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Storm Front

Hurricane experts push to improve intensity forecasts

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10:38am, May 18, 2012
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Anyone waiting for Hurricane Irene on North Carolina’s coast last August might have been a little disappointed. As the storm barreled toward the Outer Banks, parka-clad TV meteorologists lined the beaches in anticipation. But instead of grinding ashore as powerfully as expected, Irene wimped out, hitting land with wind speeds about 10 percent weaker than predicted.

Just as easily, hurricanes can do the opposite, strengthening when they’re not expected to. Take Charley, which jumped two categories on the hurricane scale in five hours before slamming into Florida in 2004. Or 2007’s Felix, which intensified quickly into a Category 5 storm, the highest possible, before devastating much of Nicaragua.

Why some storms spin up with deadly force and others putter along, or even weaken, remains something of a scientific mystery. And so hurricane forecasters have made this problem a top priority for the next decade.

Their effort got a big shot of science in 2010, when three research groups flew planes into a series of Atlantic storms as they grew from tropical depressions to tropical storms and on to full-fledged hurricanes. Findings from the flights, just now being analyzed and reported at scientific conferences, suggest new ways that forecasters might finally conquer the challenge of understanding what makes hurricanes rev up.

After looking at the embryonic beginnings of tropical depressions, one team thinks that hurricanes may get their start from pouches of moist air whose ability to stay intact allows them to intensify into stronger storms. Another group has found, at least in the case of 2010’s Hurricane Karl, that a strange warm spot at a hurricane’s center may help it strengthen. Meanwhile, hurricane hunters have begun comparing storms that intensify quickly with others that don’t, finding that the way winds and rainbands move may account for some of the difference.

Soon, scientists hope, the research will help them more accurately predict what coastal residents should expect. During this year’s Atlantic hurricane season, beginning June 1, forecasters will be testing a new approach fine-tuned by the last few years of discovery.

“This is a huge deal,” says Frank Marks, head of the National Oceanic and Atmospheric Administration’s hurricane research division in Miami. “In the next couple of years we’re going to see rapid increases in our ability to forecast peak wind. That’s the way we’re going.”

Just in time, some say, to better understand how hurricane risks may change as rising global temperatures heat the oceans and the atmosphere.

PREDICTions

The basic physics of how hurricanes form is deceptively simple. Thunderstorms over the tropical ocean begin to organize themselves, with water vapor condensing to form rain. Warm air begins to rise, creating more condensation and a feedback loop in which the storm’s center warms and an area of low pressure develops. Eventually the hurricane becomes a monstrous swirling storm with rainbands stretching hundreds of kilometers across. But exactly what happens during that early heating and condensation can vary dramatically from storm to storm — with very different consequences for what comes next.

Knowing which storms will strengthen dramatically requires understanding processes on many scales, from individual clouds to mammoth thunderstorm complexes. It’s a lot harder than predicting where a particular storm will head, which is driven mainly by steering currents in the atmosphere such as the jet stream.

Imagine trying to figure out how a rubber ducky will move across a bathtub when pushed, says Edward Zipser, a meteorologist at the University of Utah in Salt Lake City. “If you know which way you’re pushing and how hard you’re pushing, you have a pretty good idea of where that duck will be in another three to five seconds,” he says. But imagine trying to figure out how the duck is spinning throughout the journey, especially if the duck also has an internal motor whirling it around like a top. “Events on different scales of motions and dimensions affect the intensity in very complex ways,” Zipser says.

With nearly 100 million Americans living within 50 miles of a coastline, NOAA wants to solve the riddle of hurricane intensification sooner rather than later. The agency has set specific goals to reduce errors in its seven-day forecasts (by 20 percent by 2014, and 50 percent by 2019) of both where a storm goes and how intense it will be at any given point along that path. Intensity is what drives the category rating, and it’s determined based on the highest wind speed sustained for one minute anywhere within a storm at a height of 10 meters above the water. To reach Category 1 status, the sustained speed has to be 119 kilometers per hour, and Category 5 winds exceed 252 kilometers per hour.

