The TurbEFA experiment - analysing turbulence and vegetation structure across a forest clearing

Detailed knowledge of the turbulent flow and vegetation structure around flux measurement sites, especially within forests, is essential for the evaluation of flux measurements and the capacity for absorption and emission of atmospheric trace gases. The vegetation layers of forests act as storage for energy and gases and contain significant advective fluxes which cause remarkable uncertainties in the measured fluxes (Aubinet 2008; Feigenwinter et al. 2008). The direction of the total transport and the partitioning on turbulent and non-turbulent fluxes changes with meteorological conditions (e.g. radiation, wind speed and direction) but also canopy structure. The influence of the canopy structure on the fluxes is rarely investigated. To address this topic and to improve the parameterisation of unresolved exchange effects in numerical models the TurbEFA experiment was designed. TurbEFA is the acronym for the interdisciplinary project “Turbulent Exchange processes between Forested areas and the Atmosphere”, which encompasses the work of five groups with responsibilities for: terrestrial laser scanning, meteorological field measurements, wind tunnel measurements, boundary layer modelling and large eddy simulation. Subject of investigation is the FluxNet site ‘Anchor Station Tharandt' which is located about 20 km southwest of the city of Dresden in Germany (N 50_57'49", E 13_34'01", 380 m a.s.l.). From May 2008 to May 2009 intensive measurements took place to investigate the forest structure and the turbulent flow in the vicinity of the FluxNet site. The forest structure was recorded applying a terrestrial laser scanner capturing an area of 450 m × 250 m and including a forest clearing (approx. size 50 m × 90 m). The scans were filtered and combined to one point cloud to derive a 3D plant area distribution of the stands (Bienert et al. 2010; Queck et al. 2012). The turbulent flow across the clearing was recorded by Sonic anemometers at 32 measurement positions distributed at 4 towers (heights: 40m, 40m, 40m, 30m) and five ground level positions (2 m). The contribution describes the field measurements, extracts features of the turbulent flow field, and interprets them using the complex canopy structure. It also compares selected variables with results from a boundary layer model (Harmansa et al. 2012) and large eddy simulations (Schlegel et al. 2012). The relationship between wind speed, drag coefficient (C_D) and plant area density (PAD) was investigated to improve the canopy representation in numerical models (Queck et al. 2012). Beside a weak dependence of the C_D on wind speed, the observed variation of C_D could most likely be attributed to specific plant structures. Consequently, height-averaged drag coefficients also show a strong dependence on wind direction. The TurbEFA project is supported by the Deutsche Forschungsgemeinschaft (DFG) within the priority programme 1276 MetStröm: Multiple Scales in Fluid Mechanics and Meteorology. The data are part of a reference data set available at the project homepage and are currently subject to investigations of several Metström projects. One aim of this presentation is to make the dataset public and to invite modellers to further data analysis. References Aubinet M (2008) Eddy covariance CO₂ flux measurements in nocturnal conditions: an analysis of the problem. Ecol Appl 18:1368–1378 Bienert A, Queck R, Schmidt A, Bernhofer C, Maas H-G (2010) Voxel space analysis of terrestrial laser scans in forests for wind field modelling. IAPRS XXXVIII, Part 5:92–97 Feigenwinter C, Bernhofer C, Eichelmann U, Heinesch B, Hertel M, Janous D, Kolle O, Lagergren F, Lindroth A, Minerbi S, Moderow U, Mölder M, Montagnani L, Queck R, Rebmann C, Vestin P, Yernaux M, Zeri M, Ziegler W, Aubinet M (2008) Comparison of horizontal and vertical advective CO2 fluxes at three forest sites. Agric For Meteorol 148:12–24 Harmansa S, Goldberg V, Vowinkel B, Queck R, Bernhofer C (2012) Comparison of different closure schemes for an eddy diffusion model applied to a highly resolved forest clearing and an idealised forest edge. Theor Comp Fluid Dyn submitted Queck R, Bienert A, Maas H-G, Harmansa S, Goldberg V, Bernhofer C (2012) Wind fields in heterogeneous conifer canopies: parameterisation of momentum absorption using high-resolution 3D vegetation scans. Eur J Forest Res 131:165–176 Schlegel F, Stiller J, Bienert A, Maas H-G, Queck R, Bernhofer C (2012) Large-Eddy Simulation of Inhomogeneous Canopy Flows Using High Resolution Terrestrial Laser Scanning Data. Bound-Layer Meteorol 142:223–243