NASA Loosens GRIP on Atlantic Hurricane Season
October 06, 2010
NASA wrapped up one of its largest hurricane research efforts ever last
week after nearly two months of flights that broke new ground in the
study of tropical cyclones and delivered data that scientists will be
able to analyze for years to come.
While the 2010 hurricane season has been a rather quiet one for
coastal dwellers, the churning meteorology of the Atlantic Ocean and
Caribbean Sea seemed to cooperate well with the science goals of the
Genesis and Rapid Intensification Processes (GRIP) experiment. Those
goals were designed to answer some of the most fundamental yet still
unanswered questions of hurricane science: What ultimately causes
hurricanes to form? Why do some tropical depressions become strong
hurricanes, while others dissipate? What causes the rapid strengthening
often seen in hurricanes?
Mission scientists, including personnel from NASA's Jet Propulsion
Laboratory, Pasadena, Calif., which had two instruments flying in the
campaign, wanted to capture data on hurricanes as they formed and
intensified. Ideally, the NASA planes – the manned DC-8 and
WB-57, and the remotely piloted Global Hawk – would also fly over
systems that were weakening, or that were expected to form into
hurricanes yet did not. When the flights had ended, all those goals had
been met.
"It was successful beyond my reasonable expectations. It requires
cooperation with the weather and good luck with the aircraft," said
Mission Scientist Ed Zipser of the University of Utah, Salt Lake City.
"It's not so much a logistical challenge as it is a toss of the dice by
Mother Nature during our time available. But it takes a good airplane,
a skillful crew and good luck with the equipment."
Flying to Hurricanes
Hurricanes Earl and Karl each became important objects of
observation for scientists during GRIP. The DC-8 flew to Earl four
times, criss-crossing the storm as it intensified to a category 4
hurricane and then weakened. On the final Earl flight, as the storm was
breaking down and losing strength, the Global Hawk made its debut
hurricane flight and passed over Earl's eye in concert with the DC-8,
providing valuable comparison measurements for the instruments on board
both aircraft. The WB-57 flew over Earl as well as Karl.
At the outset, scientists hoped that several aspects of GRIP would
help gather important data as well as complete a couple of technical
accomplishments. First, collaboration with the Air Force, NOAA and the
National Science Foundation would allow scientists to observe a single
storm system with as many as six aircraft. Second, GRIP featured the
debut of NASA's Global Hawk drone in a hurricane research capacity. The
unmanned plane's 24-hour flight range gave scientists the ability to
observe a hurricane directly as it changed over time and distance, in a
way that conventional planes and satellites have not done before.
Both of these aspects of GRIP were used to great effect during the
two major hurricanes observed during the campaign, Earl and Karl.
"We're all very pleased we were able to get the Global Hawk over a
hurricane," said Mission Scientist Gerry Heymsfield of NASA's Goddard
Space Flight Center, Greenbelt, Md. "There was a question about that.
That's a major accomplishment both on the science side and the
capability side. It really paves the way for future research."
As the campaign went on, Global Hawk pilots, based remotely at
Dryden Flight Research Center, near Palmdale, Calif., grew more
comfortable with the drone's capability at 18,288 kilometers (60,000
feet) and over a hurricane. On Sept. 16 and 17, the Global Hawk made a
25-hour flight that included 20 passes over the eye of Karl as it was
evolving into a hurricane – precisely the type of formation and
storm development that scientists hoped to capture during GRIP.
"None of our other planes can do that," said GRIP Project Manager
Marilyn Vasques of NASA's Ames Research Center, Moffett Field, Calif.
"They've been learning the capabilities of this aircraft at every
flight."
On that same flight, the collaboration with the other agencies
reached full steam, as six aircraft flew over Karl. The DC-8 was even
able to follow the storm after it made landfall in Mexico and began to
deteriorate. It is unusual to get the clearances to fly over a
hurricane once it has reached land, making the scientific payoff all
the more valuable. "We were able to capture some rare detail once it
made landfall," Zipser said.
What's in the data?
For all the logistics involved in coordinating flights and using a
drone designed for military purposes in a scientific campaign, the
chief purpose of the experiment remained getting good data. The
instruments on board the GRIP planes provided 3-D observations of storm
cloud and precipitation structures, measurements of wind speed in the
horizontal and the vertical dimensions, data on lightning strikes, and
lidar measurements of clouds and aerosols in and around hurricanes.
