Energy Dissipation by Tornadoes in Heavily-forested Landscapes

Studies that characterize the role of surface roughness in the development and maintenance of tornadoes typically involve numerical simulations that represent surfaces with only modest roughness values, such as relatively smooth fields. Yet urban areas and forests present larger obstacles to tornadic winds and hence exhibit much greater surface roughness. Two 27 April 2011 long-track tornadoes through heavily-forested terrain provide a way to characterize aspects of this increased surface roughness through estimates of the drag force and the net mechanical work accomplished by the tornadoes as they toppled trees.  This first attempt at such a characterization relies on GIS analyses of aerial photos to provide forest damage severity maps and on ground surveys to provide information on tree density and the typical size structure of the tree populations.  Previous tree winching experiments provide estimates of the mass distribution and the lateral force necessary to topple a variety of trees. Damage severity measurements along 200-m segments spanning the width of each tornado track and spaced every 500 m allow reasonable estimates of the number of downed trees and, together with the information from ground surveys and tree-winching experiments, yield estimates of the force exerted by the wind and the net mechanical work accomplished within each segment. Estimates of the total mechanical work accomplished in each tornado track are compared with the yield of small, tactical nuclear weapons. These estimates may provide numerical models with empirically-grounded values that will allow a more realistic characterization of the influence of natural surface roughness on tornadoes.