@article {bnh-7499, title = {Determining threshold conditions for extreme fire behaviour - annual report 2019-2020}, number = {626}, year = {2020}, month = {11/2020}, institution = {Bushfire and Natural Hazards CRC}, address = {MELBOURNE}, abstract = {

At this phase the project was focused on development of a new method to test flammability of live vegetation in dynamic conditions and understanding influence of climatic changes on the 2019/20 bushfire season in New South Wales (NSW), Victoria, and South Australia (SA).

Understanding live vegetation fuel properties and how they behave when exposed to radiant heat and flame allows us to better predict fire behaviour in forested areas. This study aims to determine a more effective, replicable and accurate method of testing flammability in live vegetation by comparing the impact different radiant heating regimes have on the ignitibility of live vegetation samples. Current methodologies are limited in their ability to provide accurate quantification of flammability due to their reliance on static heat flux exposure, which does not accurately replicate how live plants experience radiative heat flux during a wildfire in their natural environment. Two heating regimes were tested for this study {\textendash} a static heat flux to reflect current methods and a dynamic (increasing) heat flux to more accurately replicate real conditions of an approaching fire front. Piloted-ignition and unpiloted-ignition were also tested for both of these heating regimes. A Variable Heat Flux (VHFlux) Apparatus was used to study flammability of Acacia floribunda, Cassinia arcuata, Pinus radiata and bark from Eucalyptus obliqua. Time to pyrolysis (production of volatile products), smouldering, flaming ignition, complete consumption and radiant exposure (the radiant energy received by a sample over a time of heating, He) were used as ignitability measures. It was observed that time and radiant exposure required to reach flaming ignition (and the other ignitibility metrics) was higher under a dynamic heating regime. It was also observed that the presence of a pilot igniter greatly increased the number of samples that reached flaming ignition, and decreased the time and He required to reach flaming ignition (and the other ignitibility metrics). These results suggest clear differences observed between heating regimes for time and Herequired for ignition and other ignitibility measures, which supports the validity of using dynamic heating regimes and the VHFlux apparatus as a standardised method. Adoption of this methodology is recommended to ensure more realistic data on flammability of individual plant species and plant communities, which will ultimately lead to better informed and more accurate wildfire behaviour modelling.

There is no doubt that the fire season of 2019/20 was extraordinary. A total of 18,983,588hectares were burned, 3113 houses and 33 lives lost in 15,344 bushfires in Black Summer fires. NSW had the highest number of fires, area burned, houses and lives lost for the last 20 years. Two mega-blazes occurred in NSW and burned more than in any fire season during the last 20 years. Victoria had the highest number of fires, area burned, and houses lost (except for the Black Saturday fires). SA had the highest number of houses lost in the last 20 years. Relationships between the burned area and number of fires, the houses and lives lost had positive trend for all states irrespective of the dataset. A negative relationship between the houses and lives lost for SA was the only exception. Multiple studies show that fire weather will become more severe in many regions around the world. Based on this and observed positive trends for all categories for NSW and Victoria, it is likely that the values will continue to increase in these states in the future. SA before 2019/20 was in a relatively good position showing negative trends for almost all categories. However, the 2019/20 fire season changed that for the worse. The magnitude of effect from increased fire weather may depend on how these conditions alter vegetation across Australia, however the indications shown in this analysis are concerning for fire managers.

Smoke from bushfires significantly impacted on people with cardiovascular and respiratory problems and increased mortality. It also had indirect impact on the economy by disrupting communities. The total impact of the 2019/20 bushfire season to the economy is estimated to be as much as A$40 billion. Due to the record burned area, at least 1 billion vertebrate animals were lost. It will take many years to restore the economy in impacted areas, and for animal and vegetation biodiversity to recover. Understanding of high-level trends of number of fires, area burned, houses and lives lost for the last two decades in south-eastern Australia will provide useful insights to fire managers for future strategies and policies.

}, keywords = {Behaviour, conditions, extreme fire, threshold}, issn = {626}, author = {Alex Filkov and Thomas Duff and Trent Penman} } @article {bnh-6392, title = {Evolution of an extreme Pyrocumulonimbus-driven wildfire event in Tasmania, Australia}, journal = {Natural Hazards and Earth System Sciences}, year = {2019}, month = {12/2019}, abstract = {

Extreme fires have substantial adverse effects on society and natural ecosystems. Such events can be associated with intense coupling of fire behaviour with the atmosphere, resulting in extreme fire characteristics such as pyrocumulonimbuscloud (pyroCb) development. Concern that anthropogenic climate change is increasing the occurrence of pyroCbs globally is driving more focused research into these meteorological phenomena. Using 6-minute scans from a nearby weather radar, we describe the development of a pyroCb during the afternoon of 4 January 2013 above the Forcett-Dunalley fire in south-eastern Tasmania. We relate storm development to: (1) near-surface weather using the McArthur Forest Fire Danger Index (FFDI), and the C-Haines Index, a measure of the vertical atmospheric stability and dryness both derived from gridded weather reanalysis for Tasmania (BARRA-TA), and (2) a chronosequence of fire severity derived from remote sensing. We show that the pyroCb rapidly developed over a 24-minute period in the afternoon of 4 January, with the cloud top reaching a height of 15 km. The pyroCb was associated with a highly unstable atmosphere (C-Haines 10-11) and Severe-marginally Extreme (FFDI 60-75) near-surface fire weather, and formed over an area of forest that was severely burned (total crown defoliation). We use spatial patterns of elevated fire weather in Tasmania, and fire weather during major runs of large wildfires in Tasmania for the period 2007-2016 to geographically and historically contextualise this pyroCb event. Although the Forcett-Dunalley fire is the only known record of a pyroCb in Tasmania, our results show that eastern and south-eastern Tasmania are prone to the conjunction of high FFDI and C-Haines values that have been associated with pyroCb development. Our findings have implications for fire weather forecasting and wildfire management, and highlight the vulnerability of southeast Tasmania to extreme fire events.

}, keywords = {C-Haines, extreme fire, fire severity, Fire weather, McArthur Forest Fire Danger Index, pyrocumulonimbus, smoke plume injection, Tasmania, wildland fire}, doi = {https://doi.org/10.5194/nhess-2019-354}, url = {https://www.nat-hazards-earth-syst-sci-discuss.net/nhess-2019-354/}, author = {Mercy Ndalila and Grant Williamson and Paul Fox-Hughes and Jason J. Sharples and David Bowman} }