Cities Intensify Storm Violence

As the violent electrical storm and tornado that ripped through Manhattan and Brooklyn last week illustrated, summer thunderstorms become much more violent when they collide with a city than they would otherwise be in the open countryside, according to research led by Princeton engineers.

During last weeks storm in New York City -- which played havoc with subways, street traffic, and airports – about three inches of rain fell in one hour. In the 2004 Baltimore storm that the researchers studied, about six inches fell within two hours.

Alexandros A. Ntelekos (above in front of Baltimore skyline) and James A. Smith of Princeton University’s School of Engineering and Applied Science based their conclusion on computer models and detailed observations of an extreme thunderstorm that hit Baltimore in July of 2004. Observational data shows that, during the 2004 storm, parts of Baltimore experienced as many lightning strikes in the space of two hours as they normally receive during the course of a year.

While thunderstorms are thought of as being purely forces of nature, the Princeton research suggests that man’s urban environments can radically alter a storm’s life cycle. A storm of the intensity of the 2004 event in Baltimore is extremely rare, occurring only once every 200 years or so. However, climate change is expected to make such events more frequent, according to the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC).

“Precipitation events like gully-washing rainstorms are expected to increase in intensity as the world warms due to the buildup of greenhouse gases,” said Michael Oppenheimer, a lead author on the IPCC report and the Albert G. Milbank Professor of Geosciences and International Affairs at Princeton. “This is just the sort of research that combines science, engineering, and social response that may allow us to better cope with the future, warmer climate. I hope it will also serve as a warning about the complexity of adaptation, and therefore, as a goad to policymakers to act more urgently to stabilize the climate.”

The storm that occurred last week in New York City "is an example of the sort of event that we expect more of in the future,” said Oppenheimer. “The disruptive effect was quite obvious.”

Exactly how does the urban environment alter the evolution of thunderstorms" The researchers described three mechanisms:

Urban heat islands: Cities produce heat and are often 2 to 5 degrees Fahrenheit hotter than the surrounding environment. In milder storms, this “heat island” can provide fuel for a modest thunderstorm. But in their study of the 2004 thunderstorm, the Ntelekos and Smith found that the heat island had little effect because high winds leveled temperatures.

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 Ntelekos and Smith believe their research points to the possibility that urban aerosols actually increase rainfall.

Posted by Casey Kazan