Measuring a storm's intensity

Hurricane Lili, which hit the Louisiana coast earlier this month, was the kind of storm that gives forecasters sleepless nights long after the winds have died down.

Forecasts of the storm's track were right on the money. But no one predicted that Lili would jump from a minimal hurricane just south of Cuba's western tip to a roaring storm 24 hours later, bearing down on the Gulf Coast with maximum sustained winds nudging 140 miles an hour. Nor could forecasters give a heads-up that the storm's fiercest sustained winds would quickly drop to less than 110 miles an hour just before Lili struck the coast.

Now, scientists are poring through data from a unique set of experiments on Tropical Storm Isadore and Hurricane Lili that could help sharpen storm-intensity forecasts.

The work is focused on increasingly subtle interactions between the ocean, the storm, and high-altitude weather features that may appear 1,000 miles or more away from the storm's core.

"Improvements in track forecasts have been slow but steady over the past 30 years," says Joseph Cione, a research meteorologist at the National Hurricane Center in Miami. Intensity forecasts, he adds, haven't kept pace, despite the critical role storm strength plays as public-safety officials prepare evacuation orders for people living in the storm's path. A minimal hurricane creates a temporary rise in the sea level roughly four to five feet above normal. A storm striking with Lili's maximum strength could add 10 to 15 feet to that storm surge.

Dr. Cione notes that today's intensity forecasts display about as much skill as track forecasts did 20 years ago.

Researchers trace the lag to the scientific complexity of the problem, the lack until recently of hardware that allows for more detailed studies of storm conditions, and to too few of the right kind of storms to study. Virtually every storm has something to teach about factors that steer it along its path, researchers say, but not every storm exhibits sudden changes in strength.

This season, scientists have focused their efforts on two key factors affecting hurricane intensity – ocean temperatures and the presence of abrupt changes in wind speed and direction at storm-top altitudes.

Over the past seven years, much attention has focused on unusually deep pools of warm water that can spin off from currents between Mexico and Cuba and move into the Gulf. Hurricanes draw their energy from warm water.

In 1995, Hurricane Opal moved over one of these spin-offs, known as a warm-core ring. Opal's wind speeds nearly doubled to 134 miles an hour over a 14-hour period, according to Lynn Shay, a University of Miami atmospheric scientist, and Peter Black, a researcher with the National Hurricane Center.

Typically, as a hurricane stirs up the ocean beneath it, cooler water wells up to act as a brake on strengthening.

Warm-core rings, however, provide reservoirs of heat to depths of 200 to 300 feet. "They act as big gas tanks for hurricanes," Dr. Black says.

Using special buoys dropped from aircraft to measure the ocean's heat content ahead of Isadore's and Lili's paths, the two scientists hope to get a more detailed picture of the environmental conditions that can contribute to sudden intensity changes.

Yet conditions ahead of a storm provide only part of the story, researchers say.

Cione, for instance, is researching the change the storm brings to ocean temperatures directly beneath the core, the area drawing the most energy from the ocean.

The goal, he says, is to develop an ability to drop sensors along the predicted path of a storm and forecast the extent of ocean cooling that would likely occur as the storm's center passed over those spots. That would provide a way to gauge the relative importance of ocean conditions in a storm's intensity.

"We already know there's a lot of energy there," he says. But it's also important to get a storm's-eye view of conditions at the center, he adds.

Looking at past storms, Cione and University of Miami atmospheric scientist Eric Uhlhorn say the smaller the temperature difference between water ahead of the storm and the same spot as the storm center passes over it, the more likely the storm is to intensify. They also find that subtle differences can have a large effect – temperature changes as small as 0.5 degrees Celsius could alter the amount of energy feeding the storm by 25 percent.

Lili provided an ideal testing ground, he says. Using the National Hurricane Center's track forecasts, Cione was able to drop ocean-temperature sensors in a grid ahead of the storm. As Lili's center passed over the sensors, he and colleagues flew through the storm, dropping more sensors to gauge the temperature difference. Compiling these kinds of data over time, he says, should allow researchers to get a more detailed accounting of energy transfer in the heart of the storm, which they can then feed into forecasting models.

Meanwhile, up around 40,000 to 50,000 feet, another factor also may have played a role in the rise and fall of Lili's clout, researchers say.

An upper-level, low-pressure system moved in from a direction that may have allowed its counterclockwise rotation to spin briefly up the hurricane near its top. Measurements taken from a research jet operated by the National Oceanic and Atmospheric Administration should shed light on the significance of this "upper-level trough" in giving Lili a kick.

The trough may also have played a role in softening Lili's blow. Once the trough passed a certain point in its journey, researchers speculate, its winds would cease to reinforce Lili's circulation and begin to buck it. At the same time, the trough brought drier air, which would have drawn moisture out of the storm system.

"It's a complicated problem," Cione says.