Jason J. Sharples

An overview of mountain meteorological effects relevant to fire behaviour and bushfire risk

Many of the processes that can occur in mountainous landscapes have the potential to significantly affect fire behaviour and bushfire risk in general. These processes can lead to otherwise unexpected fire behaviour and escalation in fire size and severity that could endanger firefighting crews and compromise suppression activities. Interaction of upper winds with rugged terrain can often result in highly variable and turbulent wind patterns and variations in temperature and humidity that can affect fire regimes in the long and short term. More generally, the effect of rugged terrain on atmospheric flows can give rise to complex dynamics and emergent properties that are discontinuous in nature. Hence, the ‘fire weather continuum’ that is often assumed in fire management practices is of reduced validity in mountainous or hilly landscapes. This paper presents an overview of the main elements of mountain meteorology relevant to fire weather and discusses the potential roles they may play in bushfire behaviour, development and risk. As such, the paper is intended to promote understanding, across the wide range of professions concerned with bushfire, of how mountain meteorological effects might contribute to fire potential and fire behaviour.

A simple index for assessing fire danger rating

Fire danger rating systems are used to assess the potential for bushfire occurrence, fire spread and difficulty of fire suppression. Typically, fire danger rating systems combine meteorological information with estimates of the moisture content of the fuel to produce a fire danger index. Fire danger indices are used to declare fire bans and to schedule prescribed burns, among other applications. In this paper a simple fire danger index F that is intuitive and easy to calculate is introduced and compared to a number of fire danger indices pertaining to different fuel types that are used in an operational setting in Australia and the United States. The comparisons suggest that F provides a plausible measure of fire danger rating and that it may be a useful pedagogical tool in the context of fire danger and fire weather.

A simple index for assessing fuel moisture content

Assessing fuel moisture content to within a reasonable degree of accuracy is an important part of wildland fire management. In this paper we introduce a fuel moisture index that provides a simple and intuitive method for assessing fuel moisture content. The method can be quickly and easily applied in a field setting to provide a dimensionless measure of fuel moisture content. We compare the index with predictions from several models for fuel moisture content and conclude that it provides an equivalent measure of fuel moisture content for a number of fuel types. We go on to briefly discuss how the index could be used to construct a simple and intuitive fire danger index.

On the Horizontal Scale of Elevation Dependence of Australian Monthly Precipitation

Abstract Determination of the scale of the interaction between precipitation and topography is important for the accurate interpolation of rainfall in mountainous areas and also provides insight into the physical processes involved. In this paper, trivariate thin-plate smoothing splines are used to investigate the scale of interaction between monthly precipitation and topography by interpolating monthly rainfall over three subregions of the Australian continent, incorporating different climatic conditions and rainfall types. The interpolations are based upon elevations derived from digital elevation models (DEMs) of various resolutions. All of the DEMs are local averages of version 2.0 of the 9-s-resolution DEM of Australia. The results suggest that the optimal scale of the interaction between precipitation and topography, as it pertains to the elevation-dependent interpolation of monthly precipitation in Australia, is between 5 and 10 km. This is in agreement with results of similar studies that addresse...

Wind–terrain effects on the propagation of wildfires in rugged terrain: fire channelling

The interaction of wind, terrain and a fire burning in a landscape can produce a variety of unusual yet significant effects on fire propagation. One such example, in which a fire exhibits rapid spread in a direction transverse to the synoptic winds as well as in the usual downwind direction, is considered in this paper. This type of fire spread, which is referred to as ‘fire channelling’, is characterised by intense lateral and downwind spotting and production of extensive flaming zones. The dependence of fire channelling on wind and terrain is analysed using wind, terrain and multispectral fire data collected during the January 2003 Alpine fires over south-eastern Australia. As part of the analysis, a simple terrain-filter model is utilised to confirm a quantitative link between instances of fire channelling and parts of the terrain that are sufficiently steep and lee-facing. By appealing to the theory of wind–terrain interaction and the available evidence, several processes that could produce the atypical fire spread are considered and some discounted. Based on the processes that could not be discounted, and a previous analysis of wind regimes in rugged terrain, a likely explanation for the fire channelling phenomenon is hypothesised. Implications of fire channelling for bushfire risk management are also discussed.

