Influence of antecedent soil moisture conditions on the synoptic meteorology of the Black Saturday bushfire event in southeast Australia

Abstract

The dynamics and large-scale drivers of heat wave (HW) events in Australia are well documented. However, the influence of soil moisture in modulating HWs is largely unexplored. We focus here on a recent significant HW event in southeast Australia that preceded the Black Saturday bushfires (3rd to the 7th of February 2009). During this period, the southeast of Australia experienced unprecedented warm conditions, which, in conjunction with high fuel load and mesoscale weather conditions, led to devastating bushfires. We examine how different initial soil moisture conditions with lead times of 5, 10, and 15 days prior to the event would have altered its overall dynamics at the continental scale. We show that at short lead times (5 days), the influence of perturbing soil moisture is mostly linear. Decreasing (increasing) soil moisture increases (decreases) maximum temperatures, associated with an intensification of the upper-level anticyclone. The effect of increasing soil moisture is more non-linear than decreasing soil moisture with increasing lead time; namely, increasing soil moisture can also lead to an increase in maximum temperature over some parts of the domain, rather than a decrease everywhere. At lead times of up to 15 days, the imposed perturbation in soil moisture, mostly confined to the tropics, is essentially lost such that the impact on maximum temperatures on the day of the event cannot be related to the sign of the imposed perturbation in soil moisture. Our results highlight the importance of accurate soil moisture estimates in capturing the intensity and spatial extent of HW events in southeast Australia, but only at relatively short lead times.

Amplification of Australian Heatwaves via Local Land‐Atmosphere Coupling

“…Kala et al . (Kala, Evans & Pitman, ) focused on a single event and examined the sensitivity to soil moisture conditions 15 days prior to that event. In contrast, Herold et al () used a 3‐month timescale when examining the potential role of antecedent soil moisture conditions on heatwaves.…”

“…Research on Australian heatwaves demonstrates some uncertainty on what timescale is most appropriate to diagnose a land surface contribution. Kala et al (Kala, Evans & Pitman, 2015) focused on a single event and examined the sensitivity to soil moisture conditions 15 days prior to that event. In contrast, Herold et al (2016) used a 3-month timescale when examining the potential role of antecedent soil moisture conditions on heatwaves.…”

The influence of soil moisture deficits on Australian heatwaves

“…Soil moisture, synoptic meteorology and large scale modes of variability constitute the three primary elements that influence HWs (Perkins 2015). For example, Kala et al (2015) showed that an increase in soil moisture over northern Australia can in fact lead to increases in maximum temperature in southeast Australia after 10-15 days, as the perturbation acts to change the structure of the upper-level anticyclones. However, the large local heterogeneity observed here suggests factors operating at smaller spatial scales than synoptic meteorology and modes of variability must be influential.…”

“…Nicholls and Larsen (2011) showed that daily maximum temperatures can be up to 1°C-3°C higher after droughts over southeast Australia when winds are northerly (from over the land), typical of the extreme high temperatures experienced during early February 2009 in the state of Victoria in southeast Australia. Kala et al (2015) investigated the effects of soil moisture perturbations applied 5, 10 and 15 days prior to the same HW event in February 2009 in Victoria. They showed that the influence of soil moisture perturbations is only predictable at a lead time of 5 days, with drier soils leading to higher maximum temperatures and the converse for wetter soils.…”

“…The study of Nicholls and Larsen (2011) only used temperature data at a single station in Victoria, and did not explicitly relate soil dryness to the number of HW events, as has been done elsewhere (e.g., Hirschi et al 2011). Kala et al (2015) only considered a single HW event and focussed on soil moisture up to 2 weeks before that event, rather than longterm drought. Perkins et al (2015) examined the influence of precipitation anomalies 5 months prior to the Austral summer on HW events in Australia.…”