Global Hawks: Unmanned Aircraft in Hurricane Science
Our understanding of the formation and intensity of hurricanes is still evolving. Even after hurricanes have formed, it is difficult for scientists and meteorologists to predict the intensity of a storm when it makes landfall. Further investigation into the life processes of hurricanes will be essential to produce more reliable predictions of intensity when storms reach shore.
Hurricane science is usually limited by a several factors, and scientists who use instrumentation to study hurricanes must rely on a perfect storm of circumstances. Most essentially, the storm must be in the right place. If the scientist’s research station is land-based, the hurricane has to make landfall nearby. More commonly, the hurricane must be brewing within range of the scientist’s instrument-bearing aircraft, which usually only has the range to overfly the storm for a few hours at a time over the ocean. However, collaboration between NASA and Northrop Grumman has produced an aircraft that will make the study of hurricanes much easier.
Global Hawk is the name given to the unmanned aerial vehicle (UAV), and it has proven its utility to hurricane scientists in the 2010 Genesis and Rapid Intensification Process (GRIP) mission. GRIP, a project coordinated to complement field experiments by the National Science Foundation (NSF) and the National Oceanic and Atmospheric Administration (NOAA), employed two aircraft to collect hurricane data: the manned DC-8 and the unmanned Global Hawk. Because of the Global Hawk’s high-altitude capabilities and long flight time, scientists were able to collect data to create wind maps and study the evolution of the storm’s core.
The Global Hawk is well-suited to study hurricane science because of its high-altitude capabilities, long range, and long flight time. A typical hurricane tops out at around 55,000 feet, but a Global Hawk can exceed this altitude by almost 10,000 feet, enabling scientists to collect data from overhead. The aircraft can fly for about 26 hours at a time and has a range of 11,000 nautical miles, more than enough to reach the coast of Africa (where Atlantic hurricanes form) from the continental United States. The Global Hawk employs a satellite communication link to exchange information with ground control. Although the aircraft’s primary navigation is based on pre-programmed routes fed into its navigation system, scientists can request changes to the aircraft’s course based on real-time data about the storm from their instruments. This allows hurricane scientists to direct the plane to a more interesting section of the storm for data collection and can provide for increased safety, since pilots can avoid large thunderstorms within hurricanes that may pose a threat to the aircraft.
The Global Hawk can carry 1900 pounds of useful payload. For the GRIP mission, this included DropSondes (which collect vertical profiles of pressure, temperature, humidity, and winds), a Lightning Instrument Package (LIP) for the measurement of lightning and other electric fields, as well as two more complex instruments called HAMSR (High Altitude MMIC Sounding Radiometer) and HIWRAP (High Altitude Imaging Wind and Rain Airborne Profiler). HIWRAP Principal Investigator Gerry Heymsfield described the variety of instruments as “complementary,” since they all measure different parameters of the storm.
HAMSR is a microwave atmospheric sounder whose data is used to glean information about the warm core structure and precipitation structures of a storm. Developed by NASA’s Jet Propulsion Laboratory (JPL), HAMSR was one of the first graduates of the Earth Science Technology Office (ESTO) Instrument Incubator Program. It collects data by measuring thermal radiation from the atmosphere and the surface below it, yielding information on temperature, water vapor, and precipitation.
HIWRAP, also an ESTO-funded instrument, is a unique radar system used to measure wind within storms. HIWRAP sends microwave pulses into a storm and collects data on the backscattered energy and Doppler shift from clouds and precipitation in different ranges along the radar beam. These data are used to reconstruct the wind structure and determine the type, height, and amount of precipitation in a storm. HIWRAP uses a carefully planned scanning pattern to yield the best collection of information. The radar system transmits two beams simultaneously at different incidence angles, as illustrated in Figure 1.
The instrument rotates continuously, resulting in a conical scanning pattern. Conical-scan radar is a new addition to precipitation and cloud study—only used in the past to study wind patterns close to the ocean’s surface, rather than in the higher precipitation regions of a storm. Instruments on the Global Hawk are constrained by size and weight, so the scientists developing HIWRAP decided to use a smaller, lighter, solid-state transmitter rather than heavier, more bulky transmitters that are usually used in conventional weather radar.
The Global Hawk, HAMSR, and HIWRAP also are part of another NASA hurricane science mission, called the Hurricane and Severe Storm Sentinel project, or HS3. HS3 is an ongoing five-year mission to investigate the formative processes of hurricanes. HS3 seeks to collect data that will help scientists address the roles of the Saharan Air Layer and deep convection in the inner core in hurricane formation. The mission uses two Global Hawks with distinct payloads specialized for different types of data collection. One Global Hawk carries the “environmental payload,” with instruments geared toward collecting data on environmental factors that may contribute to the hurricane’s formation. The other Global Hawk carries an over-storm payload, and its instruments are key to investigating the inner core structure and processes. Both HIWRAP and HAMSR are part of the over-storm payload.
Heymsfield says the Global Hawk is revolutionary for hurricane science, citing the increase in flight times and its abilities compared to more traditional aircraft.
“[It] allows us to capture the development of the storm, whereas before, we maybe got a few snapshots of the storm … It really opens up a different way to look at any kind of phenomena.”
Heymsfield, who is involved with GRIP and HS3, predicts that aircraft with an endurance of up to five days will be ready within five years.
The data collected by Global Hawk from the GRIP and HS3 missions will help scientists further develop their models of hurricane intensification. As the volume of hurricane data grows, scientists will be able to better predict the strength and size of storms as they encounter the North American coastline. More accurate predictions mean better preparation and, surely, more lives saved.by