitation than their rural surroundings. Seattle, Kyoto, and Jakarta are examples of cities where nearby geographical features influence precipitation patterns, reducing urban rainfall.
One of the primary reasons for increased rainfall in cities is the presence of tall buildings, according to Liang Yang, professor at the Jackson School and co-author of the study. These structures reduce wind speeds, causing air to converge toward city centers, which enhances the upward motion of air, promoting cloud formation and precipitation.
Researchers also identified population size as the strongest factor correlating with urban rainfall anomalies. Larger populations often lead to denser cities with taller buildings and more greenhouse gas emissions, amplifying the urban heat island effect, according to Niyogi.
Looking to the future, this phenomenon has significant implications as climate change progresses. Yang emphasized the need to find innovative solutions to manage increased rainfall and the associated risks of flash flooding, particularly in cities with extensive impervious surfaces that exacerbate flood risks.
University of Georgia Professor Marshall Shepherd also contributed to the study. The research was funded by NASA.
Research Report:Global Scale Assessment of Urban Precipitation Anomalies
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