Dry Intrusions and Their Role in Extreme Wildfires

A multitude of factors influence the size and intensity of a wildfire and the extensive damage it will cause. Among these are the landscape features, soil moisture and type and amount of vegetation. Atmospheric conditions such as heat waves, ‘correct’ wind speed, wind direction and changes in the wind direction, as well as atmospheric instability are key factors in the spread of the wildfire.

Dry Intrusions are slantwise descending airstreams that descend over 400hPa in 48 hours. They form as part of mid-latitude cyclones bringing to the surface dry upper tropospheric air and may bring to the surface either a cold or warm temperature anomaly. Dry Intrusions are known to destabilize the lower troposphere, lead to stronger winds at the surface and have been found to strengthen the cold front in cases of extreme wildfires in Australia.

In this work we globally analyze the co-occurrence of large wildfires and Dry Intrusions, using a Global Fire Atlas (GFED, Andela et al., 2019) and a calculated Dry Intrusion trajectory database based on ERA5 reanalysis (Hersbach et al., 2020). We find that in fire prone areas of the world there is a strong connection between the most extreme wildfires and the occurrence of Dry Intrusions. We examine this relation in different regions in Europe, Asia, America and Australia, areas in which the frequency of Dry Intrusions vary, and show that the connection is season dependent. Identifying processes that influence the spread of wildfires can improve their current forecast and future predictions in a changing climate.