Science News - June 2, 2012

Danger scale the saffir-simpson scale ranks hurricanes based on their sustained wind speed and also provides an estimate of the type of damage expected from the storm.

Category Wind speed (km/h) Damage at landfall

1 119–153 some damage to roofs and trees

2 154–177 Major home damage, uprooted trees

3 178–208 Major damage, weeklong power outages

4 209–251 severe damage, monthlong isolation

5 252+ total roof failure and wall collapse source: national hurricane center/national weather service

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 mois ture on its journey westward across the Atlantic. “Conditions in here are favor able 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 tropi cal storm status (with winds of 63 kilo meters 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 intensi
fication 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 typi cal hurricanehunter airplanes as well as unmanned Global Hawk aircraft, the first time drones had been used for hur ricane science.

The biggest success: tracking Hur ricane Karl for more than a week, with more than 20 flights capturing its evo lution. Karl took many days to develop from a strong lowpressure system, and scientists don’t understand why it took so long. Then Karl weakened while cross ing the Yucatán Peninsula, and intensi fied to Category 3 in the Gulf of Mexico before making its second landfall.

Using a radiationmeasuring device on board a Global Hawk, GRIP research ers got data every halfhour 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 hurri cane is about to get more powerful.

For Karl, the spot started out around
3 degrees Celsius warmer than the sur
rounding 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 moun
tains of central Mexico.

For the first time in 2010, an unmanned Global Hawk was used for hurricane science. The Hawks fly far above a storm (sample path shown) to collect information on storm intensity and evolution.