Rapid Surface Drying During the Black Summer Bushfires in Australia: Insights From High-Resolution Simulations

Abstract

This study investigates the meteorological processes that contributed to rapid drying at the surface and extreme wildfire conditions (forest fire danger index >100) in southeastern Australia between 29 December 2019 and 2 January 2020. This period was part of the so-called Black Summer 2019/2020. The underlying physical mechanisms and the origin of the near-surface dry air are explored with simulations from the Unified Model (UM) l with 2.2-km horizontal grid spacing and with back trajectories using winds from the UM and ERA5. We find that a near-surface dry region forms prior to the passage of a strong front. The near-surface dry air originates in the upper atmosphere over the Indian Ocean and Southern Ocean, before circulating around an anticyclone, descending as a dry slot onto a deep mixed layer, and finally rapidly reaching the surface through convective rolls. A second dry region forms after the front passes. Again, much of the dry air originates in the upper atmosphere near Antarctica, before descending to the top of the boundary layer and forming a postfrontal dry slot. The dry air then mixes rapidly to the surface. These findings have potential applications for fire weather forecasting and fire management.