2020-03-01_Australian_Geographic

FACT

More than 50

pyroCbs have been

witnessed this

bushfi re season

Stratospheric injection

Upper air pushes plume downwind

away from heat source, replacement

plume continues to form

Smoke layer expanding laterally

Overshooting top in stratosphere

Fire-driven turrets

Anvil (at 14km) as low as -55oC

Rugged forested terrain

Indraft

Surface at 40oC

Cloud base at 4km (5oC)

Leeward-slope eddy winds allow

fi re to spread sideways by vortici-

ty-driven lateral spread (VLS)

Dark brown smoke

column due to VLS

Fire tornado formation

from cloud base

Lightning

possible at

-40oC

WEST

Mid-level

winds

NNW

Upper level

winds

NW

Surface

winds

50 Australian Geographic

A

LONG WITH WEATHER that creates opti-

mum conditions for destructive bushfi res, the fi res

themselves can create their own weather. When

a number of factors come together, including the type of

terrain, extremely hot bushfi res that feature intense areas

of fl aming that extend a long way back from the fi re front

can develop into deadly fi restorms. Extreme heat generated

from such fi res sends dense plumes of smoke as far as 15km

up into the atmosphere. Depending on atmospheric condi-

tions at these altitudes, smoke plumes can trigger a type of

fi re thunderstorm known as a pyrocumulonimbus, or py-

roCb for short. Research suggests that bushfi res can be made

more severe by feedback mechanisms driven by the atmo-

spheric instability that results in the formation of pyroCbs.

PYROCUMULONIMBUS Bushfi res can create their own deadly weather.

PyroCbs are visible as tall, billowing smoke clouds with a fl at base

and anvil-shaped head. If the bushfi re is power f u l enoug h a nd the

atmospheric conditions unstable, thunderstorms can lead to down-

burst winds, strong infl ow, fi re tornadoes and lightning strikes.

They can cause dangerous and unpredictable bushfi re behaviour

with strong, rapidly changing winds and large ember attacks

that help spread the fi re quickly, making it diffi cult for fi refi ght-

ers to manage. The incidence of pyroCb events is increasing as

Australia’s bushfi re seasons lengthen and the intensity of fi res grows.

In this 2019–2020 fi re season, more than 52 pyroCbs have so

far been observed, far exceeding any previous records. Climate

modelling indicates that unabated global warming will cause

further increases in the potential for the type of extreme bushfi res

that lead to pyroCb events in the future.

DATA SOURCES AND ACKNOWLEDGMENTS

Infographic design Mike Ellott; Graphics Mike Rossi; Cartography Will Pringle; PyroCb Illustration Michael Payne; Infographic concept

and main text Chrissie Goldrick; text annotations Mike Rossi; Data research Chrissie Goldrick and Mike Rossi; additional research

Rebecca Cotton; Climate and weather graphs, information and data courtesy of the Australian Bureau of Meteorology; Other data

courtesy of The Bushfi re and Natural Hazards CRC; ARC Centre of Excellence for Climate Extremes; Bushfi re statistics: Dept of Home

Affairs, NSW Rural Fire Service, SA Country Fire Service, Victoria Country Fire Authority, WA Dept of Fire and Emergency Services,

The Air Quality Index, NSW Dept. of Energy and the Environment, Copernicus: European Union's Earth Observation Programme, UNEP,

Meat and Livestock Australia, ABC Fact Check Unit, University of Sydney (School of Life and Environmental Sciences), Insurance Coun-

cil of Australia. AG thanks Rick McRae of the ACT Emergency Services Agency for his generous assistance with the preparation of the

pyroCb graphic and Alvin Stone, ARC Centre of Excellence for Climate Extremes, for his assistance with fact checking.

Figures and statistics correct at time of printing but subject to change.

ILLUSTRATION BY MICHAEL PAYNE

A typical pyroCb event

Smoke plume forming

central core while

expanding

AG