Hurricane trackers catch storm's 'second eye'
Modelling the eyes of a hurricane should help predictions of intensity.
Scientists have documented for the first time how the eye of a hurricane dies, and is replaced by a new one. The observations, made by radar-equipped aircraft during the hurricane season of 2005, could be used to improve forecasts of hurricane intensity.
It's well known that there's calm in the eye of a storm. But the eye is in fact a highly dynamical zone that constantly interacts with the rotating bands of rain clouds surrounding it.
Eyes have been seen dying and re-forming several times during the lifetime of cyclones, abruptly altering their strength. 'Eye replacement' temporarily reduces the spin of a hurricane. But as a new eye forms and contracts, the cyclone gathers spin again, like a swirling figure skater who folds his arms, and wind speed increases once more.
Understanding the behaviour of the outer boundary of the eye (or 'eyewall'), where the strongest winds occur, is critical for forecasting storm intensity, says Robert Houze, an atmospheric scientist at the University of Washington in Seattle. A study published in this week's Science1 gives new insight into just that, and should help jazz up deficient forecasting methods.
Predicting the track hurricanes are likely to follow as they approach the coast has made great advances in recent years. But it has proven much harder to predict a storm's strength at landfall — a detail that is just as important to storm preparations.
So researchers set up an ambitious collaboration to address this problem — the Hurricane Rainband and Intensity Change Experiment (RAINEX). The National Science Foundation-funded project uses three Doppler radar-equipped aircraft, aided by high-resolution numerical modelling, to investigate the eyes of a storm.
Houze and his team looked at radar data taken by manned aircraft that flew through Hurricanes Rita, Katrina and Ophelia in 2005. They found that the processes that eventually lead to the creation of a new eye occur in what they call the 'moat', a region of dry air between the new and the old eyewall.
"The moat can join with the original eye of the storm to form a wider eye," says Houze. "The original eye dies out and the new wider eyewall takes over." A wider eye makes for a calmer storm, until the new eye also contracts and the storm intensifies again.
Small but important
Past studies have only shown brief glimpses of this process, because it takes place on a scale too small to have been spotted or modelled before.
RAINEX was the first time that aircraft pilots flying into a hurricane were directed from crews on the ground in real time, so that they could remain in the moat region throughout the flight. This allowed them to collect data from the source during eye replacement.
In addition, RAINEX also supported the development of a computer model that could capture such small-scale processes. Shuyi Chen, a meteorologist and physical oceanographer at the Rosenstiel School of Marine and Atmospheric Sciences in Miami, Florida, created such a model, with a resolution of 1.67 kilometres — three times better than previous ones. It successfully showed the process of eye replacement, in agreement with the data collected by the planes.
The model is a first step towards computer simulations intended to help forecast hurricane intensity changes, says Houze.
"This may lead to increased hurricane predictability in future years," says Christopher Landsea, a meteorologist at the National Hurricane Center in Miami. "One sobering conclusion, however, is that the current generation — and those available in the next few years — of hurricane computer models here at the National Hurricane Center may not have the resolution needed to depict these crucial changes."
RAINEX has provided a massive data set that hurricane researchers have only just begun to use. Destructive though they were, hurricanes Katrina and Rita supplied scientists with a rare opportunity for study. Among other things, Houze and his team are now trying to understand why, despite similar paths, Katrina didn't seem to undergo eyewall replacement whereas Rita did.
- Houze R., et al. Science, 315 . 1235 - 1239 (2007).
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