Effect of dust and smoke on Western drought ‘likely similar’ to African study, author says
A University of Alabama at Huntsville (UAH) student says his research on the combined influence of dust and smoke on the surface atmosphere temperature in sub-Saharan Africa likely applies to conditions in the American West, now in the United States. worst drought in over 1200 years.
Dust occurs naturally, but most smoke comes from human activity or possibly man-made factors such as wildfires.
In the West, dust and desertification likely form a positive feedback loop that leads to dehydration because of less variation between daytime and nighttime temperatures, said Christopher Phillips, a doctoral student in atmospheric sciences at UAH, part of the University of Alabama System.
In a study published in the Journal of Geophysical Research: Atmospheresthat’s what Phillips found is happening with atmospheric surface temperatures in Africa, where the Sahara is growing in size.
“Smoke is a bit different, though. Smoke cools the day without warming the night. This takes away some of the feedback,” said Phillips, who is advised by associate professor of atmospheric science Dr Udaysankar Nair. “We found that dust dominates the smoke in Africa, so I don’t think it would change the overall picture of surface warming.”
The tiny dark carbon particles of smoke are very good at blocking sunlight, which scientists call shortwave radiation. Being less than two-tenths of a micron in size, the tiny particles don’t interact much with what scientists call Earth’s long-wave radiation, the planet’s re-radiation of solar energy.
“So when smoke is dense, it cools you down a lot during the day because it blocks out sunlight, but it doesn’t change the nighttime energy budget because smoke aerosols don’t interact with Earth’s long-wave radiation emissions,” Phillips says.
While his work focuses on surface warming, Phillips also commented on the implications of the results.
“Conceptually, both dust and smoke stabilize the atmosphere during the day, reducing the chances of cloud formation,” he says. “Without clouds, you won’t find rain. Furthermore, other studies show that African rainfall occurs in a seasonal cycle linked to the temperature of the Atlantic Ocean. When dust and smoke are blown over the ocean, they cool the water, increasing the chance on seasonal rainfall in sub-Saharan Africa.”
When the dust gets warmer at night, the amount of dew that forms on plants can be reduced, causing them more stress, Phillips says.
“As plants die, more dust can get into the atmosphere, stabilizing the atmosphere and warming the night even more. This is the heart of the feedback loop,” he says.
Dust has much larger particles than smoke. They can be several microns in size and dust blocks sunlight like smoke does, but it also absorbs a fraction of the Earth’s thermal radiation.
“That absorbed fraction doesn’t go out into space, but is instead beamed back to the surface. Essentially, dust acts like a blanket insulating the ground overnight.”
This is comparable to how the greenhouse gas CO2 behaves, and that was one of the drivers for Philips’ research.
“How do we separate the impact of dust from global warming in these dry, dusty places? We’ve found that you can quantify how much nighttime warming is caused by dust, using weather models, but you have to look at combinations of smoke and dust,” Phillips says.
“That’s because too much smoke means you’re cooling your days without the blanket effect at night,” he says. “We found that when the smoke is more than about half that, the daytime cooling becomes greater than the nighttime warming.”
To study these effects in Africa, Phillips used a computer modeling approach with a single-column weather model that only looks at vertical processes such as energy transfer in the atmosphere. He used UAH’s Matrix high-performance computing cluster.
“It’s not quite a supercomputer like the big national labs, but it’s close,” Phillips says. “Overall, I think we had to analyze almost 5,000 simulations, which wouldn’t have been possible without some kind of powerful computer.”
The model was developed by combining weather balloon and aerosol measurements from the 2006 Dust Biomass burning Experiment, which used aircraft to measure the vertical structure of dust smoke plumes over Africa, and weather balloon data from two US Department of Energy and NASA research sites. in Nigeria.
“We found two broad processes that alter surface temperature. First, there are the radiative processes we talked about earlier, where aerosols block sunlight but trap the Earth’s thermal radiation,” Phillips says.
“The second is related to how aerosols stabilize the atmosphere. The air near the surface is known as the planetary boundary layer. It grows when heat at the surface creates turbulent vortices that mix the atmosphere, but adding dust and smoke keeps the boundary layer shallow. By trapping heat in a smaller layer, the surface temperatures rise.”
Phillips found that this effect competes with shade created by dust and smoke, complicating the response of daytime temperatures to dust. At night, however, the radiation and boundary layer effects combine to cause warming.
“These two processes will be the same whether you’re here in the US or in sub-Saharan Africa,” Phillips says.
While careful to note that the atmosphere is a very complex and constantly evolving system, Phillips says ongoing drought in the western U.S. creates many positive feedback loops that perpetuate it, such as a decrease in plant cover that affects both the movement of water vapor from ground to air reduces and also increases the atmospheric dust load. Both effects can reduce the chances of rain.
“I would certainly expect prolonged drought to cause an expansion of the American deserts, perhaps even turning semi-arid areas into arid regions,” he says.
On the plus side, western agriculture tends to be well irrigated, and Phillips says research shows the practice helps increase rainfall.
Large climate patterns will also help, he says. Cycles such as el Nino and la Nina significantly alter rainfall in the US, and they can overwhelm local factors such as dust and smoke.
“It goes without saying that nature is incredibly complex and that dust and smoke are just one piece of the puzzle,” Phillips says.
“That’s why it’s so important to continue to drive scientific progress and conduct increasingly comprehensive studies that look at each component together,” he says. “I’m proud to say that our paper is a step in that direction by bringing together how aerosols affect both radiative transfer and boundary layer growth.”
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Christopher Phillips et al, The influence of dust-smoke mixtures on boundary layer processes and nighttime warming in the Sahel, Journal of Geophysical Research: Atmospheres (2022). DOI: 10.1029/2021JD036349
Quote: Effect of dust and smoke on western drought ‘probably similar’ to African study, says author (2022, June 23) retrieved June 23, 2022 from https://phys.org/news/2022-06-effect-western-drought – similar-african.html
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