Sandy holds lessons for forecasters
She was born, like all hurricanes, as a faintly inauspicious stirring of winds. But she didn’t come from off the coast of Africa, as many tropical Atlantic storms do. She was a child of the Caribbean.
On the evening of October 19, a trough of low air pressure drifted slowly in the Caribbean Sea, east of Costa Rica and south of Haiti. The U.S. National Hurricane Center gave this tropical wave only a 20 percent chance of strengthening into a named storm.
But the surrounding atmosphere was full of water vapor, possibly thanks to the recently departed Hurricane Rafael. Weak winds on either side of the trough began slowly to pull it into a counterclockwise rotation.
Over the next few days, thunderstorms sucked up heat energy from the warm Caribbean waters and became better organized. Surface air pressures began to drop. The storm coalesced. On October 22, Tropical Depression Eighteen was officially born.
Soon the storm gathered enough strength to become Tropical Storm and then Hurricane Sandy. It shuffled north through Jamaica, Cuba and the Bahamas. Its heavy rains left at least 69 dead across the Caribbean — mostly in Haiti, which it did not even hit directly.
Then arrived a combination of meteorological factors popularly known as the perfect storm, a set of conditions guaranteed to deliver devastation to the U.S. Northeast. Down from the north and west came a low-pressure system that dug in its heels farther south across the continent than such systems normally do. Out to the east, over the Atlantic, a ridge of high pressure nestled down. Between them, these two systems blocked the usual west-to-east flow of the jet stream.
Up from the south came Sandy. The jet stream could not push her out harmlessly to the east as it normally would. Instead, the ridge system merged with tropical Sandy to create a rare hybrid storm.
“One gave you the circulation, and the other gave you a lot more warm ocean water,” says Shuyi Chen, a hurricane modeler at the University of Miami. “To me the interesting thing is all the multiple players that are involved and the odds that are required for them all to be in the same place at the same time.”
That, in a nutshell, is the lesson of Sandy: She showed how rare meteorological conditions can combine just so to generate a once-in-a-lifetime superstorm. To better predict any future Sandys, scientists are working to understand exactly how she was born, grew and died.
Tropical storms often develop in the Caribbean in October, Chen says, but most fizzle out long before making landfall. To probe why Sandy took a different path, researchers have run sophisticated computer simulations and even taken to the skies themselves.
Some computer models fared better than others. The European Center for Medium-Range Weather Forecasts predicted Sandy’s sharp westward turn days before the U.S. equivalent did. The European model uses higher-resolution data, farther in advance, than the leading American model does.
To help improve such models, “hurricane hunter” teams flew planes into the developing storm. A group from the National Oceanic and Atmospheric Administration uses turboprops to collect Doppler radar data on a storm’s structure. “It’s kind of like a CT scan of the inside of the storm,” says Robert Rogers, a meteorologist with NOAA’s hurricane research division in Miami.
Seven turboprop flights into Sandy showed how the storm weakened at first as it ran into wind shear — differing wind speeds at different altitudes — as well as dry air flowing off the continent and from behind the blocking ridge. But then Sandy passed over the warm waters of the Gulf Stream and got another boost as she interacted with the blocking ridge, which seems to have forced her rotating winds back into vertical alignment, Rogers says.
Once Sandy moved a bit farther north, NOAA sent out a Gulfstream-IV jet to explore the merger with the blocking ridge. Instead of drawing energy from warm sea surface temperatures as hurricanes do, Sandy took on characteristics of what’s called an extratropical cyclone — powered by temperature contrasts in the atmosphere. Scientists on the Gulfstream flights measured a jet of strong winds on Sandy’s south side that is often seen in these cyclones as they get stronger, Rogers says.
Overhead, satellites were gathering their own views of the storm’s astounding power. NASA’s Tropical Rainfall Measurement Mission spotted Sandy dumping more than 2 inches of rain an hour into the waters off Florida. Satellite images also showed how the storm’s eyewall — the bands of clouds immediately surrounding its eye — recovered after partially breaking apart north of Cuba.
“Sandy just didn’t have the energy to support a very strong storm in its inner core,” Chen says. Because of that, Sandy never got above Category 2, the second-lowest hurricane classification possible based on the storm’s wind speed.
Even so, the records she set are astonishing. When Sandy lurched ashore near Atlantic City, N.J., on the evening of October 29, the barometric pressure was 946 millibars, tying the 1938 Long Island hurricane for lowest recorded landfall pressure north of Cape Hatteras, N.C. The storm engulfed nearly the entire eastern third of the United States, some 1.8 million square miles. (Though neither is a global record: 1979’s Super Typhoon Tip recorded pressures of 870 millibars as it spun in the Pacific Ocean. Tip could have covered nearly the entire western half of the United States.)
Whether Sandy is a harbinger of future storms remains to be seen. Climate scientists have calculated that globally rising temperatures could bring more intense Atlantic hurricanes in the future, and one recent paper in the Proceedings of the National Academy of Sciences uses storm-surge records along the Atlantic coast to argue that large surges have become more common since 1923. Other work has suggested that losing Arctic sea ice, which has been declining in recent decades, could lead to the presence of more blocking ridges in the Atlantic like the one that steered Sandy into New Jersey. But the link between climate change and hurricanes remains murky, and the link between climate change and any individual storm is impossible to draw.
For now, as the East Coast mops up from this legendary superstorm, scientists are focusing on how Sandy might help them improve future hurricane predictions. One big lesson may be that researchers need to look at the broader environments in which hurricanes form, Chen says. “If you can’t forecast the very broad area — the trough to the west, the high pressure to the east, and Sandy itself,” she says, “you can’t really get the whole picture right.”
National Hurricane Center [Go to]