Correlating Tree Vibration Frequencies with Turbulent Events to Interpret Gust Impressions

The primary limitation to understanding the spatial scales of canopy-driven turbulent coherent structures is the spatial measuring framework. In a forested ecosystem, the trees themselves respond directly to the wind, providing a potential data source for understanding the turbulence motions driving momentum exchange. To examine this idea, a field study was conducted in the Howland Research Forest in Howland, ME, USA.  The experimental setup included six three dimensional sonic anemometers (3 at each of two tower locations) and 149 tree sway sensors to monitor vegetative motions. All instruments were monitored at 10Hz to capture small-scale turbulent motions. A technique that uses changes in tree vibration frequency to identify the horizontal scale of eddies is presented. Measurements of winds taken at heights above, below, and within the live crown are used to identify the time-scale of coherent eddies. This time-scale is applied to measurements of tree bole motion taken simultaneously for 149 trees surrounding the main meteorological tower. Changes in dominant tree frequency were mapped over time to show the spatial size of the gust impression. Results from high and low wind speed periods will be presented and indicate that the most coherency in stand-scale motion occurs when frequency changes are mapped at the same time resolution of the dominant eddy size.