Characterizing Rainfall and Land Surface Conditions Associated with Lightning-Initiated Wildfires

Abstract This study examines 11 years of lightning-initiated wildfires (LIWs) that burned 1000+ acres over the conterminous United States spanning 2010–20 to identify the associated rainfall and land surface characteristics. The U.S. Forest Service database is used to examine LIWs along with Multi-Radar Multi-Sensor 1-km resolution hourly rainfall, a NASA Land Information System observations-driven simulation on a 3-km grid for historical soil moisture, 4-km resolution satellite-derived green vegetation fraction, and 5-km resolution evaporative stress index (ESI) of live vegetation health to examine land surface conditions surrounding LIWs. The study objectives are to quantify antecedent land surface conditions, compare how conditions vary between wildfires reported the same day (immediate) versus smoldering LIWs reported one or more days after ignition (holdover wildfires), and determine rainfall and land surface characteristics that favor LIWs. Composite results indicate that a steady decline in antecedent soil moisture occurred for the 3 months leading up to LIWs, with similar but not as pronounced declines in evaporative demand given by ratios of evapotranspiration (ET) to potential ET (PET) and ET/PET anomalies (ESI). Rain rates were considerably lower at flash locations igniting wildfires compared to noninitiating flash locations ( p = 100%). Additionally, rainfall tended to be highly associated with holdover LIWs than in immediate LIWs. Antecedent soil moisture and ET/PET were incrementally higher preceding holdover wildfires. Shallow soil moisture was modestly drier (wetter) for lightning flashes igniting wildfires (holdover wildfires), deep soil moisture was slightly wetter in LIWs, but few statistically significant differences were found in the ET/PET and ESI results. Significance Statement This study examines the importance of antecedent land surface properties in conjunction with rain rates associated with lightning-initiated wildfires in the United States. Steadily declining soil moisture and evaporative stress of live vegetation preceded lightning-initiated wildfires. Rain rates were the most important factors in determining whether lightning will ignite wildfires; however, drier surface soil moisture and slightly wetter deep soil moisture occurred at lightning flash locations that initiated wildfires. Rain rates and soil moisture were notably higher for holdover wildfires that were identified at least a day or more after lightning ignition, whereas vegetation was less stressed on average in holdover wildfires.