 |
| Dean: David Paulison, Administrator for FEMA, briefs reporters on preparations for Hurricane Dean on Sunday, August 19. The
National Hurricane Center began using new forecasting software this year. Charles Dharapak/AP |
Next forecasting challenge: predicting hurricane's wallop
Despite big improvements in predicting a storm's track, researchers struggle to peg its intensity.
from the August 23, 2007 edition
Page 2 of 2
Page 1 | 2
Hurricanes can kick spray 100 feet or more into the air, where the droplets start to evaporate. The question is: What happens to the exchange of heat between the ocean and the atmosphere. The droplets cool as they evaporate and fall back and cool the ocean surface. A cooler surface would tend to sap a storm's strength. But a portion of the airborne droplet evaporates, releasing water vapor, which carries heat to the rest of the storm. In principle, this heat should help sustain a storm's strength. Researchers are divided over whether, on balance, the heating or cooling effect is the most dominant.
Even if researchers zero in on the right answer, they still have to cope with the fact that the processes take place to differing degrees in different parts of the storm and often in locales so small that models can't represent them.
For its part, foam whipped up by high winds may help reduce the friction between the swiftly moving air and the rough ocean surface. That would help keep hurricanes strong.
But scientists are trying to sort out whether the friction rises, then reaches a plateau as winds exceed a certain speed, or whether friction actually decreases as the speed increases. Results from buoys in the path of hurricane Ivan in 2004 suggest that friction does decrease, allowing the storm to spin up and draw more moisture into itself. Here too, the effect can vary widely across a storm's expanse.
As if scientist didn't have enough to think about, Alexander Khain is suggesting another wrinkle to the intensity problem – the presence of tiny particles called aerosols from natural and human causes on land. At a presentation earlier this year at a hurricane and climate conference in Crete, Dr. Khain, a researcher at The Hebrew University in Jerusalem, offered data suggesting that just before hurricane Katrina struck the Louisiana coast in 2005, aerosols got sucked into the outer reaches of the storm, weakening it just before landfall. In principle, as a storm inhales aerosols, this would enhance storm activity at the hurricane's outermost boundary but weaken its center.
The idea remains controversial, notes NCAR's Dr. Davis. Other factors, such as how dry the air might have been, could play a bigger role.
As scientists continue to tackle the intensity-forecast problem, "we have a much better handle on what matters" than 25 years ago, he says. Some aspects may inherently be unpredictable, he says, adding, "There's not an infinite reservoir of improvement out there."
But new models are beginning to reproduce realistic storm structures, although the intensity forecasts they yield are still off. As models start to simulate more processes with increasing levels of accuracy, "you're bound to move closer to your potential for intensity prediction." At the least, he says, "we'll be able to better quantify what the limit" of predictability is.
1 | Page 2
RSS feed
