Pyroconvection Risk in Australia: Climatological Changes in Atmospheric Stability and Surface Fire Weather Conditions

Abstract

Extreme wildfires with strong convective processes in their plumes have recently led to disastrous impacts on various regions of the world. The Continuous Haines index (CH) is used in Australia to represent vertical atmospheric stability and humidity measures relating to pyroconvective processes. CH climatology is examined here using reanalysis data from 1979 to 2016, revealing large spatial and seasonal variations throughout Australia. Various measures of severity are investigated, including regionally specific thresholds. CH is combined with near‐surface fire weather conditions, as a type of compound event, and is examined in relation to environmental conditions associated with pyroconvection. Significant long‐term changes in CH are found for some regions and seasons, with these changes corresponding to changes in near‐surface conditions in some cases. In particular, an increased risk of pyroconvection is identified for southeast Australia during spring and summer, due to decreased vertical atmospheric stability and humidity combined with more severe near‐surface conditions.

Evolution of an extreme Pyrocumulonimbus-driven wildfire event in Tasmania, Australia

“…Southeast Tasmania is particularly prone to foehn winds in pre-frontal environments, making the region vulnerable to wind direction changes that cause fire flanks to become fire fronts, thereby rapidly escalating the size of fires. Our analysis has shown that the Forcett- Mainland Australia, and especially the south-eastern region, experiences a greater number of days of elevated values of FFDI and C-Haines than Tasmania (Dowdy and Pepler, 2018). The FFDI/C-Haines distribution in that region shows pyroCbs occurring over a wide range of FFDI but under more limited extreme values of C- Haines (Di Virgilio et al, 2019).…”

“…PyroCbs lead to highly erratic fire behaviour because of strong updrafts and downdrafts, the possibility of associated whirlwinds and tornadoes, and rapid fire growth due to heightened ember generation, long-range spotting and cloud to ground lightning strikes (Lareau and Clements, 2016;Dowdy and Pepler, 2018;Fromm et al, 2010;Tory and Thurston, 2015;Cunningham and Reeder, 2009). Specifically, downdrafts can cause erratic fire spread, driven by sudden wind gusts impacting the surface in multiple directions, endangering firefighters near the pyroCb (Johnson et al, 2014;Potter and Hernandez, 2017) and frustrating accurate prediction of fire behaviour.…”

Understanding the changing fire environment of south-west Western Australia

“…soil dryness) to be analysed independently of the observational network which is limited across much of SWWA. Re-analysis data now provide a robust method for establishing climatological norms for meteorological variables and derived values of fire danger and potential fire behaviour (Jones et al 2009, Dowdy andPepper 2018).…”

Weather Radar Insights Into the Turbulent Dynamics of a Wildfire‐Triggered Supercell Thunderstorm

“…The complex interactions between weather, fuels, and anthropogenic influences (including relating to fire ignition and fire suppression activities) make accurate predictions of extreme wildfire difficult. The influence of climate change on wildfire risk factors presents additional challenges in relation to long‐term planning for wildfire management (Abatzoglou & Williams, ; Dowdy & Pepler, ; Flannigan et al, ). Concurrently, changes in rural activities including the urban sprawl into wildland regions (Radeloff et al, ), as well as the “tree change” phenomenon where residents seek to escape city life by moving into rural areas, are resulting in an increasing population in many wildfire prone regions.…”

“…Research of pyroCb has primarily been centered on cases in western North America and temperate regions of Australia, regions that appear to have favorable environmental conditions for their formation (Dowdy & Pepler, ; McRae et al, ; Peterson et al, ). The conditions associated with pyroCb genesis include dry fuels and an ignition source, a dry boundary layer often with moister conditions in the midtroposphere and a steep temperature lapse rate, a middle‐upper level disturbance (which can potentially help trigger the convection in some cases), and extensive heat and moisture release from combustion (Dowdy et al, ; Peterson & Wang, ; Peterson et al, ), although the exact contribution of each of these factors is not certain.…”