Analyzing Sweep-Ejection Momentum Transfer in a Forest Canopy

Understanding of intermittent turbulence interactions in the canopy roughness sub-layer (CRSL) is crucial for assessing energy and matter transfer to and from these environments. Different heights throughout a canopy may experience varying intensities of turbulence as changes in the velocity field and drag from foliage impact momentum. Pressure, in relationship with temperature, is concomitant to turbulence development as mechanical or thermal forces drive their frequency. While these processes happen on small temporal scales, deriving a longer term climatological analysis is crucial for defining potential relationships between pressure and turbulence. This study examines the relationship between surface level pressure on intermittent turbulence via sweeps and ejections within an evergreen forest canopy near Howland Forest, Maine. We examine one year of sonic anemometer observations to calculate periods of sweep dominant flow, high momentum propagating into the canopy (+w’), and periods of ejection dominated flow, strong momentum propagating out of the canopy (-w’), for longer than two minutes. These periods of strong sweeps and ejections are cross-examined with the magnitude of pressure throughout three canopy levels (e.g. below, mid, and top) for potential variations, along with a frequency and lag analysis of sweep-ejections based on pressure fluctuations.