Summer Thunderstorms Fiercer When They Collide With City

Recent summer storms caused flooding and wind damage in several areas of the city of Pittsburgh.

Now a new study by Princeton engineers may explain why Pittsburgh and other cities are seeing more damage from thunderstorms.

Researchers found summer thunderstorms become fiercer when they collide with a city than they would otherwise be in the open countryside.

Alexandros A. Ntelekos and James A. Smith of Princeton University based their conclusion on computer models and detailed observations of an extreme thunderstorm that hit Baltimore in July 2004.

Their modeling suggests that the city of Baltimore experienced about 30 percent more rainfall than the region it occupies would have experienced had there been no buildings where the city now sits.

While thunderstorms are thought of as being purely forces of nature, the Princeton research suggests that man’s built environment can radically alter a storm’s life cycle.

A recent storm in New York City played havoc with subways, street traffic, and airports. In that storm about three inches of rain fell in one hour.

“The storm that occurred in New York City is an example of the sort of event that we expect more of in the future,” said Michael Oppenheimer of the Intergovernmental Panel on Climate Change.

Neighboring cities also can affect the behavior of a storm.

The study showed that the 2004 storm over Baltimore was partially affected by the neighboring urban environment of Washington D.C. as air from the south became more turbulent when passing over it. This made the air mass particularly ripe for a storm by the time it reached Baltimore.

The researchers described three mechanisms that cause the urban environment to alter the evolution of thunderstorms:

Urban heat islands: Cities produce heat and are often 2 to 5 degrees hotter than the surrounding environment. In milder storms, this “heat island” can provide fuel for a modest thunderstorm.

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.

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 this latest research points to the possibility that urban aerosols actually increase rainfall.