Intensity Remains a Big Gap in Storm Science
September 06, 2011
By Curtis Morgan, The Miami Herald
It`s no solace but if forecasters had been right about Irene`s intensity, things could have been worse.
Irene struck North Carolina as an 85 mph Category 1 hurricane - two grades lower and more than 25 mph weaker than forecast - and the winds that raked the East weren`t nearly as strong as feared.
It wasn`t the first time the National Hurricane Center had missed the mark in trying to predict the power of tropical cyclones fueled by complex forces that can be hard to even measure.
"It`s our gap," acknowledged Bill Read, the director of the hurricane center in West Miami-Dade. "We have not improved on the skill of intensity forecasting pretty much in my career." He started with the National Weather Service in 1977.
A week after Irene, scientists can still only speculate why such a huge storm unexpectedly lost so much steam after slamming the Bahamas - though a phenomenon called an "eye wall replacement cycle" is one suspect.
Despite the over-estimation on Irene, however, there are signs the single biggest gap in forecast science may be narrowing. Some experimental computer models predicted Irene would weaken, despite the favorable environment it was entering. The models were enriched by Doppler radar aboard two research planes that provided ground-breaking images and data Read likened to a CAT scan of the hurricane`s guts.
James Franklin, chief of the center`s hurricane specialists, stressed the models will need to perform well over a string of storms before they`re formally added to the tool box. But he`s cautiously optimistic.
"This is the first thing that I have seen that really looks promising for being able to get to these structural change that would help us with intensity," said Franklin, who like Read has three decades of hurricane forecasting experience.
The hurricane center pretty much bulls-eyed Irene`s path once the hurricane cleared Hispaniola, opting to keep Florida out of watches and warnings despite a large and powerful storm whirling just miles off the coast. That call, which would have never happened in the 1990s, is a sign of increasing confidence in track forecasting advances that have cut error margins in half over a decade.
But the miss on intensity has brought grumbling of overhyping Irene, mainly in Northeastern cities fortunate enough to dodge heavy damage. In an otherwise succinct analysis, the New York Times wrote Irene had "shone an uncomfortable light" on hurricane scientists.
It was a curious turn of phrase because hurricane center directors, forecasters and researchers have been the ones training a spotlight on intensity problem, years before Irene loomed on the Big Apple`s radar. The center`s forecasters have long acknowledged struggles with intensity estimates - and emphasized the uncertainty in public warnings and news conferences.
Three years ago, the National Oceanic and Atmospheric Administration, working with NASA and the National Science Foundations, launched an ambitious decade-long research project with the official goal of reducing track and intensity errors by half. But the primary aim is to bring intensity forecasting up to the fast-improving standards of the track forecasts.
That`s a challenge. The atmospheric steering currents that guide hurricanes, Franklin said, are large and fairly easy things to observe like the "Bermuda High" or cold fronts that can measure hundreds or thousands of miles across.
But factors feeding intensity come on a smaller scale, more difficult to measure and so dense with detail that computers models choke on the data.
As Irene approached the Bahamas, both track and accuracy were on the mark but from there, models kept ramping up the storm as it moved into warm waters - overstating its future strength, according to an NHC analysis, by an average of 21 mph two days off and 32 mph five days out, worse than average forecasts.
As Irene approached the coast, forecasters repeatedly stressed that main dangers were going to be from storm surge and inland flooding, which proved to be spot on. But because evacuation zones are tied to intensity forecasts, the high-ball wind estimates also likely led to moving more people than needed, including in low-lying areas of New York City.
With lives at stake, forecasters always stress caution and urge emergency managers to plan for a storm one category up from the forecast, Franklin said.
"We would rather people evacuate than have them drown,`` he said.
In 2004, Hurricane Charley jumped two notches to a Cat 4 as it neared Southwest Florida. In 2007, Hurricane Humberto in a few hours went from depression to tropical storm to a 90-mph hurricane just before hitting the Texas coast - a rapid intensification at the doorstep of a populated area that forecasters most dread. Similar significant errors happen all the time, Franklin said, but at sea where the public isn`t so focused on the familiar Saffir-Simpson scale that lump storms in five categories.
Once a hurricane looms on the horizon, all the qualifiers that forecasters give about intensity uncertainty tend to get ignored, said Frank Marks, director of the hurricane research division at NOAA`s laboratory on Key Biscayne.
Irene seemed to defy some basic hurricane rules. With warm water and light wind shear in its path, there was time and fuel to strengthen. Instead, Franklin said, it began an eye wall replacement cycle that will typically temporarily weaken a hurricane before a new eye forms and contracts, rebuilding wind strength.
But Irene`s wall simply collapsed, turning the hurricane into weaker, but still formidable mess of rain bands. Other factors might have come into play - dry air or upwelling of deeper, cooler ocean water might have seeped into its core - but Franklin said forecasters won`t take a deep look until after what has been a busy season ends on Dec. 1.
Though hurricane science has greatly expanded in the last decades, there are still poorly understood processes - such as why some storms rapidly intensify and what leads to eye replacement cycles.
"We know it and we can see it and we can diagnose it but to forecast it is an entirely different thing," said Marks, who is leading the intensity improvement research. "There are just so many details in an eye wall replacement."
Because hurricanes can be so large, pinpointing and predicting a storm`s peak intensity poses numerous challenges.
The latest breakthroughs are the Doppler radars installed on two of NOAA`s P-3 Orion turbo-props and soon to be added to a Gulfstream IV jet. They`re delivering detailed images of storms and information into a massive computer bank NOAA built in Boulder, Colo., to crunch the flood of data.
Marks believes researchers are "no doubt on the way" to closing the yawning gaps in intensity forecast but said there will probably always be wiggle room. The single knot of wind speed, for instance, separating a tropical storm from a hurricane can`t even be measured accurately by many flight instruments.
And, as Irene proved, it doesn`t take a major hurricane to do major damage.
"I would argue that the public can`t tell the difference between 63 knots (the strongest tropical storm) and 70 knots," Marks said. "It really gets down to how thin you want to slice the onion."
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