Articles | Volume 6, issue 8
Biogeosciences, 6, 1519–1537, 2009

Special issue: Processes controlling the exchange of ammonia between grassland...

Biogeosciences, 6, 1519–1537, 2009

  07 Aug 2009

07 Aug 2009

Turbulence characteristics in grassland canopies and implications for tracer transport

E. Nemitz1, B. Loubet2, B. E. Lehmann3,†, P. Cellier2, A. Neftel4, S. K. Jones1,4, A. Hensen5, B. Ihly3,6, S. V. Tarakanov6, and M. A. Sutton1 E. Nemitz et al.
  • 1Centre for Ecology and Hydrology (CEH), Edinburgh, Bush Estate, Penicuik, Midlothian, EH26 0QB, UK
  • 2Institut National de la Recherche Agronomique (INRA), UMR Environnement et Grandes Cultures, 78850 Thiverval-Grignon, France
  • 3Institute of Physics, University of Berne, Switzerland
  • 4Agroscope Reckenholz-Tänikon Research Station ART, 8046 Zurich, Switzerland
  • 5Energy Centre of the Netherlands (ECN), Petten, The Netherlands
  • 6Institute of Silicate Chemistry, St. Petersburg, Russia
  • This paper is dedicated to Bernhard Lehmann who sadly passed away during the analysis of this dataset

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.

Final-revised paper