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

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.