Dropsondes Go Where Even Hurricane Hunters Fear to Fly
What flying machine could withstand 230-mph winds?
Name one machine that can enter the strongest hurricane you care to mention and complete its mission without going to pieces. Hurricane Hunter airplanes? Good guess, but those crews don’t fly into the most dangerous locations, near sea level and along the eyewall.
Heart of darkness: that’s where the dropsondes go. These are tube-shaped sensors about the diameter of a Pringles can, and 16 inches long. Dispatched from Hurricane Hunter aircraft flying above or through the storms, each dropsonde descends under a small squarish parachute on the way to sea level, transmitting all the way down. Dropsondes take 10 to 15 minutes to reach the ocean depending on the release altitude, which can be as high as 65,000 feet when flung from the unmanned, long-duration Global Hawk aircraft. Global Hawk uses a smaller version sized to fit the aircraft’s automatic launcher.
Twice a second, the falling sonde reports on location, pressure, temperature, humidity, windspeed, and wind direction. Dropsondes drive what amounts to a core sample through the storm, producing what researchers call a high-resolution profile. No other instrument is currently capable of this particular feat, says scientist Holger Voemel at the National Center for Atmospheric Research (NCAR). While the famous Hurricane Hunter aircraft do carry instruments into monster storms, those can be foiled by clouds and, at any rate, it’s too risky for people to fly any lower than two or three miles above the surface. And it’s wild down there: dropsondes approaching sea level have documented off-the-charts turbulence that would tear airplanes apart, and surface winds exceeding 230 mph.
Vaisala Inc. manufactures the devices under an NCAR license. Each season, hurricanes and cyclones devour more than a thousand dropsondes, which cost $700 to $800 apiece, says Holger. And it’s a one-way trip. Unlike balloon-borne “driftsonde” instrument packages, which have a return-to-sender label to use if found on land, dropsondes are used for open-ocean work, and sink after service. But NOAA has no regrets about losing them (more than 30,000 GPS-aided models have been expended since full adoption in 2005), since those models have played a starring role in raising forecast-track accuracy.
Helping scientists improve their track predictions is a perennial task for dropsondes, and now a new task is emerging: helping them understand rapid intensification. Atmospheric scientists would like to know why four of this year’s hurricanes (Harvey, Irma, Jose, and now Maria) grew so quickly into enormously destructive storms. By reporting sea-surface temperature, the fuel that feeds these monsters’ engines, a new generation of dropsondes equipped with infrared eyes might help scientists figure it out.