Remotely Visible Width and Discontinuity of 50 Tornado Damage Paths through Forested Landscapes

Abstract

Tornado damage-path width is a necessary component for calculation of area impacted, which allows estimation of hazards. To date, rarely has variation in damage path width or path discontinuity been a focus. In this paper, using a damage threshold of >25% canopy damage, we quantify width and discontinuity in 50 tornado paths in forested areas. Tornado-path starting and end points were overlaid on Google Earth imagery obtained ≤24 months after the tornadoes, and damage-path width (or absence of damage) was measured for severities >25% canopy loss, at fixed intervals. Width was measured only where both sides of the damage path were clearly defined by forest tree damage, thus many points were excluded from our analysis. Given our threshold level of forest canopy damage, no EF0 tornadoes showed remotely visible damage, and analyses were thus restricted to ≥EF1 tornado paths. Variation in remotely visible damage width was quantified as coefficient of variation, which ranged from 0.227 to 0.852, with a mean of 0.531 among the 50 paths. Discontinuity in remotely visible damage also varied among damage paths; up to 45% of the total number of measured points within a path lacked visible damage. Almost 40% of tornado damage paths exhibited such discontinuity along 20% or more of their path length. We suggest that the long, narrow EF-scale contours (particularly for ≥EF1) often reported after storm surveys may mask extensive width variation in severe damage and substantial portions of tornado paths with no severe damage.

Catastrophic storms, forest disturbance, and the natural history of Swainson's warbler

“…Recent advances in remote sensing have made it possible to document tornado tracks in unprecedented detail, overcoming previous challenges in accurately measuring tornado forest damage (Burow et al., 2020 ; Cannon et al., 2016 ; Kingfield & de Beurs, 2017 ; Molthan et al., 2014 ; Rodríguez & Bech, 2020 ; Zenoble & Peterson, 2017 ). Case examples demonstrate the significant impact of exceptional tornadoes on forest structure.…”

“…The frequency distribution of tornadoes by EF intensity scale appears to follow a power law in which the number of EF2 tornados is approximately half the number of EF1 tornadoes and so on (Elsner, Jagger, Widen, & Chavas, 2014 ). While low intensity tornadoes (F0 and EFO) are capable of stripping foliage and breaking small branches, they seldom result in windthrow or canopy damage that is visible by remote sensing (Zenoble & Peterson, 2017 ). EF1 (F1) tornadoes can uproot or topple mature trees, while EF2 (F2) and EF3 (F3) events can snap them off.…”

“…Assuming a rectangular track, the disturbance footprint can be roughly estimated by multiplying median track length by median track width (Cannon et al., 2016 ; Fujita, 1971 ). The width of individual tornado tracks often varies (Cannon et al., 2016 ; Karstens et al., 2013 ; Zenoble & Peterson, 2017 ), with patches of severe forest damage nested within less severely damaged patches that dissolve away from the center (Cannon et al., 2016 ). The degree of forest damage within the footprint will vary, from broken branches and stripped foliage at the track margins to toppled and snapped trees near the center.…”

“…Tornado tracks often exhibit gaps (zero track width) with no severe forest damage. A study of 50 tornado tracks revealed a mean gap length of 13% of total track length (Zenoble & Peterson, 2017 ). I used this percentage as a correction factor in estimates of median footprint area for tornadoes: EF1 (35 ha), EF2 (199 ha), EF3 (1115 ha), EF4 (2439 ha), and EF5 events (9850 ha) (metadata from Elsner, Jagger, & Elsner, 2014 ).…”

“…Tornado tracks often exhibit gaps (zero track width) with no severe forest damage. A study of 50 tornado tracks revealed a mean gap length of 13% of total track length (Zenoble & Peterson, 2017). footprint for all tornadoes (197 ha) was used to estimate forest disturbance gradients.…”