Foehn-Like Winds and Elevated Fire Danger Conditions in Southeastern Australia

Abstract Bushfires in southeastern Australia are a serious environmental problem, and consistently cause loss of life and damage to property and other assets. Understanding synoptic processes that can lead to dangerous fire weather conditions throughout the region is therefore an important undertaking aimed at improving community safety, protection of assets, and fire suppression tactics and strategies. In southeastern Australia severe fire weather is often associated with dry cool changes or coastally modified cold fronts. Less well known, however, are synoptic events that can occur in connection with the topography of the region, such as cross-mountain flows and foehn-like winds, which can also lead to abrupt changes in fire weather variables that ultimately result in locally elevated fire danger. This paper focuses on foehn-like occurrences over the southeastern mainland, which are characterized by warm, dry winds on the lee side of the Australian Alps. The characteristics of a number of foehn-like occ...

Review of formal methodologies for wind–slope correction of wildfire rate of spread

The effects of wind and topographic slope are important considerations when determining the rate and direction of spread of wildfires. Accordingly, most models used to predict the direction and rate of spread contain components designed to account for these effects. Over the years, a variety of different approaches have been developed. In the present manuscript, we examine the various mathematical models employed to account for the effects of wind and slope at a formal level, making comparisons where appropriate. The methods reviewed include scalar methods, which ignore the directional nature of wind and slope effects, as well as methods in which the effects of wind and slope are combined in a vectorial manner. Both empirical and physical models for wind–slope correction are considered.

Natural hazards in Australia: extreme bushfire

Bushfires are one of the most frequent natural hazards experienced in Australia. Fires play an important role in shaping the landscape and its ecological dynamics, but may also have devastating effects that cause human injuries and fatalities, as well as broad-scale environmental damage. While there has been considerable effort to quantify changes in the occurrence of bushfire in Australia, a comprehensive assessment of the most extreme bushfire cases, which exact the greatest economic and environmental impacts, is lacking. In this paper we reflect upon recently developed understanding of bushfire dynamics to consider (i) historical changes in the occurrence of extreme bushfires, and (ii) the potential for increasing frequency in the future under climate change projections. The science of extreme bushfires is still a developing area, thus our conclusions about emerging patterns in their occurrence should be considered tentative. Nonetheless, historical information on noteworthy bushfire events suggests an increased occurrence in recent decades. Based on our best current understanding of how extreme bushfires develop, there is strong potential for them to increase in frequency in the future. As such there is a pressing need for a greater understanding of these powerful and often destructive phenomena.

Large eddy simulation of atypical wildland fire spread on leeward slopes

The WRF-Fire coupled atmosphere-fire modelling system was used to investigate atypical wildland fire spreadonsteepleewardslopesthroughaseriesofidealisednumericalsimulations.Thesimulationsareusedtoinvestigate both the leeward flow characteristics, such as flow separation, andthe fire spread from an ignition region atthe base of the leewardslope.Thefirespreadwasconsideredundervaryingfueltypeandwithatmosphere-firecouplingbothenabledand disabled.Whenatmosphere-firecouplingisenabledandthereisahighfuelmassdensity,thefirespreadcloselyresembles thatexpectedduringfirechannelling.Specifically,thefirespreadisinitiallydominatedbyupslopespreadtothemountain ridge line at an average rate of 2.0kmh � 1 , followed by predominantly lateral spread close to the ridge line at a maximum rate of 3.6kmh � 1 . The intermittent rapid lateral spread occurs when updraft-downdraft interfaces, which are associated with strongly circulating horizontal winds at the mid-flame height, move across the fire perimeter close to the ridge line. The updraft-downdraft interfaces are formed due to an interaction between the strong pyro-convection and the terrain- modified winds. Through these results, a new physical explanation of fire channelling is proposed.

Evaluation of a very simple model for predicting the moisture content of eucalypt litter

Operational prediction of wildfire behaviour requires assessment of dead fuel moisture content to an acceptable degree of accuracy. Ideally, the methods of assessment should be simple enough to implement in most operational settings, including those where computational power is a constraining factor. In this short note, we describe a very simple model for estimating dead fine fuel moisture content and compare its predictions with several fuel moisture observations and the predictions of a complex process-based model and two of its simplifications. Remarkably, the very simple model is shown to fit the observational data just as well, if not slightly better, than the more sophisticated models. The result highlights the issues of engineering and parsimony of models for dead fuel moisture content. These issues are briefly discussed.