There is, of course, no average hurricane, and forecasters at Miami’s National Hurricane Center do well with some storms and poorly with others. “Maybe the better way to state the goal is to reduce the times that we get caught with our pants down,” says Zipser.

One way to keep their pants up as often as possible is to gather data on individual storms to see how each develops within specific environmental conditions. Hurricane hunters with the U.S. Air Force have been flying into Atlantic storms since the 1940s, on planes laden with instruments to measure factors such as wind speed, temperature and humidity. NOAA started flying a decade later. As technologies improved over the decades, scientists began tackling such questions as hurricane intensification (SN: 6/23/07, p. 392).

But flying the occasional reconnaissance into a single hurricane provides only a snapshot of its evolution in time, rather than a high-definition movie of its birth, life and death. Thus the unprecedented 2010 push, in which three research agencies conquered the logistics of flying multiple planes from multiple locations into multiple storms.

One experiment run by the National Science Foundation, called PREDICT, targeted storms in their earliest stages. By flying out of St. Croix in the Virgin Islands, the PREDICT team could travel across much of the Atlantic and capture tropical disturbances forming off Africa’s coast.

The idea was to test the charmingly named “marsupial paradigm” about how hurricanes are born. This theory holds that tropical disturbances sometimes form a small pouch where the air is more or less stationary. Like a kangaroo pouch that protects a baby from the elements, this pouch isolates and protects moisture on its journey westward across the Atlantic. “Conditions in here are favorable for thunderstorms to keep firing day after day,” says Christopher Davis, a team member at the National Center for Atmospheric Research in Boulder, Colo. “This isn’t sufficient to get a tropical storm, but it makes it a lot more likely.”

What exactly happens to the pouch can also drive what happens to storms later. PREDICT scientists, for instance, watched a vigorous tropical depression with all the hallmarks of a storm that would intensify. It did make it to tropical storm status (with winds of 63 kilometers per hour or greater), receiving the name Gaston. It looked like it would keep getting stronger.

But then Gaston fizzled. “You could see it unraveling,” says Davis. Part of the reason may be that Gaston’s central vortex became misaligned, shearing sideways at higher elevations instead of maintaining a straight columnar center. Dry air could then penetrate the vortex, interrupting the flow of moist air needed to fuel the storm further, Davis and colleague David Ahijevych wrote in April in the Journal of the Atmospheric Sciences. So one prerequisite for intensification may be a storm’s ability to hold its center together.

Hawk’s-eye view

As 2010’s hurricanes got closer to the Atlantic coast, a second group organized by NASA joined the fray. This team, named GRIP for Genesis and Rapid Intensification Processes, flew the typical hurricane-hunter airplanes as well as unmanned Global Hawk aircraft, the first time drones had been used for hurricane science.

The biggest success: tracking Hurricane Karl for more than a week, with more than 20 flights capturing its evolution. Karl took many days to develop from a strong low-pressure system, and scientists don’t understand why it took so long. Then Karl weakened while crossing the Yucatán Peninsula, and intensified to Category 3 in the Gulf of Mexico before making its second landfall.

Using a radiation-measuring device on board a Global Hawk, GRIP researchers got data every half-hour for 10 hours directly over Karl’s eye. The data showed details unlike any seen before of a warm spot in the upper atmosphere inside Karl. Similar warm spots have been detected in other storms right as they intensify, and may signal that a hurricane is about to get more powerful.

For Karl, the spot started out around 3 degrees Celsius warmer than the surrounding environment, then warmed about another 3 degrees as the storm spun up over the Gulf of Mexico, says meteorologist Shannon Brown of the Jet Propulsion Laboratory in Pasadena, Calif. As temperatures increased, broad swaths of clouds began to develop a sharply defined center, creating the eye. After flooding many parts of Veracruz, Karl eventually died out over the mountains of central Mexico.

Sometimes a storm’s speed-up happens very quickly, like gaining several categories in less than 24 hours. If it’s close to landfall at that point, forecasters can be caught off guard. “That’s kind of the nightmare scenario,” says Robert Rogers, a hurricane researcher at the Miami center.

