After Charley, a better crystal ball
It sat silently off the southwest coast of Florida last Friday - a vast pool of unusually warm seawater as well suited to a salt-water spa as to the open subtropical ocean. Meanwhile, high above, a filament of high-speed air had peeled away from its mother flow, the jet stream.
For hurricane Charley, heading north after clearing Cuba, meeting this patch of ocean was like turbocharging a locomotive. Fueled by the unusually warm water below and the right wind environment aloft, Charley stunned forecasters as it grew from a worrisome storm to a major hurricane shortly before landfall, packing sustained winds of more than 145 miles an hour.
"It was like a runaway train," says Peter Black, a scientist at the Atlantic Oceanographic and Meteorological Laboratory in Miami, part of the National Oceanic and Atmospheric Administration (NOAA). "It was our worst nightmare."
Charley's sudden growth spurt represents one of the most challenging aspects of hurricane forecasting. Yet atmospheric scientists say they are hopeful that over the next few years, they will be able to provide forecasters with the tools to significantly improve forecasts of hurricane intensity. New forecast models are being assembled and tested. New sensors, satellites, and field studies are being planned. And recent field and lab studies are yielding fresh insights into the conditions that give tropical cyclones more kick.
It's a long, grinding process that moves only as fast as budgets and storm frequency allow. Still, "within the next five years it will be possible to do 20 to 25 percent better than we're doing now" just by making better use of available reconnaissance data gathered from storms, says Mark DeMaria, a NOAA researcher who developed the rudimentary tools for forecasting intensity that the National Hurricane Center now uses. Over the next decade, the highly sophisticated computer models under development should boost improvements further, he says.
What makes the intensity problem so fiendishly difficult to solve? "It's far more complex" than track forecasting, acknowledges Naomi Surgi, who heads the hurricane forecasting modernization effort at the National Centers for Environmental Prediction (NCEP) in Camp Springs, Md. Scientists must identify and understand how large-scale factors, such as regional atmospheric circulation patterns, affect tropical cyclones. They also have to focus on processes within hurricanes that can occur on scales as small as a few tens of meters across. It's all about heat moving from the ocean into and through the storm and the factors that can affect the storm's momentum.
Take the boundary between air and ocean, for example. Existing forecast models assume that when a hurricane grows stronger and its wind speeds increase, the wind kicks up ever-larger waves. As the waves grow, friction increases between wind and wave, slowing the pace at which a storm strengthens.
Studies published over the past year, however, suggest that this assumption is wrong. The ocean's drag on the wind grows until the winds hit hurricane force, says Isaac Ginis, a professor of oceanography at the University of Rhode Island's Graduate School of Oceanography in Narragansett, R.I. Then the amount of drag levels off and in some cases declines, allowing the wind to intensify to whatever speeds other key conditions permit.
The mechanism affecting drag "remains an open question," Dr. Ginis says. Some researchers suggest that once winds hit about 78 miles an hour, they blast the tops off waves, sculpting a smoother sea surface and "lubricating" it with spray and foam. Ginis and others hold that the winds are moving so fast that the air has come - and gone - before it has a chance to "feel" the waves' drag. It's a bit like moving one's finger quickly through a candle flame and not getting burned.
Then there's the computational challenge. Existing forecast models are unable to simulate processes on such small scales. Nor can existing models absorb the full range of data available without spitting nonsense back out; the data are too detailed for the relatively simple equations existing models use.
This gap has led to an effort between NCEP, the military, and the National Center for Atmospheric Research in Boulder, Colo., to build a new forecasting model. Dubbed the Weather Research and Forecasting Model, it will be able to simulate processes on scales as small as 1 kilometer versus today's best operational scale of about 18 kilometers. It's currently being tested, running experimental forecasts alongside the models providing operational forecasts. When it's ready for prime time, hurricane forecasters will get their own custom version of the model.
Beyond improved models, researchers also are planning more-ambitious field campaigns to gather information on a hurricane's complete life cycle. So far the lion's share of studies has focused on mature hurricanes. But according to Dr. Surgi, next year NOAA's hurricane-hunter aircraft will take measurements of storms and their ocean and atmospheric environments from the storms' earliest stages.
Nevertheless, additional steps could be taken now to improve today's intensity forecasts, notes Kerry Emanuel, an atmospheric scientist at the Massachusetts Institute of Technology in Cambridge who specializes in tropical climate and meteorology. The key, he says, is a well-known but underutilized type of sensor that can be dropped from hurricane reconnaissance planes.
The sensors measure ocean temperatures to depths of roughly 1,000 meters. Research has shown how hurricanes, passing over unusually deep pools of warm water, can dramatically strengthen. The storms' fury churns up warm water from the depths, rather than the usual, storm-stifling cold water. Given the improved track forecasts, he says, hurricane hunters could drop these sensors along the project path of a hurricane to see if warm pools lie in its path.
Such sensors are not "silver bullets," but they could provide additional help in assessing whether future storms might suddenly intensify.