Watch Australia's Wildfires Spawn Massive Smoky Thunderclouds
2020-01-13 12:13

The wildfires currently tearing through Australia aren't just unprecedented—they're catastrophic, as one fire researcher put it. Climate change and fierce heat waves have dried the landscape into swaths of tinder, and all it takes is a single spark to unleash wildfires so powerful.


Researchers have been catching Australia's fires in the act of producing pyrocumulonimbus clouds, or pyroCbs. These ominous phenomena take two ingredients: a mass of hot air that produces an updraft, in this case columns of smoke-filled air. And two, they need an unstable atmospheric environment that allows the updraft to continue rising higher than it otherwise would, says Scott Bachmeier, a research meteorologist at the University of Wisconsin, Madison. As the mass climbs higher, the smoky air cools and forms into a pyrocumulonimbus cloud, a soup of water and smoke particles towering miles into the sky. (Though to be clear, the cloud itself isn't on fire.)

研究人员一直在追踪澳大利亚的火灾,以及火灾产生高温积雨云(pyroculonimbus clouds,简称pyroCbs)。威斯康星大学麦迪逊分校的气象学专家,斯科特巴赫梅尔称,这些可怕的现象有两种成因:第一,一团产生上升气流的热空气,这种热空气类似一个充满烟雾的空气柱。第二,它们需要一个不稳定的大气环境,使上升气流继续上升,高于它本来应有的高度。随着质量的上升,烟雾弥漫的空气冷却并形成一团焦积雨云,一团水和烟雾颗粒的混合物高耸在数英里外的天空中。(要说清楚的是,云本身并没有着火。)

Video: Scott Bachmeier/University of Wisconsin-Madison 

视频:Scott Bachmeier/威斯康星大学麦迪逊分校

If you take a look at the GIF above, you can see two pyroCbs sprouting, captured by satellite in the infrared spectrum. In the top panel we see the fires, and the bottom panel shows cloud-top temperatures. Red pixels indicate the heat of the wildfire, whereas violet indicates a cloud-top temperature of -70 degrees C. The colder it is, the higher the altitude. The low temperatures reveal that a pyroCb is soaring into the sky.


And another one forming here.


Video: Scott Bachmeier/University of Wisconsin-Madison 

视频:Scott Bachmeier/威斯康星大学麦迪逊分校

For hundreds of millions of years, thunderclouds and their lightning have been natural partners to fire. Any wildfire is likely to spread when strong surface winds carry embers perhaps miles ahead. “But what's carried aloft by the pyroCb updraft are copious amounts of smoke particles, which then get carried up to or even ejected above the pyroCb cloud top,” says Bachmeier. If wildfires are sprouting pyroCbs that strike the landscape with lightning without also dumping water, the conflagration will spread all the more readily.


So a pyroCb isn't directly floating embers downwind to start new fires, but it could well be depositing lightning strikes into dry vegetation. The cloud could also contribute extremely strong downdrafts that push embers along at the surface. In addition, pyroCbs deposit smoke pollutants miles high in the upper atmosphere. Luckily, that keeps at least some of the wildfire's smoke away from human lungs.


Scientists are also monitoring the movement of the wildfires' smoke from space. In this image, the massive plume of brown-gray smoke is at lower left.


Video: Scott Bachmeier/University of Wisconsin-Madison 

视频:Scott Bachmeier/威斯康星大学麦迪逊分校

And in this haunting clip, the GOES satellite shows that the high-altitude smoke has actually crossed the Pacific and reached South America. (Red flashes being smoke detection.)


Video: Scott Bachmeier/University of Wisconsin-Madison 

视频:Scott Bachmeier/威斯康星大学麦迪逊分校

These fires have been devastating for the people and wildlife of Australia. But they also give scientists the opportunity to watch in real-time as smoke spews off the continent. The goal of studying pyroCbs, Bachmeier says, is to better understand the behavior of smoke: to see what impact it “is having on short-term weather near and downstream of the fires, and perhaps the smoke's long-term effects on climate change." How much carbon, for instance, might pyroCbs contribute to the atmosphere? “It's a relatively young science, but interest in the phenomenon is rapidly growing.”


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