Researchers conducted a study to identify precipitation anomalies in 1,056 cities around the world and discovered that over 60% of these cities experience more rainfall than the rural areas nearby. This trend could lead to significant issues, with the most critical being increased flash flooding in densely populated urban locales.
The impact of urbanization on temperature is fairly well-known: cities are generally warmer than their surrounding countryside, a phenomenon referred to as the urban heat island effect. However, less is known about its counterpart, the urban precipitation anomaly, which indicates that urban development influences the amount of rainfall in a location.
In a recent study published in Proceedings of the National Academy of Sciences, researchers from The University of Texas at Austin examined data for 1,056 cities globally and found that over 60% of these urban areas receive more rain than their surrounding rural regions. In certain instances, the rainfall difference can be quite substantial. For example, Houston receives an average of nearly 5 more inches of rain annually than its rural surroundings.
This can have far-reaching consequences, with the most serious being the potential for increased flash flooding in heavily developed urban areas.
While variations in urban rainfall have been acknowledged by scientists for decades, this study is unique in its global approach. Prior research focused on specific cities or storm events, as noted by study author Xinxin Sui, a doctoral candidate at the Cockrell School of Engineering. For this research, she and her colleagues analyzed precipitation data from satellites and radar systems, monitoring daily rainfall anomalies in these 1,056 cities from 2001 to 2020.
“Overall, we found that more than 60% of these cities worldwide experience more rainfall compared to the surrounding countryside. We also observed that cities in hotter and wetter climates tend to have greater rainfall anomalies than those in cooler, drier regions,” Sui said.
Alongside Houston, other major cities with notable precipitation anomalies include Ho Chi Minh City in Vietnam; Kuala Lumpur in Malaysia; Lagos in Nigeria; and the Miami-Fort Lauderdale-West Palm Beach area.
Study co-author Dev Niyogi, a professor at the Jackson School of Geosciences and Cockrell School of Engineering, clarified that urban areas tend to redistribute precipitation from one area to another, akin to how a sponge releases water when squeezed.
“If you were to compress one section of the sponge, water would flow more forcefully from that end,” he explained. “The total amount of water remains constant, but the process of compressing the atmosphere allows for more efficient rain extraction from that section.”
Interestingly, some urban regions receive less rainfall compared to their rural counterparts, a phenomenon usually seen in cities located in valleys and lowlands, where nearby mountains influence precipitation patterns. Notable examples include Seattle, Washington; Kyoto, Japan; and Jakarta, Indonesia.
There are multiple reasons why many cities experience higher rainfall than the surrounding countryside. Co-author Liang Yang, a professor at the Jackson School, noted that one important factor is the height of buildings, which reduce wind speed. This results in air converging towards the city center.
“The buildings enhance this convergence by slowing the winds, leading to a more vigorous upward movement of air. This upward air motion is conducive to water vapor condensation and cloud formation, which are essential for generating rainfall and precipitation,” Yang stated.
Researchers found that urban precipitation anomalies are most closely linked to population density compared to other environmental and urban development factors. Higher populations typically result in denser, taller urban areas with increased greenhouse gas emissions, which contribute to higher temperatures, according to Niyogi.
This issue is particularly relevant for cities as they face the future challenges of climate change, Yang remarked, emphasizing how the increased likelihood of rainfall in urban areas, combined with non-absorbent surfaces common in cities, heightens the risk of flash flooding.
“These combined factors necessitate the development of innovative strategies to prepare for flash flooding,” Yang concluded.
