Technology behind hurricane forecasts starts with a drop from a plane

This Sunday, Sept. 17, 2017, GOES East satellite image provided by NASA taken at 7:45 p.m EDT, shows Hurricane Maria as it approaches the Lesser Antilles. Maria swiftly grew into a hurricane Sunday, and forecasters said it was expected to become much stronger over the coming hours following a path that would take it near many of the islands wrecked by Hurricane Irma and then on toward Puerto Rico, the Dominican Republic and Haiti. (NASA via AP) less This Sunday, Sept. 17, 2017, GOES East satellite image provided by NASA taken at 7:45 p.m EDT, shows Hurricane Maria as it approaches the Lesser Antilles. Maria swiftly grew into a hurricane Sunday, and ... more Photo: HOGP Photo: HOGP Image 1 of / 1 Caption Close Technology behind hurricane forecasts starts with a drop from a plane 1 / 1 Back to Gallery

At its core, the science of hurricane hunting is pretty simple: a plane flies over a storm and researchers drop a probe into the storm to measure its intensity.

That overly simplified explanation begins with that probe which, in the meteorology field, is called a dropsonde.

Weighing less than a pound and measuring just over 16 inches long, a dropsonde is a series of sensors inside a tube resembling the cardboard core of a paper towel roll that’s about the width of a Pringles can. The tube is attached to a white, papery parachute by a thin metal stem.

It’s “designed to be ejected out of an aircraft and fall through the atmosphere and take measurements that are critical to understanding hurricanes or tropical cyclones,” said Dr. Kevin Petty, chief science officer of Vaisala, the company that manufactures dropsondes. “And those measurements include temperature, relative humidity, wind speed, wind direction, and pressure.”

During Hurricane Harvey, about 70 dropsondes were released over 13 hurricane hunter missions. The one-time use reconnaissance probes are used in both operational and research capacities by the National Oceanic and Atmospheric Administration and the agency’s National Hurricane Center. NOAA has fleets of specialized aircraft, known as hurricane hunters, that fly into approaching storms to launch the dropsondes. Both the dropsondes, which cost $800 each, and the planes are funded through federal budget appropriations.

The measurements, collectively called an “atmospheric sounding,” are conveyed directly to a computer system on the plane, where a meteorologist checks the quality of the dropsonde data before transmitting it to forecast modeling centers and the National Hurricane Center.

Hurricane hunters

The planes that scientists use to track — or hunt — hurricanes are based in Mississippi and Florida.

“The workhorse of the operational hurricane hunter fleet is the U.S. Air Force WC-130J aircraft,” said Dennis Feltgen, a spokesman for the National Hurricane Center. “The planes and crews are with the U.S. Air Force 403rd Wing, 53rd Reserve, located at Keesler AFB in Biloxi, Mississippi.”

Additionally, the NOAA Aircraft Operations Center in Lakeland, Fla., has the P-3 Orion hurricane hunter aircraft, used mostly for research, and the G-IV jet, used for high-altitude operations, he said.

In the 2017 hurricane season, a total of about 1,701 dropsondes were deployed by the 53rd Reserve and the NOAA Aircraft Operations Center, according to NOAA. For Hurricane Harvey, about 70 dropsondes were released by the 53rd Weather Reconnaissance Squadron over 13 missions, while several hundred were dropped by NOAA over eight missions.

There is no standard number of dropsondes used to track a storm, but a general drop pattern targets key points of each storm.

“For tropical storms and hurricanes, they typically drop in the eye wall or maximum wind band on one or both sides of the center and they drop in the center,” said Maj. Ashley Lundry, the squadron’s aerial reconnaissance weather officer.

Making the weather report

In storm reports broadcast to the public, dropsonde data is included in the form of real-time information about a storm’s surface pressure, near-surface wind speeds, and models that forecast a storm’s path and intensity. During storms, the hurricane center includes the data as part of an advisory package issued to weather stations every three hours. When a hurricane is within radar range, hourly updates are issued.

Jeff Lindner, a meteorologist with the Harris County Flood District, said that Harvey was notable in how quickly it escalated from a tropical storm system into a Category 4 hurricane. This sudden and rapid strengthening was reflected in a combination of dropsonde data, aircraft observations, Doppler radar from Corpus Christi, and buoys on the ocean surface, all of which contributed toward a more accurate forecast.

Petty said that dropsondes allowed forecasters to get a better sense of Harvey’s structure.

“That was really key,” he said. “Because Harvey’s translation speed, or the speed at which it moves forward, was tied to the amount of rainfall it dropped and the flooding that occurred.”

Technological growth

Vaisala has been manufacturing dropsondes since the late 1990s, but the technology was developed in the mid-1970s, said Nick Potts, a software engineer with the National Center for Atmospheric Research.

The dropsonde was developed as part of the same vein of technology as the radiosonde, which is commonly known as a “weather balloon.” Both sondes measure atmospheric parameters, but while the radiosonde floated up into the atmosphere, the new sonde was dropped from an aircraft — hence the name “dropsonde.”

The current dropsonde technology is owned by the University Corporation for Atmospheric Research, a nonprofit consortium of more than 100 colleges and universities that does research and training in the atmospheric and Earth system. The technology has been licensed to NOAA and the U.S. Air Force, which contracted with Vaisala to manufacture dropsondes for their own use. Vaisala also has a separate license from UCAR to produce the dropsonde for independent researchers and continues to work with UCAR to improve the device, Potts said.

One area for the dropsonde’s technological growth is in intensity forecasts, especially when it comes to rapid storm intensification, as with Hurricane Harvey.

“It’s very difficult to collect data in that eyewall because the conditions are so extreme,” Lindner said. “You know, a dropsonde is just one point in the big storm. So there’s a lot going on between the atmosphere and what we believe is the ocean surface that we don’t maybe fully, completely understand.”

UCAR is looking to add features to future dropsondes that will allow them to take measurements in water in addition to the current measurements taken when they fall through the air.

alex.park@chron.com