Tree-sway frequency and the turbulent co-spectral gap in a dense canopy environment

摘要

Today, a great deal of the knowledge collected concerning canopy turbulence comes from merely field observations of movement. However, measurements or field observations that are taken at a specific location within the canopy cannot accurately capture the interaction of the wind and the canopy elements that it comes across. Without this complete picture of the atmosphere, the temporal fluctuations that exist in turbulent flows cannot be understood. From an atmospheric perspective, the dynamic and complex build of forested ecosystems has significant influences on turbulence structures which exist in the atmospheric boundary layers (ABL) by consistently imposing both mechanical and thermal forces. The temporal and spatial characteristics of tree-sway motions and their aerodynamic interactions with coherent turbulence wind fields in a forest (Howland Forest, ME) are examined. Flux calculations from tower data, such as in this experiment, require the researcher to choose a timescale to define fluctuations, however since the atmosphere typically contains motions and coherent vertical transports on a multitude of timescales; the selection of it is not always straightforward. This study will aid in answering whether or not there is a correlation between average stem sway frequency and the turbulent co-spectral gap in a forest environment. Results were achieved by using a multi-resolution decomposition (MRD) technique to find a day-time specific time scale and then examining the tree's frequencies at that time scale through a Fourier transform to determine if MRD can find an appropriate time scale of coherent motions. Through a mapping comparison, the sub-mesoscale motions of a canopy atmosphere and their effect on the tree's movement as well as fluxes of energy were better understood. Less coherence was seen when examining motion on time scales greater than the co-spectral gap, which would include meso and sub meso scale motions. Frequency proved to be a good variable for mapping the wind. Overall this work highlights the usefulness of dynamic maps for displaying rapidly changing spatial patterns. This will eventually lead to incorporating canopy motion physics into bigger climate models, as well as providing an explanation to the uneven calculations of many natural cycles, such as the carbon flux cycle.