Identification and characterization of wind storm events using tree-fall patterns

Background/Question/Methods: Windstorm events cause millions of dollars in structural damage and related losses every year. These events also cause significant damage and impact on ecological systems, which have spurred efforts to study the ecosystem resilience. The first step in understanding the windstorm impact on ecosystems is to accurately estimate the near-surface wind speeds. Due to the difficulty of obtaining in-situ and/or radar measurements of near-surface (< 10 m AGL) wind speeds in windstorm events, a promising method of estimating these speeds was developed using patterns of tree-fall. One of the essential parameters used in this method is the critical wind speed at which trees fall. There is an important relationship between this critical wind speed (and related risk) and tree properties. The information on tree properties can be incorporated in this method to assist in determining the characteristics of windstorm events.

Results/Conclusions: The near-surface wind field of an actual tornado event (Naplate, IL) was estimated using this method. During ground survey, a number of fallen and standing trees were sampled and their properties were documented. From this sampling, the critical wind speed of tree-fall and near-surface wind characteristics were estimated. The method estimated a maximum wind speed corresponding to an EF-2 tornado and the result was compared to wind speed estimated from residential houses and other infrastructure such as street signs. Other method such as estimating EF rating based on the tree-fallen percentage was also applied and compared.