GPS Tracks Wind in the Eye of A Storm
GPS is great for getting us to unknown destinations, and now scientists have figured out a new use for this multipurpose tool. When used in the eye of a hurricane, GPS receivers can collect valuable information about the storm’s behavior.
Hurricane Hunters are an elite crew of storm chasers from the Air Force and the National Oceanic and Atmospheric Administration who routinely fly into the fiercest storms to collect information. They collect barometric pressure, wind speed and other readings that allow forecasters to warn us about threatening hurricanes.
In recent years, scientists have rigged the Hurricane Hunters’s planes to carry GPS receivers that are able to track windspeed inside the storm in almost real time. The work was published in the journal Radio Science.
The best way to get wind measurements historically have been using dropsondes, which are tiny tracking devices attached to balloons. Hurricane Hunters usually drop these devices at a few places inside the weather system. The devices collect as much information as possible as they float toward the surface of the ocean. But these devices are expensive so the planes only drop a few per storm, which means the conditions of the hurricane are monitored intermittently.
In contrast, the GPS system works almost continuously as the plane flies through the storm, giving researchers a list of readings.
The planes are essentially collecting satellite signals that hit the ocean’s surface and then get reflected back. When the oceans are choppy due to high wind speeds, the waves reflect the GPS radio waves back in various directions.
“Imagine you blow on a hot bowl of soup,” said Stephen Katzberg, a researcher at the NASA Langley Research Center and author of the study, in a press release. “The harder you blow, the bigger the ‘waves’ are in the bowl.”
Scientists figured out how to measure these reflected radio waves and use the measurements to decode the speed of the wind.
The first comprehensive test run happened in 2000 when a plane flew into Hurricane Michael, a Category 2 storm, and came back with a receiver loaded with data. Since then, NOAA scientists have refined the system every year and finessed their equations to get better predictions of wind speed. The system as it exists today is almost always accurate for wind speeds close to zero to 40 meters a second.
The GPS system is still less accurate than dropsondes but GPS is cheaper, lower weight, and needs less power. It could even be installed on a drone. Scientists plan to use both the GPS and dropsondes together to get a better picture of these storms.
Image: a Hurricane Hunter walks to his storm chasing aircraft. Credit: Manuel Martinez