Impact of Vertical Atmospheric Structure on an Atypical Fire in a Mountain Valley

Abstract

Wildfires are not only a natural part of many ecosystems, but they can also have disastrous consequences for humans, including in Australia. Rugged terrain adds to the difficulty of predicting fire behavior and fire spread, as fires often propagate contrary to expectations. Even though fire models generally incorporate weather, fuels, and topography, which are important factors affecting fire behavior, they usually only consider the surface wind; however, the more elevated winds should also be accounted for, in addition to surface winds, when predicting fire spread in rugged terrain because valley winds are often dynamically altered by the interaction of a layered atmosphere and the topography. Here, fire spread in rugged terrain was examined in a case study of the Riveaux Road Fire, which was ignited by multiple lightning strikes in January 2019 in southern Tasmania, Australia and burnt approximately 637.19 km2. Firstly, the number of conducive wind structures, which are defined as the combination of wind and temperature layers likely to result in enhanced surface wind, were counted by examining the vertical wind structure of the atmosphere, and the potential for above-surface winds to affect fire propagation was identified. Then, the multiple fire propagations were simulated using a new fire simulator (Prototype 2) motivated by the draft specification of the forthcoming new fire danger rating system, the Australian Fire Danger Rating System (AFDRS). Simulations were performed with one experiment group utilizing wind fields that included upper-air interactions, and two control groups that utilized downscaled wind from a model that only incorporated surface winds, to identify the impact of upper air interactions. Consequently, a detailed analysis showed that more conducive structures were commonly observed in the rugged terrain than in the other topography. In addition, the simulation of the experiment group performed better in predicting fire spread than those of the control groups in rugged terrain. In contrast, the control groups based on the downscaled surface wind model performed well in less rugged terrain. These results suggest that not only surface winds but also the higher altitude winds above the surface are required to be considered, especially in rugged terrain.

Dynamic Wind Effect on Fireline Intensity in Rugged Terrain

“…(1) Although, each cross section between preceding isochrone was linear, due to current constraints in the capabilities of Prototype 2, only a single ignition is allocated along the valley axis of each fire isochrone in order to reduce bias. (2) other limitations follows our previous study (Ozaki et al, 2022).…”

“…The main dataset used as the basis of this study is the Bureau of Meteorology Atmospheric highresolution Regional Reanalysis for Australia for the Tasmanian region (BARRA-TA), which consists of various weather parameters, such as cloud coverage, precipitation, temperature, dew point temperature, geopotential height, soil moisture, and relative humidity represented at various atmospheric pressure levels (Eizenberg et al, 2021;Su et al, 2019). In addition, the surface wind in this dataset is employed to produce another surface wind field downscaled by WindNinja, a diagnostic tool (Firelab, n.d., 2020;Ozaki et al, 2022). These parameters, including two types of these surface wind fields, are used to parameterize the Prototype 2 fire simulator, and used to produce wind vector maps and numerical weather model vertical soundings (NWMVS).…”

“…Prototype 2 is a fire simulator that incorporates various fire models of fire spread for different vegetation types and provide two types of wind field: BARRA-TA and downscaled winds (Ozaki et al, 2019). The prototype 2 then maps simulation results on either of five geometries: Delaunay, diamond, hexagon, square and Voronoi (Ozaki et al, 2022). Primarily, fire propagation is simulated by Prototype 2.…”

“…Fractions skill score (FSS) is employed to verify the simulated fire propagation against actual recorded isochrons (Ozaki et al, 2022). The FSS provides a threshold value representing "usefulness" which is used to confirm the accuracy of fire spreading.…”

“…Three specific types of plots are analyzed: wind vector plots, which show the air flow with topography, and is expected to identify terrain forced flow (TFF), numerical weather model vertical soundings (NWMVS), which indicate various weather changes through the depth of the atmosphere in a time series, and fireline intensity, which shows the spatially explicit intensity of fire based on hotspot images from two types of satellite system, also in a time series. Fire spread simulations were performed using Prototype 2, a fire simulator implemented based on the specification of Australian Fire Danger Rating System (AFDRS) (Ozaki et al, 2022). Prototype 2 allows not only simulation of fire propagation but also recording the fireline intensity and detecting potential lateral fire channeling (LFC) in each prediction grid when simulating the fire propagation.…”