Atmospheric scientists are equally interested in sampling near the equator, because that’s where greenhouse gases fountain up into the stratosphere before falling back toward the poles. For example, scientists would like to measure stratospheric water vapor—a greenhouse gas that is more insulating than carbon dioxide—to understand its effect on Earth’s surface climate.
But flying at high altitudes near the equator presents a risk: fuel freezing. “With our atmosphere, at 60,000 feet it’s actually colder at the equator than it is at the North Pole,” says Naftel. “The Air Force will fly around the equator for only about an hour, and now we have scientists that want to take eight or 10 hours of data there.” For such long-duration equatorial stays, NASA intends to take advantage of the Global Hawk’s heat exchange system, in which fuel is warmed through direct contact with the avionics equipment. The agency also hopes to utilize fuels that freeze at lower temperatures, says Naftel.
Ironically, one of the special requests scientists made during the Pacific mission was to have the high flier occasionally fly low. “We’re interested in other altitudes because some of the important things that we need to do are to take a cross-section of the atmosphere,” says Hall. “We’ll come down and loiter at a little lower altitude, maybe around 43,000 [feet] or so, and then go back up to altitude.”
The aircraft itself was also modified for the Pacific mission. While the Air Force had typically flown two sensing instruments, says Naftel, “we had 11 instruments on GloPac, and they were all over the place: some in the nose, some under the bottom, but some were on the back of the airplane, some on the sides of the airplane.” Northrop Grumman designed a modular honeycomb pallet system for two of AV-6’s payload areas that allowed instruments to be easily added or taken out and returned to the scientists for data analysis or repairs. Among the instruments flying on the demonstration mission: a cloud physics LIDAR (Light Detection And Ranging) instrument on the nose to profile dust and aerosols, ozone and water vapor samplers, and chromatographs for measuring greenhouse and ozone-depleting gases like nitrous oxide, methane, and CFCs (chlorofluorocarbon compounds).
For the 2010 hurricane surveillance mission, the Global Hawk carried four instruments—including a radar for profiling wind and rain, and a microwave radiometer for measuring temperature and moisture—and faced new challenges. Because Global Hawk’s endurance allows it such a long outbound leg, it faces a lengthy voyage home, with diminishing fuel and uncertain weather. (Flights to observe Atlantic Ocean hurricanes took about eight hours out, with eight on station, leaving eight to return.) But because the unmanned vehicle can fly only over sparsely populated areas, it has fewer landing options in emergencies. “When you’re coming back and facing weather diverts, you’re always wondering, ‘Do I got enough to make it home, or am I going to have to go into one of these emergency fields?’ ” says Miller.
Hurricane surveillance also involves less-than-friendly skies: turbulence, lightning, and precipitation, all of which the crew was eager to avoid. “If you look at that airplane, with its long wing, it’s designed for a flight envelope that does not have high amounts of turbulence,” says Hall. “So we’re not permitted to fly in moderate or severe turbulence.”
Hall, who was at the controls when the Global Hawk overflew its first full hurricane—Earl, off the South Carolina coast—says the experience was more dynamic than the GloPac demonstration mission: As the team assessed the storm’s severity, they called for more rapid changes. At first Hall avoided the hurricane’s eye, but after the team decided it was calm enough, he transected it, crossing the eye seven times on that mission; he flew near it another two. But Hall still had to keep alert for cloud tops that could provoke turbulence, as well as for lightning—a mission rule forbade coming within 20 miles of it. “It was a fun flight because it was busy and kept us challenged,” says Hall.
Global Hawk proved to be an excellent hurricane observer, says research meteorologist Gerry Heymsfield, one of three GRIP scientists. (On a later flight, over Hurricane Karl in the Gulf of Mexico, the Global Hawk made 20 passes, including 12 over the eye. By contrast, an ER-2 typically does about six passes.) He points out that duration is especially important in studying hurricanes, which can intensify rapidly within 12 to 24 hours. In the past, long-term storm coverage required multiple airplanes.
The hurricane observation mission, which also involved DC-8 and WB-57 aircraft, focused on gathering clues about how tropical storms originate, and why some blow into hurricanes while others fizzle out. The team investigated how temperature, wind, and moisture in the environment around storms affect their direction and intensification; the mechanisms that cause a cluster of clouds to start spinning; and the role of convection in intensifying storms. The mission also studied easterly waves, weather disturbances off the west African coast that trigger thunderstorms, which sometimes become hurricanes. “We’re trying to learn why some waves develop and some don’t, because they all look similar,” says Heymsfield.
NASA officials have a long list of what they’d like to do next with their Global Hawks. A portable ground station, which would allow a pilot to launch and land the aircraft in the East Coast while the payload operators remain at Dryden, is in the works for next year. Cutting out the cross-country transit would allow future missions to observe hurricanes that are farther away and for longer periods, which means earlier in their development. NASA is planning a Hurricane and Severe Storm Sentinel mission for 2012 that will fly two Global Hawks, one over the storm and another to study its environment.