Hurricane Karl gave the GRIP team one of the best opportunities to study a tropical storm.
Hurricane Karl gave the GRIP team one of the best opportunities to study a tropical storm. On Sept. 16, all three NASA aircraft flew above Karl simultaneously as it approached the coast of Mexico. This image is a screen grab from GRIP’s Real-Time Mission Monitor. Image credit: NASA
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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."

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Written by Patrick Lynch and Alan Buis

Alan Buis 818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif.