Tracing the formation of exceptional fronts driving historical fires in Southeast Australia

Abstract

Extreme cold fronts are closely associated with the spread of large wildfires in Australia. The strength of a front is determined by the drop in temperature across it, which will in turn be determined by the warm and cold temperature anomalies on either side of the front. Here, we examine the Black Saturday and Ash Wednesday fires in southeast Australia through a Lagrangian decomposition framework, exploring the origin of the potential temperature anomalies that formed these extreme cold fronts. We identify the contributions of three processes: an initial anomaly at the origin, adiabatic transport of climatologically different air, and diabatic heating along the air-parcel trajectory. We find that on both sides of the cold front descending trajectories contribute to the extreme anomalies. In the warm sector, positive anomalies arrive with descending trajectories from the Indian Ocean. In the cold sector, negative anomalies are dominated by strongly descending dry intrusions forming as part of the cyclonic system. The dry intrusions advect colder air, overcompensating for its adiabatic warming during its descent. Identification of the precursors and the mechanisms contributing to extreme cold fronts associated with large wildfires can improve the forecast of such events and help evaluate them in future climate projections.

Decomposing the role of dry intrusions for ocean evaporation during mistral

“…DIs usually originate below the tropopause, upstream of an upper-level trough, and descend slantwise towards the cold sector of the surface cyclone. These dry and cold airstreams interact with the surface and generate a wide range of weather extremes, ranging from wind and precipitation compound extremes (Raveh-Rubin & Wernli, 2015, 2016 to dust storms (Fluck & Raveh-Rubin, 2023a, 2023b, temperature extremes (Klaider & Raveh Rubin, 2023), and the spread of wildfires (Magaritz-Ronen & Raveh-Rubin, 2023). DIs were shown to enhance vertical mixing in the boundary layer (Ilotoviz et al, 2021), and enhance ocean evaporation (Rai & Raveh-Rubin, 2023;.…”

Extended Influence of Midlatitude Cyclones on Global Cold Extremes

“…Cold air advection is known to govern regional cold extremes (e.g., Bieli et al., 2015). Recently, the global Lagrangian‐based quantification by Röthlisberger and Papritz (2023) provided a climatological perspective on the mechanisms driving the cooling of the airmasses comprising near‐surface extremes. They have shown that advection is a main contributor to near‐surface extremes and hypothesized that DIs are related.…”

“…Compared to near‐surface equatorward flow, DI airmasses are not heated diabatically by surface fluxes. Rather, DI air initiates in anomalously cold regions in the upper‐troposphere and is further cooled diabatically, so despite their adiabatic warming during their descent, the cold anomaly is often maintained all the way to the surface (Magaritz‐Ronen & Raveh‐Rubin, 2023; Raveh‐Rubin, 2017; Röthlisberger & Papritz, 2023). Thus, unlike purely vertical adiabatic descent (resulting in warm anomalies), the slantwise‐descending DIs often induce cold anomalies (Ilotoviz et al., 2021).…”