Defining confidence classes for lightning discharges igniting wildfires

The projected increase in lightning-ignited wildfires (LIWs) during the 21st century highlights the need to improve our understanding of the mechanisms and processes governing these natural fires. However, results from large-scale LIW studies are often limited by uncertainty in identifying the specific lightning discharge responsible for each ignition. Here, we present a simple and flexible classification system that ranks LIWs according to the level of confidence in the lightning event causing the ignition.

We first used a probabilistic index to identify the most likely lightning event igniting each wildfire. This index was combined with a set of filters based on eight criteria, including holdover time (the time between lightning-induced ignition and fire detection) and the distance between the reported lightning location and the fire ignition point, to define four confidence classes. The lowest-confidence class applied no filters and retained all lightning events selected by the probability index (one per fire). The remaining three classes applied increasingly strict filters, yielding progressively higher confidence levels. This classification framework was applied to LIWs from four study regions: Switzerland, Catalonia (Spain), California and Nevada (United States), and the whole continental United States. In addition, two LIWs with ignitions documented by video footage were used for validation.

Relative to the unfiltered class, intermediate confidence classes retain approximately one-quarter to two-thirds of lightning discharges, whereas the highest-confidence class retains only 5-20%. This reflects a trade-off between sample size and confidence. The proposed confidence classification provides an initial framework that can be further refined, and offers a way to increase the robustness of LIW analyses, thereby supporting improved investigations of the factors controlling lightning-induced wildfire ignitions.