Directional wind shear within an old-growth temperate rainforest: observations and model results

Observed and modeled estimates of wind speed and direction within an old growth temperate rainforest are analyzed and compared. The UCD Advanced Canopy–Atmosphere–Soil Algorithm (ACASA) predicts strong directional shear of the horizontal wind velocity within and above the canopy. The modeled shear is less during the day than at night, due to diurnal variations in buoyancy-controlled turbulent coupling. Diurnal patterns of observed wind direction shear depart from the predicted shear in the lowest 15% of the canopy (z/hc<0.15), likely related to local mountain/valley breezes. The modeled and observed directional wind shear has significant implications regarding the interpretation of many footprint analyses and models. Model estimates of shear are sensitive to the vertical distribution, and the total amount, of leaf area index (LAI). Tall and/or dense vegetation, especially if concentrated high in the canopy, is associated with greater integrated ageostrophic (antitriptic) flow and directional shear than for shorter and/or less dense canopies. These results have implications for the role of vegetation type and distribution on simulated convergence patterns at the regional and synoptic scales within the surface-layer. Sensitivity analyses confirm prior findings that the pressure gradient force is largely responsible for the presence of a simulated subcanopy wind speed maximum, with the direction approaching that of the antitriptic wind in the understory given a strong pressure gradient force. Results also show that second-order diabatic processes influence the shear by affecting the amount of turbulent mixing in thermodynamically stable and unstable conditions.