Rogers is involved in the third and usually annual project, NOAA’s Intensity Forecasting Experiment, which since 2005 has been flying P-3 turboprops and occasionally a Gulfstream-IV jet into hurricanes approaching the U.S. coast. The jet flies in a pattern around the outside of the storm, to gather data on the environment surrounding a hurricane. The turboprops fly through the eye of the storm. Among many other instruments, they carry Doppler radar in their tails. The radar is the sort that monitors thunderstorms on your local television station, allowing scientists to build a three-dimensional picture of how winds and rain are moving in the storm.

In 2010, Hurricane Earl revved up quickly to Category 4 off the U.S. East Coast. “We had an aircraft in there almost continuously,” Rogers says. The storm was a classic case of “rapid intensification,” in which maximum winds increase by at least 46 kilometers per hour over 24 hours. Rapid intensification is fairly rare, but nearly every storm that gets to Category 4 or 5 goes through this phase at some point in its history. “You don’t just get something building up slow and steadily,” says Rogers.

With some 15 years of detailed radar observations in hand, Rogers is now trying to draw broader conclusions about storm behavior from how individual hurricanes act. For instance, he is comparing 14 flights into storms that went through rapid intensification, including Earl, with 14 flights into storms that didn’t. So far, he’s seeing differences in factors such as the range of winds around the storm, how those winds flow into the center at different heights above the sea surface and how the strongest thunderstorm activity is arranged around the hurricane.

Exactly how these differences translate into being able to forecast intensity better isn’t clear yet, but “now we’re starting to get some good information out of the data,” says Rogers. He reported his findings in April at a tropical meteorology conference in Ponte Vedra Beach, Fla.

Real-time results

After all the excitement of 2010, the following year saw more Atlantic storms than usual, but Irene was the only one to cause major damage. Now scientists are preparing for what 2012 might bring.

Along with NOAA’s usual flights this summer, NASA will also be busy testing its Global Hawks to see if they are a useful — if expensive — tool to add to the hurricane-hunting repertoire. The agency will have two drones based at its Wallops Flight Facility in eastern Virginia. One will fly over a hurricane’s surrounding environment, while the other will fly over the storm’s inner region. Unlike the manned NOAA P-3 flights, which enter hurricanes at altitudes of up to 8 kilometers, the more fragile Global Hawks fly far overhead, some 19 kilometers above the sea’s surface.

Global Hawks offer a key advantage in that they can stay in the air for up to 28 hours, says project leader Scott Braun of NASA’s Goddard Space Flight Center in Greenbelt, Md. A typical manned hurricane-hunter flight can spend only around six to eight hours flying in a storm before it has to return for refueling and to change out crews. The drone’s extra hours allow more continuous monitoring. “To really try to understand what’s happening in a storm, you can’t just go look at it intensely for six hours and then leave it alone for 20,” Braun says. “It might change significantly in the meantime.”

All these data have been helping Marks and his colleagues develop a better approach that hurricane forecasters plan to use in real time this summer. The new method for predicting hurricane path and intensity builds on years of tweaking computer codes to better simulate how hurricanes progress. This year, for the first time, NOAA forecasters will be running this experimental approach alongside their old one, to see which might give them more accurate information about a storm.

The technique will embed a detailed computer simulation, at a resolution of just 3 kilometers, inside the coarser-resolution one used until now. Marks’ team recently did a three-year retrospective run, plugging in data on how and when Atlantic hurricanes started and seeing how well the simulation reproduced their tracks and intensity. “In our vernacular, it kicked butt,” Marks says.

In the long term, forecasters need to better understand intensification to better prepare for the outcomes of climate change. In theory, warmer sea surface temperatures provide more fuel for hurricanes to start and feed off of.

Scientists say it’s too early to know whether rising temperatures of the last few decades have already affected hurricane activity in the Atlantic. But if the climate warms as much as expected by the end of this century, hurricanes could increase globally in strength by 2 to 11 percent, according to one middle-of-the-road projection from the Intergovernmental Panel on Climate Change. 

Forecasters can expect an intense time ahead. 


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