Hurricanes are loath to give up their secrets. Cloaked in thick rain that scatters light, the storms’ interiors defy the gaze of weather satellites. For decades, one of the only ways to gauge their winds has been with aircraft—and pilots—brave enough to plunge inside.
A new NASA mission, the Cyclone Global Navigation Satellite System (CYGNSS), is poised to lift that rainy veil, with eight identical microsatellites set to launch on 12 December from Cape Canaveral, Florida. If successful, the mission could improve hurricane wind speed forecasts and help explain how certain clouds and storm systems take shape.
Until now, the United States has relied on its "hurricane hunter" aircraft to measure storm winds. But the limited range of these planes means they cannot reach many hurricanes, especially as they are forming over the tropical Atlantic Ocean. And cyclones in the western Pacific Ocean, which differ from hurricanes in name only, rarely get monitored at all, says Sharan Majumdar, a member of CYGNSS's science team and a hurricane scientist at the University of Miami in Florida. "The vast majority of tropical cyclones are not benefiting from an aircraft flying into it," Majumdar says.
But now, nearly all hurricanes will be seen by CYGNSS, which measures the storms’ internal wind speeds with a tool common to any traveler—GPS. The long radio wavelengths of GPS signals, continuously emitted from a constellation of satellites in medium-Earth orbit, can pass through heavy rain—one reason your phone can easily locate you in a storm. By gathering up these GPS signals after they bounce off the ocean, CYGNSS can measure the surface roughness of the sea, which in turn can be used to calculate wind speeds. Typical weather satellites can’t do the job because they rely on microwave-based sensors, which get scrambled by rain.
It's a reliable method that the hurricane hunters have already verified. Although the CYGNSS satellites, in tropical low-Earth orbits, won’t be able to “see” the ocean roughness as clearly as the planes, the enhanced spatial coverage more than makes up for it, says Christopher Ruf, CYGNSS's principal investigator and an atmospheric scientist and engineer at the University of Michigan in Ann Arbor.
In recent decades, the accuracy of hurricane forecasting has improved markedly, especially when it comes to predicting the path of a storm. But predictions of intensity, the maximum sustained winds in a cyclone, have not kept pace. Winds are not uniform in a storm, and they can simply be missed during aircraft assessments. There is also considerable uncertainty about how these winds link with the warm ocean, which fuels storms. Based on analyses using simulated data, CYGNSS should tangibly improve intensity forecasts, says Frank Marks, who leads hurricane research at the Atlantic Oceanographic and Meteorological Laboratory, a National Oceanic and Atmospheric Administration (NOAA) lab in Miami.
Technically, CYGNSS is a research mission, not something that NOAA should depend on for its operational forecasts. It's slated to last 2 years, though its shortest-lived components should last 5 years, and the satellites themselves won't fall into the atmosphere for 7 to 9 years. If NOAA scientists see that the mission could improve real-time forecasts, they will find a way to tap into the findings, and get busy proposing a follow-up mission. "They can scramble and try to integrate it into operations if it will save lives," Ruf says.
A lot can go wrong with satellite missions, and CYGNSS will face hurdles in verifying the accuracy of its data, adds Michael Kozar, a hurricane modeler at Risk Management Solutions in Tallahassee. But if it pans out, he's interested not just in wind speeds from the hurricane's interior eyewall, but also those from its exterior, which airplanes can sometimes neglect. Getting a fuller picture will help measure not just a storm's highest winds, but the overall energy spread throughout its system, he says.
Beyond hurricane forecasts, CYGNSS could help measure wind speeds hidden beneath dense tropical rain clouds, Majumdar adds. That, in turn, could lead to a better understanding of how clouds form, and how rising temperatures may shift their abundance. Similarly, it could help scientists studying wind bursts in the western Pacific Ocean that are tied to the Madden-Julian Oscillation, a set of storms that periodically marches around the equator, influencing global weather patterns.
The $157 million mission also represents a few firsts for NASA: It is the first to come out of the agency's Earth Venture program, which targets low-cost, quickly built, and riskier missions using smaller satellites. And it is one of the first NASA earth science mission that will have its science operations run out of a university rather than a NASA center. The agency has put a lot of trust in them, Ruf says. "Hopefully we'll show that was a good idea when we get up there," he says.
For more coverage on hurricanes and other natural hazards, visit our topic page.
Correction, 3:21 p.m.: A previous version of this story stated that CYGNSS was the first NASA earth science mission to run its science operations out of a university, rather than a NASA center. The Solar Radiation and Climate Experiment, run out of the University of Colorado, Boulder, precedes it.