Predicting fire thunderstorms

By Rick McRae. This article first appeared in Issue One 2018 of Fire Australia.

Global monitoring of the atmosphere began in 1978. For most of this era, bushfires were not on the agenda of the atmospheric scientists involved, with volcanoes an initial focus after a British Airways 747 flew into a volcanic ash cloud in 1982, stalling all four engines. Before this, atmospheric scientists had been intensively watching for the effects of nuclear weapons tests, as these events show up clearly in the clean stratosphere.

And yet there were cases of stratospheric aerosol injections that could not be explained. By running weather models backwards, it was deduced that large bushfires in Canada were the source. Now the search was on for a fire event to confirm this. In 2001, the Chisholm fire in Alberta, Canada, formed a fire thunderstorm (pyro-cumulonimbus or PyroCb for short). A number of papers were written on this fire that raised key questions, and another detailed case study was sought.

On 8 January 2003 a dry lightning storm lit scores of fires across the Australian Alps. The only previous consideration of a fire thunderstorm was the Berringa fire in Victoria in 1995, which addressed the threat to firefighters if the smoke plume collapsed. It started firefighters and fire managers thinking about what was going on over our heads - that the fire plumes had to monitored, not just the flames.

When the January 2003 alpine fires formed violent pyro-convection, no-one was prepared. There was no predictive capability. These events are still the subject of scientific studies - I go as far as saying that they are the most scientifically important bushfires ever.

From 1978 to 2001, Australia recorded two minor pyroCbs. Since 2001 we have 56 on record (to November 2016), including some of the most intense events, globally. PyroCbs have long been a problem in the forests of the United States, Canada, eastern Russia and Mongolia. They became of problem in Australia in 2001, in western Russia in 2010 and in Europe in 2017. The Black Saturday fires in 2009 had the record for the most intense pyro-convection until August 2017, when the Chezacut fire in British Columbia, Canada, erupted.

How pryoCbs develop

Australia has entered the “era of violent pyro-convection”. Bushfires are modelled and predicted based on the assumption of steady-state spread. This includes weather, terrain and vegetation inputs to predict a fire's behaviour. For any inputs, the model gives a unique prediction of what the fire will be doing. This is the basis for all fire service preparedness, fuel management and community protection.

PyroCbs are now known to occur when a fire forms deep flaming under an unstable atmosphere. Deep flaming is the depth of the active fire, as in how far back from the front there is strong heat release. This can be some kilometres, and is not to be confused with the fire front.

A Canberra-based research group is developing a list of conditions when deep flaming can occur. Two of these occur during steady-state fire spread - high rate of spread and a wind change. Others are now grouped under dynamic fire spread. Dense spotting creates deep flaming as the spotfires merge. Vorticity-driven lateral spread (VLS, or fire channelling), is the most effective source of deep flaming known. Eruptive growth is a concept that emerged from a fire in London's Kings Cross Underground railway station. Prof Domingos Viegas from the University of Coimbra in Portugal has shown how the flame attachment involved can very easily lead to fire crew burn-overs. Sebastien Lahaye at the University of NSW in Canberra has recently extended our knowledge of dynamic burn-over causes. Interior ignition is a new concept resulting from staggered flammability of different fuels. A final cause of deep flaming is inappropriate use of a drip torch on a bad fire day.

The unstable atmosphere is a difficult concept for fire agencies to handle. The Haines Index and its continuous variants are the main tools that have been in use for decades, but they do not pick up the key elements needed. A better tool must be developed or found. Researchers in the Bureau of Meteorology, through the Bushfire and Natural Hazards CRC, including Dr Mika Peace, Dr Jeff Kepert and their colleagues, are looking closely at the instability above fires.

Predicting pryoCbs

An extreme bushfire is defined as a fire that, on one or more occasions over its duration, will form deep flaming in an atmospheric environment conducive to the fire coupling with the atmosphere and the plume punches through the cloud base (a blow-up). Our group developed a process model, called BUFO - the Blow-Up Fire Outlook, which takes fire behaviour analysts through a series of questions, the answer to which determine which question is next, or if the analyst loops back to the beginning to wait for conditions to change, before starting again. Most questions seek to anticipate deep flaming, with raised fire danger a prerequisite condition.

It is an open research topic that vegetation (fuel for the fire) does not currently have a role in the model. The same coupled fire-atmosphere events occur over the vast range of fuel types found in Alpine Ash, Siberian steppe, Albertan boreal forests, or the Great Victoria Desert.

The BUFO model has been formally tested in NSW and the ACT. I conducted the trial with formal oversight from A/Prof Jason Sharples (University of NSW) and Laurence McCoy (NSW Rural Fire Service). Over three fire seasons we got enough data to confirm the model, with a number of blow-up events predicted and incident management teams alerted. No pyroCbs occurred in the trial domain, but we did informally anticipate some pyroCbs elsewhere in Australia. The formal statistical results were sufficient for the model to be declared successful, and it is now operational.

Our group is seeking to expand its implementation into jurisdictions beyond the trial area - the model is most useful south of the Tropic of Capricorn.

The Sir Ivan fire

A key part of the BUFO is that on a day of widespread raised fire danger with many fires burning, the one or two fires that have the potential to develop a pyroCb can be distinguished. On 11 February 2017 a bad fire day was predicted in north east NSW, with an even worse day forecast on the 12th. There were many fires burning, with the BUFO model successfully predicting that most would not blow-up to a pryroCB, and that only the Sir Ivan fire had the potential, through VLS, to do so. An alert was issued for this fire late on 11 February, and by mid-afternoon the next day it had formed a PyroCb just as a trough-line passed (producing peak instability). Other fires nearby did not blow-up.

The Sir Ivan fire is the only fire, globally, for which a formal operational forecast of a blow-up and subsequent pyroCb has occurred. While not wishing for more pyroCbs, the goal is to be able to anticipate their formation. The researchers require discussions with fire services to see how the model might be implemented elsewhere.

It has long been thought that climate change's impact on bushfires would involve turning up the dial with temperatures on the rise. Now it is clear that there is a big switch as well, and that in Australia it was flicked to ‘on’ in 2001. Blow-up pryroCb fires are the cause of much of the impacts on the Australian community, and they are poorly handled by the primary fire prediction tools in use currently.