Abstract
This study examines 95 lightning-initiated wildfires and 1170 lightning flashes in the western United States between May and October 2017 to characterize lightning and precipitation rates and totals near the time of ignition. Eighty-nine percent of the wildfires examined were initiated by negative cloud-to-ground (CG) lightning flashes, and 66% of those fire starts were due to single stroke flashes. Average flash density at the fire locations was 1.1 fl km−2. The fire start locations were a median distance of 5.3 km away from the maximum flash and stroke densities in the 400 km2 area surrounding the fire start location. Fire start locations were observed to have a smaller 2-min precipitation rate and 24-h total rainfall than non-fire start locations. The median 2-min rainfall rate for fire-starting (FS) flash locations was 1.7 mm h−1, while the median for non-fire-starting (NFS) flash locations was 4.7 mm h−1. The median total 24-h precipitation value for FS flash locations was 2.9 mm, while NFS flash locations exhibited a median of 8.6 mm. Wilcoxon–Mann–Whitney rank sum testing revealed statistically different Z-Scores/p-values for the FS and NFS flash populations. These values were −5.578/1.21 × 10−8 and −7.176/3.58 × 10−13 for the 2-min precipitation rate and 24-h total rainfall, respectively. Additionally, 24-h and 2-min precipitation rates were statistically significantly greater for holdover versus non-holdover fire events. The median distances between the fire start location and greatest 2-min precipitation rate and greatest 24-h precipitation total were 7.4 and 10.1 km, respectively.
Highlights
Lightning-initiated wildfires (LIWs) are responsible for 56% of the total acreage burned by wildfires within the continental U.S from 1992 to 2012, resulting in an average of 2.3 million acres consumed per year [1,2]
These exclusions may be due to the lightning occurring outside of the 2 km radius used to associate lightning to the wildfire, an incorrect classification of the cause of initiation, or they may be long duration holdover events that are beyond the 10 days used in this study as shown in the previous study by Schultz et al [12]
When the 30 random samples of the NFS flash database are compared with the FS flash database, Z-Scores range from 3.656 to 6.820 with an average of 5.343, continuing to indicate that the FS and NFS flash samples are from different distributions. These findings indicate that the lightning strikes causing forest fires occur in regions where 24-h rainfall is less than in NFS flash locations
Summary
Lightning-initiated wildfires (LIWs) are responsible for 56% of the total acreage burned by wildfires within the continental U.S from 1992 to 2012, resulting in an average of 2.3 million acres consumed per year [1,2]. A general characteristic of lightning flashes that ignite wildfires is a long continuing current (LCC) [3,4]. Continuing current (CC) is the length of time (longer than ~40 ms) during which charge flows through the lightning channel to the surface. +CG flashes are thought to be responsible for most natural wildfires [3,6,10,11]. Recent research indicates that 90% of lightning-initiated wildfires between 2012 and 2015 were ignited by –CG flashes [12]
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