TOPICS IN THIS EDITION l RISK l TECHNOLOGY l NATURAL ENVIRONMENT ISSUE 137 DECEMBER 2014 MODEL PINPOINTS FACTORS DRIVING EXTREME BUSHFIRE BEHAVIOUR CONTEXT The WRF-Fire coupled atmosphere-fire model was used to investigate physical interactions between bushfire, local terrain and atmosphere, that drive lateral fire spread across steep leeward slopes. This dynamic mode of fire spread was observed in the 2003 Canberra bushfires and can result in extreme fire behaviour. BACKGROUND On 18 January 2003, bushfires in and around the Australian Capital Territory exhibited some of the most extreme fire behaviour ever witnessed in Australia. Using remote- sensing technology, including thermal and multispectral linescans, satellite imagery and aerial photographs, McRae (2004) presented Researchers used the WRF-Fire coupled atmosphere-fire model to examine dynamic fire evidence of extreme fire behaviour during spread observed in the ACT 2003 bushfires. the 2003 Canberra bushfires. He included a description of an “asymmetric wind-terrain SUMMARY interaction event” that occurred in a number In the 2003 Canberra bushfires, a number of unusual fires were observed in which of bushfires. In those events, the fire advanced bushfire spread sideways in a diagonal or crosswise direction to background winds. It rapidly across a leeward slope in a direction occurred alongside rapid and intense downwind fire spread due to spotting. The atypical approximately transverse to the background fire occurrence was on steep leeward slopes (a sheltered slope facing away from the wind) winds. In addition to this atypical lateral fire where background wind was mostly perpendicular to the terrain. spread, embers were launched by atmospheric The WRF-Fire coupled atmosphere-fire model was used extensively to examine both eddies and travelled downwind, resulting in a the physical mechanism driving the atypical lateral fire spread, as well as its sensitivity rapid and intense downwind propagation of to the fire environment conditions. The numerical simulations qualitatively match the the fire. fire spread seen in the 2003 Canberra bushfires, and provide new insights into dynamic modes of fire spread in rugged terrain. The results have important implications for The atypical lateral and downwind fire spread firefighter safety and fire management. observed in the 2003 Canberra bushfires was subsequently studied by Sharples et al. ABOUT THIS PROJECT (2012), who termed the phenomenon “fire This research originated from a Bushfire CRC funded PhD project entitled ‘Dynamic channelling”. Sharples et al. (2012) ruled interactions between wildland fires and the surrounding atmosphere’. Dr Simpson out several possible physical mechanisms continues to research this topic in his new position as a postdoctoral fellow at the as the driver of the atypical lateral fire University of New South Wales. spread. In addition, they discovered that the AUTHOR phenomenon was sensitive to the terrain slope and background wind direction relative to Dr Colin Simpson (right), Postdoctoral Fellow, School of the terrain orientation. Based on the lateral Physical, Environmental and Mathematical Sciences, University fire spread events in the Canberra bushfires, of New South Wales, Canberra, Australia. Formerly a Bushfire they estimated that a leeward slope of around CRC funded PhD candidate (2009-2013) at the Department of 26-30˚ or higher, and a wind direction within Geography, University of Canterbury, Christchurch, New Zealand. around 40˚ of the terrain aspect (i.e. winds For more information contact [email protected] approximately perpendicular to the terrain) © BUSHFIRE CRC LTD 2014 All Fire Notes are available at bushfirecrc.com/firenotes SUBSCRIBE Share on 1 was required for the lateral fire spread to occur. 1200 This atypical fire spread challenges the assumption commonly made in operational 925 fire spread prediction tools that the rate and direction of forward fire spread remains 650 constant for non-varying fire environment 6000 conditions. If the appropriate fire environment 375 5400 4800 thresholds are met, it is possible for a fire 4200 3600 on a leeward slope to exhibit asymmetric 100 and extreme fire behaviour with little or no 3000 change in the local fire environment required. 3900 It is this non quasi-steady state nature of the 4200 atypical fire spread, which can occur at a rapid 4500 rate, that makes the phenomenon a potential 4800 danger to firefighters. 