The Meteorology of the Tathra Bushfire

Abstract

The meteorological conditions over the South Coast of New South Wales, Australia, are investigated on 18 March 2018, the day of the Tathra bushfire. We present an analysis of the event based on high-resolution (100-m and 400-m grid-length) simulations with the Bureau of Meteorology’s ACCESS numerical weather prediction system and available observations. Through this analysis we find several mesoscale features that likely contributed to the extreme fire event. Key among these was the development of horizontal convective rolls, which emanated from inland and aided the fire’s spread toward Tathra. The rolls interacted with the terrain to produce complex regions of strongly ascending and descending air, likely accelerating the lofting of firebrands and potentially contributing to the significant lee slope fire behaviour observed. Mountain waves, specifically trapped lee waves, occurred on the day and are hypothesised to have contributed to the strong winds around the time the fire began. These waves may also have influenced conditions during the period of peak fire activity when the fire spotted across the Bega River and impacted Tathra. Finally, the passage of the cold front through the fireground was complex, with frontal regression observed at a nearby station and likely also through Tathra. We postulate that interactions between the strong prefrontal flow and the initially weak change resulted in highly variable and dangerous fire weather across the fireground for a significant period after the change initially occurred.

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

“…Although this conceptual model is based on eastern Victorian events, the physical ingredients of direction of the synoptic wind flow, the orientation of the topographic barrier and the orientation and distance from the topographic barrier of a coastline would also generate complex wind changes in other regions. Wilke et al (2022) point to complex variations in wind speed and direction at AWS locations on the southern NSW coast during Black Summer, and Mills (2007) also documents interactions between synoptic-scale cold fronts and the topography in south-east NSW leading to geographically varying wind change behaviour.…”

The Destructive Sir Ivan Fire in New South Wales, Australia; Simulations Using a Coupled Fire—Atmosphere Model

“…Boundary layer rolls (counter-rotating vortex rolls in the atmospheric boundary layer that develop parallel to the direction of the flow) are evident in the wind speed as faster and slower bands (the banding is also present in wind direction but not shown), with the simulated roll distribution comparable to the orientation of the convective cloud seen in Figure 8. The boundary layer roll phenomenon is a feature of hot windy days (see, for example, [22][23][24]). Figure 7 shows wind speed variability of up to 10 m s −1 due to boundary layer rolls within 10 km of the fire, with the range increasing to 4-20 m s −1 (>50 km h −1 ) adjacent the fireline in response to fire-modified winds.…”