Turbulence characteristics in grassland canopies and implications for tracer transport

Abstract

In-canopy turbulence is a required input to study pollutant cycling and chemistry within plant canopies and to link concentrations and sources. Despite the importance of grasslands worldwide, most previous work has focused on forests and crops. Here, turbulence parameters in a mature agricultural grassland canopy were measured with a combination of a small ultrasonic anemometer, hotwire anemometry and a radon (Rn) tracer technique, as part of a measurement to study ammonia (NH3) exchange with grassland. The measurements are used to derive vertical profiles of basic turbulent parameters, for quadrant-hole analysis of the two-parametric frequency distributions of u'-w' and to derive in-canopy eddy diffusivities as input for models of in-canopy tracer transport. The results are in line with previous measurements on taller canopies, but shows increased decoupling between in-canopy flow and above-canopy turbulence. The comparison of sonic anemometry and Rn measurements implies that Lagrangian time-scales must decrease sharply at the ground, with important implications for estimating the magnitude of ground-level and soil emissions from concentration measurements. Atmospheric stability above and within the canopy has little influence on the standard deviation of vertical wind component inside the canopy. Use of the turbulence parameters in an analytical Lagrangian framework, which is here validated for heat transfer, suggests that measured in-canopy profiles of NH3 are consistent with a ground-level source, presumably from senescent plant parts, which is recaptured by the overlying canopy.