5100 Figure 1: A three-dimensional visualisation of the wind flow associated with the fire whirls that drive the atypical lateral fire spread across steep BUSHFIRE CRC RESEARCH leeward slopes. The red surface colour shows the extent of the fire line Following the studies by McRae (2004) starting from a circular ignition towards the bottom of the leeward slope, and Sharples et al. (2012), two important which has advanced upslope and then spread laterally close to the ridge. questions about the atypical fire spread events The flow streamlines show the spatial extent of the fire whirls and the observed in the 2003 Canberra bushfires corresponding wind speeds (m/s, as shown in colour scale). remained. The first asked what physical demonstrated that WRF-Fire could simulate of ignited fuel in the fire model produces mechanism drove the lateral fire spread? The atypical lateral fire spread across steep leeward sensible and latent heat, which is then second was what was the relationship between slopes in a qualitatively similar manner to injected back into the atmospheric model, the atypical fire spread and fire environment that observed in the 2003 Canberra bushfires. and can therefore affect the local atmospheric conditions, in particular the terrain slope, However, the rapid and intense downwind dynamics and subsequently the fire spread, the fuel properties, and the background wind propagation of the fire discussed by McRae hence providing the fire to atmosphere speed and direction? (2004) was not reproduced, most likely due to coupling. the absence of a spotting model in WRF-Fire. Initially part of a Bushfire CRC funded PhD Simpson et al. (2014) noted that for those project investigating the dynamic interactions WRF-Fire is one of a small number of WRF-Fire simulations that exhibited atypical between wildland fire and the atmosphere, models that can directly model the dynamic lateral fire spread, three distinct stages of fire the research discussed in this fire note was interactions between a wildland fire and spread were evident. First, the fire advanced undertaken to address those two questions. the atmosphere. It combines the Weather upslope due to the combined effects of wind More broadly, the research fitted into Bushfire Research and Forecasting (WRF) numerical reversal along the leeward slope and the CRC research into fire behaviour modelling weather prediction model with a two- positive upslope contribution to the rate and the physical mechanisms responsible for dimensional implementation of Rothermel’s of spread. Second, a thermal, or pyrogenic, extreme fire behaviour. semi-empirical fire spread model. The rate of wind developed along the leeward slope that RESEARCH OUTCOMES spread is calculated at sub-second intervals increased the upslope rate of spread and could Simpson et al. (2013, 2014) and Sharples at multiple points along the fire line, and is trigger the formation of fire whirls over the et al. (2013) used the WRF-Fire coupled dependent on the local fuel properties, the fire region. A fire whirl is a rotating column atmosphere-fire model, described below, to terrain slope and the mid-flame height winds. of air above the fire region and is pyro- perform idealised numerical simulations of The mid-flame height winds are calculated convectively driven, which distinguishes it the two-dimensional fire spread following using the atmospheric model, hence providing from atmospheric vortices such as dust devils. the ignition of a fire on a leeward slope. They the atmosphere to fire coupling. Combustion Third, if a fire whirl moves across the fire line, WRF-Fire is one of a small number of models that can model dynamic interactions between a bushfire and the atmosphere. 2 END USER STATEMENT A major impetus for research into wildfire dynamics was observations from the January 2003 alpine fires. With rapid advances still being made in physical drivers of fires, it remains crucial to make detailed observations of fires to support research. The long-term benefits to fire agencies are immense, but require the collection of detailed field and airborne data, as well as weather and satellite data. As knowledge of dynamic modes of fire spread improves, fire agencies will be better placed to provide researchers with rich data on a broad range of fire events. This could potentially lead to improved operational predictability and management of bushfires. It is worth noting that there have been a number of incidents in recent decades where firefighters have been injured or killed when fires in rugged terrain suddenly became more severe. This has been largely due to unexpected changes in local wind conditions or through some dynamic interaction between the fire and its local environment. This specific work on VDLS improves our understanding of extreme fire behaviour, and in turn could help reduce the risk to firefighters working in extreme bushfire conditions. – Rick McRae, ACT Emergency Services Figure 2: Plan view of a fire whirl driving the lateral fire spread between a time of 50 and 53 min in Agencies one of the WRF-Fire simulations. The wind vectors and colours show the mid-flame height winds across the leeward slope. It is evident from the black lines, which depict the changing position of the fire line at 1 min intervals, that rotating winds are responsible for driving lateral and diagonally background wind speed of around 5 m/s or downslope fire spread during this three minute period. lower. More recent WRF-Fire simulations, which will be published in due course, thereby providing a local wind that is aligned when they move laterally along the slope or also suggest that the simulated lateral fire approximately transverse to the background diagonally downslope.