As a tropical cyclone churned the Atlantic Ocean this month, a drone watched from above, dropping a paper-towel-roll-sized set of sensors attached to a parachute through the clouds on a 20-minute, 10-mile journey.
The instruments revealed dry air low in the storm's center — something scientists from the NASA Goddard Space Flight Center suspect was the nascent eye of Hurricane Edouard. The storm went on to become the Atlantic's first to reach winds of more than 110 mph since Sandy in 2012, though it never threatened the United States.
Scientists hope those observations — and others gathered by the Goddard mission over the past three years — will bring a better understanding of why some hurricanes gather strength and pack a wallop while others appear menacing, only to lose strength. Ultimately, scientists hope the research will help to improve forecasts and disaster preparedness.
While manned reconnaissance aircraft routinely fly into the heart of hurricanes to gather data used in storm forecasting, the Goddard mission took a different view. From a computer at the NASA Wallops Flight Facility, pilots have flown the unmanned Global Hawk aircraft to altitudes twice the level at which commercial airliners fly to scan the storms from above.
As part of the Hurricane and Severe Storm Sentinel mission, scientists will fly the Global Hawk for the last time this weekend before spending the next year analyzing what they have seen.
They want to understand how air moisture and wind speed and direction influence tropical cyclones, and why the tropics are quiet some years, as they have been in the past two, and stormy others. The hurricane season concludes at the end of November.
"It's been kind of an unusual year," said Michael Black, a research meteorologist with the National Oceanic and Atmospheric Administration who is working on the Goddard mission. "The question is why. We don't have all the answers to that."
The Global Hawk aircraft, a windowless jet that looks like a cross between a plane and a space shuttle, carries three instruments to measure humidity levels, temperatures and winds within cyclones. Along with the dropped sensors, the other instruments use a laser and infrared scanning to explore through the clouds.
The scientists gathered data by flying the aircraft in a snakelike pattern hundreds of miles across a tropical cyclone's area, and also by taking triangle-shaped passes across the eyes of storms. The flights take an entire day.
Tropical cyclones are, by definition, counterclockwise-rotating, organized systems of clouds and thunderstorms that originate in waters close to the equator. They thrive on warm waters and humid air, and generally fall apart or fail to strengthen if they lack enough of either. The mission will also explore the role of dry, dusty Saharan air that blows from the Cape Verde islands off Africa to the Lesser Antilles in the Caribbean
The past two hurricane seasons in particular have been subdued relative to high activity over the past decade or two. Just five tropical storms formed in the Atlantic this year, four of them reaching hurricane status. In 2013, only two of 14 tropical storms became hurricanes.
On average in any given year, the Atlantic sees about a dozen tropical storms and a half-dozen hurricanes, according to National Hurricane Center climatologists.
"We like to joke about how we've sort of proven our deterrent effect on hurricanes," said Scott Braun, a research meteorologist at Goddard and the study's principal investigator.
During the more active 2012 hurricane season, the researchers had a better observation opportunity. Hurricane Nadine, a zombie-like storm, became the fourth-longest-lived Atlantic cyclone on record, maintaining strength even as it moved into colder northern waters.
Some of the mission's observations back then, when shared with National Hurricane Center forecasters, prompted the storm to be reclassified as a tropical cyclone even after it was briefly downgraded to a non-tropical area of low pressure, Braun said.
"We were catching a storm right at a point where it was recovering," he said.
Perhaps the mission's best opportunity came this month, when, as Hurricane Edouard developed in the middle of the Atlantic, they were able to take four flights during the storm. Scientists watched it turn from what is known as a tropical wave, an area of low pressure that moves westward off Africa, to a tropical storm that drifted around the middle of the Atlantic and eventually became the Atlantic's first major hurricane in two years.
In one of the flights, they captured data as the storm quickly intensified from a weak Category 1 storm to a strong Category 2 — information that could prove useful when the scientists analyze the data they have collected to learn why storms intensify.
"That was sort of the ideal target we had been hoping for," Braun said.
The researchers haven't come to any conclusions yet, but already they have noted some surprising findings. For example, even when satellite data at times indicated ample moisture over the central Atlantic, the Global Hawk's sensors found that it was limited to low levels of the atmosphere close to the ocean. Above that, the air was remarkably dry, Black said.
Most people assume the air above the tropics is laden with moisture, but that's true only to a certain altitude, he said. Dry air can stunt a hurricane's intensity.
"Just how dry it was was really kind of eye-opening," he said.
Even in mild weather, the data collected by the mission could prove useful. This weekend the aircraft will survey what's expected to be a quiet area across the middle of the Atlantic. The scientists hope the trip will provide new insights why that is.
"Mother Nature didn't quite cooperate, but regardless we got a lot of valuable and unique data," Black said.