Why some hurricanes stay strong after landfall
When Hurricane Charley barreled up the Florida peninsula and struck Orlando with 105-mph gusts, forecasters were stunned the storm retained so much power after landfall.
Another unpleasant surprise came with Hurricane Wilma, which came ashore on Florida’s west coast and traveled across the state to hit South Florida with Category 2 winds, shearing off roofs and shattering windows at downtown office buildings.
The course of hurricanes over land may be the least-studied period of their life cycle. Hurricane hunter planes don’t enter them for safety reasons. Wind stations get knocked out by strong gusts. And atmospheric scientists have tended to focus their research on hurricanes’ life spans over the ocean, where they form and achieve their peak strength.
“In some ways we know less about what happens over land than what happens over water,” said Michael Brennan, branch chief of the hurricane specialist unit of the National Hurricane Center. “We’re not able to fly aircraft into the storm over land. And a lot of the observation systems over land are either very sparsely placed or fail during very high winds. It’s an irony that once a storm hits land, we sometimes know less about it.”
Aware of the threat hurricanes can pose to inland cities, scientists are devoting more attention and more technology to this period of hurricanes’ lives, when they’re weakened but still dangerous.
“We’ve had storms that have gone well inland and generated significant damage,” Brennan said. “It’s not just a coastal issue. It’s certainly an inland issue, maybe more so in Florida than in any other state. It’s something people need to be aware of, even if you do live in Orlando or somewhere else relatively far from the ocean.”
This much is known: When hurricanes hit land, they begin to weaken immediately. Having left the ocean, they lose contact with the warm water that provides their energy. And over land they tend to encounter drier air that further weakens them.
By seven hours after landfall, the typical major hurricane — one with a wind speed of at least 111 mph — falls below hurricane strength, which means wind speeds less than 74 mph, Brennan said. By 20 hours after landfall, it’s below tropical storm strength, its winds diminishing below 39 mph.
But in their final spasms, some hurricanes cause more destruction than others, either by lasting longer or moving faster.
After coming ashore at Port Charlotte in southwest Florida, Hurricane Charley moved so fast it confounded experts who thought it would lose strength. The storm raced up the peninsula at 25 mph, more than twice the average speed of a hurricane, forcing theme parks to close, knocking out most wind-measurement instruments, and striking Orlando’s airport after midnight with gusts measured at up to 105 mph. The storm retained hurricane strength as it departed Florida near Daytona Beach.
“The faster a storm is moving, the farther inland those winds can go before they start to die off,” Brennan said. “Since the rate of decay is pretty fast, if the storm is moving slowly, those winds aren’t going to penetrate very far. If a storm is moving more quickly, those winds can make it much farther inland.”
Hurricane Wilma hit southwest Florida at Category 3 strength and sped east at up to 28 mph, crossing the state in 4 ½ hours. Barely weakened by its passage over land, the storm lashed Broward, Miami-Dade and Palm Beach counties with winds of Category 1 and 2 strength, the region’s worst hurricane hit since Andrew.
“Everyone, myself included, was very surprised by how strong the impact of Wilma was in the MiamiFort Lauderdale area,” said David Nolan, chairman of the Department of Atmospheric Sciences at the University of Miami. “The idea was it’s hitting the land over there, it’s going to weaken drastically by the time it gets to us, and that really was not the case. Wilma barely weakened at all. It was very fast-moving. That’s a very good case of where a poor understanding of decay over land led to a surprise.”
Hoping to be less surprised by future storms, Nolan and other scientists are devoting more time and instruments to measuring hurricane wind speeds over land. Nolan’s lab is using measurements in the field, computer simulations and wind-tunnel experiments to measure the effects of terrain and human-built structures on hurricane winds. The aim is to improve forecasts of hurricane wind speeds at the ground level.
A key factor, he said, is terrain. As anyone who watches hurricane forecasts knows the mountains of Haiti, the Dominican Republic, Jamaica and Cuba can severely weaken a hurricane.
Florida, where the major topographical features are landfills, has nothing like the mountains to disrupt a hurricane after landfall. South Florida’s wall of oceanfront towers may impress tourists but they don’t impress hurricanes, amounting to less than a speed bump for storms at landfall. An area immediately behind a tower may get shielded from some of the wind, but that’s about it, Nolan said.
“In terms of weakening a hurricane and benefiting people miles inland, there’s no effect whatsoever,” he said.
Forests of tall, thick trees, such as those that rim parts of the northern Gulf Coast, can weaken a hurricane. But Florida doesn’t have much of that, increasing the state’s vulnerability to storms that travel inland.
“In places where we have very high concentrations of trees, like the Gulf Coast, those cause the winds to be reduced considerably,” Nolan said. “In places where there’s less concentration of tall, thick trees, like Florida or southeast Texas, then you could get the effect of the strong winds penetrating further inland.”
At the University of Florida, a team of scientists and engineers operate mobile wind-measurement towers that can be set up in 30 minutes and withstand gusts of up to 200 mph. They’re gathering data that could help design stronger buildings by learning what really happens at the ground-level from the winds of powerful hurricanes.
They’re trying to connect the phenomena familiar to scientists, such as developments in the eyewall, with winds at the ground level, where roofs get ripped off and windows blown out. The work began after a series of storms including Hurricane Andrew exposed the weakness of many current construction standards.
“It was a wake-up call for the structural engineering community and forced us to reevaluate how we design buildings,” said Forrest Masters, associate dean for research and facilities at UF’s Herbert Wertheim College of Engineering.
They may get a lot of chances to gather information this year. An above-average hurricane season is predicted, with up to 10 hurricanes, according to a forecast released by the National Oceanic and Atmospheric Administration. An outlook by Colorado State University assesses the likelihood of a landfall by a major hurricane in the United States this year at 69%.