Feather phenomenon: the radar indicates stronger


image: Radar from Tropical Storm Beta after making landfall near Corpus Christi, Texas, September 22, 2020. White oval marks a biodiffusion signature, indicating the presence of birds. Research from the University of Nebraska-Lincoln has shown that more intense hurricanes seem to trap and carry more birds in their relatively calm eyes, which could have ecological implications by displacing species.
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Credit: Remote sensing in ecology and conservation / John Wiley & Sons, Inc.

Whether birds are caged in the eye of a hurricane may depend on the intensity and totality of the chaos beyond the stillness, according to a new study by Matthew Van Den Broeke of the University of Nebraska-Lincoln .

Reports of birds trapped in the center of hurricanes date back to at least the 19th century, when crews observed the phenomenon from the bows of ships and saw their ships become mobile harbors for exhausted birds.

“It was really fun reading some of those older observations from the 1800s of getting a ship through the eye of a hurricane and watching the birds land on it,” said Van Den Broeke, professor. Associate of Earth and Atmospheric Sciences. “So we’ve known for a long time that this is happening.

“But it’s really only since (the advent of) radar observations that we have any idea of ​​how many of these systems actually carry birds and insects.”

The technology that allowed meteorologists to really begin to differentiate the weather of organisms – dual polarimetric radar, which added a second vertical dimension to previously one-dimensional observations – has only become widely available in the past 10 years. Which means that there is still a lot to be known about when, how often, and under what conditions a hurricane turns a free bird into a whirling bird.

Van Den Broeke set out to analyze double-pol data from 33 Atlantic hurricanes that hit the US coast or Puerto Rico between 2011 and 2020. These 33 included some of the fiercest hurricanes in recent memory: Irene in 2011 , Sandy in 2012, Harvey and Irma in 2017, Dorian in 2019.

He was specifically looking for biodiffusion signatures – the electromagnetic waves of the speed of light that bounced back to a radar station not from precipitation circling the eye, but from birds or even insects within. In each of the 33 cases, Van Den Broeke identified at least some biodiffusion.

But the signatures differed. Tellingly, these differences generally corresponded to differences in the hurricanes themselves. The higher the wind speed of a hurricane, the denser and sometimes larger the biodiffusion signature, indicating the presence of more birds in the eye. Van Den Broeke also categorized hurricanes according to whether their eyes were closed, open, or somewhere in between. A closed eye was surrounded by 100% thunderstorms, while a mostly closed eye was surrounded by 75% to 99%, and so on. As with wind speed, greater thunderstorm cover was generally correlated with greater numbers of birds.

Of course, stronger winds and more thunderstorms tend to go hand in hand, Van Den Broeke said, making it difficult to distinguish the precise influences of wind speed versus precipitation. What is clear: The more severe the hurricane, the more intimidating the prospect of abandoning the relative safety of the eye, even if that can mean spending thousands of miles and a week in the air.

While the intensity of a hurricane may have the biggest influence, Van Den Broeke has found evidence that timing and geography matter too. The largest biodiverse signatures appeared in hurricanes that occurred between July and October, when many species of birds migrate south to the tropics, suggesting that native seabirds weren’t the only ones swept away. Biodiversity was also higher and denser, on average, in hurricanes that hit the Gulf Coast and Florida, which have a higher concentration and diversity of birds than other areas affected by recorded hurricanes.

This could have ecological implications, Van Den Broeke said, especially if the strength and frequency of Atlantic hurricanes increase in response to global warming. Population dynamics could hypothetically be distorted, or invasive species introduced, by a hurricane of the right intensity crossing a migration route at the wrong time.

“Environmentalists think a lot about transport,” he said. “How do you get organizations from one point to another? How many organisms are transported? Are you able to transport new species that were not present before? Can you transport pests that could pose a threat to agriculture? “

The question of how many birds could be carried by a single hurricane is obscure and likely unanswered without knowing which species are trapped in the eye, Van Den Broeke said. Other lingering questions, however, should prove to be more resolvable.

In his recent study, Van Den Broeke found that the cruising altitude of trapped birds also increases with the intensity of hurricanes. He doesn’t know why, although it may have to do with the thermal structure of a typical hurricane. Warm, humid air near the surface of the water tends to rise up to the eye until it reaches a limit, called inversion, beyond which the higher and higher air dry tends to go down. The inversion altitude can vary widely between hurricanes and could affect that of birds, Van Den Broeke said.

It is now analyzing inversion data collected by parachuted instruments, called dropsondes, which are released in hurricanes from planes piloted by the US Air Force and the National Oceanic and Atmospheric Administration.

“I compare the inversion height observations to biodiffusion signatures,” said Van Den Broeke, whose work is supported by the National Science Foundation. “Does that match?” Do the birds fly over it? Below? And can we say something, then, about the changes in intensity of tropical cyclones, and relate that to how the biodiffusion signature behaved?

“It is possible that there is some sort of systematic effect there.”

If so, the biodiffusion altitude could eventually become a radar indicator of hurricane characteristics that currently can only be measured by airdrops. In the meantime, Van Den Broeke, who has previously considered a career in ecology and still has a penchant for it, said he relishes the chance to bridge two rarely overlapping areas.

“I am always fascinated by ecosystems and the interactions of organisms with their environment,” he said. “But my expertise is in meteorology, so being able to combine the two areas is really exciting for me. “

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