Hyperactive Hurricane Season Provides Wealth of Research Data

The 2024 hurricane season, predicted to be “above-normal,” began with the earliest-ever category 5 hurricane in the Atlantic, Hurricane Beryl, in early July, and ended with Rafael, the westernmost major November hurricane on record. Saildrone sailed into both.

“In a lot of ways, this was the most successful hurricane mission that we’ve ever had,” said Greg Foltz, an oceanographer at the National Oceanic and Atmospheric Administration (NOAA)’s Atlantic Oceanographic and Meteorological Laboratory (AOML) and one of the principal investigators on the mission. 

The most exciting footage captured by Saildrone USVs throughout the 2024 Atlantic hurricane season.

In 2021, NOAA and Saildrone launched a multiyear mission to deploy Saildrone Explorer uncrewed surface vehicles (USVs) into tropical storms and hurricanes to measure near-surface atmospheric and upper-ocean data to better understand how ocean-atmosphere interaction affects hurricane intensity and improve hurricane prediction models. 

With 12 Saildrone Explorers stationed in five operational areas in the Tropical Atlantic and Gulf of Mexico, NOAA and Saildrone had three intercepts with sustained hurricane-force winds—the most ever—in Hurricane Ernesto, Hurricane Helene, and Hurricane Milton, and nine intercepts with at least 50-knot sustained winds.

The threshold for “hurricane-force” winds is 64 knots. The National Hurricane Center measures winds at a height of 10 meters (30 feet) above the surface. Saildrone Explorers equipped for hurricane monitoring measure wind speed at 3.5 meters (11 feet) above the surface. “Wind speed decreases closer to the ocean surface. At 10 m, windspeed can be 15% higher, so it’s even more impressive that we are measuring sustained hurricane-force winds so close to the surface,” said Foltz.

In another first, two Saildrone Explorers intercepted different parts of the eyewall of the same hurricane, within a couple of hours of each other. 

“We did that in Hurricane Ernesto, and then we did it again in Hurricane Helene. The big research question is, how does the ocean interact with the hurricane in different parts of the storm?” asked Foltz.

The area in a hurricane with hurricane-force winds is a relatively small target to hit. For hurricanes that originate in the eastern Atlantic and track west, the wind field is wider on the “right” side or northern side of the storm, where the storm, winds, and waves are all moving in the same direction.

“We almost always go for the ‘right’ side of the storm, which is where the winds are strongest, and it’s a bigger target to shoot for, so we have a lot of data on the right side. Ocean waves and hurricane winds affect the exchanges of heat and momentum. On the ‘left’ side, the winds are usually weaker, but the wind and the waves are going in the opposite direction, and we really want to understand that better,” said Foltz. In 2023, multiple Saildrone Explorers intercepted Hurricane Lee, but the intercepts were at least a day apart, so the structure of the storm had changed.

Several of this season’s intercepts, notably Hurricane Helene and Hurricane Milton, occurred in the Gulf of Mexico, where the ocean is much shallower, less than 100 meters (300 feet) deep. In a shallow area of the ocean, the wave height can nearly match the ocean depth. That friction with the seafloor can have a big impact on when waves break and the structure of the waves, which can, in turn, influence the winds in a hurricane. 

“That’s really badly understood and not understood in models at all; the models predict hurricane intensity. I’m really looking forward to analyzing the data from the Gulf of Mexico and, in fact, using the photos and video captured by Saildrone to look at the roughness of the ocean and the drag effect on the winds,” said Foltz. “Those images are valuable not only for warning coastal communities about the oncoming storm but also for research.”

The mission was initially designed as a purely research mission, but over time, constituents in other NOAA departments, including the National Hurricane Center (NHC) and National Weather Service (NWS), have integrated the real-time Saildrone data into forecasting and advisories. The data is transmitted via satellite and fed into models every six hours to provide a better idea of the initial conditions, improving the forecast.

“We’re still focused on the research, but it’s been a happy surprise to see the value of the Saildrone data for real-time forecasters,” said Foltz.

In total, Saildrone USVs traveled over 27,000 nautical miles (32,000 miles) and gathered 768 total days of observations. Saildrone intercepted hurricanes 16 times and spent 102 hours in tropical storm-force wind conditions, providing near-real-time data from above and below the sea surface. Saildrone measured a maximum 3-second wind gust of 99 knots (114 mph) and a maximum significant wave height of 13.5 m (44 feet).