Using GPS to Measure Hurricane Winds
Echoes
“I’ve been doing meteorology for 30 years and the jet stream the last three years has done stuff I’ve never seen.”
— Jeff Masters, meteorology director at Weather Underground, on the jet stream’s recent erratic behavior. The jet stream, which usually moves from west to east in a generally straight direction, lately has been undulating north and south. Some scientists attribute certain extreme weather events—like June downpours that led to historic floods in Alberta, Canada; record-breaking early summer heat in parts of Alaska; the path of Hurricane Sandy; and snowstorms in May in the Midwest and Plains—to this prolific buckling of the jet stream. While some scientists also relate the shift to climate change, others disagree and say this relatively new phenomenon could be random weather or long-term cycles at work. Whatever the case, they agree it’s a change they’re not yet able to fully understand. “The fact that the jet stream is unusual could be an indicator of something,” Masters goes on to say. “I’m not saying we know what it is.” (SOURCE: The Associated Press)
Current methods of measuring wind speeds in hurricanes generally involve dropwindsondes—instrument tubes known as dropsondes that also use GPS tracking to determine wind speed and direction. These measuring devices attached to parachutes are highly accurate but also quite expensive (about $750), and a typical storm-hunting mission utilizes only around 20 dropsondes, which means their limited distribution is unable to measure all parts of a storm. This lack of coverage could be redressed through a novel use of GPS systems, which in testing have been found to successfully and inexpensively augment dropsonde surface wind speed measurements.
The reflection of a radio wave sent by a GPS satellite becomes distorted when the reflective surface is turbulent, such as with ocean wave roughness. If a hurricane is blowing over the water, the wave surface distress is intensified, leading to more agitation of the reflected radio signals.
“Imagine you blow on a hot bowl of soup,” explains Stephen Katzberg of the NASA Langley Research Center, the lead author of a study about this GPS utilization that has been accepted for publication in Radio Science. “The harder you blow, the bigger the ‘waves’ are in the bowl,” which in such a small area equates to surface roughness. With the roughness greater, Katzberg says, “the reflections get more disturbed and that’s what we measure.”
GPS receiver chips located within an aircraft flying through a hurricane collect the distorted signals reflecting off the ocean, as well as those originating from satellites, and a computer compares both sets of signals to approximate the storm’s surface wind speed. Measurements taken during test flights were found to be accurate to within 5 meters per second (11 mph); by comparison, dropsondes are accurate to within 0.5 meters per second (1.1 mph), but because aircraft are already equipped with GPS systems, they can be used continuously during a flight to supplement the dropsondes’ measurements, with no appreciable additional cost.
The system does have a couple of limitations: Because it involves the movement of waves in a large body of water, it cannot be used over land, and it would not be useful in the eye of the storm, where the waves would be choppy but there would be no wind to agitate the rolling sea surface. (Source: American Geophysical Union)