Cities Attract Hurricanes

Aug. 28. 2010— -- Five years ago, Hurricane Katrina slammed into the Gulf Coast, devastating New Orleans and other regions along the Mississippi River Delta. Hurricane forecasting has steadily progressed over the intervening years, which should help cities and states better prepare for devastating cyclones.

Now researchers have added another piece to the forecasting puzzle by determining how the texture of landscapes can affect a storm's motion.

New research shows that rough areas of land, including city buildings and naturally jagged land cover like trees and forests can actually attract passing hurricanes. The research found also that storms traveling over river deltas hold together longer than those over dry ground.

As a result, the city of New Orleans might feel a greater impact of hurricanes coming off the Gulf of Mexico than existing computer models predict.

A team from the University of Hong Kong modeled the effects that different terrain has on the paths of tropical storms to determine how cities that lie in the path of a hurricane change a storm's motion.

"Cities impose greater friction on the swirling flow because of the tall buildings," said Johnny Chan, a professor of meteorology at the university. "Our results show that tropical cyclones tend to be 'attracted' towards areas of higher friction. So it is possible that cities could cause tropical cyclones to veer towards them."

Rough cityscapes and forests trap air. This compresses the air and forces it up into the atmosphere, adding energy to the storm and pulling the center of the hurricane toward the rough region. As a result, a city can cause a hurricane to swerve from its predicted path by as much as 20 miles.

The change is comparatively small for hurricanes that can reach widths of hundreds of miles, but according to Chan, "The main implication from this study is that in any computer prediction of the track of a hurricane, the representation of the land surface is important."

The researchers also found that river deltas contribute to the longevity of hurricanes. There is more heat-carrying moisture available to evaporate from the wet deltas than dry ground, prolonging the life of the storm.