Hurricanes are the most violent, awesome storms on earth, according to the National Aeronautics and Space Administration (NASA), and are monitored carefully as they spin and slow-dance across the ocean, because they can be a dangerous risk to humans and property.
Yet the science of how the huge storms form and intensify in their warm, ocean spawning grounds is not well understood. Satellites can view them for short periods as they soar past high above the earth. Hurricane watcher planes can fly monitoring missions – but are limited in time and distance, and flights can be risky for their crews. NASA has turned to a high-altitude, unmanned aircraft to improve research capabilities – the Northrop Grumman Global Hawk, powered by a single Rolls-Royce AE 3007 turbofan engine.
Global Hawk, piloted by ground-based aviators, can stay aloft for up to 31 hours and fly up to 65,000 feet high, with a range of 11,000 miles. Equipped with high-tech sensors – including tiny, launchable dropsondes with transmitters and parachutes – the surveillance aircraft have been applied to storm chasing.
Scientists can monitor pressure, temperature, humidity, wind direction and speed, cloud and dust particles, and other conditions – and receive the data in near real-time, within as little as ten minutes.
Originally intended, and still primarily used, as an unarmed military surveillance aircraft, the Global Hawk system has grown its workload in recent years into humanitarian missions – from wildfire monitoring and disaster relief and now to hurricane study. It is the long endurance, high-altitude capability that has made the aircraft so well suited to the new task.
‘Global Hawk gives you the ability to get to storms that are far out in the eastern Atlantic, and still be out there for long periods of time. It gives us a lot more time on station than we could ever get with a manned aircraft. This year really showed that,’ says Dr Scott Braun, NASA’s principal investigator for the study programme, which is known as the Hurricane and Severe Storm Sentinel or HS3.
The HS3 programme includes two NASA Global Hawks that are based at Dryden Flight Research Center at Edwards Air Force Base in California. Some initial study missions were flown in 2010 from Edwards, requiring a long, cross-country flight before even getting to the east coast. This year, the programme placed portable control trailers at NASA’s Wallops Flight Facility on the eastern coast of Virginia to get the aircraft closer to the storms.
‘Flying from the east coast, you get to a storm pretty quickly and spend a lot more time over them. It is a big advantage,’ Braun says.
Manned hurricane flights can last up to ten hours, but with travel time to reach the storm, might provide only four to six hours of collected data. A Global Hawk can collect 16 to 20 hours of data – or even as much as ten hours’ worth for the most distant storms off the coast of Africa.
‘That wealth of data will prove invaluable in studying how hurricanes intensify,’ Braun says. Within a hurricane, there are continuous, evolving and chaotic interactions between large- and small-scale winds, and the clouds within the storm.
‘Studying those has been a really challenging problem because of the difficulties with getting the observations you need from within storms for a long period of time,’ he says. Most data has been collected from satellites – but the collection time is fleeting, ‘and it’s not enough to monitor these small-scale processes.’
Manned flights can stay near a hurricane longer – but still only for a short duration, ‘and you have to wait for a storm to come close.’
Now, he says, ‘instead of waiting for the storm to come to us, we can go wherever the storm is and stay there a long time.’
The first hurricane to be monitored this year was named Leslie in early September. Then along came Nadine, forming on 11 September – ‘in the middle of nowhere,’ off the west coast of Africa near Cape Verde.
Nadine intensified into a hurricane, then weakened due to colder waters and wind shears; it faded, and the National Hurricane Center in the US downgraded it and even stopped tracking it.
But the NASA Global Hawk was still monitoring the storm – and the data showed it was re-intensifying, with a well-defined warm core and tropical storm force winds. The NASA team shared its data and the hurricane centre reacted by reclassifying the storm back into a tropical storm and tracking it again. Nadine went on to intensify into a strong Category 1 hurricane, but eventually fizzled out with minimal impact.
In addition to the weather data collected during the flights, the programme learned many valuable lessons about operating the aircraft, according to the National Oceanic Atmospheric Administration’s (NOAA) Commander Philip Hall, NASA’s Global Hawk deputy project manager and a pilot for the aircraft.
‘A lot of people didn’t think you could fly an aeroplane like Global Hawk over a hurricane,’ Hall says. ‘The big benefit of this aeroplane is it goes above the weather.’ Being an unmanned aircraft also requires close co-ordination with the Federal Aviation Administration; flight plans filed in advance – challenging due to the unpredictable nature of storms; and a manned chase plane during take-off and landing operations. One flight issue the programme didn’t have to worry about – the Rolls-Royce AE 3007 engine in the aircraft, Hall says.
‘I think about it a lot – as a single engine aeroplane. Obviously having an extremely reliable engine is very important. We put a lot of faith in that technology.’ The NASA Global Hawk is operated as a partnership. Both NASA and Northrop Grumman contributed pilots, mission planners, maintenance and engineering support at both Wallops and NASA Dryden for the missions. By operating as an integrated team with NASA, the flight operations have been controlled from both locations during the flights.
‘Able to fly as high as 65,000 feet for periods up to 31 hours, Global Hawk provides the unique combination of high-altitude and long endurance performance capabilities that allow the science community to study and deepen our understanding of how hurricanes form and what processes control their intensity,’ says Fred Ricker, Vice President and Deputy General Manager for Advanced Programs and Technology for Northrop Grumman's Aerospace Systems sector.
‘Under our agreement, Global Hawk is able to meet many demanding payload and mission requirements, allowing it to host various instruments and sensors to conduct different science missions and research campaigns for both the NASA/NOAA science community and other Northrop Grumman customers,’ Ricker adds.