Windblown trees as a palaeoclimate indicator: The character and role of gusts

Regional studies of the fall-directions of windblown trees preserved in peals, lignites and coals offer the possibility of widening knowledge of palaeowind fields beyond arid regions to areas where precipitation exceeded evaporation. For the successful interpretation of tree fall-directions, however, an improved understanding is required of tree-wind interactions and of the sources of variance in regional patterns of fall. The distribution of tree fall-directions in a swathe created during the Great Storm of October 1987 in southeast England provides the first direct and detailed evidence for the structure and role in tree-damage of canopy-scale coherent vortices (with a gust part) in the atmospheric, rough-wall, turbulent boundary layer. The fall pattern appears to confirm the inference, based so far on purely meteorological evidence, that the vortices, in which the strongest part is described as a gust, combine features of Townsend's double-roller model of turbulence with Theodorsen's concept of the horseshoe vortex. The gust part, as registered by the spread of tree fall-directions in the swathe analysed, appears to contribute 20-25% of the total variance of tree fall-directions due to storms over a region in a geologically significant interval of time.