Thunderstorms not cut out for city living

A team of US scientists have found that thunderstorms can become much fiercer when they collide with a city.

Research suggests cities can enhance the power of thunderstorms

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

Hydrologists have observed evidence in the past that urban environments alter the behaviour of storms, but they have mostly noted average increases in rainfall over long periods of time. Until now, they have not made observations of specific extreme storms because they lacked the right tools to do so.

Lead researcher Alexandros Ntelekos, of Princeton University’s School of Engineering and Applied Science, said: “Previous studies basically came from cities where the terrain was simple, where you had a town in the middle of nothing - no mountains, no water. But most of the hub cities are close to either mountains or water as well as being close to other cities. So we have to understand how extreme thunderstorms behave over complex terrains.”

Ntelekos and colleague, James Smith, based their conclusion on computer models and detailed observations of a specific thunderstorm that hit Baltimore in July of 2004. After combining observational information on lightning strikes, rainfall, clouds and aerosols with analyses based on computer models of the atmosphere, they came up with three main mechanisms describing the effect of a city landscape on a storm.

Firstly, the heat produced by a city can provide fuel for the storm. Secondly, the height and placement of buildings alters a storm’s low-level wind field, a key ingredient in its behaviour - the tall buildings increase wind drag on the city, resulting in vertical velocities that can enhance rainfall. Thirdly, minuscule particles in the atmosphere - elevated in urban environment - can enhance the storm. Traditionally, researchers have thought that air pollution tends to suppress precipitation, however Ntelekos and Smith believe their research points to the possibility that urban aerosols actually increase rainfall.