Directional wind shear within an old-growth Douglas Fir forest: observed and modeled results

Observed and modeled estimates of directional vertical wind shear in the old-growth Douglas Fir forest at the Wind River Canopy Crane Facility for 1998 and 1999 are presented. Simulated wind field estimates were generated using the UCD Advanced Canopy-Atmosphere-Soil Algorithm (ACASA). ACASA is a vertical column model that simulates the turbulent canopy microenvironment and associated fluxes of heat, mass, and momentum using higher order closure principles. Both observed and model mean velocity profile estimates exhibit a classic Ekman-like configuration, where increasing drag within the canopy disrupts geostrophic balance. The amount of both observed and modeled turning of the mean wind field approaches 90 degrees, indicating nearly pure ageostrophic flow, in the lower canopy. Tests involving simulated alterations to the canopy structure reveal that the magnitude of directional wind shear increases with vegetation density and canopy height. These results imply that canopy architecture may play a role in influencing near-surface atmospheric convergence over sufficiently large areas.