A three dimensional Lagrangian analysis of the smoke plume from the 2019/2020 Australian wildfire event

Abstract

During the 2019/2020 Australian bushfire season, intense wildfires generated a rising plume with a record concentration of smoke in the lower stratosphere. Motivated by this event, we use the atmospheric wind reanalysis model ERA5 to characterize the three dimensional atmospheric transport in the general region of the plume following a dynamical system approach in the Lagrangian framework. Aided by the Finite Time Lyapunov Exponent tool (FTLE), we identify Lagrangian Coherent Structures (LCS) which simplify the three-dimensional transport description. Different reduced FTLE formulations are compared to study the impact of the vertical velocity and the vertical shear on the movement of the plume. We then consider in detail some of the uncovered LCS that are directly relevant for the evolution of the plume, as well as other LCS that are less relevant for the plume but have interesting geometries, and we show the presence of 3D lobe dynamics at play. Also, we unveil the qualitatively different dynamical fates of the smoke parcels trajectories depending on the region in which they originated. One feature that had a pronounced influence on the evolution of the smoke plume is a synoptic-scale anticyclone that was formed near the same time as, and close to the region of, intense wildfires. We analyze this anticyclone in detail, including its formation, the entrainment of the smoke plume, and how it maintained coherence for a long time. Transport paths obtained with the inclusion of the buoyancy effects are compared with those obtained considering only the reanalysis velocity.