Summer storms tend to occur more frequently, with greater intensity and concentration in urban areas compared to rural regions, according to recent in-depth observations of eight urban areas and their adjacent environments. These findings could significantly influence how city planners prepare for potential flooding, particularly as urban areas grow and climate change impacts weather patterns globally.
Summer storms tend to occur more frequently, with greater intensity and concentration in urban areas compared to rural regions, according to recent in-depth observations of eight urban areas and their adjacent environments. These findings could significantly influence how city planners prepare for potential flooding, particularly as urban areas grow and climate change impacts weather patterns globally.
The latest study indicates that urban areas and their adjacent regions experience a higher frequency of storm formation compared to the surrounding rural zones, and that larger cities tend to lead to increased rainfall intensity more than smaller ones. This research was published in Earth’s Future, an open-access journal by AGU that covers cross-disciplinary studies on our planet’s past, present, and future.
“As cities are projected to grow in population and size in the next few decades,” stated Herminia Torelló-Sentelles, an atmospheric scientist at the University of Lausanne and the lead author of the study. “Quantifying urban flood risks is vital for urban planning and designing drainage systems effectively.”
Although previous studies have noted rainfall effects within single cities, this research identified patterns and variations among multiple cities. The differences in urban rainfall behaviors underscore the necessity for ongoing studies of storm activities in cities worldwide, according to Torelló-Sentelles.
Transforming storms in urban settings
Some storms deliver rain evenly, similar to a sprinkler’s output, while others unleash concentrated downpours akin to a fire hose. The new study reveals that cities can convert storms into these powerful downpours, distributing heavy rain across limited urban sections rather than more broadly. Such concentrated rainfall can heighten flood risks if the municipal infrastructure fails to manage the surge.
Urban areas largely produce more of these intense downpour-type storms than rural locales. Additionally, cities are generating a greater number of storms compared to their rural surroundings, with larger cities giving rise to even stronger storm activity.
“Rainfall intensity is not the sole factor to consider when assessing flood risks; the spatial distribution of that rainfall is also crucial,” Torelló-Sentelles explained. “If an enormous volume of rain falls on a small area, it can overwhelm the drainage systems in an urban setting.”
Several elements may contribute to the creation and intensification of storms in urban centers, as stated by Torelló-Sentelles. Urban areas are typically warmer than their surrounding cooler, moisture-rich, and vegetation-abundant environments, potentially causing air to be drawn toward these cities and uplifted. This warm, rising air can subsequently condense into rain clouds over metropolitan regions.
Storms often develop when air is uplifted along mountain ranges, forming rain clouds at their summits. Similarly, city skylines can create conditions conducive to lifting air masses and generating storm formations.
“You can think of a city as an obstacle,” Torelló-Sentelles mentioned. “When a storm approaches, the air may be lifted up and around it.”
Aerosol pollution suspended above urban areas might also influence rainfall, either enhancing or suppressing it.
The researchers analyzed seven years of high-resolution weather data collected from eight cities in Europe and the U.S. (Milan, Italy; Berlin, Germany; London and Birmingham, United Kingdom; Phoenix, Arizona; Charlotte, North Carolina; Atlanta, Georgia; and Indianapolis, Indiana) to monitor the development and intensity of summer storms both in urban areas and their surroundings. Although these cities differed in size, climate, and urban structure, they are all located in relatively flat regions and are distant from large bodies of water, factors that could affect local rainfall patterns.
The study examined storm formation and development in and around city boundaries, noting the average direction, intensity, peak intensity, and the area affected by each storm.
The researchers discovered that cities, along with their borders, generated more storms than adjacent rural areas. Storms typically showcased the highest intensity over city centers or their edges, such as those in Berlin and Birmingham. Larger urban centers exhibited greater rainfall intensification than their smaller counterparts; rainfall intensified by 0.9% to 3.4% in smaller cities, while it surged from 5.2% to 11% in the largest cities in comparison to surrounding regions. Some cities also recorded significantly higher rainfall intensification at certain times of the day.
Moreover, rainfall became more spatially concentrated over urban regions by as much as 15%. Such concentrated rainfall can place additional strain on city water management systems compared to rainfall that is distributed evenly.
Intense storms heighten urban flood vulnerabilities
Growing urban areas are likely to generate and amplify more storms than their rural surroundings, especially as climate change continues to escalate storm severity globally. The combined effects of urban expansion and climate change may burden urban stormwater systems, leading to more frequent and severe flooding.
While the researchers noticed some consistent trends across all cities studied, each city presented its unique patterns of rainfall. For example, even though most cities experienced storms with stronger bursts of rain compared to their surroundings, Berlin and Charlotte showed more dispersed patterns. In Atlanta, storms intensified primarily during the daytime, while in Birmingham, they peaked at night. Unlike the other six cities, Berlin and Phoenix did not exhibit a higher frequency of storms initiating over the urban areas than in their rural neighbors.
These findings underscore the necessity for tailored urban planning strategies and the incorporation of additional cities in future studies, according to Torelló-Sentelles. As climate conditions evolve and urbanization continues, individual cities will need to devise specific adaptation and mitigation measures.
“It’s essential to investigate a broader range of cities to derive generalized conclusions and identify which characteristics most significantly influence rainfall modulation in urban environments,” she noted. “The factors influencing urban rainfall are quite complex, and further research is required to fully comprehend these processes.”
