Meteorological drivers of the eastern Victorian Black Summer (2019–2020) fires

Abstract

The spring and summer of 2019–2020 (Black Summer) saw the largest and most significant bushfire outbreak recorded in eastern Australia. In Victoria, the fires ran from mid-November through early autumn. In this paper, we use a high-spatial and temporal resolution 48-year fire weather re-analysis data set (VicClim5) to describe fire weather and vertical wind and stability profiles for five significant high Forest Fire Danger Index (FFDI) fire events and compare these with detailed fire reconstructions. A feature of several of these fires was very active overnight fire spread driven by topographically enhanced low-level jets and low fine fuel moisture content. The FFDI values on these nights were either the highest or near highest on record in the 48-year data set. We describe cases of lightning ignition, prefrontal fire spread and two cases of post-frontal fire spread – one into Mallacoota on the early morning of 31 December 2019 and the other a northward overnight run down the Buffalo Valley on 4–5 January 2020. On two of the days studied there were complex wind changes associated with the inland penetration of low-level south-easterly winds under the influence of locally generated pressure gradients. An elevated hot, dry mixed layer above these shallow layers also played an important role. On one occasion there is some evidence of possible mountain-wave modulation of surface wind flows. These events demonstrate a range of features of the fire weather and climate in eastern Victoria and the utility of VicClim5 in 3-dimensional climatological analyses.

Investigating the Effect of Fuel Moisture and Atmospheric Instability on PyroCb Occurrence over Southeast Australia

“…C-Haines uses fixed pressure levels for calculation, which indeed limits its application in varying topography. However, in southeast Australia, the warm summer days usually associated with significant wildfire activity are typically characterized by a deep planetary boundary layer [46]. Within this well-mixed layer, the dewpoint and temperature lapse rates are approximately constant, so that the dewpoint and temperature traces form a triangular pattern in a skew-T log P thermodynamic diagram, such as can be seen in Figure 9.…”

Future fire events are likely to be worse than climate projections indicate – these are some of the reasons why

“…The concurrent overnight fire runs at the Badja Forest, Corryong and Banana Track were driven by meteorological processes synonymous with a heatwave over south-eastern Australia, a deep atmospheric boundary layer with hot, dry temperatures, and a nocturnal low-level jet. Mills et al (2022) used the VicClim 5 reanalysis dataset to examine meteorological drivers of several of the Victorian Black Summer fires reported on by Salkin (2022). Their description includes examples of low-level inversions overlain by an elevated mixed layer that through fire-atmosphere coupling may influence fire behaviour in a way not represented by surface meteorology.…”

Introduction to the Australian Fire Danger Rating System†

“…Over the last two decades, the occurrence of significant catastrophic wildfire (bushfire) events has led to large numbers of human fatalities, significant asset losses and long term economic and social impacts in Australia (Cruz et al 2012;Nolan et al 2021;Mills et al 2022) and disparate places including Chile (Gómez-González et al 2018), Portugal (Castellnou et al 2018), Greece (Xanthopoulos and Athanasiou 2019) and the United States of America (Nauslar et al 2018). Recent variations to fire regimes including increased fire weather severity, frequency and extent have been linked to changes in climate and land use patterns in different regions around the world (Clarke and Evans 2018;Dowdy 2018;Syphard et al 2019).…”