14 July 2010

Cities may be magnets for disaster

Posted by koneil

Tropical Cyclone Monica heading for the northern Australian city of Darwin in April, 2006. Credit: NASA/GSFC/MODIS Rapid Response Team

Between the recent oil spill and Hurricane Katrina, it might seem that New Orleans is a magnet for disasters. A new study suggests that could be true – researchers have found that coastal cities could actually attract hurricanes.

Johnny Chan and Andie Au-Yeung of the City University of Hong Kong wanted to improve forecasts of where hurricanes, or tropical cyclones, will go as they approach land. This “track forecasting” is crucial for giving people advance warning of where storms are likely to hit hardest.

The researchers manipulated variables in a computer simulation called the Weather Research and Forecasting Model, which was built by a collaboration of federal agencies, laboratories and universities, including the National Center for Atmospheric Research (NCAR) and the National Oceanic and Atmospheric Administration (NOAA). One of the model’s variables is “roughness,” or the small-scale topography of the land. Farmland, for example, tends to be very flat and smooth with little to impede the wind, so it has low roughness. Cities have buildings and forests have trees that stick up that get in the way and alter winds, so they have high roughness.

In an earlier modeling study, Chan found that if a coastline is rough across its entire length, a stretch hundreds of kilometers long, cyclones would drift toward the coastline, apparently attracted by the roughness. With the new study, they wanted to see what would happen if the coastline varied, partly rough and partly smooth.

In the simulation, the researchers set up a coastline running north-south, with hurricanes approaching from the east – like Florida’s Atlantic coast, or China’s Pacific coast. Their study, which has been accepted for upcoming publication in the Journal of Geophysical Research – Atmospheres, did see an effect on the storms’ tracks due to changes in roughness.

When a simulated hurricane approached a coastline with rougher land in the north and smoother land in the south, it veered north. When the researchers swapped the land areas, so that the rough patch was in the south, the hurricane tracked south.

The reason for this effect on a storm’s track, the study found, is that as the cyclone approaches rougher land, the air in that region gets compressed more, due to increased friction from the surface. This pushes more air toward the low-pressure area in the center of the storm, where it is forced to rise up. Water vapor in the rising air then condenses, releasing heat. The heat causes the nearby air to circulate faster than the air on the opposite side of the cyclone, thereby pulling the center of the cyclone toward the rougher terrain.

The researchers say the change in direction was “minor,” since it caused a shift in the storm’s track by 30 kilometers at most. And they found no shift in hurricane tracks in cases where the land was of the same roughness all over. Nonetheless, Chan argues, the results show it is important to have a correct representation of the land surface in forecasting models. However, New Orleans’ record aside, it’s not clear yet whether this happens in real life.

“About 10 years ago, one of my former students did a study of the track changes near landfall of real tropical cyclones, but he could not find any,” Chan says. It may be that this effect is hard to discern in the records, because field measurements can’t pinpoint the exact center of the storm.

“The deviation we found [in the storms’ tracks] in the simulations is of the order of about 10 to 30 kilometers, [which] is actually within the error in determining the center of the tropical cyclone,” Chan says. “Now that we have these simulation results,” he adds, “we will try to look for such changes again in real cyclones.”

The paper, “The Effect of a River Delta and Coastal Roughness Variation on a Land falling Tropical Cyclone,” is expected to be published in the fall of 2010.

–Mason Inman, contributing science writer