Lower atmospheric drying, stability, and increased wildfire activity

In several recent notable wildfire events in Australia, abrupt and extreme decreases in near-surface relative humidity were observed near the time that the fires showed dramatically enhanced activity. Given that at least two of these events occurred on days when the wildfires led to loss of life and significant property and infrastructure damage, understanding the causes of, and predicting, these events is an important facet of fire weather forecasting. In this paper I will illustrate that one class of such drying appears to be associated with dry middle-tropospheric air being mixed to the surface, and that these dry areas can be identified in water-vapour imagery some hours prior to the surface effects being realized. It is of note that dry slots in water-vapour imagery have been linked with intrusions of low equivalent potential air into the middle troposphere, and thus tend to be associated with decreased atmospheric stability – a factor that has also been associated with increased fire activity through the Haines index and other studies.

There are other mechanisms that also have been identified as associated with local extreme surface drying in Australian fire studies – the vertical circulations associated with dry cold fronts, and the interactions of nocturnal low-level jets with mountainous terrain are two that have been identified.

These case studies have prompted a larger-scale project to identify these marked drying episodes in the meteorological record, with the aim of developing a broader understanding of, and an assessment of the predictability of, such events.