Quantifying Influences of Dispersive Motions on Observed Canopy Turbulence Response to Atmospheric Stability

The turbulent exchange between canopies of large roughness elements (e.g., plants, wind turbines, and buildings) and the atmosphere provides essential forcing to atmospheric dynamics and thermodynamics. Current understanding of canopy turbulence response to atmospheric stability is primarily based on field observational data and large-eddy simulation (LES) results, which are mostly in qualitative rather than quantitative agreement. For example, observational and numerical results consistently suggest increase of shear length scale (defined as mean wind over mean shear at canopy top) with increasing instability. With stability changing from near neutral to free convection, LES runs show up to 42% increase in shear length scale, whereas field observational data obtained in a forest canopy with similar leaf area index show less than 16% increase in shear length scale. The quantitative difference between numerical and observational results potentially arises from the fact that LES models do not typically account for dispersive motions induced by individual-plant-scale horizontal variability in stem and leaf distribution. Quantifying influences of dispersive motions on observed turbulence statistics requires reliable mean wind direction estimates, which is particularly difficult in the presence of vigorous velocity fluctuations for strongly unstable conditions. We apply a recently developed multi-sensor stationarity analysis technique (MSAT) to the Canopy Horizontal Array Turbulent Study (CHATS) data to obtain reliable mean wind direction estimates during stationary periods ranging from near neutral to free convection conditions. Preliminary results suggest that a 10-degree change in mean wind direction creates variation in the shear length scale that is comparable to variation induced by substantial change in stability. In support of model evaluation and theoretical development, MSAT enables interrogation of the observed canopy turbulence response to atmospheric stability variation in isolation from contamination by dispersive motions.