How weird, bouncing cell signals can help track smoke from wildfires

Like massive bush fires rages through Eastern australia in January 2020, a deadly haze moved over Melbourne, a clear signal for residents to stay indoors. Bouncing overhead, however, was a less visible signal: Cellular data was flying through the air in an odd pattern, a model scientists could perhaps use to better understand and predict severe smoke events in the air. ‘to come up.

Cellular signals over Melbourne were interacting with an atmospheric quirk known as a temperature inversion. Normally, you will find warmer temperatures near the ground, where the sun heats the surface, and cooler temperatures higher up in the atmosphere. But, true to its name, a temperature inversion reverses that.

As a layer of smoke rolled across the city, it absorbed the sun’s energy, preventing much of that radiation from heating the surface. This created a layer of hot, dry, smoky air that lay on the cooler air at ground level. “You have this dual process,” says Adrien Guyot, atmospheric scientist at Monash University, lead author of a new paper in the newspaper AGU Advances describing the research. “You have the warming of the layer and the fact that the soil isn’t warming up as it normally is.”

It did strange things to the signals transmitted between cellular antennas atop buildings in Melbourne. (Guyot and his colleagues were specifically interested in antenna-to-antenna communication in the network, not how people’s cell phones connect to it.) Usually, when these antennas talk to each other, the signal passes more or less directly between them. But a temperature inversion creates a kind of atmospheric cap, bending the signal considerably towards the ground.

These are called “abnormal propagation conditions”, which means that a signal is moving, well, in an abnormal way. “It will bounce off the ground, then come back up, then bounce off the ground and come back up. It will therefore find itself trapped in the inversion layer, ”Guyot explains. As the signal bounces, the travel time between the antennas is different from what it would be under normal conditions, when its path is straighter. “And since that doesn’t always happen at the same time, sometimes we have a high reception, sometimes we have a lower reception,” adds Guyot. “And it’s really clear in the signal.”

By examining this cellular data, Guyot was able to identify when a temperature inversion took hold over Melbourne as Australia burned during this wildfire season. In addition to trapping these signals, the inversion layer also trapped the smoke, creating a data log as the city’s air quality became low. the worst in the world. In the future, Guyot thinks, it may be possible to monitor these cellular signals for clues as to where an inversion might form and its severity. This would give officials a better idea of ​​how quickly air quality could deteriorate. “If you have a temperature inversion, and if that inversion gets stronger, then you’re more likely to have an increase in smoke concentration as well,” Guyot explains.

Imagine throwing food coloring in a kiddie pool versus an Olympic size pool – even with the same amount of coloring you will get darker water in the smaller body of water than the large one. The same is true for condensed smoke trapped in a thin layer of air near the ground, compared to smoke that diffuses more widely in an open atmosphere. “Having these inversions means that the smoke is not carried to a higher altitude,” says Rebecca Buchholz, atmospheric chemist at the National Center for Atmospheric Research, who was not involved in this new work. “So it stays close to the ground, gets very concentrated, and there’s more pollution on the ground that can impact humans.”

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