WEST PALM BEACH, Fla. — There’s a slice of atmosphere where ocean meets sky that is a mystery to tropical meteorologists — a chaotic place where hurricane winds tear, 50-foot waves tower and evaporation feeds intensification.
But for the first time, messages from that frothy space were transmitted to scientists from the eyewall of a Category 4 cyclone by a specially equipped vessel floated into the path of Hurricane Sam as it tore through the Atlantic basin last month.
Created by the California-based company Saildrone, the solar-powered, unmanned vehicle looks similar to a canoe but is self-righting with a lead-weighted keel and a unique wing that helps with stabilization.
While the data collected by the drone in Sam’s eyewall was experimental and not used in forecasts by the National Hurricane Center, researchers see a potential to improve weather models with measurements including near-surface temperature, humidity levels, wave heights, wind speeds and ocean salinity.
The project is a partnership between the National Oceanic and Atmospheric Administration and Saildrone.
“Hurricanes are fueled by the ocean through heat flux and we want to better understand the physical process happening in the air and sea interface,” said Jun Zhang, a meteorologist with NOAA’s Hurricane Research Division who worked on the experiment and flew missions into Hurricane Sam. “This process is one of the keys for hurricane intensification.”
Intensification, especially rapid intensification, remains a challenge for hurricane forecasters. Rapid intensification is defined as an increase in winds speeds of at least 35 mph in 24 hours.
This hurricane season, six storms underwent a rapid intensification, including Sam, which accelerated from an 85-mph Category 1 storm at 5 p.m. Sept. 24 to a 140 mph Cat 4 by 5 p.m. the next day.
While hurricane track predictions have showed consistent improvement, intensity predictions stagnated through the '70s, '80s, '90s and early 2000s. Some small improvements in intensity forecasting have been measured since 2013, according to a 2020 study in the journal of the American Meteorological Society.
Brian Connon, vice president of ocean mapping for Saildrone, said sending the 23-foot long vehicles into a hurricane wasn’t something being considered by the company in 2014 when it was founded.
“We started primarily as ocean data collection to support science, but then NOAA came to us and asked if we could modify the design to survive in a hurricane,” Connon said. “We tested them last year in the north Pacific and that was when we said, yeah, we can do this.”
Five Saildrones have been operating in the Atlantic Ocean this year with two launched from Jacksonville and three from the U.S. Virgin Islands. Operators — called mission managers — can maneuver the Saildrones from the company’s office in Alameda, California. They worked with NOAA to position the vessels in the path of storms and got close with Henri and Peter when they were tropical storms.
It wasn’t until Sam, which reached wind speeds of 150 mph, that the Saildrone proved its grit, holding up in 50-foot waves.
Connon said the drone is aimed so that the wind is at its back.
“There comes a point when sailing doesn’t really work anymore and it just waits the storm out,” Connon said.
In decades past, manned hurricane research missions into storms flew closer to the surface of the water. That was ultimately deemed too dangerous and now the lowest flights range between 8,000 and 10,000 feet depending if other hurricane flights are in the air, said NOAA flight meteorologist Nikki Hathaway.
“Now we know there are tornadoes that spin up in the eyewall,” she said. “You’re bounded by mesocyclones below and ice above.”
NOAA’s Gulfstream IV jet can fly as high as 45,000 feet, putting it above the tropical cyclone.
NOAA's flying Coyote drone lasts less than an hour in a hurricane.
But it’s the surface observations that are challenging. Dropsondes – cylinder-shaped devices shot out of the research planes that collect data on their descent before crashing into the ocean — offer a fleeting glimpse inside the storm.
A 13-pound flying drone called the Coyote was introduced by NOAA in 2014 with the ability to fly below where airplanes take measurements.
Zhang is one of the principal researchers on the Coyote project and said a drone flew as low as 300 feet above the surface during in 2017’s Hurricane Maria. But, like the dropsondes, the Coyotes end their life in the water and last only about 30 minutes to an hour, Zhang said.
The Saildrone that ventured into Sam was in the storm about 15 hours, according to Connon. It was able to capture the first surface video and photos in the eyewall of a major hurricane.
Ocean buoys can also collect surface data, but whether a storm passes by one is just luck.
“Buoys can get great information but they are anchored in one spot and never seem to be in the right spot to get the information you need,” Connon said.
The five Saildrones are still in the Atlantic basin waiting for Wanda, the next name on the 2020 hurricane list.
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