Cities Egg On Thunderstorms: Increase Flooding

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Image: Rick Morley, www.rickmorleyphotos.com

I have vivid childhood memories of visiting my grandmothers house in rural/suburban Pennsylvania, where besides chasing the fireflies, we would revel in the warm summer evening thunderstorms. Turns out that children in the city may get more than their fair share of lightning, rain, and flooding. Alexandros A. Ntelekos and James A. Smith of Princeton University’s School of Engineering and Applied Science determined that cities create conditions that encourage increased rainfall during severe storms. Looking at a severe storm in Baltimore in 2004, their research shows the city experienced 30% more rainfall than it would if there was no city structure.

Given the rise of more severe weather events, associated run-off problems found in modern cities, and the strain on drainage systems, this research spells out increased likelihood of flooding similar to what was seen in New York this past week. The scientists found two key elements that cause the increased rain -and it's not the well known heat-island effect.

Urban canopies: While forests have tree canopies, cities have building canopies. The height and placement of buildings alters a storm’s low-level wind field, a key ingredient in its behavior. The tall buildings increase wind drag on the city, resulting in vertical velocities – essentially a boiling action – that can enhance rainfall. The urban canopy had a large effect during the 2004 storm, the researchers found, which was exacerbated all the more by the presence of the Chesapeake Bay to the east.

Urban aerosols. These are essentially minuscule particles in the atmosphere that are at elevated levels in urban environments due to industrial and automobile emissions. Traditionally, researchers have thought that air pollution tends to suppress precipitation. But Ntelekos and Smith believe their research points to the possibility that urban aerosols actually increase rainfall.

The urban heat-island effect does provide fuel to smaller, more mild storms, but the stiff winds found in severe weather events tend to even out the temperature difference. The research highlights the increasing need to adjust how cities think about and handle rain water. Using green roofs, paving tiles, and landscaping the city to native habitat is a good start. What else can modern cities do to battle increased flooding?
::Eurekalert.com ::AMS Journal (abstract)

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