These are all in addition to the basic yet important measurements of
factors such as humidity, pressure and temperature that provide context
for more advanced observations.
JPL's two GRIP instruments were the High-Altitude Monolithic
Microwave Integrated Circuit Sounding Radiometer, or HAMSR; and the
Airborne Precipitation Radar (APR-2). HAMSR, which flew aboard the
Global Hawk uninhabited aerial vehicle, is used to infer the 3-D
distribution of temperature, water vapor and cloud liquid water in the
atmosphere. The dual-frequency APR-2 weather radar, which flew aboard
the DC-8, is used to help scientists understand the processes at work
in hurricanes by looking at the vertical structure of the storms.
While scientists will mine the GRIP observations for months and years,
the team knows now that it was mostly able to fly over the types of
storms and conditions that it wanted to fly over. Both Earl and Karl
provided strong examples of rapid intensification. The Global Hawk
arrived over Karl shortly after it reached hurricane status, and
continued to fly over it as it rapidly strengthened to a Category 3
storm in the next nine to 12 hours. The flights over Karl could provide
great insight into the genesis of that system, and the reasons for its
rapid intensification soon after it passed over the Yucatan Peninsula
and into the Gulf of Mexico.
HAMSR Principal Investigator Bjorn Lambrigtsen of JPL said HAMSR
turned out to be the best tool for determining the position of Karl's
eye. "The Global Hawk was able to fly over the eye 20 times in a row
over a 13-hour period," Lambrigtsen said. "HAMSR provided quick-look
data in real time, and the mission science team was able to use those
images to get an exact fix on the location of the eye as the storm
evolved, and the result was a very large number of exact hits
[bulls-eyes]." A comparison with fixes provided by the U.S. Air Force,
whose objective during the flights was to determine that information,
showed that the HAMSR-based locations were accurate to within a mile or
so.
"The number of passes over the eye and the nearly complete coverage
for such an extended period of time is unprecedented," Lambrigtsen
continued. "It has provided an extremely rich data set that will be
used to study the evolution of a tropical storm that re-forms after
passing over land – something we have not previously been able to
study – and the rapid strengthening of Karl as it approached the
Mexican mainland."
JPL's APR-2 instrument was able to show Karl's precipitation
structure as it changed from a disorganized disturbance to a tropical
storm. "APR-2 measurements of the horizontal winds were among the first
data to show the center of the newly formed storm before its first
landfall on the Yucatan peninsula," said APR-2 Principal Investigator
Steve Durden of JPL. "Wind patterns that were somewhat similar, but
less clear, were observed in a disturbance the following week, just
prior to its becoming Tropical Storm Matthew. We believe that these
data should provide insight into how storms form, one of the key goals
of the GRIP experiment."
"The flights into Karl as soon as it
emerged over the Gulf and became a hurricane gave us just a fantastic
example, and that was the day the Global Hawk did 20 passes over the
eye," Zipser said. The GRIP planes were also able to fly to tropical
systems – such as Gaston – that were forecast to strengthen
and become hurricanes but ultimately did not. In the quest to
understand why some tropical depressions become hurricanes and others
don't, these were also important flights.
The system known as Gaston formed out of an African easterly wave
– one of a number of depressions that routinely form off the
coast of Africa and often become hurricanes. It was forecast to become
a full-fledged hurricane but it did not. "It had all the elements to
become a storm, so scientifically that's very interesting," Vasques
said.
Wrapping up
With the flight portion of the campaign ended, scientists will soon
move on to analyzing the data they've gathered, Heymsfield said.
Scientists who worked on GRIP and many others will likely mine this
cache for years to come. These observations could provide insights with
great value to science, which is still trying to fully understand
hurricanes, and to society, which could eventually benefit from more
accurate forecasts of storm strength and development.
"There will be some quick results. But the rest of it, in my
experience, really takes years," Heymsfield said. "This is probably the
most timely, coupled experiment that I've ever seen. It's really worked
out well. We collected a wealth of data that the scientists are really
excited to analyze."
