Characterising the holdover phase of lightning-ignited wildfires in Catalonia

Abstract

In the present work, we analysed fuel moisture conditions during ignition, holdover, and detection of flaming combustion of lightning-ignited wildfires in Catalonia (north-eastern Spain) between 2003 and 2018. First, we identified the most probable lightning candidate for each wildfire, implementing a matching algorithm between historical lightning-caused fire data and cloud-to-ground lightning records. The bulk of lightning-fire ignitions (80%) occurred between June and September during the warm season. Conifer forests concentrate almost half of the lightning-ignited wildfires. Then, we spatially interpolated air temperature and relative humidity data from automatic weather stations to calculate a weather index describing the evolution of fuel moisture content at the specific wildfire location, from the time of the lightning-caused ignition to the time of the fire detection. Results showed that fuel moisture content drives lightning-ignited wildfires since most ignitions around midday turn into flaming combustion almost immediately, when fuel moisture content reaches the minimum of the day. The holdover duration increased in late afternoon lightning-ignited fires, which remain smouldering overnight and evolve to flaming combustion in the next solar cycle. We found that latent fires above 24 h were rare (15%), and only 3% of the fires had a holdover period above three days. Only 1 in ∼840 cloud-to-ground flashes started a wildland fire. Our outcomes provide valuable insight to improve the modeling and management of natural wildfires in the Mediterranean areas.