As in Fujita (1989), a common theme among these studies is the converging tree fall within the tornado damage path that results from the complex near-surface flow structure in tornadoes. Fujita noted that identifying patterns in tree fall would be difficult, if not impossible, without the aid of aerial photographs.įujita’s study is one of many to document and analyze patterns in tornado-induced tree fall (e.g., Letzmann 1923 Hall and Brewer 1959 Budney 1965 Fujita 1981 Bluestein 2000 Peterson 2003).
Differences in these patterns were primarily attributed to microbursts in close proximity to the tornado. From this analysis, Fujita identified converging and diverging tree-fall patterns within the tornado damage path. In addition to objectively determining the tornado’s starting point, ending point, length, and spatially varying width, Fujita used the vertical aerial photographs to map the generalized direction of fallen trees (i.e., windfall or tree fall) overlaid on topographic maps. This tornado traversed complex terrain on either side of the Continental Divide at elevations that ranged from approximately 2380 to 3270 m above sea level. The use of aerial vertical photographs for tornado documentation has been limited, however.Īs a rare example in which aerial vertical photography has been used in tornado-damage analysis, Fujita (1989) used aerial vertical photographs and stereo image pairs, in addition to oblique aerial photographs, to document an unusual Fujita-scale magnitude-4 (F4) tornado occurring on 21 July 1987 in Wyoming. In contrast, aerial vertical photographs (photographs taken at an angle of less than 3° from vertical) have an approximately constant scale throughout, allowing measurements to be made from the photograph for subsequent geospatial analysis.
Although aerial oblique photographs can reveal information that is not easily determined from the ground, their utility in spatial analysis is limited by difficulties in accurately determining distances. Fujita surmised that these marks were the result of multiple vortices orbiting the parent tornadic circulation, a hypothesis that has been investigated recently by numerical simulation (e.g., Lewellen and Zimmerman 2008).Īerial analysis of tornado damage has primarily utilized oblique photographs (photographs taken at an angle of greater than 3° from vertical). Aerial oblique photographs were critical in identifying cycloidal “suction” marks, or lines of debris deposition, within the damage paths of many tornadoes. Fujita and his colleagues, who used such photographs to remotely observe tornado damage, compose damage paths, and relate scouring patterns to near-surface tornado dynamics. This made it difficult to distinguish between tornado- and RFD-related damage and thus illustrates an ambiguity in ascertaining tornado-damage-path width in some locations.Īerial oblique photography has been used to document and assess tornado damage for several decades (e.g., Fujita et al. Here, the wind field was strong enough to produce tornado-strength damage well beyond the visible funnel cloud. Another distinct pattern of tree fall, likely not linked to the underlying topography, may have been associated with a rear-flank downdraft (RFD) internal surge during the tornado’s intensification stage.
These damage patterns are hypothesized to be the result of flow acceleration caused by channeling within valleys. Concentrated bands of intense tree fall, collocated with and aligned parallel to the axis of underlying valley channels, extend well beyond the primary damage path. A few distinct tree-fall patterns are identified at various locations along the Tuscaloosa–Birmingham tornado track. Results from these simulations suggest that both tornadoes had strong radial near-surface winds. A computationally efficient method of simulating tree fall is applied that uses a Gumbel distribution of critical tree-falling wind speeds on the basis of the enhanced Fujita scale. Normalized mean patterns of observed tree fall from each tornado’s peak-intensity period are subjectively compared with results from analytical vortex simulations of idealized tornado-induced tree fall to characterize mean properties of the near-surface flow as depicted by the model. In this study, aerial imagery of tornado damage is used to digitize the falling direction of trees (i.e., tree fall) along the Joplin, Missouri, and 27 April 2011 Tuscaloosa–Birmingham, Alabama, tornado tracks.
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