Pyrotornadogenesis
This incredible video was shared by an ABC News affiliate in California earlier this year during the Carr Fire, which hammered the community of Redding, California during one of many devastating wildfires in that state this year. During many intense wildfires, small “fire tornadoes” or “Fire whirls” are observed when rapidly rising hot air begins rotating in a column. Here’s one video I shared of one a few years ago: https://tmblr.co/Zyv2Js2Av4RcN. However, there’s a huge difference between the thin column of fire in that video and what happened during the Carr Fire. The Carr Fire produced a huge vertical column, with winds comparable to an F3 tornado. A new study of the data collected during this event suggests that in fact it was something very different from a simple fire whirl. It was in fact a pyrotornado, produced by the same type of cloud dynamics and wind shear that give rise to regular tornadoes. This event is only the 2nd on record like it – the first occurred during a firestorm in New South Wales, Australia, in 2003.
Most fire whirls are fairly thin columns of hot gas, they rotate as they rise because of angular momentum being conserved in the thin column of rising gas, but they are confined to layers of air near the surface. In this case, this fire set up a full weather system, which created properties in the whole atmosphere much more similar to a tornado.
When this fire tornado began, air was rising rapidly above the area, triggering the formation of clouds known as pyrocumulonimbus clouds. These clouds are like thunderstorm clouds – high, towering, containing rising air and forming ice crystals – but they are triggered by hot air rising above fires. In this case, the forming pyrocumulonimbus cloud took a small fire whirl and stretched it out vertically into a column just like a normal tornado. It rose to a height of 12 kilometers (over 6 miles) above the surface within about 15 minutes of it first being detected, and its rotation was fed by the same sort of shearing winds at the edge of the cloud that feed tornadoes from thunderstorms.
The combination of shearing winds and a thunderstorm cloud forming created this spectacular vortex, which tore through the city of Redding as a burning column in the air. This fire tornado occurred after a multi year drought was followed by one record wet season and another record dry season – leading to lots of growth of vegetation in the record wet year and a huge amount of burnable vegetation when it dried out. That volatile mix combined with a specific set of weather conditions to trigger this event – by studying the radar signatures of pyrotornadogenesis in Redding, weather forecasters may be able to recognize when those ingredients are present in the future.
-JBB
Video source: https://twitter.com/abc10/status/1022862397780852739?lang=en https://www.youtube.com/watch?v=x1fV5Uq_b4A
Original paper: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL080667?fbclid=IwAR3c6ubet7GxGdirZfcmpvoBQcP_9HqwTVWxfNUj38yEeS1lS76JGVKRXpM Press versions: https://blogs.agu.org/geospace/2018/12/06/scientists-find-causes-of-firenado-in-deadly-carr-fire/?fbclid=IwAR03ZH6bq7Imh1GAZmtE38ZdKwWKEiQHZxUaUrxakvkZG9Qk6iOeD_zdWLQ https://www.sacbee.com/news/state/california/fires/article222687285.html?fbclid=IwAR0Z24UYk4HwQnPfYyIYOxPBs2aACC5g8Vs0HMteos0EhXxpDKOK_0fFjDo