Precipitation associated with lightning-ignited wildfires in Arizona and New Mexico

Abstract

From 1990 to 1998, over 17000 naturally ignited wildfires were observed in Arizona and New Mexico on US federal land during the fire season of April through October. Lightning strikes associated with these fires accounted for less than 0.35% of all recorded cloud-to-ground lightning strikes that occurred during the fire season during that time. Given the high aridity of this region, why do some lightning strikes ignite fires and others not? Natural wildfire ignitions in this region are often attributed to what is referred to as ‘dry’ lightning, or lightning with little or no precipitation. This study used daily and hourly gridded precipitation derived from historical gauge data to compare the amount of precipitation associated with natural wildfires and the amount of precipitation associated with lightning strikes that were not associated with natural wildfire events. Climatology of natural ignitions tend to peak before the time of the seasonal maximum lightning flashes and precipitation, suggesting ‘dry’ ignitions in the early part (e.g. late June, early July) of the wildfire season. Observed natural wildfires were more often associated with conditions of <2 mm of precipitation on the day of the event than were cases without an associated ignition. The majority of lightning flashes that did not cause a discovered wildfire were associated with a higher precipitation rate after the timing of the event, which suggests the possibility of the precipitation extinguishing the wildfires before discovery. Most lightning flashes that were not associated with a discovered ignition had twice as many hours with some measurable precipitation than where there were discovered ignitions, which eludes to the longer-term impact of preceding precipitation on the fuels than the lightning storm-associated precipitation. These results can be applied to gridded forecasts of amount of precipitation to indicate areas where there is an increased probability of natural wildfire ignition, assuming the other factors of ignition source and fuel availability are present.