Reconstruction of Near-Surface Tornado Wind Fields from Forest Damage

Abstract

Tornado intensity is usually inferred from the damage produced. To foster postevent tornado intensity assessments, the authors present a model to reconstruct near-surface wind fields from forest damage patterns. By comparing the structure of observed and simulated damage patterns, essential parameters to describe a tornado near-surface wind field are derived, such as the ratio G max between circular and translational velocity, and the deflection angle a between peak wind and pressure gradient. The model consists of a wind field module following the Letzmann analytical tornado model and a tree module based on the mechanistic HWIND tree model to assess tree breakage. Using this method, the velocity components of the near-surface wind field, the track of the tornado center, and the spatial distribution of the Fujita scale along and across the damage path can be assessed. Necessary requirements to apply the model are knowledge of the tornado translation speed (e.g., from radar observations) and a detailed analysis of the forest damage patterns. One of the key findings of this analysis is that the maximum intensity of the tornado is determinable with an uncertainty of only (G max 1 1) times the variability of the usually well-known tornado translation speed. Further, if Letzmann's model is applied and the translation speed of the tornado is known, the detailed tree model is unnecessary and could be replaced by an average critical velocity for stem breakage y crit independent of the tree species. Under this framework, the F3 and F2 ratings of the tornadoes in Milosovice, Czech Republic, on 30 May 2001 and Castellcir, Spain, on 18 October 2006, respectively, could be verified. For the Milosovice event, the uncertainty in peak intensity was only 66.0 m s 21 . Additional information about the structure of the near-surface wind field in the tornado and several secondary vortices was also gained. Further, this model allows for distinguishing downburst damage patterns from those of tornadoes.

Tornado Intensity Estimation: Past, Present and Future

“…However, even with an established relationship, differences still are likely in ratings when compared to that of the damage-based techniques such as Alexander and Wurman (2008) have demonstrated. Another relationship may have to be established between DI-based tree damage rating method and the treefall pattern, consistent with the method discussed by Beck and Dotzek (2010) (see Fig. 6).…”

“…4). In fact, a technique to correlate observed tree fall patterns to the tornado wind field shape and intensity has been under development by Beck and Dotzek (2010). This differs from the methodology currently employed by the EF scale guidance owing to its treatment of trees in large numbers rather than individual DIs.…”

Draft final report, National Institute of Standards and Technology (NIST) Technical investigation of the May 22, 2011 tornado in Joplin, Missouri

“…As mentioned in Beck and Dotzek (2010) and by Peterson (2003) a limited number of studies have been performed on specific types of trees to test their resistance to wind. In addition, since most studies are undertaken in forests, it is difficult to apply them to tree falls in urban settings due to a number of factors including root growth, spacing between trees, and varying numbers of species (Peterson, 2003).…”

“…therefore has not been taken into account in this study. Due to these uncertainties, an average critical tree fall wind speed (i.e., the average wind speed when trees fall) as suggested by Beck and Dotzek (2010) was used in construction of the Joplin tornado wind field model.…”

Central European tornado environments as viewed from a potential vorticity and Lagrangian perspective

“…The public ESWD web interface at URL www.eswd.eu encourages submission of reports from all over Europe and the Mediterranean region. The input to the ESWD by the public and the ESWD maintenance has led to a large increase in reports over the last years, though some remarkable cases, such as the 2006 F2 Castellcir tornado in Spain (Aran et al, 2009;Beck and Dotzek, 2010), are not present in the ESWD. For further details on the ESWD and in particular its quality, see Dotzek et al (2009).…”