Since 2007, Barros’ lab has been steadily building up its slate of science-grade monitoring stations, which include rain gauges, disdrometers to measure raindrop size and vertically pointing radars to look at how the clouds are organized at different levels of the atmosphere. These instruments, located on both public and private land throughout the Carolina Smokies, have been automatically recording data since October and will continue to do so through the coming October.
But the investigation rose above ground level. An airplane flew 70,000 feet in the air, so high in the stratosphere that the pilots had to wear spacesuits, to observe the clouds from above. A smaller plane flew right through the clouds to observe the varying characteristics of water and ice particles suspended in them.
The airplanes flew everywhere but focused especially on two sites, Maggie Valley Sanitary District and Purchase Knob, that were furnished with an even more complete set of monitoring equipment than the other sites during an intensive observation period in May and June. Those instruments included a trailer that measured the concentration of aerosols — nanoparticles suspended in the air — and an array of large radars to record the weather patterns from the ground.
Those radars are especially important, said Anna Wilson, a member of Barros’ lab, “because in this region there’s a lot of influence in the lower levels of the atmosphere closer to the surface, and we’re trying to investigate in closer detail how that interacts with synoptic events, [which are weather] events that move across the country.”
The Duke group isn’t working alone, however. The study was part of a campaign to reconcile data gleaned from a group of NASA satellites, launched in conjunction with the Japanese space agency, with actual weather patterns on the ground, a process being repeated in varying areas across the country. In February, the core observing station was launched in the constellation of 10 satellites, the first of which launched in 1999. The goal is to glean data that can be used to improve human understanding of atmospheric patterns and, therefore, weather.
Seeders and feeders
As anyone who’s had a sunny-and-70 forecast turn into a thunderstorm can tell you, weather is tricky in the mountains. Local weather forecasts come largely from radars located at the Greenville, S.C., National Weather Service Station, but their signal can’t travel through mountains. In order to get radar readings in the Smokies, the Greenville station has to point its radars high enough that the beams reach above the mountains. This allows them to come up with mountain forecasts, but the high angle means they don’t get the whole story.
“Because most of the interesting weather is happening at the lower altitudes, most of the time this radar doesn’t even see what is going on the mountains because they are pointing so high,” Barros explained. “Often it’s hard to know if it rained unless you were actually there.”
This low-altitude “interesting weather” frequently has to do with the multi-layered cloud systems that characterize the Smokies. What starts out as a little drizzle coming from a high cloud picked up by radar often falls down into a lower cloud, where it picks up more moisture. Those larger raindrops can then fall down into an even lower cloud and become even bigger. Before you know it, a forecast of light drizzle has turned into the reality of a torrential downpour.
Called the “seeder-feeder” effect, these interactions have important implications for wildlife as well as for forecasts.
“Multi-level clouds and fog actually create different types of micro climates,” Barros said. “Small changes in climate can have huge impacts on whether the fog forms, on where the fog forms and all of that really interferes with how the plants really function. It can have implications for the stability of the forest, for biodiversity, even for the animals and of course implications for wildfires. All of these things are related to where is the moisture in the atmosphere, how is the rain falling, how is it moving.”
Because, as well as feeding the raindrops seeded by higher clouds, the fog of the Smokies is actually an important water source. People might dismiss that water as “just drizzle,” but a slow and steady drizzle can add up to a lot of water, and it tends to sink into the ground a lot better than a faster, heavier rain. In the cold season, Barros said, drizzle resulting from interactions between fog and higher clouds accounts for 50 to 60 percent of all rainfall in the region.
“It’s like when you leave your faucet on at home and you don’t notice it, but you wind up with a big water bill at the end of the month,” Barros said.
Where the weather hits the woods
But weather isn’t just a product of the atmosphere. It’s a product of the terrain, too, especially when it comes to storms. Another goal of the project is to synthesize the data for a better understanding of how clouds and terrain interact to dictate severe whether, such as thunder, lightning and flash flooding.
“You get these very intense cells that are very localized and cause these major events in very specific locations in the mountains,” Barros said. “We’re trying to understand how these cells evolve and how do they become so intense and so heavy.”
It has to do with the shape of the terrain, the direction of the wind and the stability of the atmosphere, Barros said.
But in order to get raindrops at all, she said, you have to have aerosols, nanoparticles that hang suspended in the atmosphere. They’re the nuclei around which cloud droplets, which grow into raindrops, form. With no aerosols, the air can become quite humid but will never form a cloud, much the same as how with no particles to anchor to, water can become frigid but will never turn to ice. Aerosols deserve much of the credit for the Smokies’ ample clouds and fog.
The question for Barros lab, then, is where do local aerosols come from? Are they arriving as blown-in pollution from some other place? Are they dust that’s been kicked up in the air? Or, are Smokies aerosols possibly a product of the trees themselves?
“Forests and vegetation, when they transpire, they emit these volatile organic carbon compounds,” Barros said. “They can change phase, and they can actually become aerosols.”
If the vegetation actually turns out to be the source, that would point to a self-fulfilling prophecy of life and lushness in the Southern Appalachians.
“It’s basically showing how the plants themselves are creating the conditions to harvesting water resources,” Barros explained.
It will take a year or two for the lab to run the data and reach any solid conclusions — the field stations will keep recording data through October — but they’ve already pulled out plenty of themes worth pursuing and plenty of hope for better forecasts down the road. And not just here — while the research happened in the Smokies, the world at large will reap the benefits.
“It’s beautiful to study the Smokies, it’s beautiful to explain what’s going on there, it’s beautiful to explain the whole transition from fog to rain, but at the end, from a scientific point of view, it’s all about the processes,” Barros said.
The “seeder-feeder” data can be used to explain precipitation patterns anywhere from the coast of California to the United Kingdom. The data gleaned about the interaction between terrain and atmosphere can be applied to low-elevation mountains ranging from the foothills of the Alps to those of the Himalayas. And the picture as a whole will likely be of great use to forecasters in tropical cloud forests throughout South America.
“We’re going to really try to put all of this information together,” Barros said, “and come up with a big picture.”