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Pyrocumulonimbus and firestorms

September 30th, 2020 No comments

Meteorologists call Cumulus (Cu) to cotton-like clouds, and Cumulonimbus (Cb) to denser and bigger clouds carried by powerful upward air currents. When these clouds are originated by a fire (or a volcano), we call them pyrocumulus (PyroCu) and pyrocumulonimbus (PyroCb) clouds.

Pyrocumuloninbus (= Cumulonimbus flammagenitus) are dense towering vertical clouds carried by powerful upward air currents generated by the heat of a wildfire [1]. These fires are also called plume-dominated fires, super-heated wildfires, or wildfire-driven thunderstorms. The origin is typically tied to very high and continuous fuel loads and extreme fire weather that produces great heat and strong convection currents. In most cases, they remain in the troposphere, but when the heat produced by the fire is very high, they can cross the tropopause and inject a large amount of smoke into the stratosphere; in those cases, wildfires contribute to the global carbon and nutrients redistribution [2]. These plumes often collapse when temperatures drop with altitude and create extreme winds. That is, these high intensity fires generate feedback processes between the atmosphere and the fire that produce strong surface winds, tornadoes, and even pyrogenic lightning ignitions that further expand the fire (firestorms).

Firestorms are wildfires with extreme, sometimes erratic behavior, typically beyond the capability of fire services to contain; the term has been used to describe very different fire types such as wind-driven fires (e.g., Santa Ana wind firestorms in California) and pyrocumulonimbus plume fires [1].

Examples of pyrocumulonimbus clouds (click in the picture for enlarge it).

A: Benaixama fire, Alicante (Spain), 7/2019, by Servicio de Vigilancia de PrevenciĆ³n de Incendios Forestales, Observatori Caperutxo, Generalitat Valenciana.
B: Llutxent, Valencia (Spain), 8/2018, by Empar.
C: Tasmania (Australia), 2013, by Janice James.
D: Creek fire, California (US), 9/2020, by Thalia Dockery.
E: La Pampa (Argentina), 1/2018, Earth Observatory, NASA.
F: Funny river, Alaska (US), 5/2014, Earth Observatory, NASA.

References

[1] Pausas J.G. & Keeley J.E. (in press). Wildfires and global change. Front. Ecol. Environ.

[2] Pausas J.G. & Bond W.J. 2020. On the three major recycling pathways in terrestrial ecosystems. Trends Ecol. Evol. 35: 767-775. [doi | sciencedirect | pdf]