Single and two-point statistics of turbulence and tree-sway measurements

Exchange processes of heat, water vapor and greenhouse gases between forests and the overlying atmosphere are driven primarily by turbulence in the canopy roughness sublayer (CRSL). Honami (cereal waves) is an observed phenomenon signifying organization of gusts in plant canopies on windy days. These large coherent gusts dominate turbulence dynamics in plant canopies. Previous studies over short crops confirm there is a mutual relationship between plant motions and turbulence gusts, and the movements of plant stalks have significant impacts on CRSL turbulence. However, group tree-sway has not been formally considered in most CRSL turbulence models of forest-atmosphere exchanges. In addition, forest structure, morphology, and the aeroelasticity of trees are significantly different from short crops.

Results presented here are based upon simultaneous measurements of a large array of tree-sway motions and CRSL turbulence in the summer of 2009 at an AmeriFlux forest site in Howland, ME. One overall goal of this unprecedented field experiment is to quantify the spatial and temporal characteristics between CRSL coherent gusts and group tree-sway. Preliminary analyses indicate patterns of tree sway movement highly correlate with CRSL coherent gusts. Time series and integral time scales show tree sway and turbulence gusts are on the same order of magnitude (~ 1 min). In addition, there is an identifiable spatial pattern to gust-driven tree-sway, which indicates group tree-sway patterns may be used to delineate the spatial and temporal characteristics of coherent gusts. Single-point and two-point statistics of both tree sway and turbulence measurements (10 Hz) are analyzed under a range of atmospheric conditions (stability, wind direction, wind speed). These results are compared with previous observations of honami over short crops.